ROAD RESEARCH LABORATORY

Ministry of Transport

RRL Report LR 346

SELECTION OF M A T E R I A L S FOR SUB-SURFACE DRAINS by

R. Spalding, B.Sc.

- 4 ,

Climate and Environment Section

Road Research Laboratory

Crowthorne, Berkshire

1970

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on l S t April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

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A b s t r a c t

1; I n t r o d u c t i o n . : . ~ ~ , . . . . ~ - : : . . . . ;:-....-

2 , R e q u i r e m e n t s . o f S u b - s u r f a c e D r a i n s . ~ . : , ; . . . . .-

2 , 1 F r e e d r a i n a g e o f g r o u n d - w a t e r - ' " " ~

2 , 2 Long l i f e ~ . . . . : . . . . "

2 , 3 Summary o f . r e q u i r e m e n t s f o r p i p e s a n d b a c k f i l l

3. Discussion of Design Criteria ..... :

3.i Filter action of the backfill :

3.i.i Theoretical 'basis of filtratfon Criteria

3.1.2 Results available

3.i.3 Selection of reliable criteria

5.1.4 Filter criteria for silty soil

, 5.1.5 Filter criteria for clay soils

3.i.6 Standard filter gradings "

3.1.7 Design for gap-graded soils

3.2 Permeability of the backfill

3.3 Drain pipe criteria

3.3.1 Present criteria for hole size

3.3.2 Recent research on hole size

3.3.3 The effect of backfill compaction on hole size

3.3.4 Permeability considerations

3.3.5 Pipe diameter and length

4. Conclusions and Recommendations

4.1 Filtration and permeability criteria for backfill

4.1.1 Filtration criteria

4.1.2 Permeability criteria

4.1.3 Filters for silt and clay soils

4.2 Drain, pipe design criteria

4.2.1 Hole size

4.2.2 Porosity and pipe diameter

5, F u r t h e r R e s e a r c h

6 , A c k n o w l e d g e m e n t s

7, R e f e r e n c e s

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CONTENTS (Contd)

Appendix 1

8.1 Uni t ed S t a t e s Waterways Experiment S t a t i o n t e s t s

8 . 1 . 1 De te rmina t ion o f l i m i t i n g s i z e o f f i l t e r m a t e r i a l

8 .1 .2 Tests o f f i l t e r s over pipe openings

8 . 1 . 5 F u l l s c a l e t e s t s on va r ious types of p ipe

8.2 Uni ted S t a t e s Bureau of Reclamation t e s t s

8.5 American Railway Engineers Assoc ia t ion t e s ~ , s

8.4 Un i t ed S t a t e s Waterways Experiment S t a t i o n and Ohio River D i v i s i o n Labora to r i e s t e s t s

Appendix 2 , Assessment o f F i l t e r Design c r i t e r i a

9 . 1 Newton ~ Hurley c r i t e r i a

9 ,2 Waterways Experiment S t a t i o n c r i t e r i o n (1941)

9 .5 Corps o f Engineers c r i t e r i a

9,4 Uni t ed S t a t e s Bureau of Reclamation c r i t e r i a

9.5 S tandard Dev ia t ion des ign method

9.6 Uni ted S t a t e s So i l s and Paving Laboratory D e s i g n c h a r t

9.7 Modi f ied S o i l s and Paving, Laboratory Method

Appendix 3 - F i l t e r des ign examples

Appendix 4.- Calculation of flow in drains

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Q CROWN COPYRIGHT 1970

Extracts from the text may be reproduced provided the source is acknowledged

SELECTION OF MATERIALS FOR SUB-SURFACE DRAINS

ABSTRACT

The report discusses the part played by the various components of a sub-surface drain in achieving ade- quate permeability and in preventing clogging of the drain by silt. Present criteria for the grad- ing limits of the backfill and the porosity of the pipes in a drain are reviewed, and conclusions are reached as follows:-

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(i) l~ne most reliable criteria for backfill in sands and gravels are those of the United States Waterways Experiment Station, which are based on ratios of the particle sizes of the backfill and the surrounding soil;

(ii) For drains in silts and clays, B.S.882, zone 2 concrete sand is recommended as a backfill;

[iii) Maximum pipe hole sizes should be equal to the 85% size of the backfill for circular holes, and the widths of slots should be slightly less.

More experiments are needed to confirm the filter design chart proposed by the Waterways Experiment Station.

Further research is suggested to determine the number and size of holes required in a pipe, pipe rough- ness values, and also the effect of compaction on the allowable hole sizes in piPes.

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i. INTRODUCTION

"The importance of good sub-surface drainage for roads in our climate is well known, and sub-surface drains are provided on most roads in Britain. The cost ofsuch drainage is in the region of £2OOO per kilometre for a sub-surface drain consisting of a ISO mm pipe laid i metre below ground level. Doubling the depth of the drain approximately doubles the cost. Furthermore, the road can suffer extensive damage, which may not be immediately apparent, if a drain fails to conduct water properly.

I t i s t h e r e f o r e d e s i r a b l e to u n d e r s t a n d c l e a r l y t h e f a c t o r s which a f f e c t t h e c h o i c e o f p i p e s and b a c k f i l l f o r a s u b - s u r f a c e d r a i n , and t o have r e l i a b l e d e s i g n c r i t e r i a f o r t h e m a t e r i a l s . Th i s r e p o r t i s i n t e n d e d

to meet these needs. It summarises the functions of a sub-surface drain, reviews literature on the subject¢ and suggests desigTl criteria for construction. It also indicates some aspects of sub-surface drainage requiring further research.

2. REQUIREMENTS OF SUB-SURFACE DRAINS

The sub-surface drains used in road construction usually consist of a pipe embedded in a backfill of permeable filtermaterial in the bottom of a trench. The upper part of the trench is filled by returning part of the original soil. Alternatively, filter material may be extended nearly to surface level, and the trench sealed with turf or impervious soil (see Fig. 1 (A)). Sub-surface drains are used either to lower a fairly static water table or to intercept a sub-surface flow of water.

It is importantto distinguish between sub-surface drains, as described above, and combined surface and sub-surface drains which are also widely used for road drainage.

The combined drain is primarily intendedto dispose of fairly large amounts of dirty run-off water from the road and verges. It is back- filled with a coarse permeable material, which is exposed at surface level (see Fig. 1 (B)). The large pores in the backfill are intended to allow the silty water to pass straight into the pipes without clogging the drain. Silt is removed from the drain by means of traps in the pipe. The combined drain will also intercept some sub-surface water, and can become clogged by silt from the surrounding soil washing into the back- fill. Because of the large amounts of silt entering such drains the backfill should be cleaned or replaced every five to seven years.

The sub-surface drain should remain permanently free from silt blockage, and so a fine grained backfill (or filter) is provided to hold the surrounding soil in place.

This report deals with design criteria fop sub-surface drains but some of the principles discussed can be applied to combined surface and sub-surface drains.

The basic characteristics required of any sub-surface drain are:-

i. Free drainage of groundwater,

2. Long life, i.e.

(i) it must not clog ' .

and (ii) it must not be disrupted.

The factors involved in achieving these characteristics are discussed below.

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2 . 1 F r e e d r a i n a g e o f g r o u n d w a t e r

Each p a r t o f a s u b i s u r f a c e d r a i n should c o n t r i b u t e to t h e e f f i c i e n t removal o f g roundw a t e r .

The purpose of thebackfill is to enhance the interception ability of the drain, and to provide easy passage of water to the drain pipe. The pipe should collect the water intercepted by the backfill, and conduct it away.

The least permeable part of a drain limits the rate at which water can flow from the surrounding soil. Insufficient permeability in any part of a drain will affect the speed with which the water table is lowered. It will also increase the time taken by the drain to dispose of percolating rainwater, causing temporary high water tables.

To obtain free drainage of a soil, the pipe s and backfill of a drain must be designed to conduct the maximum flow of water likely to move through the soil. This consideration dictates the permeability of the backfill, and the porosity and diameter of the pipe to be used in a sub- surface drain.

2.2 Long life

The life of a sub-surface drain can be limited by clogging. Clogging occurs when a water flow carries fines from the soil around the drain into the backfill and pipe. In time the backfill and pipe become partially or even completely blocked, usually by silts, and fail to conduct water properly. Clogging of the drain can be prevented by providing a backfill that will not allow migration of the soil particles.

A s u i t a b l e b a c k f i l l has p o r e s too smal l to admit t h e l a r g e r s o i l g r a i n s , so t h a t as w a t e r moves i n t o t h e d r a i n , t h e l a r g e r p a r t i c l e s in the s o i l become lodged ove r the po res in t h e b a c k f i l l . The e f f e c t i v e s i z e o f t h e b a c k f i l l p o r e s i s t h e r e b y r educed and s u c c e s s i v e l y f i n e r p a r t i c l e s in t h e s o i l a r e s u p p o r t e d . In t h i s way a zone i s b u i l t up n e a r t he s i d e s o f t h e b a c k f i l l t h a t p r e v e n t s t h e m i g r a t i o n o f most s o i l p a r t i c l e s . B a c k f i l l t h a t suppo r t s a S o i l in t h i s way i s c a l l e d a f i l t e r m a t e r i a l .

