module5: site investigation using in situ testing -...

SITE CHARACTERIZATION & INSTRUMENTATION MODULE 5

Dr. P.Anbazhagan

Module5: Site investigation using in situ testing

Topics:

Introduction

Penetration testing

1. Standard penetration test

2. Cone penetration test

Strength and compressibility testing

1. Field vane shear test

2. Pressure meter test

3. Plate loading test

4. Marchetti dilatometer

Permeability testing

1. Rising and falling head tests

2. Constant head tests

Keywords: In-situ testing, Penetration tests, shear test, permeability tests

5.1 Introduction:

The physical survey is that part of site investigation which aims to determine the physical

properties of the ground. These are required:

1. To classify the soil into groups of materials which will exhibit broadly similar engineering

behaviour; and

2. To determine parameters which are required for engineering design calculations.

Some soils, for example clays may readily be sampled. If good-quality samples can be

obtained, then laboratory testing offers the best method of determining soil and rock

parameters under carefully controlled conditions. But other types of ground are either

difficult or impossible to sample and test successfully. In such cases, in situ tests should be

used.

Information may be obtained in situ in at least three ways:

By using geophysical techniques;

By using in situ soil testing techniques, such as those described in this chapter; and

By making measurements using field instrumentation, as described in the further

chapters.

The following types of ground conditions are examples of those where in situ testing is either

essential or desirable.

Very soft or sensitive clays


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Stoney soils

Sands and gravels

Weak, fissile or fractured rock

In situ tests may be classified in a number of ways - cost, ease of use, method of

interpretation, soil types in which they may be used, parameters which can be determined,

etc. In this module only relatively common in-situ tests are presented and divided according

to their purpose, i.e. to obtain:

Penetration resistance;

Strength and/or compressibility, or

In situ permeability.

A classification can be established on the basis of the degree to which tests can be analysed in

a fundamental way to obtain real soil parameters, which is a function not only of how the test

is applied to the soil, but also of the type of data collected. On this basis, the tests can be

classified as explained below:

Wholly empirical interpretation: No fundamental analysis is possible. Stress paths,

strain levels, drainage conditions and rate of loading are either uncontrolled or

inappropriate. (Examples: SPT, CPT.)

Semi-analytical interpretation: Some relationships between parameters and

measurements may be developed, but in reality, interpretation is semi-empirical

either, because both stress paths and strain levels vary widely within the mass of

ground under test, or drainage is uncontrolled, or inappropriate shearing rates are

used. (Examples: plate test, vane test.)

Analytical interpretation: Stress paths are controlled and similar (although strain

levels and drainage are not). (Example: self-boring pressuremeter.)

5.2 Penetration testing:

Many forms of in situ penetration test are in use worldwide. Penetrometers can be divided

into two broad groups.

The simplest are dynamic penetrometers. They consist of tubes or solid points driven by

repeated blows of a drop weight. ‗Static‘ penetrometers are more complex, being pushed

hydraulically into the soil.

The two most common penetration tests, which are used virtually worldwide, are

The dynamic Standard Penetration Test (SPT) , and

The Static Cone Penetration Test (CPT).


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5.2.1 The standard penetration test (SPT):

The standard penetration test (SPT) was developed circa 1927 and is perhaps the most

popular field test.

• The standard penetration test is done using a split- spoon sampler in a borehole / auger hole.

This sampler consists of a driving shoe, a split- barrel of circular cross-section (longitudinally

split into two parts) and a coupling. Figure 5.1 shows split-spoon sampler used in SPT test.

The procedure for carrying out the standard penetration test is discussed as follows:

Figure 5.1: Spit-spoon sampler

Standard penetrometer

SPT uses a thick-walled sample tube, with an outside diameter of 51 mm and an inside

diameter of 35 mm, and a length of around 650 mm. This is driven into the ground at the

bottom of a borehole by blows from a slide hammer with a weight of 63.5 kg (140 lb)

falling through a distance of 760 mm. Figure 5.2 shows schematic representation of SPT

setup and testing

The sample tube is driven 150 mm into the ground and then the number of blows needed

for the tube to penetrate each 150 mm up to a depth of 450 mm is recorded. The sum of

the number of blows required for the second and third 150 mm. of penetration is termed

the "standard penetration resistance" or the "N-value".

In cases where 50 blows are insufficient to advance it through a 150 mm (6 in) interval,

the penetration after 50 blows is recorded. The blow count provides an indication of the

density of the ground.

A borehole is dug to the required depth and the bottom of the hole is cleaned. The split-

spoon sampler, attached to the drill-rods of required length is lowered into the borehole

and is relaxed at the bottom.

The sampler is then, driven to a distance of 450 mm in three intervals of 150 mm each.

This is done by dropping a hammer of 63.5 kg from a height of 762 mm (BIS: 2131,

1981). The number of blows required to penetrate the soil is noted down for the last 300

mm, and this is recorded as the N value. The number of blows required to penetrate the

sampler through the first 150 mm is called the seating drive and is disregarded. This is

because the soil for the first 150 mm is disturbed and is ineffective for the SPT- N value.

The sampler is then pulled out and is detached from the drill rods. The soil sample, within

the split barrel, is collected taking all precautions not to disturb the moisture content and


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is then transported to the laboratory, for tests. Sometimes, a thin liner is placed inside the

split barrel. This makes it feasible for collecting the soil sample within the liner, by

sealing off both the ends of the liner with molten wax and then taking it away for

laboratory test of the contained soil.

Figure 4.2 shows schematic representation of SPT setup and

The standard penetration test is, performed at every 0.75 m intervals in a borehole. If the

depth of the borehole is large, however, the interval can be, made 1.50 m. In case the soil

under consideration consists of rocks or boulders, the SPT- N value can be recorded for

the first 300 mm. The test is stopped if:

1. 50 blows are required for any 150 mm penetration

2. 100 blows are required for any 300 mm penetration

3. 10 consecutive blows produce no advance

However, it should be noted that the SPT- N value obtained from the above set of

procedures has to be corrected before it can be used for any of the empirical relations.

These corrections and their values for certain conditions are as follows:

4.2.1.1 Corrections Applied for SPT “N” Values:


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• The SPT data collected is field ‗N‘ values without applying any corrections. Usually for

engineering use of site response studies and liquefaction analysis, the SPT ―N‖ values have to

be corrected with various corrections and a seismic bore log has to be obtained.

• The seismic bore log contains information about depth, observed SPT ‗N‘ values, density of

soil, total stress, effective stress, fines content, correction factors for observed ―N‖ values,

and corrected ―N‖ value.

• The ‗N‘ values measured in the field using Standard penetration test procedure have been,

corrected for various corrections, such as:

1. Overburden Pressure (CN),

2. Hammer energy (CE),

3. Borehole diameter (CB),

4. Presence or absence of liner (CS),

5. Rod length (CR) and

6. Fines content (Cfines)

• Corrected ‗N‘ value i.e., (N1)60is obtained using the following equation:

(N1)60 = N × (CN ×CE× CB× CS× CR)

Correction for Overburden Pressure:

The effective use of SPT blow count for seismic study requires the effects of soil density and

effective confining stress on penetration resistance to be, separated. Consequently, Seed et al

(1975) included the normalization of penetration resistance in sand to an equivalent of one

atmosphere as part of the semi empirical procedure.

• SPT N-values recorded in the field increases with increase in the effective overburden

stress, hence, overburden stress correction factor is applied (Seed and Idriss, 1982). This

factor is, commonly calculated from equation developed by Liao and Whitman (1986).

