I ABSTRACT

REINFORCED At~1) PRESTRESSED MASONRY EARTH RETAINING WALLS - ACOST STUDY

Stephen W Garrityl and Richard D. Nichol!2

A cost study in which reinforced concrete and brick-faced reinforced concrete eanh retaining walls were compared with reinforced and prestressed masonry altematives is described . Three metre and five metre high walls were considered, each having been designed to resist the lateral forces produced by active earth pressure and a live load surcharge of 10 kN/m2

; costs were provided by an independent quantity surveyor. The results indicate that, where a plain concrete finish is acceptable, reinforced concrete block walls may cost less than reinforced concrete construction. If, however, an exposed clay brick finish is required, reinforced or prestressed brickwork may be cheaper altematives to brick-faced reinforced concrete.

2. INTRODUCTlON

Reinforced concrete is probably the most common form of construction in the UK. for vertical cantilever earth retaining walls greater than about I .Sm high. Often the exposed face of these walls is clad with brick or block masonry to improve their aesthetic appeal. As retaining walls are generally subjected to very severe exposure conditions, most engineers specifY c\adding composed of very durable bricks or blocks laid in a high

Keywords Reinforced Masonry; Prestressed Masonry; Retaining Walls: Clay Brick; Concrete Block.

ILecturer, Department ofCivil Engineering, University ofBradford, Bradford, West Yorkshire, England. BD7 !DP.

2Engineer, Ferguson and McIlveen, Consulting Civil and Structural Engineers, Beechill House, Beechill Road, Belfast, Northem Ireland. BT8 4RP.

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cement content mortar. Even though the ciadding made from such materiais usually has a comparatively high compressive strength, any contribution it could make to the strength of the wall is usually ignored in the design_

In the last thirty years, various structurally efficient forrns of reinforced and prestressed brickwork and blockwork have been developed[1] and some of these have been used for a number of laterally loaded structures such as earth retaining walls and bridge abutments[2,3,4]. In spite of this, many engineers appear to be reluctant to adopt reinforced or prestressed masonry construction as an alternative to masonry-c1ad reinforced concrete_ There are probably many reasons for this, a Iikely one being a concern that reinforced or prestressed masonry construction may be prohibitively expensive Such a concern initiated the cost study described in this paper in which reinforced concrete earth retaining walls were compared with reinforced and prestressed masonry alternatives_ Three metre and five metre high walls were designed to represent the likely range of heights encountered in practice_ For each wall height, the following forms of construction were compared :-

a) Insitu Reinforced Concrete plain finish ; c1ay brickwork-c1ad_

b) Clay Brickwork reinforced grouted cavity; reinforced pocket-type; reinforced cellular or "diaphragm" wall; prestressed cellular or "diaphragm" walL

c) Concrete Blockwork reinforced grouted cavity; reinforced pocket-type; reinforced cellular or "diaphragm" wall ; reinforced hollow blockwork.

3_ DESIGN AND DETAILS

3 _1 Design Assumptions

The structures were designed in accordance with the limit state-based Codes of Practice currently in force in the UK, namely, BS 8110 : part 1[5] for reinforced concrete and BS 5628 : part 2[6] for reinforced and prestressed masonry_ It was assumed that ali the masonry units would comply with the requirements described in BS5628 : part 2 for the "special" category of manufacturing controL The material parameters assumed in the design study are described in 3 _2_

The following design assumptions were made:-

a) Foundations_ A reinforced concrete foundation was designed for the plain finish reinforced concrete wall and the same construction was assumed for ali

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lhe O1her waiis. ln pracüce, minor differences in lhe widlh of lhe foundaüon for each wall will be possible, however. such differences are uniikely to have a significant effect on the total cost .

b) Stability. Each wall was designed with adequate factors of safety against sliding, overturning and tilting failure .

c) Backfi/l material. Each wall was assumed to be backfilled with a well compacted, free-draining, cohesionless material. The angle of shearing resistance (<p) and unit weight of the backfill and surrounding ground were assumed to be 30° and !8 kN/m', respectively

d) Rear face drainage Each wall was specified with a rear face drainage system of the type shown in Figure 1 to prevent the build-up of any significant pore water pressure.

e) Earth pressures. It was assumed that sufficient lateral movement of the wall would occur during backfilling to perrnit the use of "active" rather than "at-rest" earth pressures in the designo Earth pressures were calculated using a triangular Rankine earth pressure distribution for a cohesionless material. The coefficient of active earth pressure (Ka) used in the design calculations was 0.33.