In practice, filter materials may allow a certain amount of silt migration, and if incorrectly designed, may well become less permeable than their surroundings. Such reductions in permeability will, however, be comparatively small in a properly designed filter, and will not seriously affect the efficiency of the drain.

The soils most likely to cause clogging of a sub-surface drain are coarse silts and fine sands, Such materials have pore sizes large enough to permit fairly high flow velocities, while their particles are small enough to be transported. Coarser soil particles (e.g. medium and coarse sands) are too large to be readily transported by groundwater flow. Finer soils (such as fine silts and clays) are less permeable, and the velocity of flow in such soils is usually insufficient to cause excessive particle migration.

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Where a drain runs through clay soils the protective action of the backfill is somewhat different from that in silts and fine sands. Firstly, particles are less likely to wash out of clay soils, because of the cohesive forces in the soil, and the low flow velocities involved. Secondly, the cohesion between the clay particles allows only groups of particles to wash away from the soil round the drain. The pore sizes in the backfill may therefore be designed to exclude aggregates of clay particles, rather than individual grains.

Another cause of failure in sub-surface drains is disturbance of either the filter material, or the drain pipe.

The filter material in a drain may be washed through the holes in the drainage pipe. As a result the filter subsides, and the pipe may become blocked either by the backfill material itself, or by soil pene- trating the disturbed filter. To prevent failures of this sort, the size of the holes in the walls of a drainage pipe must be related to the particle size of the filter material.

Failure of a drain may also be caused by dislocation of the pipe due to uneven settlement of the backfill. The water flowing in the pipe can then erode the filter material, leading to subsidence and eventual clogging of the pipe. Settlement of the pipe can be prevented by adequate compaction of the backfill.

2.3 Summary of requirements for pipe and backfill

The main concern of sub-surface drain design is to find a backfill permeable enough to allow easy drainage , yet fine enough to act as a filter.

The drain pipe used must be pervious enough to accept all the ground- water entering the drain, and have perforations small enough to prevent serious backfill infiltration.

3. DISCUSSION OF DESIGN CRITERIA

In order to find reliable design criteria for sub-surface drainage materials, a literature review of experimental investigations and design recommendations was made. Many different criteria were discovered for every aspect of drain desig n .

To assess which of the criteria is the most satisfactory, the various recommendations are discussed below and compared with experimental results. The discussion is divided into three sections, corresponding to the three main requirements of a sub-surface drain. The sections are:

I. Filter action of the backfill

2. Permeability of the backfill

3 Drainpipe diameter and porosity

Criteria con~rollingeach of these aspects of drain design are examined and compared in t u r n .

3.1 F i l t e r aCtiOn Of the b a c k f i l l

3.1.1 The0retical~basis of'filtration criteria. The aim of filter design is to ensure that the pores in the filter are fine enough to prevent the migration of coarser soil particles, which will support the soil mass. Filter desig n criteria therefore need to relate the pore size of the backfill to the particle sizes of the soil around the drain.

C. Terzaghi was the first to formulate design criteria on this . basis. He assumed that the eighty-five percent size of the soil (D85S) * was representative of its larger particles, and that the fifteeh percent size of the filter (DISF)* was a measure of its pore size. Terzaghi stated that provided DISF ~ 4 x D85S, the base soil would not wash into the filter. Most of the more recent filter design criteria are based on Terzaghi's ratio.

Terzaghi's ratio involves severalsimplifying assumptions. For example, the use of the fifteen percent size of the filter as a guide to its pore sizes takes no account of the shape of the filter grading curve, which will affect the pore size distribution.

Similarly, the eighty-five percent size of the base soil is only a rough measure of the size of the larger soil particles. The number of particles of similar magnitude will obviously affect the operation of a filter, and hence some accountshould be taken of the grading of the base soil. The shape of particles in the soil and filter will also affect the filtration process.

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It is difficult to assess the effect of all these variables on ~:: filter performance. More recent investigations tend to take account of more and more of the factors.mentioned above.

3.1.2 ReSUlts available. No single comprehensive series of filter tests has ever been undertaken. However, several investigators have conducted tests on particular ranges of materials.

The earliest filter tests are those of Newton and Hurley I (1938) who tested graded base soils with fairly uniform filters. Next Bertram 2 (1940) carried out tests on uniform materials. The United States Waterways Experiment Station 3,4,5 (1941, 1948, 1953) then began a series of investigations into filter materials, using mainly fairly uniform base soils. Finally, the United States Bureau of Reclamation 6

*The eighty-five percent size of the base soil, or D85S, is used to des ignat:6 the size of the sieve that allows eighty-five percent by weight of the base soil to pass through it. Similarly, DISF designates the size of sieve that allows fifteen percent by weight of the filter material to pass through it. Particle sizes smaller than the 75~m sieve refer to Hygrometer analysisresults. 5

(1947) c a r r i e d ou t t e s t s u s i n g a w i d e r v a r i e t y o f f i l t e r and s o i l g r a d i n g s . D e s c r i p t i o n s o f t h e B u r e a u ' s t e s t s , and t h o s e o f the Waterways Exper iment S t a t i o n in 1941 a r e g i v e n i n Appendix 1, be ing t y p i c a l o f the e x p e r i m e n t a l methods u~ed .

Each series of tests gave rise to design criteria, and a summary of the various recommendations is given in Table i. The table also includes some other proposed design criteriaT,8, 9.

Al though t h e e x p e r i m e n t s men t ioned above i n v o l v e d a t o t a l o f ove r a h u n d r e d f i l t r a t i o n t e s t s , t hey d i d n o t form p a r t o f a s y s t e m a t i c i n v e s t i - g a t i o n . C o n s e q u e n t l y t h e r e s u l t s do n o t cove r a comple te v a r i a t i o n o f a l l f i l t e r p a r a m e t e r s . Most f i l t e r t e s t s i n v o l v e d e i t h e r un i fo rm f i l t e r s f o r u n i f o r m s o i l s , o r g r a d e d f i l t e r s f o r g raded s o i l s . However, enough r e s u l t s e x i s t t o p r o d u c e r e l i a b l e , i f c o n s e r v a t i v e , de s ign c r i t e r i a .

3.i.3 selection of'reliable criteria. In order to find which of the criteria in Table 1 was most reliable, a range of base soils was selected for which experimental filtration results were available. Filters predicted for these soils by the various criteria were then compared with successful and unsuccessful filters from the tests; Experimental results available for the comparison were those of the Waterways Experiment Station3, 5 (1941, 1953), and_the Bureau of Reclamation 6 (1947). The range of soils tested in these experiments is shown in Fig. 2. The criteria obtained by Bertram 2 (1940)were not included in the comparison, since they were obtained for very uniform materials only.

Considerable variation was found to exist between the filters pre- dicted for each soil. For example, Fig. 5 shows the Various filters predicted for a fine sand.

Comparison with the experimental results showed that the Waterways Experiment Statlon!s criteria5 (1953) were the only ones in agreement with all the test results. The other design methods were all inadequate in some respect. Some comments on the accuracy of the other criteria are given in Appendix 2.

The Experiment Station's criteria may be stated as follows:

DISF~5 x D85S ) )

D15F~20 x D15S ) ., )

D50F~25 x D5OS )

Filtration criteria

DI5F>s5 x DI5S - P e r m e a b i l i t y c r i t e r i a

e x c e p t :

For uniform soils (i.e. D6OS/DIOSgI.5) use DI5Fz6 x D85S

For w e l l g r a d e d s o i l s ( i . e . . D6OS/D1OS)4.O) use D15F440 x D15S

F u r t h e r , : t h e f i l t e r m a t e r i a l s h o u l d no t be g a p - g r a d e d . 6

TABLE 1

SUMMARY OF FILTER DESIGN CRITERIA

Recommended .by Fi itration .Criteria

K Terzaghi (1921) D15F..<4 x D85S

m b

Bertram 2 (1939) DISF~6 x D8SS, DISF.<9 X DISS

Newton & Hurley I (1940) DISF.<IS x DSOS, DI5F~<32 x DISS

3 United States Waterways Experiment Station (1941)

United States Bureau of Reclamation6 (1947)

United States Waterways Experiment Station4 (1948)

United States Waterways Experiment Station5 (1953)

United States Soils & Paving Laboratory 8 (1942). Also U.S. Waterways Experiment Station (1953)

U.S. Corps of Engineers Manual 9 (1955)

A. Pillsbury 7 (1967)

Di5F~5 x D85S'

I) for uniform filters 5 x D5OS4DSOF410 x D5OS

2) for graded filters 12 x D50S~D5OF~58 x DSOS 12 x DI5S~DI5F~40 x DISS

DISP45 x D85S ~ DI5F~20 x DI5S

D5OF425 xD5OS

as above, except whem: i) D6OS/DIOS.<I.5 (uniform. soils)

then DISF.<6 x D85S 2) D6OS/DIOS>~4.O_ (well graded soils)

then DI5F,<40 x DI5S

Design chart of ratio DI5F/DI5S against uniformity coefficient. See Figs. 13 & 4

DISPel5 x D85S D5OF.<25 x DSOS

Design chart of ratio D5OF/DSOS against standard deviation. See Fig. II

Permeability Criteria

DI5F~4 x DISS

combined with fi Iter criteria

DISF~4 x DISS

i

DISF~5 x DI5S

*Throughout this report, DI5F is used to designate the fifteen percent size of the filter material, that is, the size of the sieve that allows fifteen percent by weight of the filter material to pass through it. Similarly, D85S designates the size of sie~e that allows eighty-five percent by weight of the base soil to pass through it. Particle sizes smaller than the 7S~m sieve refer to the results of Hygrometer analysis.