• However Kayen et al. (1992) suggested the following equation, which limits the maximum

CN value to 1.7 and provides a better fit to the original curve specified by Seed and Idriss

(1982):

CN = 2.2 / (1.2 + σ υo/ pα )

• Where, σ υo = effective overburden pressure, Pα = 100 kPa, and CN should not exceed a

value of 1.7. This empirical overburden correction factor is also, recommended by Youd et al

(2001). For high pressures (300kPa), which are generally below the depth for which the

simplified procedure has been, verified, CN should be estimated by other means (Youd et al,

2001).

Correction for hammer energy ratio:


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Another important factor, which affects the SPT ‗N‘ value is, the energy transferred from the

falling hammer to the SPT sampler. The energy ratio (ER) delivered to the sampler depends

on the type of hammer, anvil, lifting mechanism and the method of hammer release.

Approximate correction factors to modify the SPT results to a 60% energy ratio for various

types of hammers and anvils are, listed in Table 5.1 (Robertson and Wride, 1998).

Table 5.1 :Hammer correction factors for three type of hammar

Type of hammers Notation Range of correction Donut hammer CE 0.5-1.0

Safety hammer CE 0.7-1.2

Automatic trip donut hammer CE 0.8-1.3

• Because of variations in drilling and testing equipments and differences in the testing

procedures, a rather wide range in the energy correction factor CER has been, observed as

noted in the Table 5.1. Even when procedures are carefully monitored to confirm the

established standard, some variation in CE may occur because of minor variations in testing

procedures.

• Measured energies at a single site indicate that variations in energy ratio between blows or

between tests in a single borehole typically vary by as much as 10%. The workshop

participants of NCEER 1996 & 1998 (Youd et al, 2001) recommend measurement of the

hammer energy frequently at each site where the SPT is used.

• Where measurements cannot be, made careful observation and notation of the equipment

and procedures are required to estimate a CE value. Use of good quality testing equipment

and carefully controlled testing procedures will generally yield more consistent energy ratios.

• For, Liquefaction calculation, Yilmaz and Bagci (2006) had taken the CE value as 0.7 for

SPT hammer energy, donut type for soil liquefaction susceptibility and hazard mapping in

Kutahya, Turkey. Similar kind of hammer is used for soil investigations and hence, the value

of 0.7 is taken for CE.

Other correction factors:

The other correction factors adopted such as correction for borehole diameter, rod length and

sampling methods modified from Skempton (1986) and listed by Robertson and Wride

(1998) are, presented in Table 5.2.

• Correction for borehole diameter (CB) is, used as 1.05 for 150 mm borehole diameter, Rod

length (CR) is taken from the Table 5.2, based on the rod length the presence or absence of

liner (CS) is taken as 1.0 for standard sampler.

• The corrected ―N‖ Value (N1)60 is further corrected for fines content based on the revised

boundary curves derived by Idriss and Boulanger (2004) for cohesion-less soils as described

below

(N1)60cs = (N1)60 + Δ (N1)60


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Δ (N1)60 = exp

FC = percent fines content (percent dry finer than 0.074mm).

Table 5.2 : Other correction factors for SPT N values

Factor Equipment variable Notation Correction

Bore hole dia 65-115mm CB 1.0

Bore hole dia 150mm CB 1.05

Bore hole dia 200mm CB 1.15

Rod length <3m CR 0.75

Rod length 3-4m CR 0.80




Sampling method Standard samplers CS 1.00

Sampling method Samplers without liners CS 1.1-1.3

5.2.1.2Interpretation of SPT N30:

The following factors can affect the SPT results:

1. Nature of the drilling fluid in the borehole,

2. Diameter of the borehole,

3. The configuration of the sampling spoon and the frequency of delivery of the hammer

blow.

• Therefore, it should be, noted that drilling and stabilisation of the borehole must be carried

out with care. The measured N-value (blows/0.3 m) is the so-called standard penetration

resistance of the soil. The penetration resistance is, influenced by the stress conditions at

the depth of the test.

• The resistance (N30) has been, correlated with the relative density of granular soils. Sand

and gravel can be, classified as shown in Table5.3.

Table 5.3 : Classification of sand and gravel based SPT N values

The sources of some of the common errors while carrying out SPT tests are listed in

Table 13.5 (Kulhawy and Mayne, 1990).

Table 13.5: Source of errors in SPT test

SPT N value Relative density Classification

0-4 0-15 Very loose

4-10 15-35 Loose

10-30 35-65 Medium dense

30-50 65-85 Dense

>50 85-100 Very dense


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Cause Effects Influence on

SPT-N value

Inadequate cleaning of hole SPT is not made in original in-situ soil.

Therefore, spoils may become trapped in

sampler and be compressed as sampler is

driven, reducing recovery

Increases

Failure to maintain adequate

head of water in borehole

Bottom of borehole may become quick

and soil may rinse into the hole

Decreases

Careless measure of hammer

drop

Hammer energy varies

Increases

Hammer weight inaccurate Hammer energy varies Increases or

Decreases

Hammer strikes drill rod

collar eccentrically

Hammer energy reduced Increases

Lack of hammer free fall

because of ungreased

sheaves, new stiff rope on

weight, more than two turns

on cathead, incomplete

release of rope each drop

Hammer energy reduced Increases

Sampler driven above

bottom of casing

Sampler driven in disturbed,

artificially densified soil

Increases

greatly

Careless blow count Inaccurate results Increases or

Decreases

Use of non-standard sampler Corrections with standard

sampler not valid

Increases or

Decreases

Coarse gravel or cobbles in

soil

Sampler becomes clogged or

impeded

Increases

Use of bent drill rods Inhibited transfer of energy of sampler Increases

5.2.1.3Advantages of SPT


Dr. P.Anbazhagan

Many existing correlations

Most contractors are capable of SPT testing

Obtain sample by using the split spoon sampler of material and that can be tested to

get soil properties

Relatively cheap

Robust

Suitable for most soils

Only investigation provides soil strength with soil sample; one can feel the soil

5.2.1.4 Disadvantages of SPT

Ground at the base of borehole is disturbed by drilling process

Prone to errors by drillers (e.g. water head, depth measurement errors)

Device imposes very complex strain paths to the soil and no theory at present is capable

of predicting what are the most influential factors affecting the N value.

5.2.2 The cone penetration test (CPT):

• Cone Penetration Test (CPT) is an in-situ test done to determine the soil properties and to

get the soil stratigraphy. This test was initially developed by the Dutch Laboratory for Soil

Mechanics (in 1955) and hence it is sometimes known as the Dutch cone test. On a broad

scale, the CPT test can be, divided into two – Static Cone Penetration Test (BIS-4968, Part -

3, 1976) and Dynamic Cone Penetration Test.

Static Cone Penetration Test: The cone with an apex angle of 60° and an end area of 10 cm2

will be pushed through the ground at a controlled rate (2 cm/sec)

• In static test, the cone is pushed into the ground and not driven. During the penetration of

cone penetrometer through the ground surface, the forces on the cone tip (qc) and sleeve

friction (fs) are measured.

• The measurements are, carried out using electronic transfer and data logging with a

measurement frequency that can secure the detailed data about soil contents and its

characteristics. The Friction Ratio (FR = fs/qc) will vary with soil type and it is, also an

important parameter.

Dynamic cone Penetration Test: Dynamic test will be, conducted by driving the cone using

hammer blows. The dynamic cone resistance will be, estimated by measuring the number of

blows required for driving the cone through a specified distance.

