Laboratory testing of soils
7.10 Comments on soil testing
Although the routine soil tests described in this chapter are relatively simple there is a lot that can, and often does, go wrong with soil tests. Probably the most signifi- cant sources of error in measurements of soil parameters and behaviour in laboratory tests are:
1. Malfunctions and errors in the apparatus and in the instruments. 2. Incorrect detailed procedures in performing the tests.
3. Doing the wrong test or measuring the wrong parameter for a particular application. The last of these is simply a matter of sound understanding of the basic theories involved, rather than blindly following a cookery book approach. The purpose of this book is to develop this sound understanding. The first two are largely a matter of care and attention and experience. In assessing the quality of a set of test results it is essential to distinguish very carefully and clearly between the accuracy and the resolution of the instruments. The resolution (or precision) of an observation is the smallest increment that can be discerned, while the accuracy is the limit within which you can be absolutely confident of the data. For a typical dial gauge measuring small displacements, the resolution and accuracy are both about 0.001 mm, but the resolution and accuracy of electronic instruments are often very different.
For a typical electronic load cell, pressure transducer or displacement transducer the resolution is linked to the electronics which converts an analogue signal (usually a small voltage) to a digital signal. For a 16-bit converter, using 1 bit for the sign, the resolution is 1 in 215(≈30 000) of the full-scale reading, so for a pore pressure transducer with a range of 0 to 1000 kPa the resolution is about 0.03 kPa. The accuracy depends on the linearity (or non-linearity) of the calibration constant between pressure and voltage and on the stability of the electronic signals. With most instruments commonly used in soil testing you will be doing well to achieve an accuracy better than±1 kPa, which is very different from the resolution.
The most difficult measurements to make are of small strains less than about 0.1% in triaxial and shear tests. With conventional instruments for measurement of axial and volumetric strain like those shown in Figs. 7.3 and 7.7 errors arise due to leakage and compliance (movements) in the apparatus and often these errors are greater than the measurements being made. Measurement of small and very small strains using local gauges and dynamic methods are described in Chapter 13.
Another factor is in detection of malfunctions in instruments. It is usually fairly easy to see whether a dial gauge or proving ring is not working properly, but it is much less easy to detect malfunctions in electronic instruments provided that they
continue to produce reasonable output signals. The consequence of this is that use of electronic instrumentation in soil testing does not necessarily improve the accuracy of the results compared with old-fashioned instruments and may even reduce the accuracy considerably unless the instruments are frequently checked and recalibrated. The moral of all this is that you should always be suspicious of the accuracy of all laboratory tests.
7.11 Summary
1. Laboratory tests are carried out for description and classification of soils, to inves- tigate their basic mechanical properties and to determine values for the stiffness and strength parameters.
2. The principal tests for description and classification are grading by sieving or sedimentation and the Atterberg limit tests which determine the liquid and plastic limits.
3. The principal loading tests are one-dimensional compression (oedometer) tests, shear tests and triaxial tests. These may be drained or undrained and they may be stress controlled or strain controlled.
4. Special loading or unloading stress path tests are carried out in hydraulic triaxial cells. In these tests the axial and radial stresses or strains and the pore pressure can be varied independently to follow the desired stress path.
Worked examples
Example 7.1: Interpretation of a constant head permeameter test A constant head
permeameter has a diameter of 100 mm and the standpipe tapping points are 150 mm apart. Results of a test on a relatively coarse-grained soil are given in Table 7.1.
Table 7.1 Results of constant head permeability test – Example 7.1 Volume of water collected
in 1 min (cm3) Difference in standpipe levels (mm) 270 75 220 60 160 45 110 30
The seepage velocity V is given by Eq. (7.5) and the hydraulic gradient i is given by Eq. (7.6). For the first observation,
i= 75 150= 0.5 V = Q At = 270× (0.01)3 (π/4) × 0.12× 60 = 5.7 × 10−4m/s
Figure 7.11 shows values of V plotted against i. These fall close to a straight line through the origin, which demonstrates that the basic form of Darcy’s law (Eq. 7.4)
Figure 7.11 Results of constant head permeability test – Example 7.1.
is correct. The coefficient of permeability given by the gradient of the line is
k≈ 1 × 10−3m/s
Example 7.2: Interpretation of a falling head permeameter test A falling head perme- ameter has a diameter of 100 mm, the sample is 100 mm long and the area of the standpipe is 70 mm2. Results of a test on a relatively fine-grained soil are given in Table 7.2.
Table 7.2 Results of falling head permeability test – Example 7.2 Time (s) Height of water in standpipe
above overflow (m) (P0/P) ln(P0/P) 0 1.60 1 0 60 1.51 1.06 0.06 120 1.42 1.13 0.12 240 1.26 1.27 0.24 480 0.99 1.62 0.48
At any instant the potential P is the height of water in the standpipe (above the overflow) and P0= 1.60 m at t = t0. Figure 7.12 shows the ln(P0/P) plotted against time. The data points fall close to a straight line. Hence, from Eq. (7.9) the coefficient of permeability is given by k=aL A ln(P0/P) t = 70× (0.001)2× 0.1 (π/4) × (0.1)2 × 0.1 100≈ 1 × 10 −6m/s
Figure 7.12 Results of falling head permeability test – Example 7.2.