These filter criteria were proposed by t h e Waterways Experiment Station in 1953, after analysi~ ~f the results of all the filtration investigations mentioned above ~- (about one hundred filter tests).

The criteria are slightly conservative. Several soil-filter combinations that have been shown to be safe are excluded, particularly those involving well graded base soils. The relaxed limit DISF¢40 x DISS for very well graded soils (D6OS/DIOS>~4.O) reflects the conservatism of the criteria for well graded soils with D6OS/DIOS ratio just less than 4.0.

The supplementary limit of DISF46 x D855 for uniform soils is also fairly conservative. Bertram 2 (1940) obtained limiting ratios between 6.5 and 11.5 for uniform soils. A factor of safety is, however, desirable in filter design in view of the complexity of filter variables.

G a p - g r a d e d f i l t e r s were d i s a l l o w e d b e c a u s e t h e y t e n d to s e g r e g a t e d u r i n g p l a c e m e n t . F u r t h e r , t h e p a r t i c l e s i z e c r i t e r i a recommended a r e n o t a d e q u a t e t o c o n t r o l t h e p o r e s i z e s o f such m a t e r i a l s .

Some examples of filter design by these criteria are given in Appendix 3.

A second design method was proposed by the Experiment Station in 1953, in the form of a chart (see Fig. 4). The chart was obtained by plotting all the available experimental results as shown in the figure. Except for small anomalies, the graph is reliable for all the materials that have been tested for filter stability. In some places however, the demarkation between suitable and unsuitable filters is not clear. It is thought that before the chart is widely used, a more thorough experimental investigation should be made of filters near the design curve (see Appendix 2).

3.1.4 Filter criteria for silty soils. The Waterways Experiment Station's criteria above are directly applicable to silty soils. However, some tests have been madeSto investigate the use of concrete sand (to Corps of Engineers specification) as a filter material for all silty soils.

It was found that the concrete sand was a suitable filter for all the soils tested, which ranged from a fine sand to a medium silt. The sand was also considered suitable for use with soils finer than the silts tested beeause of the low flow velocities in such soils.

3.1.5 Filter criteria for clay soils. Kassiff et al IO, state that, for highway drainage in clay soils (where only low hydraulic heads are involved), gravel filters may be used. They successfully tested gravels (2 - 12 mm) in conjunction with heavy clay soils.

However, many clay soils contain non-cohesive particles, which will be liable to migrate into the drain. To preclude any chance of the drain clogging in clayey soils, a sand backfill would be advisable. Such a material would prevent the migration of silts in the soil around

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the drain, while providing adequate support for the clay fractions. A sand backfill would usually be permeable enoug h to conduct all the flow draining from clay soils

3.1.6 Standard filter gradings. Other filter design criteria have been proposed in the form of standard gradings.

For example, in Pumerica, some authorities 5'7 consider concrete sand suitable for all fine• grained soils.

ii In the United Kingdom, the Ministry of Transport Specification

for filter materials is very wide, see Fig. 5. This leaves the choice of the most suitable material to the engineer. The Air Ministry 12 specifies two standard filter gradings, one for use in clay soils, and one for gravels and sands. The grading limits of these filters are also shown in Fig. 5.

An examination of the various standard gradings was made, and no single filter material was found that will act for all soils.

The Waterways Experiment Station tests 5 indicate that Corps of Engineers concrete sand will act as a filter for all silts and finer soils. The C.E. concrete sand is very similar to B.S.882, zone 2, concrete sand, and the sand filter recommended by the Air Ministry.

Since the fifteen percent size of a material largely determines its effectiveness as a filter,• it seems likely that gradings at the fine end of the Ministry of Transport specification would also be satisfactory filters for silts and clays (see Fig. 5).

During the preparation of this report attempts were made to design ' a standard filter material for use with all soils coarser than silts that would comply, with United States Waterways Experiment Station criteria. However, it was found that any standard filter material would only protect a narrow range of soils. For example, on Experiment Station criteria only a limited range of soils can be supported by the Air Ministry's coarse filter material. (See Fig. 6). It is therefore not worthwhile to give standard filter gradings for soils coarser than silts, because the filter requirements of such soils are very variable. Filters for these soils must evidently be designed by application of design criteria to the base soil involved.

3.1.7 DeSign for gapl-graded soils. Where some particle sizes of a soils gradation are scarce, or missing altogether, most authorities 3,13,14,15 recommend that filter materials should be designed on the basis of the finer soil particles only. Similar recommendations are made for use when layered soils are encountered.

Such precautions are intended to ensure that the finer soil cannot migrate through the coarser particles and so clog the drain.

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3.2 Permeability of the backfill

The permeability required in the filter for a drain should theoretically be determined from considerations of the groundwater flow, and the propor- tions of the drain in question. The filter should be made permeable enough at the circumference of the drain pipe to transmit all the water into the pipe, so that water does not build up in the backfill.

It is, however, difficult to control the permeability of a filter by use of grain size limits alone. Consequently the criteria for adequate permeability are estimations intended to give filters suitable for all sub-surface drainage conditions.

The most comprehensive investigations into this aspect of filter design are those Of the United States Bureau of Reclamation 6. They have developed criteria, given in Table i, for two purposes - one for use when designing graded filters, and one for use with uniform filters. The U.S. Corps of Engineers have adopted a different criterion based on the work of the Waterways Experiment Station3to ensure adequate filter permeability (Table i). In order to make a comparison between the two approaches the finest filters allowed for a uniform base soil and a well graded base soil were designed; the gradings used are shown in Fig. 7.

For uniform soils the limit given by the Corps of Engineers agrees well with the United States Bureau of Reclamation uniform criteria. There is poor agreement with the Bureau's graded filter, but it has been found that the Bureau's criteria for graded filters are inapplicable to uniform base soils. (Appendix2);

In the case of the well graded base soil the Corps of Engineers criteria are fairly close to the Bureau's 'well graded' filter.

The difference between the two methods is larger when predicting filters for well graded soils. This may be because the Corps of Engineers criteria has a theoretical basis involving approximations which are less valid for well graded soils.

3.3 Drain pipe criteria

Two main factors must be considered when selecting suitable pipes for sub,surface drains.

Firstly, the length and diameter of pipe must be chosen so that the pipe does not run full near its outlet and flood the surrounding filter material. Secondly, the pipe, whether perforated or porous, must be able to intercept all the water entering the drain without causing high heads in the filter material. Backing-u p of water in the filter material of a drain is undesirable, because it reduces the depth to which the water table can be lowered, and i~s rate of lowering.

In the case of perforated pipes, there is a further restriction on the size of the holes in the pipe, which must be small enough to prevent the filter material washing into the pipe and clogging it.

i0

Most of the criteria discovered in a literature review of drain, pipe design concerned the prevention of backfill infiltration into the pipe. These Criteria are discussed below, and some comments are also made on the size and-interception ability of pipes.

3.3.1 Present criteria for hole size. in the United Kingdom the accepted criterion to prevent backfill washing into a pipe i s that -

size of holes 4~ x D85F (16)

The term 'size of holes' refers to the minimum dimension of the openings in a pipe, for example the width of a slot, the gap between lengths of pipe or the diameter of a circular hole.

The criterion given above ensures that there are plenty of particles in the backfill that cannot pas s through the holes in the pipe. If infiltration of the backfill starts, one of these large particles will eventually become lodged across the hole in the pipe, and prevent further collapse.

However, recent tests indicate that more liberal criteria can be applied.

3.3.2 • Recent reSearch on hole size. TeSts on sand infiltration into pipes by the Armco Drainage and Metal Products Inc.17 used pipe perfora- tions Of about the same size as the larger backfill particles. Similar criteria have been recommended by others. For example, E.S. Barber 9 states:-

Circular hole diameter < D85F

or slot width 0.83 x D85F

When the openings in a pipe are designed by these criteria, only a few backfill particles are large enough to lodge directly across the holes. It is•therefore evident that the backfillcan be excluded from the pipe by particles forming arches over the perforations, as well as by some particles lodging across the holes.

When using the Criteria given above, circular holes are allowed to be wider than slots, for any given backfill material. The reason for the different limits is that particles can form interlocking arches in any direction over• a circular hole, but in only one direction over a slot. Slots must therefore be somewhat smaller than holes to ensure that the necessary arches will form.

It has been suggested 18 that the particle arches mentioned above are formed with the help of fines migrating within the filter toward holes inthe drain pipe. Near the holes the fines may clog the pores of the backfill, forming an interlocking matrix of particles, and thereby aiding the formation of arches over holes in the pipe.

Brief investigations by Nettles and Schomaker 19 indicate that low

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frequency vibrations can disrupt the arching action ofbackfill particles, resulting in infiltration into the pipe. Where there is a danger of such vibrations, a pipe with smaller holes should be used.