• Usually, this test will be, performed with a 50 mm cone without bentonite slurry or using a

65 mm cone with bentonite slurry. The hammer weighs 65 kg and the height of fall is 75 cm.

The test will be, done in a cased borehole to eliminate the skin friction.


Dr. P.Anbazhagan

• There are lots of correlations available to evaluate soil properties based on the CPT value

(either static or dynamic).

There are several variants of the basic cone penetrometer. Figure 5.3 shows different types of

cone penetrometer. Most popular three of them are

• Piezocone

• Seismic cone and

• Vision cone.

Figure 5.3: Schematic representation of CPT cones

(http://geosystems.ce.gatech.edu/Faculty/Mayne/Research/devices/cpt.htm)

Piezocone penetrometer:

The Piezocone has porous elements inserted into the cone or sleeve to allow for pore water,

pressure measurement. The measured pore water pressure depends on the location of the

porous elements. The piezocone is a very useful tool for soil profiling and estimation of in

situ shear strength, bearing capacity and consolidation characteristics of soils.

Seismic Cone Penetration Test (SCPT): The seismic cone penetration test uses a

standard cone penetrometer with two geophones. One set of geophones is located behind the

f = sleeve friction

q= measured tip stress

q =corrected tip stress

u= u2 shoulder porewater pressure (behind the tip)

U1= u1=

midfacepor

ewater

pressure

Porous filter

element made of

plastic, ceramic

or sintered metal

10-cm2Fricition- Type

cone penetrometer

10—cm2

StandardPiezocone

10- cm2

Type 1 piezocone

15 – cm2 Type 2

piezoconePenetrometer


Dr. P.Anbazhagan

friction jacket and the other set is located one meter above the first set. Typical seismic cone

penetration test is, shown in Figure 5.4.

• The test method consists of measuring the travel time of seismic waves propagating

between a wave source and ground surface. These waves will comprise of shear waves (S

waves) and compressional or primary waves (P-waves). The velocity of seismic waves in the

ground will give the properties like shear modulus and Poisson‘s ratio and soil profile.

Figure 5.4: Schematic represetation of Seismic Cone Penetration Test

Site Characterization by Cone Penetration Testing:

• Cone penetration testing (CPT) is a fast and reliable means of conducting site investigations

for exploring soils and soft ground for support of embankments, retaining walls, pavement

subgrade, bridge foundations etc. The CPT soundings can be used either as a, replacement or

a complement to conventional rotary drilling and sampling methods.

• In CPT, an electronic steel probe is hydraulically pushed to collect continuous readings of

point load, friction, and pore water pressures with typical depths up to 30 m (100 ft) or more

reached in about 1 to 11/2

h.

• Data are, logged directly to a field computer and can be, used to evaluate the geo-

stratigraphy, soil types, water table, and engineering parameters of the ground by the

geotechnical engineer on-site, thereby offering quick and preliminary conclusions for design.

With proper calibration, using full-scale load testing coupled with soil borings and laboratory

testing, the CPT results can be, used for final design parameters and analysis.


Dr. P.Anbazhagan

uaqq ct )1(

cubff st )1(

• In its simplest application, the cone penetrometer offers a quick, expedient, and economical

way to profile the subsurface soil layering at a particular site. No drilling, soil samples, or

spoils are generated; therefore, CPT is less disruptive from an environmental standpoint.

• The continuous nature of CPT readings permit clear delineations of various soil strata, their

depths, thicknesses, and extent, perhaps better than conventional rotary drilling operations

that use a standard drive sampler at 5-ft vertical intervals. Therefore, if it is, expected that the

subsurface conditions contain critical layers or soft zones that need detection and

identification; CPT can locate and highlight these particular features.

Corrections to CPT:

• For electric cones that record pore pressure, corrections can be made to account for unequal

end area effects. Baligh et al. (1981) and Campanella et al (1982) proposed that the cone

resistance, qc, could be corrected to a total cone resistance, qt, using the following expression:

Where, u is pore pressure measured between the cone tip and the friction sleeve and a, is net

area ratio. It is often assumed that the net area ratio is given by

α =

where, d is diameter of load cell support and D is diameter of cone. However, this provides

only an approximation of the net area ratio, since additional friction forces are developed due

to distortion of the water seal O-ring.

• Therefore, it is recommended that the net area ratio should always be determined ‗in a small

caliion vessel (Battaglio and Mankcalco, 1983; Campanella and Robertson, 1988). A similar

correction can also be applied to the sleeve friction (Iunneezet al., 1986; Konrad, 1987).

Konrad (1981) suggested the following expression for the total stress sleeve friction, ft:

where: sb

st

A

Ab

, s

sb

A

Ac

, u

us

• Ast is end area of friction sleeve at top, Asb is end area of friction sleeve at bottom, As, is

outside surface area of friction sleeve, and us, is pore pressure at top of friction sleeve.

• However, to apply this correction, pore pressure data are required at both ends of the

friction sleeve. Konrad (1987) showed that this correction could be more than 30% of the

measured fs, for some cones. However, the correction can be significantly reduced for cones

with an equal end area friction sleeve (i.e., b=1.0).

• The corrections in cone resistance and sleeve friction are only important in soft clays and

silts where high pore pressure and low cone resistance occur. The corrections are negligible

in cohesion-less soils where penetration is generally drained and cone resistance is generally


Dr. P.Anbazhagan

large. The author believes that the correction to the sleeve friction is generally unnecessary

provided the cone has an equal, end area friction sleeve.

CPT Profile, Down-hole Memphis:

• By recording three continuous measurements vertically with depth, the CPT is an excellent

tool for profiling strata changes, delineating the interfaces between soil layers, and detecting

small lenses, inclusions, and stringers within the ground.

• The data presentation from a CPT sounding should include the tip, sleeve, and porewater

readings plotted with depth in side-by-side graphs.

• The total cone tip resistance (qt) is always preferred over the raw measured value (qc). For

SI units, the depth (z) is presented in meters (m), cone tip stress (qt) in either Pascal (MPa

orkPa), and sleeve resistance (fs) and porewater pressure (um) in kPa. Figure 4.5 shows

typical CPT profile

• If the depth of the water table is, known (Zw), it is convenient to show the hydrostatic pore

water pressure (u0), if the groundwater regime is understood to be an unconfined aquifer (no

drawdown and no artesian conditions).In that case, the hydrostatic pressure can be calculated

from:

u0 = (Z – Zw) γw

• Where γw = 9.8kN/m3, in some CPT presentations, it is common to report the um reading in

terms of equivalent height of water, calculated as the ratio of the measured pore water

pressure divided by the unit weight of water.

Figure 5.5: Example of Conductivity Piezocone Test at Mud Island, Memphis,

Tennessee


Dr. P.Anbazhagan

CPT Soil Behavioural Classification:

• A new soil behaviour type classification system , has been presented using normalized cone

penetration test parameters. The new charts represent a three-dimensional classification

system incorporating all three pieces of data from CPT.

• The charts are global in nature and can be, used to define soil behaviour type. Factors such

as changes in stress history, in situ stresses, sensitivity, stiffness, macro fabric, and void ratio

will also influence the classification.

• A guide to the influence some of these variables have on the classification has been

included on the charts. Occasionally, soil will fall within different zones on each chart. In

these cases, the rate and manner in which the excess pore pressures dissipate during a pause

in the penetration can significantly aid in the classification.

