Friday, December 14, 2012

Field (site) Testing of Soils.

No matter how carefully soil samples are taken, stored, transported to a laboratory and tested, some disturbance is possible and even likely – and therefore many engineers prefer the alternative of testing the soil in situ. As with  sampling techniques there have been advances in sophistication and variety of field testing techniques and the most common types are briefly described here.

Site testing has come a long way from kicking the clay at the bottom of a trial pit with the heel of the investigator’s shoe – though this can still be a useful, if crude, assessment when carried out by an experienced engineer familiar with local conditions.

In foundation design less is known of soil as a structural material than is known of concrete and steel. It is not possible to analyse and forecast, with certainty, the stresses in the soil or the soil’s reaction to those stresses, since the foundation loading can only be a reasonable assessment.

Foundation design is therefore based not solely on analysis but also needs the application of sound engineering judgement.

In a sensible and valuable search to understand the material it must be tested and some researchers have devoted their careers to this essential cause. In each of the following field and laboratory tests there has been extensive research, literally thousands of learned papers and many international conferences – some devoted to just one test, for example, see References 5 and 6. It is not possible therefore in a  book on foundation design to discuss fully in depth any one test; discussion is limited to the broader considerations.

Furthermore the site and laboratory testing of soils is  the contractual responsibility of the soil survey specialist.

Hence the following sections outline and summarize the tests and the main references are given for designers wishing for more detailed information. Experience is necessary to estimate what and how to test, the test results need engineering judgement in assessing their application and relevance and in forecasting estimated behaviour – for none  of the tests give scientifically accurate results applicable to the actual strata under the real pressure. The theories, as in structural theory, are based on simplifying assumptions not fully related to the reality of practice. But to dismiss tests and theory and rely on outdated  rules of thumb  methods is inappropriate to modern structures and is as foolish as blind faith in science.

1 Standard Penetration Test (SPT)  The SPT is a useful method of indicating the relative  density of sands and gravels. It is based on the fact that the denser the sand or gravel the harder it is to hammer a peg into it. A standard weight is dropped a defined distance on a tube, with either a split tube or a cone head (cone penetration test, CPT), placed in the borehole. The tube is driven 450 mm into the soil and the number of hammer-blows taken to drive the tube into the last 300 mm of soil is termed its  N  value. Care in interpreting the result is particularly  necessary where boulders, very coarse gravel or bricks in backfill may be present, for the measurement may be of the resistance of the obstruction and not of the soil.

Approximate values of the relationship between sand properties and  N  values are given in Table 3.3 and a
summary of the test is given in Table 3.4. CIRIA Publication  The Standard Penetration Test (SPT): Methods and Use(7) is a comprehensive reference.

2 Vane test  If a garden spade is driven into clay and then rotated it will effectively shear the clay and the higher the shear resistance of the clay then the greater the force (torque) required to rotate the spade. This is the principle of the vane test.

The vane is a cruciform of four blades fixed to the end of the boring tube’s rod. It is pushed into the undisturbed soil at the base of the borehole or trial pit and the torque required to rotate the vane is measured. Table 3.5 gives a summary of the test.

When the height of the vane is twice its diameter, D(m), the relationship between shear strength of the soil, τ, and the maximum applied torque, M(kN m), is generally:

3 Plate bearing test  A plate, of known area, can be placed at the bottom of a trial pit or borehole and loaded. The settlement of the soil under load can be measured and also the pressure required to cause
shear failure of the soil. The test is summarized in Table 3.6.

4 Pressuremeters  A pressuremeter could be considered as basically a vertical plate test. If an expanding cell is placed in a borehole and pumped up to exert pressure against the sides of the bore then the stronger the soil the greater the pressure required to expand the cell. Summaries of different pressuremeters are given in Table 3.7.

5 Groundwater (piezometers and standpipes)  The presence of moisture in, and the magnitude of moisture content of, soils has a pronounced effect on soil properties and behaviour. Since the moisture content can vary so too can the soil. It is essential therefore to investigate the ground-water conditions and possible variation. Groundwater variations are likely on coastal, estuarine and tidal river sites; sites subject to artesian conditions and variable water-table levels; sites with permeable granular soil where bored piles or bentonite diaphragm walls are to be used, and particularly sites founded on fills.

The rate of seepage of groundwater into pits and bores together with level and variations in level should be
recorded. Piezometers or standpipes should be employed when groundwater problems are anticipated.

A standpipe can at its simplest be the open borehole, and the outline of the test is summarized in Table 3.8.
Piezometers, of varying sophistication, are basically perforated tubes lined internally with porous tubing, and details are summarized in Table 3.9.

6 Other field tests  There are a number of developments, refinements and adjustments to the above tests as well as geophysical  tests, aerial infra-red photography, video photography in boreholes, etc. These newer tests can sometimes be less expensive, less time-consuming and yield more information than the traditional tests. The interested reader should refer to specialist soil mechanics literature for details.

 Table 3.3 Relationship between N values and sand properties

 Table 3.4 Standard Penetration Test (Weltman, A.J. & Head, J.M.,  
Site Investigation Manual, CIRIA (1983)

 Table 3.5 Vane test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

Table 3.6 Plate bearing test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

 Table 3.7 Pressuremeter test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

 Table 3.8 Open borehole test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

Table 3.9 Piezometer test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

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