3.3.3 The effect of backfill compaction on hole Size. Very much larger hole sizelimits than those given above have•been success•fully used when compaction is applied to the filter material.

In Guillou's tests 18 for the American Railway Engineers Association, (See Appendix I) compaction to about 44% voids ratio (1.55 Hg/m 3) was found to give the best stability of a backfill against infiil~ation into the pipe. At this density hole sizes up to six times as large as the maximum particles in the backfill were found to be satisfactory.

Increasing the density of the filter to a voids ratio of 40% (1.60 Mg/m 3) was found to cause instability when the drain pipe was laid with its holes upward. Failure in this case may have been due to the position of the perforations, rather than to over compaction of the backfill. When the pipe was laid•with its holes down, no failure occurred at higher densities. Particle infiltration due to over- compaction has not beenrecorded by any other experimenters.

The t e s t s by N e t t l e s and Schomaker 19 (see Appendix 1) on b o t h u n i f o r m f i n e sand and g r a d e d s a n d c o n f i r m t h a t c o m p a c t i n g t h e f i l t e r m a t e r i a l a l l o w s t h e use o f l a r g e r h o l e s ~ z e s . The sands were t e s t e d a t a v o i d s r a t i o o f a b o u t 42% (1 .55 Mg/mO).

The compactions used in both the experiments mentioned above are quite low. In •most sub-surface drains the densities given would be achieved without mechanical compaction.

3 . 3 . 4 Permeability considerations. All the drainage pipes, both perforated and porous, that have been tested in the experiments reviewed have ample porosity3, 18 and could conduct the flow arising from a saturated backfill. In practice, the soil around a drain cannot usually supply enough water to saturate a backfill.

Appendix 4 contains an estimation of the size and number of holes per metre of pipe theoretically required in a sub-surface drain. Although the example does not take account of any abnormal sub-surface water flows (such as might occur in side-long ground), the figures i~dicate that many pipes in use today have either too large or too many perforations.

Large numbers of perforations are, of course, useful as a precaution against localised clogging of the backfill. However, it would appear from the results of the experiments considered that many small holes in the pipe are to be preferred to a few large ones. Small holes reduce

• the inwashing of filter material, and improve the interception ability of the pipe by inducing more uniform flow through the backfill. Smaller perforations also allow fine filter material to be used in contact with the pipe. The number of occasions where two layers of filter material are needed are thereby reduced.

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There is a danger• however, that very Small holes ~say 2 n~ diameter) could become blocked by slime inside the pipe. Also in some circumstances Ce.g. when a spring is intercepted by a drain) sections of pipe having larger perforations would have to be provided:.

Measurements of the flow from existing sub-sugface drains could give a more definite guide to suitable hole sizes.

• 3, 9 17, 18 All experimenters ' agree thatlholes in ~. the lower part of a

pipe are mos.t effective giving both the maximum groundwater interception, and the least amount of filter material washing into the pipe.

If perforated pipes are laid in the 'holes down' position across an area where the water-table is lower than the pipe, water can flow out of the pipes into the surrounding soil. The same effect can occur where porous pipes are used. Some perforated pipes are, therefore, laid with their holes up where the water-table is likely to fall below the level of the pipes. Porous pipes are sometimes provided with an impervious invert in these circumstances.

Such precautions appear to be unnecessary in this country. The highest level to which a watertable can be raised by water escaping from the drain is the level of the pipes, provided that the pipes are not surcharged. In the United Kingdom , the water;-table must rise to within 600 rmn Ctwo feet) of th~oformation , before the strength of the road is sig%xificantly affected . Since sub'surface drains are usually more than 600 mm deepj water at the depth of the drains will rarely endanger;-the road structure.

In drier cotultries, however, free water in the sub-grade could seriously affect the strength of the road and precautions to prevent water escaping from the drains: are necessary.

5.S.5-Pipe~diameter and length. For a given diameter of pipe there is a limit to the length of drain that can be allowed, if the pipe is not to run full and cause high water levels in the backfill. The combinatians of diameter and length allowable depend on the flow entering the drain, and the gradient and roughness of the pipe. These quantities yar~y from drain to drain, and so no single recommendation can be made for pipe diameters and length. However, it may be seen from the examples in Appendix 4that the flows generally occurring in sub-surface drains are ~ery small, except when springs or other abnormal flows are encountered.

Research to determine roughness coefficients of perforated and porous pip.es weuld be valuable in providing a better basis: for drain- pipe desig~, since no accepted roughness values have been found during the literature review.

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4. CONCLUSIONS AND RECOMMENDATIONS

4.1 Filtratio ~ and permeability criteria for baCkfill

4..1.1 Filtration criteria. T h e only filter design criteria apparently reliable for all granular soils are those of the United States Waterways Experiment Station 5 . ..

For that reason the Experiment Station's criteria are recommended, as being the best available at the present time.

T h e criteria m a y b e s t a t e d as follows:-

DIbF.<5 x D85S and DIbF,<20 x DIbS

also D50F<25 x D50S

except where the ratio D6OS/DIOS for the base soil is less than 1.5

when DISF~6 x D85S [instead of 5)

and where the ratio D6OS/DIOS for the base soil is greater than 4

when DISF~40 x DIbS [instead of 20)

The filter must not be gap-graded

The designchart, also proposed by the Waterways Experiment Station, may offer a less complicated design method. However, further tests should be made before it is extensively used.

Where the soil around a drain is gap-graded (i.e. when some sieve fractions are scarce or missing altogether), filter design should be based only on the particles finer than the gap in the grading (see Fig; 8).

If a soil contains layers of fine material, filters should be designed from the grading of the finer soil.

It is also recommended that a filter material should never have more than five per cent of its weight passing through the 75~m sieve to prevent migration of fines from the filter into the drainl3.

4.1.2 Permeability criteria. The criterion recommended for use is that of the ~ited States Corps of Engineers 5 because it is in agreement with other results, and is simple to apply. The criterion is that

DIbF~5 x DIbS

4.1.3 Filters for silt and Clay soils. Concrete sand to B.S.S.882, Zone 2 or similar material is recommended for all silt and clay soils. The concrete sand is fine enough to act as a filter for silts, and it will protect the drain from any fine non-cohesive particles "in clays.

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4.2 Drain pipe design criteria

4.2.1 Hole size. On the basis of the experimental results discussed, and drainage practice in the United States, it is recommended that the following hole size criteria4, 9 be used:

maximum diameter of circular holes = D85F

maximum width of slots = 0.83 x D85F

Special compaction of the backfill is not required if the above criteria are applied.

It is possible that hole size limits could be increased if the backfill were compacted. Further investigation is required to decide the extent of any increases.

4.2.2 P0rositY and Pipe . diameter. It is recommended that hole diameters in drainage pipes,k or the width of slots, be 3 - 5 mm, in order to decrease the possibility of backfill entering the pipe.

It is also recommended that perforated pipes should have holes in the lower half of their circumference only, in order to increase the interception ability of the pipes, and to reduce the inwashing of filter material. Holes in the lower half of a pipe also reduce the amount of water trapped below the perforations in the bottom of a trench. When an impermeable bedding is used care should be taken to ensure that there are an adequate number of holes above the bedding.

Porous concrete pipes are particularly good for preventing filter material entering the pipe line, and they should be laid as tight together as possible.

Open-jointed impervious drainage pipes should also be laid with the joints as close as possible. Tight joints prevent the inwashing of filter material, while they are unlikely seriously to affect the inter- ception ability of the drain.

5. FURTHER RESEARCH

More experiments are needed to confirm the shape of the design chart for filters proposed by the Waterways Experiment Station 5. The tests should be arranged to check the portions of the chart not fully substantiated by experimental results. They should also verify that a fifteen per cent size ratio is an adequate control for filter design. The chart could then become the basis of filter design in many engineering fields.

A thorough investigation of limiting pipe hole size criteria for compacted backfills should also be undertaken. Theexperimental results reviewed indicate that the two layers of filter material at present required in some sub-surface drains could be replaced by one material, at higher density.

A laboratory determination of the roughness coefficients of various

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drainage pipes should be made, in order to allow more. rational, design of pipe diameters and lengths. Measurement of the flow..from.,exist%ngsub- surface drains would .enable.better specifications to be made for the diameter and length of pipes, and for the size and number of perforations required in a pipe.

6. ACKNOWLEDGEMENTS

This report was prepared in the Climate and Environment Section of the Design Division of the Road Research Laboratory.

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7 . R E F E R E N C E S

NEWTON, C.T. and H.W. HURLEY. Investigation to Determine the Practical Application Of Natural Bank Gravel as a Protective Filter for an Earth Embankment. Massachusetts Institute of Technology 1940. Abstract of a Thesis.

BERTRAM, G.E. An Experimental Investigation of Protective Filters. Pub. No.267 Soil Mechanics No.7,Harvard Graduate School of Engineering. 1940.

U.S. CORPS OF ENGINEERS, Waterways Experiment Station. Filter Requirements for Underdrains. Technical Memorandum 183,1. Vicksburg, Mass., Nov. 1941, revised December 1941.