f) Effects of compaction planto The use of mechanical compaction plant elose to the rear face of a retaining wall is known to generate lateral pressures in excess of active earth pressure. To take account of this transient effect, each wall was designed to support a live load surcharge of J O kN/m2 Although this surcharge is also used to represent standard highway live loading in the U.K. , the walls were not designed to comply with the more onerous Department of Transport requirements for structures supporting a public highway.

g) Construction . It was assumed that ai! construction will be carried out by experienced contractors in accordance with BS8 J 10 and BS 5628 : part 3[7].

h) Prestressed brickwork. When designing the vertical cantilever stem of the prestressed brickwork walls, sufficient prestress was provided to ensure that the brickwork was in a state of vertical compression throughout, even under design ultimate load conditions. Not onl)' does this ensure that lateral deflections will be reduced and that no cracking wiIl occur under design service conditions but it also justifies the authors use of the full depth of the "web" or "cross-rib" of the wall when estimating its resistance to shear. A total loss of prestress of 20% was assumed in the design

i) Reinforced masonry - crack control and dejlection The reinforced masonry wall stem designs complied with the deflection and crack control provisions of BS5628 : part 2[6]. Under normal service conditions the wall stem wiIl only be subjected to permanent earth pressure loading from the backfill . Hence the tensile stress in the steel reinforcement, and therefore the tensile strain in the concrete or masonry at the foot of the rear face of the wall stem, will be less than that when full lateral loading is applied This is another form of deflection

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and crack contrai and a further good reason for designing the wall stem to resist the surcharge effects described in "f' above.

3.2 Materiais

The following materiais parameters were assumed in the design and included in the cost study materiais specifications :-

i) Steel ties and reinforcement. Ali reinforcement was assumed to be high yield carbon steel with a characteristic yield strength (fy) of 460 N /mm2 Ali bed joint reinforcement and steel ties were specified as austenitic grade 316 stainless steeI. Cover to all carbon steel reinforcing bars was assumed to be 50mm.

ii) Prestressing bars . Carbon steel prestressing bars with waterproof grease and tape corrosion protection have been used in the UK. for masonry retaining walls and bridge abutments. It was assumed that the retaining walls in this study could be built adjacent to a highway where exposure to high chloride ion concentrations from de-icing salts would be very likely. To account for this the threaded prestressing bars, couplers, anchorage plates, nuts and washers used in the construction of the prestressed brickwork wall were ali assumed to be precipitation hardened martensitic stainless steel with a characteristic breaking strength of 1000 N /mm2

; in the design, it was assumed that the bars were initially stressed to 70% of their characteristic strength.

iii) Concrete and grout. Ali insitu concrete for the foundations and the reinforced concrete retaining walls was assumed 10 have a characteristic compressive strength of 40 Nlmm2 to comply with the durability requirements of BS 8110. The insitu concrete and grout used for the reinforced masonry construction was assumed to have a characteristic compressive strength of 30 N /mm2

iv) Mortar. In all cases, designation (i), or type M, 1:1;4:3 cement:limesand mortar was specified.

v) Concrete blocks. Ali solid and hollow blocks were assumed to be the dense aggregate concrete type with a rninimum compressive strength of 15 N /mm2

vi) Clay bricks. Ali bricks were assumed to be frost resistant, low soluble salt content (durability designation "FL" to BS 3921[8]) units with a rninimum compressive strength of 50 N /rnm2 and a maximum water absorption of 7% .

vii) Waterproofing The rear faces of the reinforced insitu concrete walls were assumed to be coated with two coats of biturninous paint Ali the other walls were specified with a biturninous sheet waterproof membrane.

viii) Movement joints. Ali movement joints were assumed to consist of a closed cell polyethylene foam joint filler sealed with a polysulphide rubber sealant Joints were specified at 6m centres for the insitu concrete and concrete blockwork walls and at 12m centres for the brickwork walls.

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3.3 Design Detaiis

General àetaiis common to ali the walis considereà in this study are shown in Figure 1. A horizontal section through the vertical stem of each wall is given in Figure 2.

precast concrete coping ______ _

retaining waU stem - see Figure 2 ____ --' rear face

draioage

//0//

fully compacted, freely draining, cobesionless backfill material

, , /

I I

/

weep boles

\

1-+-__ waterproof membrane /'

/

,

concrete

: I

reinforced concrete foundation

Figure 1. Masonry earth retaining walls - general details

4. RESULTS

assumed extent of excavation

Detail drawings and specifications were prepared for each of the twenty walls described above and sent to Moore, McDade and Dowse, an experienced firm of Quantity Surveyors of Belfast, Northem Ireland, for costing. The estimated costs per linear metre of each of the 3m and 5m high walls are summarised in Tables I and 2, respectively. A1though the costs are based on March 1992 prices, the authors have been advised by the quantity surveyors that, because of the recent stagnant econornic conditions in the UK, construction costs have changed very Iittle since March 1992 and the figures presented in Tables 1 and 2 are Iikely to reflect current (December 1993) costs .