Example 7.3: Interpretation of a drained triaxial test The first three columns of Table 7.3 give data from a drained triaxial compression test in which the cell pres- sure was held constant atσc = 300 kPa and the pore pressure was held constant at u= 100 kPa. At the start of the test the sample was 38 mm in diameter and 76 mm long and its specific volume was v= 2.19.
Table 7.3 Results of drained triaxial test – Example 7.3 Axial force Fa(N) Change of length L (mm) Change of volume V (cm3) εs εv v q(kPa) p(kPa) 0 0 0 0 0 2.19 0 200 115 −1.95 −0.88 0.022 0.010 2.17 100 233 235 −5.85 −3.72 0.063 0.042 2.10 200 267 325 −11.70 −7.07 0.127 0.080 2.01 264 288 394 −19.11 −8.40 0.220 0.095 1.98 287 296 458 −27.30 −8.40 0.328 0.095 1.98 286 296
The initial dimensions of the sample were
A0= π 4D
2
0= 1.134 × 10−3m2 V0= A0L0= 88.46 × 10−6m3
At any stage of the test εa= −L L0 εv = −V V0 v= v0(1− εv) and σr= σc= 300 kPa σa= σr+ Fa A
where the current area is A= A0(1− εv)/(1− εa). From Eqs. (3.5) to (3.8), εs= εa− 1 3εv and q= (σa− σr)= q p= 13(σa+ 2σr)= p − u or q= Fa A p = p 0+13q− u
where p0= 300 kPa. The test results are given in the right-hand side of Table 7.3 and are plotted in Fig. 7.13 as O→ A.
Example 7.4: Interpretation of an undrained triaxial test The first three columns in Table 7.4 give data from an undrained triaxial compression test in which the cell pressure was held constant atσc = 300 kPa. At the start of the test the sample was 38 mm diameter and 76 mm long, the pore pressure was u0= 100 kPa and the specific volume was v= 2.19.
For an undrained testεv= 0 (by definition), but otherwise the calculations are the same as those given in Example 7.3. The test results are given in the right-hand side of Table 7.4 and are plotted in Fig. 7.13 as O→ B.
Example 7.5: Stress paths The left-hand side of Table 7.5 gives the initial states and increments of axial and radial total stresses for a set of drained and undrained tri- axial stress path tests. In the drained tests the pore pressure was u = 0. The soil can be assumed to be isotropic and elastic so that shearing and volumetric effects are decoupled.
The stress paths corresponding to tests lasting for 10 hours are shown in Fig. 7.14. The right-hand side of Table 7.5 gives the states at the start and at the end of each path. For the undrained testδp= 0 (because δεv = 0 and shear and volumetric effects are decoupled). For the drained tests the changes of qand pare found from Eqs. (7.13) and (7.14).
Figure 7.13 Results of drained and undrained triaxial tests – Examples 7.3 and 7.4.
Table 7.4 Results undrained triaxial test – Example 7.4 Axial force
Fa(N)
Change of length
L (mm) Pore pressure(kPa) εs
q(kPa) p(kPa) 0 0 100 0 0 200 58 −1.95 165 0.026 50 152 96 −4.29 200 0.056 80 127 124 −9.36 224 0.123 96 108 136 −14.04 232 0.185 98 101 148 −19.50 232 0.257 97 100
Table 7.5 Loading in stress path tests – Example 7.5
Sample σa σr dσa/dt dσr/dt Drainage σae σre q0 p0 qe pe
(kPa) (kPa) (kPa/h) (kPa/h) (kPa) (kPa) (kPa) (kPa) (kPa) (kPa)
A 200 200 10 0 Drained 300 200 0 200 100 233
B 200 200 −10 0 Undrained 100 200 0 200 −100 200
C 250 175 −10 −10 Drained 150 75 75 200 75 100
Figure 7.14 Stress paths – Example 7.5.
Further reading
Atkinson, J. H. and G. Sallfors (1991) ‘Experimental determination of soil properties. General Report to Session 1’, Proceedings of 10th ECSMFE, Florence, Vol. 3, pp. 915–956. Baldi, G., D. W. Hight and G. E. Thomas (1988) ‘A re-evaluation of conventional triaxial test
methods’, in Advanced Triaxial Testing of Soil and Rock, R. T. Donaghe, R. C. Chaney and M. L. Silver (eds), ASTM, STP 977, pp. 219–263.
Bishop, A. W. and D. J. Henkel (1962) The Triaxial Test, Edward Arnold, London.
BS 1377 (1990) Methods of Test for Soils for Civil Engineering Purposes, British Standards Institution, London.
Head, K. H. (1980) Manual of Soil Laboratory Testing, Vol. 1, Soil Classification and
Compaction Tests, Pentech Press, London.
Head, K. H. (1982) Manual of Soil Laboratory Testing, Vol. 2, Permeability, Shear Strength
and Compressibility Tests, Pentech Press, London.
Head, K. H. (1986) Manual of Soil Laboratory Testing, Vol. 3, Effective Stress Tests, Pentech Press, London.