U.S. CORPS OF ENGINEERS, Waterways Experiment Station. Laboratory Investigations of Filters for Enid and Grenada Dams. Technical Memorandum 3-150 Vicksburg, Mass. 1948.

UJS. CORPS OF ENGINEERS, Waterways Experiment Station. Filter Experiments and Design Criteria. Technical Memorandum 3-360 VicksburgMass. April i953.

KARPOFF, K. The use of Laboratory Tests to Develop Design Criteria for Protective Filters. Proc. Am. Soc. for Testing Materials. 1955, 55, i183-i198 (Paper presented at the 58th Annual Meeting of the Society, 1955).

PILLSBURY, A. Experiments with Filter Materials for Subdrains. Highway Research Record 1967. No.203,29-36.

U.S. CORPS OF ENGINEERS, Providence District. Filter Design, Tentative Design Procedure. Providence District, Rhode Island, Nov. 1942.

HIGHWAY RESEARCH BOARD, Subsurface Drainage of Highways and Airports. Bulletin 209 Washington D.C.1959 (National Research Council).

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i0. KASSIF, G., D. ZASLAVSKY and J; ZEITLEN. Analysis of Filter Requirements for Compacted Clays. Proc. of the Sixth Inter- national Conference onSoil Mechanics and Foundation•Engineering 1965, Volume ii, Division 3 - 6, pp 495-9 (University of Toronto Press).

ii. MINISTRY OF TRANSPORT. Specification for Road and Bridge Works, London 1969 (H.M. Stationary Office).

12. DIRECTORATE GENERAL OF WORKS, AIR MINISTRY. General Specification No.2Ol for Paved Areas for Aircraft, Part V, Miscellaneous. London July 1961.

13. UNITED STATES DEPARTMENT OF THE INTERIOR, Bureau to Reclamation. Earth Manual, Washington 1960 (United States Government Printing Office).

14 • WALTON, J. Ground water drainage with vitrified clay pipes, Part I. Clay • Pipe Development Association Ltd., Manchester 1968, (Cross-Courtney Ltd., Manchester).

15. CEDERGREN, H.E. Filter Design. Seepage, Drainage and Flow Nets. New York, 1967 (John Wiley & Sons, Inc.).

16.

17.

~18.

DEPARTMENT• OF SCIENTIFIC AND INDUSTRIAL RESEARCH, ROAD RESEARCH LABORATORY. Soil Mechanics for Road Engineers, London 1952 (H.M. Stationery Office).

SHAFER, G. Investigation of• Position, Size and Number of Holes• in Hel-Cor Sub-drains, Armco Drainag ~ Products Association. Middletown Ohio, 1944.

(a) GUILLIOU, J. First Progress Report on Performance of Filter Materials. Am. Railway Engineers Assoc. Bull. 1960, 61, 556 p 677-693. (b) GUILLIOU, J and R. LANYON. Second Progress Report on Performance of Filter Materials. Am. Railway Engineers Assoc. Bull. 1962, 63, 566 pp 17-38. (c) GUILLIOU, J. Third Progress Report on Performance of Filter Materials. Am. Railway Engineers Assoc, Bull. 1963, 64, 577 pp 554-565.

19. NETTLES, E., and N. SCHOMAKER. Infiltration through Pipe Joints. No.203, 37-56.

Laboratory Investigation of Soil Highway Research Record, 1967,

20. ROAD RESEARCH LABORATORY. A guide to the structural design of flexible and rigidpavements for new roads. Ministry of Transport, Road Research Laboratory and Highway Engineering Division. Road Note 29. London 1965 (H.M. Stationery office).

21. McCLELLAND, B. Large Scale Model studies of Highway Subdrainage. Proc. Highw. Res.Bd.Wash., 1943, 23, 469.

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8. APPENDIX 1

This appendi x gives a brief description of some of the major experiments carried out on the subject of sub-surface drainage materials. Only the most important aspects of the tests are described.

8 .1 U n i t e d S t a t e s ̀ Wate rways E x p e r i m e n t ' . S t a t i o n t e s t s 3. (,1941)

The experiment was directed specifically toward the design of sub- surface drains, and was conducted in three parts.

8 . 1 . 1 D e t e r m i n a t i o n o f i l i m i t i n ~ s i z e 9~ f i l ~ e r m a t e r i a l . The o b j e c t i v e o f t h e e x p e r i m e n t was t o f i n d t h e c o a r s e s t f i l t e r m a t e r i a l c a p a b l e o f h o l d i n g a f i n e s a n d b a s e m a t e r i a l . F ine sand (50 p e r c e n t s i z~ 0 .15 mm, u n i f o r m i t y c o e f f i c i e n t 1 .8 ) was u s e d as a ba se m a t e r i a l b e c a u s e i t was t h o u g h t most s u s c e p t i b l e t o w a s h i n g t h r o u g h a f i l t e r . The r e s u l t s o b t a i n e d f o r f i n e s a n d were assumed to e x t e n d to a l l n a t u r a l s o i l s .

The tests were conducted in a 75 mm diameter transparent permeameter. Filter material was compacted under water into the permeameter to a depth of 150 mm. 13 mm of fine sand was laid over the filter.

Water was p a s s e d t h r o u g h t h e s and i n t o t h e f i l t e r u n d e r a head o f 25 ram, By v a r y i n g t h e f i l t e r m a t e r i a l u s e d t he g r a i n s i z e d i s t r i b u t i o n cu rve was d e d u c e d o f a f i l t e r t h a t would j u s t p r e v e n t m i g r a t i o n o f t h e s a n d .

Recommendations for the design filter media were based on a ratio of the grain sizes of the sand and the coarsest allowable filter.

8.1.2 Tests of filters over pipe Openings. The experiment was concerned with the effect of the hole size and type of pipe used with various filters on the amount of filter material washing into the pipe.

Perforated porous plates, used to simulate various types of pipe wall, were fixed in turn to the base of a 200 mm diameter permeameter. Filter material was placed in the permeameter, sometimes under water, and sometimes in an oven dry condition. <The amount of material falling through the plates was recorded -

1) d u r i n g t h e l o a d i n g o f t h e p e r m e a m e t e r ,

2) during the flow of water through the system,

and 3) when the permeameter was tapped with a hammer.

The tests were carried out with various types of filter material. The results for each type of pipe wall were tabulated and compared.

8.1.3 Full scale tests onvarious types of pipe. The likelihood of filter material clogging various types of pipe was determined by full scale tests.

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A flume 10.8 m long, 610 mm wide and 1,2 m deep was constructed. A ISO mm diameter drainag e pipe was laid in the flume( A typical filter material [fifty per cent size 2 mm, uniformity coefficient = 6) was placed under water around the pipe to a level of about 3OO mm above the pipe.

The flume was then f l o o d e d to a l e v e l o f 610 mm above the P i P e . The we igh t o f f i l t e r m a t e r i a l t h a t the r e s u l t i n g f low washed i n t o t he p ipe was r e c o r d e d .

Various types of pipe were tested in this way and the results compared.

8.2 United States Bureau of Reclamation Tests 6 (1955)

The objective of the Bureau's tests was to develop design criteria for protective filters used in conjunction with hydraulic structures (e.g. canals, levees, etc.). Consequently very much higher heads were used than those used by the Waterways Experiment Station.

The filter material under test was compacted by about half Proctor compactive effort (i.e. half British Standard compactive effort) in a 200 mm diameter permeameter. 200 mm layers of filter and base materials were used, and flow was provided at heads of up to 9 m of water. SeVeral different base materials were tested and the shape of the grading curve of the filter material, and the mean particle size of the filters were varied.

Two limits were established for the suitability of a filter, one to ensure that it was sufficiently permeable, and the other to ensure that it would prevent migration of fines in the base material. The limits were expressed as multiples of the particle sizes of the base material conierned. Different limits were set for designing single sized and well graded filter material.

To compare the results obtained from this experiment with the Water- ways Experiment Station design criteria 3 of 1941, a check test was carried out.

8.3 American Railway Engineers Association tests 18 (1963)

The tests investigated the stability of a filter of concrete sand against washing into a drainage pipe. The experiments were carried out in a flume 1.2 m x 1.2 m x l.O5m deep. The drainage pipe and filter material under test were supported by a wire mesh box inside the flume so that they could be surrounded by water.

A.S.T.M. concrete sand was compacted into the wire mesh box around a drainage pipe. Water was then supplied to the flume at heads varying between 300 and 600 mm above the pipe. Flow was continued for about three days and the amount of filter material washed into the pipe was noted.

The variation of flow from the pipe with time was also recorded.

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The procedure was repeated for several degrees of compacZion of the filter.

The drainage pipe tested was 200 mm diameter corrugated metal pipe with 8 mm diameter holes in one third of its circumference only. Two series of tests were made with the holes in the pipe facing first upward and then downward.

Following one test the filter was frozen in order to examine the amount of particle migration that occurred over the test period.

Lastly a few tests were made to assess the interception ability of various types of pipe in conjunction with concrete sand filters.