435

.I:> w 0"1

Reinforced concrete - plain and brick-faced

;;rw: ::: :5~ Reinforced brickwork cellular (diaphragm)

+'fO I I"· (~z.)

-' (I i < < < I < I li fOZ.

main reinforcement :- 6Tl2 (5T32) ban/cell

Prestressed brickwork diaphragm

[-lZ2 7 Z-Z::;;

+to .'.~,<I < < 'I ZI!!J 1.1 .1

/I~

1 no. post-tensioned 32mm dia. bar per cell (3 no. 40mm dia. bars per cell)

Reinfon:ed brickwork - grouted cavity

màin reinforcement:- TIO -150 (T20 - 175)

.2.,5 )(

< sr '~

~tllli8ir '~I·;nr~~~g·:·2~Ph"gm) main reinforcement :- lTlO (lT40) bar/cell -' I: < < d

2J5 i. I I i Ali dimension! are in millim-;r;] "25

main reinforcement :- 5T12 (5T32) bars/cell

Figure 2. Horizontal sections showing different types of construction Details are given for 3m high walls. Figures in brackets are for 5m high walls.

.j:>. W -.....J

Wall

Type

Item of Constructioll

F ou ndations

Wall Stem

Concrete

Reinforc.ement OI'

Jlrestressing bars Formwork

Clay 8rickwOl'k OI'

Concrete Blockwork Movement Joints and other sundry items ScalTolding

-Backlill & Top Soil

'. Tota/ C'ost per Linear Metre {i - Ste,./ing}

Table 1. Summary of Costs - 3m High Masonry Eal·th Retaining Walls

ln-situ Clay Concrete COllcrete Brickwork Blockwork

Reillforced Reinforced Prestressed Reinforced !

Plain Brick- Grouted Pocket- Cellular Diaphragm Grouted Pocket- Cellular Grouted Faced Cavity Type Cavity Type Hollow Block

184 184 184 184 184 184 184 184 184 184

68 70 51 II 51 15 51 22 45 51

24 24 14 8 6 85 16 7 6 12

123 147 --- 37 --- 31 --- 18 --- ---

--- 63 155 190 167 96 92 78 104 74

39 41 31 34 35 30 38 31 31 31

11 11 11 11 11 11 11 11 11 11

71 71 71 71 71 71 71 71 71 71

520 611 517 546 525 523 463 422 452 434

.l:> w 00

Wall

Type

Item of Construction F oun([at;ons

Wall Stem

Concrete

Reinforcement or Jlrestressin~ bars Formwork

Clay Brickwork or Concrete Blockwork Movement Joints and other sllndry items Scaffolding

Backfill & Top Soil

Total Cost per Linear Metre fi - Sterlinlâ

Table 2. Summary of Costs - Sm High Masonry Earth Retaining Walls

In-situ Clay Concrete Concrete Brickwork Blockwork

Reinforced Reinforced Prestressed Reinforced

Plain Brick- Grouted Pocket- Cellular Diaphragm Grouted Pocket- Cellular Grollted Faced Cavity Type Cavity Type Hollow Block

420 420 420 420 420 420 420 420 420 420

,

150 ISO 113 17 77 17 128 77 77 77

77 77 140 41 68 123 49 38 26 32

223 273 --- 45 --- 31 --- lO --- ---

--- 100 233 275 230 138 156 101 158 127

57 60 49 46 56 43 56 48 48 48

18 18 18 18 18 18 18 18 18 18

169 i69 169 169 169 169 169 169 169 169

1,114 1,267 1,142 1,031 1,038 959 996 88/ 916 891

5. LEvíITATIONS OF Ttn= STUDY

Only one finn of quantity surveyors was asked to provide costs. To obtain more data, the authors aim to extend the study by seeking prices from at least one more quantity surveyor and two contractors from different locations in the UK

The costs were based on the assumption that the contractor building each wall would be equally experienced in ali fonns of construction This is unlikely in practice and prices tendered by contractors for reinforced and , in particular, prestressed masonry earth retillning walls are likely to be higher than those presented in this paper, reflecting a generallack of experience in masonry construction.

A particular type of construction will often be specified if the other elements of work on the same scheme require the use of similar items of plant, specialist labour and materiaIs. Hence if, for example, a project involves the construction of several reinforced concrete structures, the designer is also likely to specifY any earth retaining walls required in reinforced concrete. Considerations such as this cannot be accounted for in a cost study of this type.

