In the USA the Unified Soil Classification System is commonly employed. This was developed by the Bureau of Reclamation and the Corps of Engineers. BS5930 defines the British soil classification system for engineering purposes. Both are useful systems and have close similarities. However, neither takes full acount of the in-situ condition of a soil since they are based on index properties of disturbed samples. The insitu density, strength, moisture content and fabric are not taken account of in these classification systems and tire ol fundamental importance to many aspects of insitu geotechnical behaviours. Therefore, classification systems are generally more relevant to the performance of disturbed soil used in earthworks than to perform­ ance of the same material in-situ.

Classification systems have also been developed for rock mass characteristics. These are used for the design of tunnels or underground chambers but may have application to other rock engineering schemes. They generally involve an assessment of all relevant charac­ teristics of the rock mass, including strength, fracture state and weathering, according to a predetermined weighting scheme, and yield an index of rock mass quality which can be used for design purposes.

2.7 Laboratory tests

Laboratory tests can be carried out to measure a wide range of soil and rock characteristics. It is therefore tempting to employ the results of tests for a particular parameter directly in soil or rock mechanics formulae to obtain a design solution to specific engineering problems. This should only be done after the test results have been critically examined and possibly modified, using engineering judgement, because there are a number of factors which may limit the ability of laboratory tests to predict field behaviour. The most common ones are as follows.

Samples will inevitably have suffered some mechani­ cal disturbance during, sampling, transport and test specimen preparation. This generally reduces the undrained strength and increases the compressibility of a sample. It also reduces permeability. Stress relief is unavoidable, even in the best sampling operations. It has similar but normally less severe effects to distur­ bance. It can be very important in fissured materials.

Samples used for laboratory testing should be large enough to include a representative amount of the fabric of the material sampled. Many commonly used sample sizes of 100 mm diameter or less are too small to meet this criterion which is particularly important in per­ meability and consolidation tests. Sample size effects are also important in strength testing of rocks and fissured soils. Finally, and most importantly, the samples tested must be representative of the strata encountered.

The standard soil and rock mechanics tests do not model simultaneously all the factors which may

influence ground behaviour under field conditions. Since geological materials exhibit complex inelastic behaviour, it is therefore inevitable that field behaviour will differ somewhat from that observed in the labora­ tory. It is, however, best to conduct laboratory tests according to accepted standard procedures.

There are methods of mitigating or making allow­ ance for some of the above limitations where they are recognised to exist. Also, empirical design procedures often implicitly make an allowance for the influence of standard testing techniques under normal conditions. There, if samples arc judiciously chosen, the test methods arc appropriate and the tests arc carefully carried out, then useful results can be obtained from laboratory testing. However, as previously noted, the results should always be critically reviewed to see that they are consistent with all other data obtained for the site and any published information for comparable geological materials.

Laboratory tests fall into the following categories — index or classification tests, chemical tests may be used to categorise the materials encountered within a standard soil classification system. They may also be used to guide the selection of representative samples for engineering tests and so interpret the results of the latter.

A useful summary of the common laboratory tests on

• soil and rock is contained in Table 4 of BS5930.

2.8 Factual reports

It is imperative that a good quality factual report is compiled as a record of all the investigations made for any specific site. In terms of the desk study this might be a fairly brief typewritten account of the various matters examined and the more important facts which emerged. Copious data records should not be discarded but rather consigned to references or appendices where they will remain available for inspection by the few people who may require to study them in detail. It is important to remember that some projects and sites do not immediately proceed to further development stages but they may re-emerge at a much later date. Then the factual report of the earlier work is the only data readily to hand, and is consequently of great value at that time.

If the geotechnical site work has been undertaken by a contracting firm, it is common practice to specify that it produces a factual report of all its operations, includ­ ing the laboratory testing. Site plans are an important element of reports as it is often significant to know exactly where certain data were obtained. Commonly the site plans should be at two scales — at a small scale to define the site area relative to its surroundings, and at a sufficiently large scale to be able to pinpoint the locations of certain items, e.g., a borehole or the buried foundations of a demolished building. •