8.4 United States Waterways Experiment Station and Ohio River Division Laboratories testsl9 (1967)

The purpose of these experiments was to investigate the factors influencing the infiltrations of soil into the joints of underground pipes (e.g. stormwater sewers). Tests were carried'out in a rectangular permeameter. Adjustable slots were provided in the base of the permea- meter to simulate the openings in pipe joints. Four soils (two fine sands, a silt and a lean clay) were used in the permeameter, and a range of slot sizes was tested with each soil. At every size of slot opening, water was passed through the sample at heads increasing up to 8.5 m. Flow through the sample was maintained for at least three hours at each increment of head.

The soil samples were tested at several different densities, and combinations of slot opening, water head and soil density resulting in least material washing through the slots were noted.

9. APPENDIX 2

Assessment of Filter Design criteria

The following comments are made on various recommended filter criteria, after comparing them with the results of tests by the United States Bureau of Reclamation 6 (1947) and the Waterways Experiment Station ~'5 [1941 and 1953).

9. i Newton and Hurley criteria (1940)

The d e s i g n r a t i o s recommended by Newton and Hur l ey a r e : -

DISF~I5 x DSOS and DI5F~32 x DISS

These criteria were obtained in tests using uniform filters with well-graded soils. It was found that when these filter limits were applied to uniform base soils, they allowed filters that were too coarse. For example, Fig. 9 shows a filter that was found to be too coarse in the

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Waterways Experiment Station tests. The limitsproposed by Newton and Hurley allow an even coarser filter. Consequently, this method is not suitable for use with uniform base soils.

9.2 Waterways Experiment S t a t i o n c r i t e r i o n 3 (1941)

The criterion suggested by the Experiment Station in 1941 is still frequentlyused, although it has since been modified. The criterion is:-

DISF~S x D8SS

The Experiment Station have found this criterion too restrictive when applied to very uniform base soils 5.

Furthermore, in the case of well graded base soils, this simplified criterion predicts materials that will not act as filters. For example Fig. i0 shows a well graded base soil and some filters recommended for it. The filter designed according to thesimplified Experiment Station criterion (1941) failed under test (see Appendix I, U.S.B.R. tests). Although the test was conducted usingheads up to twelve metres, partial failure of the filter occurred even at low heads so that the high head alone was not the cause of failure.

9.3 Corps of Engineers c r i t e r i a

The design criteria used by the U.S. Corps of Engineers are recommended by E.S. Barber 9. The Criteria are similar to those proposed by the Waterways Experiment Station, except that they do not include the supplementary limits on the fifteen per cent size of filters (see Table i)'. These limits :especially affect the fifteen per cent size of filters for well graded soils, as may be seen from Fig, I0, by comparing the grading of the Corps of Engineers filter with that of the Waterways Experiment Station (1953).

The fifteen per cent limits were added by the Waterways Experiment Station after their second series of filter tests in !9484 . Unfortunately, details of the tests carried out by the Experiment Station are not avail- able, and so no direct comparisons can be made. However, without the extra limits, the filters predicted by the Corps of Engineers for well graded soils may be too coarse.

9.4 United States Bureau of Reclamation criteria • L . •

The Bureau of Reclamation criteria 6 for designing filters are in two parts: one for designing uniform filter materials, and one for designing we 1 l~:graded fiiters.

The uniform f i l t e r s p r e d i c t e d by the method are g e n e r a l l y f i n e r than those• given by o the r c r i t e r i a (see Fig. 5). This i s because t h e Bureau 's c r i t e r i a fo r f i l t e r f a i l u r e were s t r i c t e r than those of o the r expe r imen te r s , (See Appendixl, U.S.B.R. tests).

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Only one tes t was made by the Bureau to confirm the suitability of the single sized filter criteriafor well-graded base soils. The test tends to confirm the Bureau's limits, but the filter tested was not the coarsest that the criteria will allow.

Well graded filters designed by this method for uniform base soils are too coarse. For example, Fig. 9 shows a filter material that became clogged when tested with the base soil indicated. The filter lies within the limits permitted by the Bureau's criteria. Apparently the well graded filter limits are not applicable to all soils, possibly because well graded filters were not tested with single sized base soils when the criteria were being derived.

In general, the method is valid when predicting well graded filters for graded soils, and single sized filters for uniform soils.

6 The Bureau also recommends that any filter should contain less than

five per cent by weight passing the 75urn sieve, in order to prevent fines from the filter clogging the drain.

9.5 Standard deviation design method 7

This method is different from others in that it considers the standard deviation of particle sizes in the filter as a variable in filter design.

In order to obtain the standard deviation of the filter material, the sieve analysis of the material is plotted on logarithmic probability paper. From the best straight line drawn through these points, the five per cent and the ninety-five per cent sizes of the material are noted.

The standard deviation of the sample is then given by

D9SF i (standard deviation) = DS--~ x 3.2----9-

Knowing the standard deviation of the filter material, and the ratio of the fifty per cent size of the filter to that of the base soil, the suit- ability of a filter may be checked from the design chart shown in Fig.ll.

For uniform soils the standard deviation design method appears to allow filter materials that are too coarse. For example, Fig. 12 shows both the best graded and the most uniform filters predicted by this method for a uniform base soil. The figure also shows materials that failed in Waterways Experiment Station tests. In both cases the materials predicted by the standard deviation design method are similar to the unsuitable materials, especially in the finer grain sizes.

No t e s t r e s u l t s have been found t h a t can be used t o a s s e s s t h e a c c u r a c y o f t h i s method when d e s i g n i n g f i l t e r s f o r w e l l graded s o i l s .

9.6 U n i t e d S t a t e s S o i l s a n d P a v i n g L a b o r a t o r y Design Char t 8

The United States Soils and Paving Laboratory design method is also given in the form of a chart, Fig. 13. From Fig. 3 it may be seen that

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the Soils and Paving Laboratory design method is in agreement with the other criteria, when used for fairly uniform base soils. However, when it is applied to well graded base Soils, it predicts filters that are almost certainly unstable. Fig. I0 shows a filter material designed by the simplified Waterways Experiment criteria [1941) which was found to allow migration of the base soil. The Soils and Paving Laboratory criteria allows a material even coarser than the unsuitable filter.

The method seems to make too large an allowance for the stabilizing effect of a well graded base soil.

9.7 Modified Soils and Paving Laboratory Method S

A modified design chart (Fig. 4) was obtained by the Waterways Experiment Station by transferring the results of all the experiments made in the field of filter stability onto the design chart given by the Soils and Paving Laboratory. The results included some that were not available to this review.

The new cha r t i s t h e r e f o r e g e n e r a l l y r e l i a b l e fo r a l l t h e m a t e r i a l s t h a t have been t e s t e d for f i l t e r s t a b i l i t y .

The main effect of the modification is that the maximum ratio of the fifteen per cent sizes of the filter and base soil that is allowable from the design chart is forty.

Some areas of the new design chart are not very well substantiated by experimental evidence (Fig. 4) and further filter tests would be needed to confirm the exact position of the design curve.

Some assessment of the effect of vibration on the stability of filters was made in preparing the chart. A second design curve was tentatively drawn from an analysis of the results of experiments that involved vibra- tion of the filter material. The curve is shown in Fig. 4.

I t w i l l be seen tha t the des ign cha r t does not t a k e account of the grading o f the f i l t e r m a t e r i a l . Only the f i f t e e n p e r cen t s i ze o f the f i l t e r i s p r e d i c t e d . This i n f e r s t h a t m a t e r i a l s o f s i m i l a r f i f t e e n per cent s i z e s have s i m i l a r pore s i z e s , and may be c o n s i d e r e d e q u i v a l e n t fo r f i l t r a t i o n purposes .

Such a hypo thes i s is suppor ted by the form o f o t h e r des ign methods. For example, as shown in F i g . 14, the s t anda rd d e v i a t i o n method p r e d i c t s a range of f i l t e r s wi th s i m i l a r f i f t e e n per cen t s i z e s fo r any p a r t i c u l a r base s o i l . F u r t h e r t e s t s by the Waterways Experiment S t a t i o n S showed tha t a r t i f i c i a l l y graded m a t e r i a l s o f s i m i l a r f i f t e e n pe r cen t s i z e s have s i m i l a r p e r m e a b i l i t i e s .

It may therefore be sufficient to specify the fifteen per cent size of a filter irrespective of its particle size range, provided that the grading curve is smooth. However, the results available at present do not allow accurate determination of the limiting fifteen per cent ratios for a wide range of filter gradings. The inconsistent results in the

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design chart at uniformity coefficients of 1.7 and 3.•2 maybe caused by the effect of variable filter gradings.

Some more experiments, varying the filter grading 0ffilter-base soil combinations near the limiting curve would resolve any anomalies, and enable a safe design curve to be drawn.

iO. APPENDIX 3

Filter design examples

Exampie I

Suppose a sub-surface drain is to be constructed in a base soil of the type•shown in Fig. 3.