• Some of the most comprehensive recent work on soil classification using electric cone

penetrometer data was presented by Douglas and Olsen (1981). One important distinction

made by them was that CPT classification charts cannot be, expected to provide accurate

predictions of soil type based on grain size distribution but can provide a guide to soil

behaviour type. Table 4.4 shows Soil Classification Type from CPT Classification Index, Ic

The CPT data provides a repeatable index of the aggregate behaviour of the in-situ soil

in the immediate area of the probe.An example of a soil classification chart for electric

CPT data is shown in Fig. 5.6 and details are given in Table 5.4

Fig 5.6: Simplified soil behavior type classification for standard electric friction

cone (Robertson et al. 1986)


Dr. P.Anbazhagan

Table 4.4: Soil Classification Type from CPT Classification Index, Ic(after Jefferies and

Davies, 1993) *Notes: Zone number as per Robertson SBT (1990)

Soil Classification Zone Number* Range of CPT Index Ic Values

Organic Clay soils 2 Ic>3.22

Clays 3 2.82<Ic>3.22

Silt Mixtures 4 2.54<Ic>2.82

Sand Mixtures 5 1.90<Ic>2.54

Sands 6 1.25<Ic>1.90

Gravelly Sands 7 Ic<1.25

CPT Tests to Evaluate Seismic Ground Hazards:

• A series of cone penetration tests (CPTs) are conducted for quantifying seismic hazards,

obtaining geotechnical soil properties and conducting studies at liquefaction sites. The

seismic piezocone provides four independent measurements for delineating the stratigraphy,

liquefaction potential and site amplification parameters.

• At the same location, two independent assessments of soil liquefaction susceptibility can be

made using both the normalized tip resistance (qc1 N) and shear wave velocity (Vs1). In lieu of

traditional deterministic approaches, the CPT data can be processed using probability curves

to assess the level and likelihood of future liquefaction occurrence.

• The cone penetrometer system used in these tests included an anchored truck-mounted

hydraulic rig with field computer data acquisition and three geophysics-type penetrometers

(5-, 10-, and 15-ton capacity). Each penetrometer consists of a 60° angled apex at the tip

instrumented to measure five independent readings: tip resistance (qc), sleeve friction (fs),

vertical inclination (i), penetration porewater pressure (either midface u1 or shoulder u2), and

down hole shear wave velocity (Vs). Shear waves are recorded at 1-m depth intervals,

whereas, the other readings are obtained at a constant logging rate, generally set between 1

and 5 cm/s.

• The tip resistance (qc) is a point stress related to the soil strength and the reading must be

corrected for pore water pressure effects on unequal areas, especially in clays and silts. The

corrected value is termed qT. The sleeve resistance relates to the interface friction between the

penetrometer and soil. Magnitudes of pore water pressure depend on the permeability of the

medium and the shoulder filter element (or u2 position) is required for the tip correction.

• The tip resistance (qT), sleeve friction (fs) and pore pressure (u2) are used together to

characterize the subsurface layering, soil behavioural type, and strength properties.


Dr. P.Anbazhagan

Particularly important in seismic investigations, a cyclic stress-based analysis of liquefaction-

prone sediments is available using the qT data.

• The seismic piezocone test (SCPTU) includes both penetration readings and down hole

geophysical measurements in the same sounding, thus optimizing data collection at a given

location.

• In the test procedure, the shear waves are generated by striking a horizontal steel plank that

is coupled to the ground under an outrigger. The downhole geophone is oriented parallel to

the plank to detect vertically propagating, horizontally polarized shear waves. From the

measured wave train at each depth, a pseudo-interval shear wave velocity (Vs) is determined

as the difference in travel distance between any two successive events divided by the

difference in travel times.

• The travel times are determined in two ways: (1) by visually inspecting the recorded wave

traces and subjectively identifying the first arrival, and (2) by a rigorous post-processing

technique known as cross-correlation to determine the time shift between the entire wave

trains from successive paired records.

Interpretation and use of CPT results:

The basic measurements made by a cone are:

1. The axial force necessary to drive the 10 cm2 cone into the ground at constant velocity; and

2. The axial force generated by adhesion or friction acting over the 150 cm2 area of the

friction jacket.

For piezocones, the basic measurement is the pore pressure developed as penetration

proceeds.

Routine calculations convert these measurements into cone resistance, local side friction and

friction ratio.

Cone resistance, qc (normally in MPa) can be calculated from:

qc =

where Fc = force required to push the cone into the ground, and Ac, plan area of the cone, i.e.

10cm2.

Local side friction, fs (normally in MPa), can be calculated from:

fs =

where Fs= shear force on the friction sleeve, and As = area of the friction sleeve, i.e. 150 cm2.

Friction ratio, Rf (in %), can be calculated from:


Dr. P.Anbazhagan

Rf=

Because of the geometry of the electric cone, where pore water pressure acts downwards on

the back of the cone end, the cone resistance will be under- recorded. When used in deep

water, for example for offshore investigations, the force exerted by groundwater will be

significant and if pore pressures are measured (with the piezocone), cone resistance can be

corrected for this effect. The corrected ‗total‘ cone resistance, qt is:

qt = qc + (1-α)u

where α = ratio of the area of the shaft above the cone end to the area of the cone (10 cm2),

typically 0.15 to 0.3, and u=pore pressure at the top of the cone.

Because the pore pressure is not always measured at the top of the cone, but is sometimes

measured either on the face, or on the shoulder, a factor must be applied to the measured pore

pressure. This factor (β) is based upon pore pressure distributions calculated using the strain

path method. Thus:

qt = qc + (1-α)(u0 +βΔu)

where β = ratio between the calculated excess pore pressure at the top of the cone and at the

point of measurement, u0 = hydrostatic pore pressure, and Δu = excess pore pressure caused

by cone penetration. Pore pressure distributions measured and calculated around piezocones.

In soft cohesive soils, at depth, much of the cone resistance may be derived from the effect of

overburden, rather than the strength of the soil. In these circumstances the ‗net cone

resistance‘ may be calculated:

qn = qc - συ

whereqn = net cone resistance, and συ = vertical total stress at the level at which qn is

measured. Net cone resistance can only be calculated once the distribution of bulk unit

weight with depth is known, or can be estimated.

In cohesive soils, the CPT is routinely used to determine both undrained shear strength and

compressibility. In a similar way to the bearing capacity of a foundation, cone resistance is a

function of both overburden pressure (σv) and undrained shear strength (cu):

qc= NkCu+ συ

so that the undrained shear strength may be calculated from:

Cu =

Provided that Nk is known or can be estimated. The theoretical bearing capacity factor for

deep foundation failure cannot be applied in this equation because the cone shears the soil

more rapidly than other tests and the soil is failed very much more quickly than in a field

situation such as an embankment failure.


Dr. P.Anbazhagan

Advantages of CPT:

• Many existing correlations

• Measurements allow soil classification but calibration boreholes preferred

• qc values etc. , are computer logged and not drilling or driller dependent

• Capable of picking up the presence of thin sand/clay lenses.

• Measurements may be related theoretically (at least qualitatively) with soil parameters such

as OCR and Dr

• Allows in-situ determinations of the (reloading) horizontal coefficient of consolidation

• Relatively cheap and very quick

Disadvantages of CPT:

• Need to provide reaction for insertion of cone (typically ≈5t)

• Not ideally suited to Stoney ground

• De-saturation of the pore pressure sensor in dilatant clays

• Upkeep of instruments (+ their calibration): time consuming/expensive.

5.3 Strength and compressibility testing:

Because strength and compressibility parameters are generally required for engineering

calculations, many forms of test have been developed with the specific purpose of

determining them in particular soil or rock types. These tests are not as widely used as the

penetration tests described in the previous section, but nonetheless many are in common

usage. Below we describe the most popular tests in use at the time of writing.