The range of materiaIs considered in the study was limited with no attempt being made to estimate the cost of building retaining walls from natural stone, concrete or calcium silicate brickwork or architectural quality concrete blocks. In spite of this, it should be possible to estimate the approximate cost of construction with different masonry units by substituting the cost quoted for the brickwork or blockwork in Tables I and 2 with that for the altemative . In addition, other wall types such as prestressed concrete blockwork, fin or "1" section walls and brickworklblockwork composite walls were not investigated In the extended study referred to above, the authors will inc!ude prestressed concrete blockwork diaphragm wall designs.

6. SUMMARY

The results of a cost study of this type must be treated with caution, even though they tend to confinn those from previous studies[9, I 0, 11,12]. A1though the results of this study indicate that reinforced and prestressed brickwork walls can be less costly than plain finish reinforced concrete walls, it would be foolish to draw such a conc!usion without more data, in particular, details of actual construction costs. What is evident from the study is that designers should seriously consider reinforced or prestressed masonry as possible altematives to reinforced concrete construction for earth retaining walls. Furthennore, bearing in mind the limitations described earlier, unless gross errors have been made in the design calculations or cost estimates it is likely that-

a) Reinforced blockwork may be more economical than reinforced concrete construction for earth retaining walls where a plain concrete finish is acceptable .

b) Where an exposed clay brickwork finish is required, reinforced brickwork may be less costly than brick-faced reinforced concrete; prestressed brickwork is likely to be more economical for tall walls.

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7. ACKc.'\iOWLEDGEMENTS

The authors wish to thank the following people for their help and encouragement :-

i) Lowry Manson and Winston Bell, partner and associate respectively, of Ferguson and McIlveen, Consulting Engineers, Belfast.

ii) Richard Dowse, partner of Moore, McDade and Dowse, Quantity Surveyors, Belfast

8. REFERENCES

1. "Reinforced and Prestressed Masonry" , (Ed. AW.Hendry), Longman Scientific and Technical, Harlow, nK, 1991.

2. Garrity, SW, "Clay Brickwork - An Altemative Material for New Highway Structures", The Masonry Society Joumal, Boulder, Us.A, Volume 11 , Number 2, February 1992, pp. 53 - 65 .

3. Ganity, S.W , Bel!, SE and Cox, DT, "Prestressed Clay Brickwork Bridge Abutments: Researeh, Design and Construetion", Bridge Management 2 -Inspection, Maintenance, Assessment and Repair (Ed. lE.Harding, G.ARParke and M.JRyall), Thomas Telford Limited, London, nK, 1993, pp. 180 - 189.

4. Beck, JK, Shaw, G. and Curtin, W.G., "The Design and Construetion of a 3m High Post-tensioned Conerete Bloekwork Diaphragm Earth Retaining Wal! in a Residential Landscaping Seheme", Paper 16 in The Praetical Design of Masonry Structures, Thomas Telford Limited, London, nK, 1987, pp. 225 - 236.

5 BS8110 part 1, "British Standard Struetural Use of Concrete. Part 1. Code of Praetice for Design and Construction", British Standards Institution, London, U.K , 1985.

6. BS 5628 : part 2, "British Standard Code of Praetiee for Use of Masonry. Part 2 . Structural use of reinforced and prestressed masonry", British Standards Institution, London, UK, 1985.

7. BS 5628 part 3, "British Standard Code of Praetice for Use of Masonry. Part 3. Materiais and Components, Design and Workmanship" , British Standards Institution, London, nK, 1985.

8. BS 3921 , "British Standard Speeifieation for Clay Bricks", British Standards Institution, London, nK, 1985.

9. Maurenbreeher, AH.P., Bird, AP., Sutherland, R.JM. and Foster, D., "Reinforeed Briekwork: Vertical Cantilevers I" , Publication No. SCP 10, Structural Clay Products Limited, UK, 1976.

10. Pastemaek, S.e. and Simon, AL. , "Feasibility Analysis of Brick and Concrete Masonry in Highway Structures", Proeeedings of the 4th Canadian Masonry Symposium, Fredrieton, Canada, 1986, pp. 203 - 215.

11. Haseltine, B.A and Tutt, lN , "Briekwork Retaining Walls" , The Briek Development Assoeiation, Winkfield, nK, 1985.

12. Haseltine, BA and Tutt, lN , "The Design ofBrickwork Retaining Walls" , Design Guide No.l, The Brick Development Assoeiation, Winkfield, nK, 1991.

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