Laboratory testing of soil samples is the most common technique to obtain geotechnical engineering properties necessary for design. A laboratory-testing program in geotechnical engineering normally consists of index tests to obtain general information on material consistency and performance tests to measure specific properties (e.g., shear strength, compressibility, hydraulic conductivity) for design and constructability assessments of projects.

Both laboratory and field soil testing programs are necessary for most geotechnical engineering projects and it is therefore necessary to properly select the types and quantities of both types of tests required for any engineering project. This often demands a careful review of all data obtained during the field investigation to develop an appropriate laboratory-testing program. Unless otherwise stated, all laboratory testing should be performed in accordance with relevant and appropriate specifications for laboratory testing as contained in AASHTO, ASTM and BS standards. Table 1 provides a listing of commonlyperformed laboratory tests. Table 2 provides a summary of typical soil index tests while Table 3 gives a summary of typical performance tests,

Laboratory Index Tests for Soils

Index tests provide an inexpensive way to assess soil consistency and variability among samples collected from a project site. Information obtained from index tests can be used to select samples for engineering property testing as well as to provide an indication of general engineering behavior (e.g., high plasticity clay based on plasticity index (PI) may indicate high compressibility, low hydraulic conductivity, and high swell potential). Commonly used index tests discussed in this manual include moisture content, unit weight, specific gravity, Atterberg limits, particle size distribution, and organic content. Index testing should be conducted on soil materials from every project. Information from these tests should be assessed prior to a final decision on the selection of specimens for subsequent performance testing.

Laboratory Performance Tests for Soils

These are the tests that are used to estimate design soil properties for deformation, shear strength, and permeability characteristics of soils. The tests are usually more costly and time consuming in carrying out than most index tests. The results, however, provide specific data regarding engineering performance. These tests include consolidation, shear strength, and permeability testing.

The Cone Penetration Test for estimating the consistency of clayey soils is included among the index tests while the West African Compaction is one of the standard methods of soil densification and improvement highlighted in the manual. The Kansas Test Method (KT-51) for measuring the "in-place" density and moisture content of soils by the nuclear gauge method is in Appendix D.

The laboratory and field test results are only meaningful when the tests are carried out properly, the resulted are well-assessed and properly interpreted. The results should be interpreted and compared to project expectations and requirements. It is very necessary that the full integration of field and laboratory test results must be coupled with the site-specific design. If test results are not completely consistent, the reason(s) should be evaluated, poor data should be eliminated, and similarities and trends in data should be identified. It may be necessary to return to the laboratory and conduct an additional review of sample extrusion, selection, and testing. In particular, selection of the correct engineering property tests, their interpretation, and summarization are indispensable in the design of geotechnical projects.