1. The field vane shear test: This is used exclusively to measure the undrained shear strength

of soft or firm clays.

2. The Pressuremeter test:This is used routinely in France to determine strength and

compressibility parameters for routine design, for all types of soil and weak rock, but (in its

self-boring form) used for special projects in over-consolidated clays, to determine undrained

strength, shear modulus, and coefficient of earth pressure at rest, K0.

3. The plate loading test:This is used primarily to obtain the stiffness of granular soils and

fractured weak rocks.

4. The Marchetti dilatometer: This is not yet used commercially but is becoming more widely

used in other parts of the world.


Dr. P.Anbazhagan

In situ strength and compressibility tests are sometimes very much more expensive than

laboratory tests. They suffer from the disadvantage that the soil under load has no drainage

control (i.e. the true state of drainage during the test is not normally known because, unlike a

triaxial test, there is no far drainage boundary), but they are often used because of the many

types of soil which do not lend themselves to good- quality sampling.

5.3.1 Vane shear test:

Early geotechnical engineers found difficulty in determining the shear strength of very soft

and sensitive clays by means of laboratory tests as a result of the disturbance induced by

poor-quality samplers. These difficulties led to the development of the vane shear test.

This device made it possible for the first time to determine the in situ shear strength and

sensitivity of soft clay. The vane shear test, is an in-situ geotechnical testing method used to

estimate the undrained shear strength of fully saturated clays without disturbance.

The test is relatively simple, quick and provides a cost-effective way of estimating the soil

shear strength; therefore it is widely used in geotechnical investigations.

Under special conditions, the vane shear test can be also carried out in the laboratory on

undisturbed soil specimens; however, the use of the vane shear test in in-situ testing is much

more common.

The results of the test are not reliable if clay contains silt or sand.

The vane shear test apparatus consists of a four-blade stainless steel vane attached to a steel

rod that will be pushed into the ground. The height of vane is usually twice its overall width

and is often equal to 10 cm or 15 cm.

Four types of vane are in use. In the first, the vane is pushed unprotected from the bottom of a

borehole or from ground surface.

In the second, a vane housing is used to protect the vane during penetration, and the vane is

then pushed ahead of the bottom of the vane housing before the test is started.

In the third, the vane rods are sleeved to minimize friction between the ground and the rods

during the test.

Finally, some vanes incorporate a swivel just above the blades, which allows about 900 of

rod rotation before the vane is engaged. This simple device allows the measurement of rod

friction as an integral part of the test. Figure 5.7 shows typical vane shear apparatus


Dr. P.Anbazhagan

Figure 5.7: Farnell Field vane apparatus

The test can be conducted either from the ground surface or from the bottom of a borehole or

a test pit. If conducted from the bottom of a bore hole, the test area should be at the depth of

atleast three times the borehole diameter lower than that of the borehole bottom in order to

avoid the borehole disturbance effects.

5.3. 1.1 Test procedure:

The test procedure is as follows:

1. Push the vane slowly with a single thrust from the bottom of the borehole or protected

sleeve for the distance required to ensure that it penetrates the undisturbed soil. Ensure that

the vane is not rotated during this stage.

2. Attach a torque wrench, or preferably a purpose-built geared drive unit, to the top of the

vane rods, and turn the rods at a slow but continuous rate. BS 1377:1990 specifies a rate of 6-

12°/min whilst ASTM D2573 specifies that the rate shall not exceed 6°/min.

3. Record the relationship between rod rotation (at ground surface) and measured torque by

taking readings of both at intervals of 15—30s. Once the maximum torque is achieved, rotate

the vane rapidly through a minimum of ten revolutions, and immediately (within 1 mm —


Dr. P.Anbazhagan

ASTM D2573) restart shearing at the previous slow rate, to determine the remoulded strength

of the soil. Figure 5.8 shows working concept of vane shear test

Figure 5.8: Schematic working concept of Vane shear test

5.3.1.2 Interpretation of vane shear test results:

The vane test is routinely used only to obtain, ‗undisturbed‘ peak undrained shear strength,

and remoulded undrained shear strength. Figure 5.9 shows, schematic representation of

geometry assumed for conventional interpretation of the vane test results. The undrained

strength is derived on the basis of the following assumptions:

1. Penetration of the vane causes negligible disturbance, both in terms of changes in effective

stress, and shear distortion;

2. No drainage occurs before or during shear;

3. The soil is isotropic and homogeneous;

4. The soil fails on a cylindrical shear surface;

5. The diameter of the shear surface is equal to the width of the vane blades;

6. At peak and remoulded strength there is a uniform shear stress distribution across the shear

surface; and


Dr. P.Anbazhagan

7. There is no progressive failure, so that at maximum torque the shear stress at all points on

the shear surface is equal to the undrained shear strength, c.

On this basis, the maximum torque is:

T =

=

=

Figure 5.9: Assumed geometry of shear surface for conventional interpretation of the

vane test.


Dr. P.Anbazhagan

For a vane blade where H = 2D:

T = 3.667D3cu

If it is assumed that the shear stress mobilized by the soil is linearly proportional to

displacement up to failure, then another simple assumption (Skempton, 1948) that the shear

stress on the top and bottom of the cylindrical shear surface has a triangular distribution is

sometimes adopted. For the rectangular vane this leads to the equation:

T =

For a vane blade where H = 2D:

T = 3.53 D3Cu

giving only 4% difference in shear strength from that obtained using the uniform assumption.

5.3.1.3 Advantages of vane share test:

• Provides direct measurement of peak and ultimate undrained strength

• Cheap to use in shallow lightly over consolidated clays

5.3.1.4 Disadvantages of vane share test:

• Only suitable for clays with undrained strengths up to about 100 kPa

• Can give erroneous measurements in very silty clays

• Not many existing correlations (apart from those proposed by Norwegians).

5.3.2 Pressure testing:

Pressuremeter tests can be carried out both in soils and in rocks.

The Pressuremeter probe, which is a cylindrical device designed to apply uniform pressure to

the ground via a flexible membrane, is normally installed vertically, thus loading the ground

horizontally. It is connected by tubing or cabling to a control and measuring unit at the

ground surface.

The aim of a Pressuremeter test is to obtain information on the stiffness, and in weaker

materials on the strength of the ground, by measuring the relationship between radial applied

pressure and the resulting deformation.

Conventional self-boring Pressuremeter cannot penetrate very hard, cemented or stoney soils,

or rocks. In these materials a borehole pressuremeter is normally used.

The Pressuremeter consists of two parts- the read-out unit which rests on the ground surface

and the probe that is inserted into the borehole (ground).


Dr. P.Anbazhagan

The major difference between categories of Pressuremeter lies in the method of installation of

the instrument into the ground. Three main types of Pressuremeter are:

5.3.2.1 The Borehole Pressuremeter:The instrument is inserted into a performed hole.

Originally developed by Menard, a borehole is formed using any conventional type of drilling

rig capable of producing a smooth-sided test cavity.

The pressuremeter has a slightly smaller outside diameter than the diameter of the hole, and

can therefore be lowered to the test position before being inflated.

In the original Menard system the probe contains a measuring cell which is fluid-filled. The

radial expansion of the probe when pressurized is inferred from measurements of volume

made at the ground surface using the control/measuring unit. A guard cell is incorporated into

each end of the probe in order to ensure as far as possible that the measuring cell expands

only radially.

5.3.2.2 The self-boring Pressuremeter:The instrument is self-bored into the ground with the

purpose of minimizing the soil disturbance caused by insertion. Figure 5.10 shows, the

Cambridge self-boring pressuremeter (after Windle and Wroth, 1977).