The criteria for filter design are:-

i) for filtration:

DI5F$ 5 x D85S; DISF $ 20 x DISS and DSOF 6 25 x DSOS

except where

Uniformity coefficient of the base soil is greater than 4, when

DI5F $ 40 x DI5S (instead of 20)

or where

Uniformity coefficient of the base soil is less than 1.5 when

DI5F ~ 6 x D85S (instead of 5)

ii) for permeability:

DI5S ~ 5 x DISS

In the case under consideration, the uniformity coefficient of the base soil (U c) is given by

D60S 0.15 U . . . . . 2 c DIOS 0.075

The exceptions to the general criteria do not, therefore, apply and

DISF $ 5 x D85S = 5 x 0.21 = 1.05mm

or

DI5F < 20 x DISS = 20 x 0.085 = 1.7mm

selecting the lowest limit, DI5F ~ 1.05mm

24

DSOF ~ 25 x D50S = 25 x 0.14 = 3.5mm

D50F .< 3.5ram.

also, for permeability

DI5F ~ 5 x DI5S = 5 x 0.085 = 0.425mm

' DI5F .< 0.43ram

A backfill material should be chosen for the drain that is within the specifications given above.

A suitable material might have an 85 per cent size of about 5 mm. The maximum allowable hole sizes in pipes used with the materlalwould be given by:

maximum diamter of circular holes = D85F 5ram

maximum width of slots = D85F. x 0.83 = 4.1mm

If the holes in available pipes are too large, a coarser filter material must be placed next to the pipe. The grading of the coarser material must prevent migration of the filter into the pipe. It should therefore be designed in the way indicated above, except that the finer filter material is considered as the base soil.

Example 2

Suppose a sub-surface drain is to be constructed in a base soil that has the grading shown in Fig. IO

Uniformity coefficient of the base soil = D6OS DIOS

0.45 i.e. U =--= 22

c 0.02

Since this value is greater than 4, the limit

DI5F ~ 40 x DI5S applies, instead of the limit

DI5F ~ 20 x DI5S

so that

DI5F 6 5 x D85S = 5 x 1.05 = 5.25mm

or

DISF ~ 40 x DI5S = 40 x 0.025 = 1.0~

Selecting the lower limit D I 5 F ~< l.Omm

25

also

D5OF $ 25 x DSOS = 25 x .3 = 7.Smm

also, for permeability

DI5F ~ 5 x DI5S = 5 x 0.025 = O.125mm

• D5OF ~< 7.5mm

• . DISF ~ O.125mm

The backfill chosen for the drain should lie within the calculated grading limits.

Exampl e 3

Suppose a sub-surface drain is to be constructed in s0il that has the grading shown in Fig. 8.

Since the soil is gap-graded, the grading curve must be replotted, using only the particles finer than the gap in the grading (i.e. finer than O. 13ram).

The filter material is then desired from the replotted curve (Fig. 8) as follows:-

Uniformity coefficient of base soil = D6OS = 0.07_____~5 = 2 1 DIOS 0.035

The exceptions to the general criteria do not apply, and so:

DISF .< 5 x D85S = 5 x 0.095 = 0.475mm

or

DISF ,< 20 x DISS = 20 x 0.039 = 0.78ram

Selecting the lowest limit

now

D50F ~< 25 x DSOS = 25 x 0.068 = 1.7rmn

• . DI5F .< O.47mm

• . D50F ~< 1.7mm

also, for permeability

DI5F ~< 5 x DI5S = 5 x 0.039 -- O.195mm

• DISF >i O.195mm

Plotting these values in the grading chart, we obtain the filter grading limits shown in Fig. 8.

26

II. •APPENDIX 4

Calculation of flow in drains

The following calculation gives an indication of the order of magnitude of the quantities of water involved in road drainage. It is not intended to be an example of drain pipe design ......

Consider the cross-section shown in Fig. 15

Let the Permeability O f the road foundation by lO-4m/sec (such as might occur in a •saturated fine sand). Also let the dimensions shown in the figure have the following values:

D = I.O m W = 12 m

The flow rate into the drains will b~ greatest if drainage has just started. Using McCelland's results z , with d/D = O106, the flow into each pipe is given by

= 0.8 where q = flow into each •pipe (m3/sec/metre of pipe) KD

or q = 8 x 10 -5 m3/sec/metre length of pipe

Now the flow intercepted per metre of pipe can be approximated to by:

where

Q = N.A.Cd ~/~.g.h•. (from Bernoulli~s equation)

Q = flow through perforations (m3/sec)

N = number of perforations in a metre of pipe

A = area of each perforation (m 2)

Cd = coefficient of discharge of each perforation

= acceleration due to gravity (9 81 m/sec 2)

h = hydraulic head to the perforations.

Now, suppose the head to the perforations is 5 ram, and take Cd = 0.8

Then

Q = N.A.(0.8) ~2 x 9.81 (.005)

Q = 0.25 N.A.m3/sec/metre of pipe

27

But, in the example considered, the flow into the pipes is q. equating q and Q we have

Q = q = 8 x 10 -5 = 0.25.N.A.

Therefore

hence the total area of perforations required in eachmetre of pipe is

N.A. = 32 x 10 -5 m2/metre of pipe

or N.A. = 320 mm2/metre of pipe

Suppose the perforations are circular holes of 5 mm diameter

2 then A = ~(2.5)2mm--

320 so N = - = 16.3

2 ~(2.5)

Sixteen 5mm diameter holes per metre of pipe would therefore be sufficient.

The value of permeability used is higher than would normally be encountered, and so sixteen 5 mmholes are the•most ever llkelv to be required in a pipe except where abnormal side•flows are entering the drain. It should however be noted that the calculation is based on idealized flow conditions, and that may simplifying assumptions have been made.

The results of the calculation should therefore be interpreted only as a rough guide to the magnitude of the quantities involved.

28

l e v e l . I m p e r m e a b l e s e a l ( e . g . c l a y )

I o ,.o -2- . : o.°. -o.I I: o..o . ' o / °.1

% . ,I , , , . ' , ° . ' / \ " / , 4 " ' o " e ' 0 ~ " - I I

/ \ ~ o o .o . . . _.o I / Fitter material W = t e r - t = b t e o . . 'o ." . . . - % 7 J o - & o d . 7 . /

° ~ ' . ° . .

V/Jo,O o , . / IDII d " 4 o o = * / - I ' , ' o " ~ : - " " / O " " ~ . . . ' { ~-~,~.. rain pipe

~" o~-° I I / ~ °.' ~ . . : - ' . \ , - -o . (wi th pervious watts) .'o.~ ..o.=O.. o . . . ° ~ t

~ . " c; • 0 " . o " . a e~ ~ 0 . ,

a. S u b - s u r f a c e dra in . Not to scale.

I iL~iiJC".. iiZ".,, i i l O ~o~ "I l ~ pllO OOl / / t ~ / i ~ 7 / l ~ i / r d :o <> o o ~ ~ ~ o,

/ . ~ ^ ° ~ i o o : o oCl I0 ~'0% o ° 0 o ° ~ . I

I°.~ °0°~ " 0 . 0 ~ ~ . ~ Coarse backfi l l I u o D ~r~ " 0 o - - 0 I

//Jo~ ° : ': o o ~ ~ Ol / / I ~ ^ ~ o ~ C, ~ " o oQI /.~l- u ~ O n 8 o, Z.I ~.~10~% o , 0"% '~ "~.dl

~x~o %b~f-%o. ~ 'o01 o°( ) ~ o :Oj Drain pipe

I% ° o , , o o o _1 (with pervious wa|tsi I U a '~ C> 'L:~ o a ' - I

/ O O o O~O ~ o 0 . I (~l

b. Combined surface and sub-surface drain. N o t to sca le .

Fig. 1. SUB - SURFACE AND SURFACE WATER DRAINS.

I s i l t I sand I gravel

100

¢-

80

60

~o

I O r

0.01

1 ,i I 2 1 r l l / / / '--" I - II

I I I 0.1 1.0

GrcLin size (ram)

f m_

.i I ti

10

Fig. 2. SOILS USED IN FILTRATION EXPERIMENTS

s a n d I g rave l

100

~, 8o r-.

6O

0

,- ~0 U

q.) a . 20

0 O05

Base soi! to / I i LLSS&R / I / I / Y u.o.w.,-~ _befi t tered'~/" I N~HI.: "'7¢--,I/' / /&DU.S.~E,,

I I / ,o<o.,r.o°!!! /St XI~;~' I;,7'

0"1 1'0 Gra in size (mm)

I II 10 50

KEY Design Method Abbreviation

Newton & Hurley (1939) N &H U.S.Waterwo,ys Experiment Station (1953) U.S.W.E.S. US Bureau of Reclamation (i) graded f i t ter U.S.B.R(G)

(it) uniform fitter U.S.B.R.(U) U.S. Corps of Engineers U.S.C.E. U.S. Soils and Paving Laboratory U.S,S.&P. Standard devio, t ion (only the coarsest S.D. and f inest of arange of predicted fitters are shown ) Valves calculated from design cr i ter ia

The grading curves shown are the coarsest mater ials that comply wi th the design cri ter ia.

