Civil and Building Engineering.

The foundation engineer must tell the testing laboratory how to test the soil so that the necessary soil properties are measured for the problem at hand.

Often the engineer must select soil tests while considering how much money is available for testing. Additionally, the engineer must recognize that meaningful testing can be accomplished only on soil samples of good quality and that testing of disturbed soil samples may obtain results that are not representative of field conditions.
Figure 3.63 Computer-controlled triaxial testing equipment (photograph courtesy of Trautwein Geotac).

that testing of disturbed soil samples may obtain results that are not representative of field conditions.
In light of these considerations, the foundation engineer understands the following limitations and capabilities of the consolidation test, direct shear test, and triaxial test:

1. The consolidation test measures the consolidation properties of the soil needed for calculations of total settlement and time rate of settlement.
2. The direct shear test can be used economically to measure the angle of internal friction of sand. The shortcoming of this test is that it is not possible to measure a stress-strain curve for sand.
3. The direct shear test on clayey soils can be used to measure the drained friction angle of clays more economically than S-type triaxial tests.
Again, as with direct shear tests on sand, the shortcoming of this test is that it is not possible to measure a stress-strain curve for the soil.
4. UU-type triaxial tests are the best tests to measure the undrained shear strength of cohesive soils. While these tests can also be used to test sands, this requires that a specimen be built for testing. This usually
involves more work to set up the specimen for testing than for a specimen of cohesive soil. One limitation of the UU-type triaxial test is that the effects of consolidation or rebound of vertical stress cannot be ad-
justed for in the test because the soil specimen is not subjected to consolidation pressures. If the effects of consolidation are important, a shearing test that permits consolidation must be used.
5. The CD triaxial test is the best and most expensive test used to measure the fully drained shearing properties of soils. The test results include the consolidation properties of the soil, the angle of friction under fully drained conditions, and the stress-strain curve. Unfortunately, time and budget constraints often eliminate the use of CD triaxial test on many projects. If only the fully drained angle of friction is required, a drained
direct shear test is a practical alternative.
6. The CU triaxial test without measurement of porewater pressures during shear is used when undrained shear strength values are needed for soils that are subject to consolidation in the field.
7. The CU triaxial test with measurement of porewater pressures during shear can be used to measure the effective stress shearing parameters of cohesive soils more economically than the CD triaxial test.