A self-boring pressuremeter incorporates an internal cutting mechanism at its base; the probe

is pushed hydraulically from the surface whilst the cutter is rotated and supplied with flush

fluid.

The soil cuttings are flushed to the ground surface via the hollow centre of the probe, as the

pressuremeter advances.

Factors affecting the amount of disturbance caused by insertion are:

(i) Soil type;

(ii) Distance of the cutter back from the lower edge of the cutting shoe;

(iii) Diameter of cutting shoe relative to the un-inflated outside diameter of the pressuremeter

membrane;

(iv)The downward force applied during drilling; and

(v) The amount of vibration during drilling.

The degree of disturbance can be minimized by attention to each of these factors at the start

of a testing programme. Regrettably this is not often done for commercial investigations.


Dr. P.Anbazhagan

Figure 5.10: The Cambridge self-boring pressuremeter (after Windle and Wroth, 1977).

5.3.2.3 Displacement Pressuremeter: The instrument is pushed into the ground from the

base of a borehole. The soil displaced by the probe during insertion enters the body of

instrument, reducing the disturbance to the surrounding soil. Figure 5.11 shows, a typical

push pressuremeter.

There are different approaches for the interpretation of results and the determination of

material properties from Pressuremeter tests. In general, these approaches rely either on

empirical correlations to allow measured co-ordinates of pressure and displacement to be

inserted directly into design equations, or on solving the boundary problem posed by the

Pressuremeter test.


Dr. P.Anbazhagan

Figure 5.11: The push-in pressuremeter

5.3.2.4 Test methods for Bore-hole pressuremeters:

Wherever, the soil is strong enough such that a borehole will stand open, uncased, the

borehole pressuremeter test may be carried out as boring or drilling proceeds or more

economically, at the completion of the hole.

In ground which will not stand unsupported (for example, sands and gravels), a special

slotted casing is sometimes used.

The borehole pressuremeter consists of two main elements; a radially- expanding cylindrical

probe which is suspended inside the borehole at the required test level, and a monitoring unit

(known as a ‗pressure-volumeter‘) which is deployed at ground level.

As noted above, the probe consists of three cells. The outer two cells are known as ‗guard

cells‘ and are normally filled with pressurized gas. The central measuring cell is filled with

water and is connected to a sight tube which records volume change in the pressure-

volumeter. Pressure is provided by means of a CO2 bottle.


Dr. P.Anbazhagan

The pressure of both gas and water is increased in equal increments of time, and

approximately equal increments of pressure. Resulting changes in measuring-cell volume are

recorded at 15 s, 30s, 60s and 120 s after each pressure increment is applied. Corrections

must be made for the following:

1. The resistance of the probe itself to expansion: The probe normally consists of both a

rubber membrane and a thin slotted protective metal cover (sometimes known as a ‗Chinese

lantern‘). A calibration test is carried out with the probe at ground surface to determine the

specific relationship (for the pressuremeter in use) between applied pressure and the

volumetric expansion of the unconfined probe. At each volume change during subsequent

tests in the ground, the calibration pressures are deducted from the measured pressure.

2. The expansion of the tubes connecting the probe with the pressure-volumeter:The required

corrections can be determined by conducting a surface test in which the probe is confined in a

rigid steel cylinder, where all measured volume change results from expansion of the leads

and the pressure-volumeter. At each pressure during subsequent tests in the ground, the

calibration volume changes are deducted from those recorded at the given pressure.

3. Hydrostatic effects: These are due to the fact, that the measuring cell and its leads are filled

with water, and therefore the pressure in the measuring cell is higher than that recorded by

the pressure volumeter. In probe/pressure-volumeter systems where the guard cells contain

air, Gibson and Anderson (1961) note that it may become necessary to use two pressure

sources in order to give equal pressures in both guard and measuring cells, when working at

depths in excess of 30m.

The Pressuremeter calibration plots and data correction are shown in Figure 5.12. After the

application of calibration corrections, the results are plotted.

Figure 5.12: Pressuremeter calibration plots and data correction


Dr. P.Anbazhagan

5.3.2.5 Test methods for Self-boring pressuremeter test:

Both stress and strain control can usually be applied to this type of pressuremeter via a

computer-controlled pressure system. The self-boring process works as follows:

Drilling:

A cutter at the foot of the instrument rotates inside an internally tapered shoe. As the

instrument is pressed steadily against the bottom of the hole, a plug of soil is extruded into

the taper much as if it were a conical extrusion die. Figure 5.13 shows, a typical low

disturbance drilling system.

The top of this plug of soil is sliced off by the cutter positioned inside the shoe such that the

pressure needed to drive the soil up the taper is made equal to the in-situ vertical stress. The

soil cuttings resulting are carried away up the inside of the instrument by a flow of flushing

fluid, normally water, supplied from the surface. This water flows, in all but the most

permeable soils, in a closed circuit and does not affect the properties of the soil outside the

instrument.

Figure 5.13 shows a typical low disturbance drilling system.

Procedure:

The Cambridge Camkometer is a self-boring Pressuremeter that minimizes soil disturbances.

It is surrounded over half its length by a suitably tough and protected elastic sleeve or

membrane, initially the same outside diameter as the cutting shoe.


Dr. P.Anbazhagan

The test is carried out by applying gas pressure to the inside of this sleeve and measuring the

resulting changes in radius of the elastic sleeve as a cavity is formed in the soil. The pressure

at which the sleeve lifts from the rigid body of the instrument gives the in-situ total stress.

Two pressure cells mounted through and moving with the sleeve as it expands, give

continuous readings of the pore water pressure. Incremental pressures are applied to radially

expand a rubber membrane that is built into the side wall of the Camkometer and a feeler

gauge measures the radial displacement. Thus, the stress-strain response of the soil can be

obtained.

Readings:

All the measurements made by the instruments are transmitted to the surface by a protected

cable passing up inside the gas supply line.

As with the borehole pressuremeter, the results must be corrected for membrane stiffness and

system compliance before being plotted. But in this case, careful additional calibrations are

also necessary for the various electronic instruments (pressure transducers and displacement

strain followers) that are used.

After application of corrections, self-boring pressuremeter test results are plotted as a curve

of corrected pressure (p) as a function of cavity strain (εc). Cavity strain is the radial strain of

the cavity,

εc =

Where d0 = original diameter of the pressuremeter just before the start of inflation, under

(ideally) the in situ horizontal total stress, and d = current diameter of the cavity, after

expansion under pressure p.

Self-boring pressuremeter results are plotted as applied pressure as a function of cavity strain.

5.3.2.6 Advantages of Pressure testing:

For the Self-boring pressure meter these can be summarised as follows:

1. The tests are performed on virtually undisturbed soil.

2. A large number of fundamental soil properties are obtained from a single test.

3. To derive these properties, no empirical correcting factors whatever are needed.

4. The test is controlled by a semi-automatic system and is largely independent of operator

influence.

5. Results can be obtained quickly.

6. Commercial operation has shown that the instruments, though more complex than

conventional site investigation equipment, are reliable and have enough redundancy to permit

useful readings even if a single fault appears.


Dr. P.Anbazhagan

Disadvantages of Pressure testing:

1. The instrument will not penetrate gravels, claystones or the like.

2. Operating in sands usually demands a cased borehole to a level one or two metres above

the desired test locations.

3. Failure planes and deformation modes are not usually appropriate to those occurring in the

final design.