Fig. 3. COMPARISON OF FILTRATION CRITERIA

O

0 i n

m o ~3

E

° ~

C

0 ~

O

100

40

20

10

4

2

I I Successful f i t ter tests o

- Unsuccesfut f i l ter tests •

I Satisfoctory f i t t e rs I 0

o O:R) o

Limit t i n e - f i t t e r v ibrated

/ I L im i t l i n e - no v i b ra t i on of f i l t e r

D o O O0 • q)

~Oo O •

• f i l t e r s

8 e q .e

, / #

o Oo~g' ,,-" 2 4 10 20 40 100 200 400

:. Rat io D15F D15S

Fig. t,. U.S. WATERWAYS EXPERIMENT STATION DESIGN CHART

(:7) O

O U L.. O (L

100

80

60

/00 I I

~011 0.05

I I Sand

] I I 1~~~ for c lays j

I ~_ .~:~ Min is t ry of I I ,#T" TZlTronsport limit'

0'1 1'0 Grain size (ram}

Graver

FI~ S J

]111 l Air Minist ry l imi ts for sands and,grave, tjs

10 50 100

Fig. 5. STANOARO FILTER SPECIFICATIONS

I sand I 9 raver l

100

80 /

~ 60

~ 40 Ministry stQndard gradin¢.l I I I I I

~. 20

0 0.1 1.0 10 100

Grain size (mm)

Fig.6. SOILS PROTECTED BY AIR MINISTRY FILTER

I s l i t I sand I 9 ravel I

100

8O

-i 60

~0

20

0

100

80

60

40

20

0 0"01

! i I i

i ;

r -

o l 0

r -

U t , , , ,

(I,,t Q.

II]lll Base soil

i

0"1

EYr .~1 J I XL ]~

i/!u i ](iiIll" 1,0 10

Grain size (mm) 50

0"01 0.1 1"0 10 50 Grain size (mm)

KEY Design met hod :Abbrev ia t ion

U.S. Corps of Engineers LLS.C.E. U.S. Bureau of Reclamation (i) Uniform cr i ter ia U.S.B.R. (U)

(ii) Grade cr i ter ia U.S.B.R.(G) Values calculated from cr i ter ia "= -' v q lW

Fig.7. FINEST FILTERS RECOMMENDEO FOR TWO SOILS

[ s i l t I sand J 9 ravel I

100

80

~- 60

• ~ ~ 0 e- ID

O . 20

0 &01

o L . L U . U J I , I1~,1 I I I I~ ' - ' . o , , II ?~?~u;u?,:~?m.~ ,I I I 11 I],teFI to be filtered i 11111111 ' , ~.~o~.o..o:.~o,II ~ I I I I L I . I II I I I I I I I I ~" I I ! | , l l l i i l l DS0S-0.068n, rn, IY Bose so1,,:,rodin,:,' . , ' i ' l i t I I I I I I I I ~ ' ~I_..-,use, d,in,~T,c,u!a,t!5,n,s. ' ' l i l l I l l l l i 7. " . I I I I

DISS-003gmmU~ ~ / 7 ] ] U~L" . . icu iated I i r ' , , L . i i i l J l i i i i ~ i l i l l l I . , T l i I \ f i l t e r i l l

~ll]~D10S'O035mmjl" I J ~ I I ]I " L Z m i t s 1 II t"~ ~ Fl l l l l l l l l l l l 0.1 H3

Grain size (mm)

For f i l te r design caiculations~see Appendix D, Example 3

I [

10

Fig.8. FILTER DESIGN FOR GAP GRAOEO SOILS

I s i l t I s a n d I g r a v e l I

lO0 .o l LL!!,l,IJo _ b e f i l tered i

~' 60 I ~ _ 0 1 i •

e-

~ 0

tl)

o . 2 0

oil, .,'. 0-025 0-1

/ III Unsuccessful filter from U.S.W.ES. ! tests " ~ /,

m /

I 1.0

'~11 / ' / I 1 I I I

~/7' U.S.B.R.g rod ed ' f i l te r Limits_

" Newton& Hurley-- I

I~'i'°r 'im"l ' I / 10 50

Grain size (ram)

Fig. 9. RECOMMENDEO FILTER LIMITS FOR A UNIFORM SOIL

l s l i t I s a n d J ElroveL

801 ase o¢=

g 60 e,

= l . o

"'., " 1 I I I I • ~ ~ " ,.,t'T~J~.d"M'] . ,L , , ' 2 "~ .~ .

0 ~ " " f i l t e r U~.W.ES.(S]

0"01 " ' . 0.1 " ' I'0 10 -Grain s ize (ram)

00

: KEY Design Me thod Abb rev ia t i on :

U.S. Bureau of RecLamat ion - success fu l f i l t e r U.S.B.R.(G) U.S..Waterways ' C r i t e r i a (19/ ,1)r .unsuccessfut f i t ter " U:S.W.ES.(S) U.S. Waterways C r i t e r i a (1953) U.S.W.ES. U.S. Corps of Engineers C r i t e r i a U.S.C.E.. U.S. SoiLs and Paving LaboratorY Cr i te r ia ~ US.S.&P.

; N~H Newton 8, HurLey's C r i t e r i a :

Fig.lO. FILTER CRITERIA FOR A WELL GRADED SOIL

E E

w

O

>

"10

10 I = , Cl

"O ¢ , -

O

¢/)

10

8

2

0 0

/

/ S a t i s f a c t o r y . / "

f i L t e r s -I <.,,o~y,,. •

. . . . ~ ' / U n s a t i s f a c t o r y / J" f i l t e r s

16 24 32 40 /,8 Rat io D 50F

D50S

Fig.11. STANDARD DEVIATION DESIGN CHART

100

,' 8 0 O e- .w

60 O1 O

0

P ID

(1.

I I

20

0 0.05

Sand I Grovel

%' /

/

Unsuccessfut filters (U.S.W.E.S. tests )

Itlll '~ ~' /1Ill J

I T ,

Ih II , ~ ~ . I~=oo0or0 0-,o,,ool

• __.== fitters I I I I I 0.1 1"0 10

Grain size (ram)

Fig. 12. STANOARO OEVIATION CRITERIA FOR A UNIFORM SOIL

100

~.@100 canT-,

5O 0

~" 20 ° ~

E L,. 0

: ,: 10 C

III

~ 2 0

U

1 10 20

Sotisfactory / filters

~ ~ a t 1 = ~ . foctory J filters

50 100 200 500 Ratio D15F

D15S

1000

Fig. 13. U.S. SOILS ANO PAVING LABORATORY OESIGN CHART

0

0

"10

o

0 0

! .

~ ',

N Z

, ~ O ' B ~

Z

l ' - - c~J

C:3 O C) O O CO CD --:t' C~I

:~eu!j aSol, ue0Je,.i

4-* O

I~ J : ) ' i i

i " i U I ~

, CNI o. O

C3 C )

during droinoge

I_ I -

Water- t ab le before ,drainage

d

,L Ii TM " ~ ' " '

q = dischorge per unit length per u n i t t i m e

t = t ime since •beginning. of drainage

.k = coeff ic ient 'of permeabi l i ty

y = volume of drainab[e woter per unit volume of-soil

Dimensionless rat ios

tkD d q ~ ' 2 D kD

0"001 0"06 0"80

0"37 0"47 0-01

0.1 0.79 o.2s

Fig. 15. DIMENSIONLESS RATIOS FOR DRAINAGE BY TWO PARALLEL PIPES (AFTER MCLELLAND)

ABSTRACT

Selection of materials for sub-surface drains: R. SPALDING, B.Sc.: Ministry of Transport, RRL Report LR 346: Crowthorne, 1970 (Road Research Laboratory). The report discusses the part played by the various components of a sub-surface drain in achieving adequate permeability and in preventing clogging of the drain by silt. Present criteria for the grad- ing limits of the backfill and the porosity of the pipes in a drain are reviewed, and con- clusions are reached as follows:-

(i)

(ii)

The most reliable criteria for backfill in sands and gravels are those of the United States Waterways Experiment Station, which are based on ratios of the particle sizes of the backfill and the surrounding soil;

For drains in silts and clays, B.S.882, zone 2 concrete sand is recommended as a backfill;

(iii) Maximum pipe hole sizes should be equal to the 85% size of the backfill for circular holes, and the widths of slots should be slightly less.

More experiments are needed to confirm the filter design chart proposed by the Water- ways Experiment Station.

Further research is suggested to determine the number and size of holes required in a pipe, pipe roughness values, and also the effect of compaction on the allowable hole sizes in pipes.

ABSTRACT

Selec:tion of materials for sub-surface drains: R. SPALDING, B.Sc.: Ministry of Transport, RRL Report LR 346: Crowthorne, 1970 (Road Research Laboratory). The report discusses the part played by the various components of a sub-surface drain in achieving adequate permeability and in preventing clogging of the drain by silt. Present criteria for the grad- ing limits of the backfill and the porosity of the pipes in a drain are reviewed, and con- clusions are reached as follows:-

(i) The most reliable criteria for backfill in sands and gravels are those of the United States Waterways Experiment Station, which are based on ratios of the particle sizes of the backfill and the surrounding soil;

(ii) For drains in silts and clays, B.S.882, zone 2 concrete sand is recommended as a backfill;

(iii) Maximum pipe hole sizes should be equal to the 85% size of the backfill for circular holes, and the widths of slots should be slightly less.

More experiments are needed to confirm the filter design chart proposed by the Water- ways Experiment Station.

Further research is suggested to determine the number and size of holes required in a pipe, pipe roughness values, and also the effect of compaction on the allowable hole sizes in pipes.