4. In practice, only two stress paths can be followed- undrained and fully drained.

5. Undrained tests must usually be performed at high rates of strain so as to prevent

introducing errors.

6. The instruments and their associated equipment are complex by conventional site

investigation standards.

7. Results obtained are, sometimes surprising and in several cases have challenged

conventional assumptions of soil mechanics.

5.3.3 Plate loading tests:

Plate loading tests provide a direct measure of compressibility and occasionally of the

bearing capacity of soils which are not easily sampled.

In the test, a plate is bedded on to the soil to be tested, either using sand/cement mortar or

Plaster of Paris. Load is applied to the plate in successive increments of about one fifth of the

design loading, and held until the rate of settlement reduces to less than 0.004mm/mm,

measured for a period of at least 60mm. Load increments are applied either until:

1. Shear failure of the soil occurs; or

2. The plate pressure reaches two or three times the design bearing pressure proposed for the

full-scale foundation.

Load is usually applied to the plate via a factory calibrated hydraulic load cell and a hydraulic

jack. The hydraulic jack may either bear against beams supporting kenledge, or reaction may

be provided by tension piles or ground anchors installed on each side of the load position.

When kentledge is used, the maximum plate size practicable may be considered to be about 1

m dia., since such a plate loaded to two and a half times a design pressure of 200 kN/m2 will

require about 40 tonnes of kentledge.

Settlement is measured using dial gauges reading to 0.05 or 0.01 mm. In order to measure

any tilt that may occur it is advisable to use four gauges on the perimeter of the largest plate.


Dr. P.Anbazhagan

Figure 5.14: Plate load testing setup

These gauges are normally supported on rigid uprights driven firmly into the ground at a

distance of at least twice the plate width from the plate centre: a necessary precaution to

avoid plate settlement interfering with the datum level.

Typical plate load test setup is shown in Figure 5.14.

At each pressure increment, a note is made of the load on the plate and dial gauge readings

are made on a ‗square of the integer‘ basis (i.e. 1, 4, 9, 16, 25 mm, etc.) after load application.

The results of these measurements are normally plotted in two forms:

1. A time— settlement curve and

2. A load—settlement curves.

Typical plots generated from plate load test is given in Figure 5.15

Owing to the natural variability of soil a single test will rarely be sufficient, but due to the

relatively high cost of the test many tests will not be possible.

The number of tests that should be carried out depends on both the soil variability and the

consequences of poor data on geotechnical design.

Tests should not normally be carried out in groups of less than three, and in order to allow

assessments of variability any plate testing should be carried out at the end of a site

investigation, or as part of a supplementary investigation.


Dr. P.Anbazhagan

Figure 5.15: Typical settlement plots from plat load test

5.3.4 Dilatometer test:

Dilatometer test is carried out by pushing or hammering a special dilatometer blade into the

soil, whilst measuring penetration resistance and then using gas pressure to expand a 60mm

dia. thin steel membrane (mounted on one side of the blade) approximately 1mm into the soil.

The operator measures various pressures during the inflation—deflation cycle, before

advancing the blade to the next test depth.

The test is generally well adapted to normally consolidated clays and uncemented sands,

where, the force required for penetration is relatively low, but it is also finding increasing use

in over-consolidated cohesive deposits.

The test equipment consists of rods and a control unit. Figure 5.16 shows, a complete setup of

Dilatometer. In most situations, the blade is pushed from ground surface without the need to

make a borehole, and drilling disturbance is therefore avoided.

The blade is 95mm wide, 14mm thick, with a base apex angle of about 12—16°. Mounted on

one side of the blade is a stainless-steel membrane, which is expanded by gas (preferably dry

nitrogen) pressure supplied through the control unit, by a small gas cylinder at ground

surface. Behind the membrane a spring-mounted electrical sensor is used to detect two

positions, when:

1. The centre of the membrane has lifted off its support and moved horizontally 0.05

mm; and

2. The centre of the membrane has moved horizontally 1.10 mm from its support.


Dr. P.Anbazhagan

Figure 5.16: Dilatometer equipment

The electrical sensor is a switch and this is generally used to sound an audible tone in the

control box. As the membrane expands away from its support the tone should cease

cleanly at 0.05 mm, returning once a deflection of 1.05 mm is achieved.

The blade is connected to the rods to the ground surface, and by a pneumatic- electrical

cable to the control box. The small control box contains a dual-range, manually read

Bourdon pressure gauge, and valves to control gas flow and vent the system. An electrical

ground cable is used to ensure continuity between the control box and the blade.

A simple calibration unit is required, in order that the pressures necessary to achieve the

0.05mm and 1.10mm membrane movements in free air may be measured. At the same

time, the displacements at which the switch is tripped can be checked.

5.3.4.1 Dilatometer Test procedure:

Calibration of the unrestrained membrane should take place at ground surface before and

after each DMT sounding. About 5 mm is required. Apart from checking the correct

functioning of the switch, two values of pressure are measured.

• ΔA is the gauge pressure necessary to suck the membrane back against its support.

• ΔB is the gauge pressure necessary to move it outward to the 1.10mm position.

The blade is pushed into the soil at between 10 mm/s and 30 mm/s. Penetration resistance

is measured (usually at the ground surface, but preferably by using an electrical load cell

mounted in the rod directly above the blade) during the last 10 mm of penetration before

stopping to carry out an inflation of the membrane.


Dr. P.Anbazhagan

During the advance the membrane should be forced back against its support, and

therefore at this stage the control box should be producing its audible signal.

Within 15s of reaching the test depth the rods are unloaded, the control-box vent is

closed, and the gas-control valve is used to pressurize the membrane. The cessation of the

audible signal indicates the point at which membrane lift-off has occurred, and the A-

pressure is then recorded. This should occur within 15—30s from the start of

pressurization.

The gas pressure is smoothly increased so that in the next 15—30s the membrane inflates

to 1.10 mm, and the audible signal returns. The B-pressure is then recorded. The vent on

the control box is immediately opened, in order to prevent damage to the membrane as a

result of over-expansion, and the gas control valve is closed.

Alternatively, a controlled depressurization may be carried out to determine the point at

which the membrane returns to its original position, which is recorded as the C-pressure.

Figure 5.18 shows schematic representation of Dilatometer Test.

Figure 5.18: Schematic representation of dilatometer test

The blade is pushed to its next test depth, and the procedure is repeated. The interval between

test depths is typically between 0.15 and 0.30m. Each test sequence takes about 2 mm, so that

a 30 m deep DMT sounding can be carried out (provided no obstructions are encountered) in

a few hours.


Dr. P.Anbazhagan

5.3.4.2 Reduction of test data:

The A- and B-pressure readings are corrected, using the calibration pressures to give:

P0 = 1.05 (A — zm + ΔA) — 0.05 (B — zm — ΔB)

P1 = B — zm — ΔB

P2 = 1.05 (C — zm + ΔA) — 0.05 (B — zm — ΔB)

where p0 = corrected pressure on the membrane before lift-off (i.e. at 0.00mm expansion),

p1= corrected membrane pressure at 1.10 mm expansion, p2 = corrected pressure at which the

membrane just returns to its support after expansion, A = recorded A-pressure reading in soil

(at 0.05mm), zm = gauge pressure reading (error) when vented, ΔA calibration pressure

recorded at 0.05 mm membrane expansion in air (a positive value), B = recorded B-pressure

reading in soil (at 1.10 mm membrane expansion), ΔB = calibration pressure recorded at 1.10

mm membrane expansion in air (a positive value), and C = recorded C-pressure, at the point

at which the audible signal returns during controlled deflation.

The corrected C-pressure can give a measure of the in situ pore pressure, u, in free-draining

granular soils, or in sand layers within clays (ID>2, approximately). In other soils the initial in

situ pore pressure (i.e. before insertion of the dilatometer) will require estimation.

The quasi-static dilatometer penetration resistance (qD) is obtained from:

qD=

where PD = measured penetration force, and AD = plan area of the dilatometer (95mm x

14mm = 13.3 cm2, as compared with the CPT plan area of 10cm

2). Approximately, qD can be

expected to equal the CPT cone resistance, qc.

From an estimate of the bulk density profile and the in situ pore pressure before DMT

penetration, the in situ vertical effective stress (σ'υ = συ - u) is calculated. Then four DMT

indices are calculated.

1. Material index (a normalized modulus which varies with soil type):

ID =

2. Horizontal stress index (a normalized lateral stress):

KD =

3. Dilatometer modulus (an estimate of elastic Young‘s modulus):

ED = 34.7(p1 – p0)

4. Pore pressure index (a measure of the pore pressure set up by membrane expansion):


Dr. P.Anbazhagan

UD =

5.3.4.3Advantages of Dilatometer Test:

• Provides a physical measurement of (disturbed) horizontal stiffness.

• Established correlations with strength and stiffness.

• May be installed using Cone truck.

5.3.4.4Disadvantages of Dilatometer Test:

• Relationships are currently empirical.

• Relatively slow due to need to log data manually.

5.3.4.5Results, interpretation and use

Results are normally processed on a portable computer (for example, using a spreadsheet

program) and therefore can be rapidly made available for use in engineering decisions and

designs.

In their relatively short life, dilatometer results have become used in a large number of

applications:

5.3.4.6Soil Profiling and Identification

Marchetti and Crapps (1981) provided the soil identification chart shown in Fig 5.19. A

particularly promising method of identifying shear surfaces below landslides in over-

consolidated soils has recently been proposed by Totani (1992).

5.3.4.7Determination of Soil Parameters

The DMT can be used to estimate unit weight (Marchetti and Crapps, 1981; see also there the

soil identification chart), undrained shear strength (Marchetti, 1980; Lacasse and Lunne,

1983; Rogue et. al., 1988), effective angle of friction (Schmertmann, 1982; Marchetti, 1985),

see Fig. 9.35, drained constrained modulus (Marchetti 1980), elastic modulus, and the very

small-strain shear modulus, Gmax.

In clays, the undrained shear strength can be estimated from a form of the bearing capacity

equation:

su =


Dr. P.Anbazhagan

Figure 5.19:Chart for determination of soil description and unit weight (Marchetti and

Crapps,1981).


Dr. P.Anbazhagan

5.4 Permeability testing:

The permeability of a soil can only rarely be obtained with sufficient accuracy from

laboratory tests on specimens from normal diameter boreholes, and therefore the in situ

permeability test is common.

In situ permeability tests can be carried out in soils or rocks, in open boreholes, in

piezometers, or in sections of drill hole sealed by inflatable packers. The two most common

types of tests, are:

1. Rising and falling head tests;

2. Constant head tests;

5.4.1 Rising or falling head tests:

The rising or falling head test is generally used in relatively permeable soils. It is usually

carried out in a cased borehole or a simple piezometer such as the Casagrande low-air entry

open-tube type. Where the groundwater level exists above the base of the borehole, the water

level in the borehole or piezometer tube may either be reduced or increased. Water level

measurements are then taken at suitable time intervals until the water level returns to

equilibrium. Figure 5.20 shows, schematic representation of rising or falling head

permeability test.

Figure 5.20: Schematic representation of rising head permeability test


Dr. P.Anbazhagan

Figure 5.21:Rising and Falling head permeability test results and Method of plotting to

find basic time lag (T)

Hvorslev‘s method (Hvorslev 1951) is used to interpret this type of test, based on the time lag

required for water pressures to equalize.

5.4.1.1 Assumptions:

Soil does not swell or consolidate. Other test errors, such as those due to air in the soil or

pipes, do not occur. There is no smear. At time t, the driving head = H. Therefore, from

Darcy‘s law the rate of flow into the piezometer is given by:

q = FkH = Fk (H0 - y)

where F= piezometer shape factor and k = coefficient of permeability of the soil.

In small time, Δt, the volume of flow into the piezometer tip equals the volume entering the

standpipe:

qdt = Ady

therefore combining we get:

=

Hvorslev introduced the concept of basic time lag. This is the time that would be taken for

equilibrium to be established if the initial flow rate were maintained throughout the test. (In

fact, since the head is reduced by the flow, the rate of flow is progressively retarded during

the test.)

For constant groundwater or piezometric level, the basic time lag is defined as:

T = = =


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Therefore:

=

=

=

where (t/T) is the time lag ratio and (H/H0)[=e-t/T

] is the head ratio.

In order to determine the coefficient of permeability, the time factor, T, must be found.

In order to determine the coefficient of permeability, the time factor, T, must be found. One

simple method which can be widely applied is shown in Fig5.21.

When the time equals the basic lag, then:

= = 0.368

If loge (H/H0) is plotted as a function of time, the basic time lag can be found from the

straight line at loge (H/H0) = -1.0.

This method, requires knowledge of the stabilized water level in order to find H0. In soils of

low permeability, the test may take so long that H0 cannot be found. Obviously, the

equalization time is a function of the volume required to reduce the driving head to zero.

Where in situ tests are carried out, but the groundwater or piezometric level cannot be

determined it may be found by inserting trial values of H0 in the above equations, and

repeatedly plotting the graph of loge (H/H0) vs. time. When the correct value of H0 is

inserted, a straight line will result: incorrect values yield curves.

Once H0 is known, the shape factor must be calculated to allow the coefficient of

permeability to be determined from the basic time lag.

5.4.2 Constant head testing:

Field tests equivalent to the laboratory constant head test can be performed in which

controlled heads or flows are applied to piezometers tip. The constant head test is more

applicable to relatively permeable soils since long test durations can result in evaporation

losses. In contrast, falling head test may be unsuitable for permeable soils because of the

inaccuracy in time measurement as water column drops rapidly. It is suggested by Terzaghi

and Peck that the constant head test is applicable for soils with a coefficient of permeability

not less than 10-3

cm/s.

Constant head testing is required in all soils where stress changes will result in significant

consolidation or swelling. When clay is subjected to an in situ permeability test the effective

stresses in the soil are modified by the increase in pore water pressure normally applied. As


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the soil swells it takes in water, and thus test records normally indicate a higher permeability

than, in fact, exists. Gibson (1966, 1970) and Wilkinson (1968) have considered the use of

the constant head test in clay strata and their objective is to find the rate of flow under steady

seepage conditions, after swelling has occurred.

For an intake formed by a cylindrical filter zone of diameter D and length L in an infinite

isotropic stratum, the coefficient of permeability may be found from Hvorslev‘s equations:

Alternatively, Maasland and Kirkham (1959) have proposed:

Under constant head conditions, the rate of water flow (q) at various times (t) after the test

start is plotted as a function of (1/√t), (see Fig 5.20.). As time passes, swelling reduces and q

decreases. After some time it should be possible to extrapolate to find the rate of flow at

infinite time (qt=∞), the steady flow. The test results may plot as concave up or down,

depending on the A value of the soil (Gibson 1966), and generally they will not give a

straight line on the (1/√t) plot.

Fig 5.20: Effect of Skempton’s ‘A’ value on in-situ permeability test results (Gibson

1970).

module5: site investigation using in situ testing -...

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