Field Tests

Plate load test and penetration tests are very useful field tests for site investigation. The set-up and other details of these  tests are given in the following paragraphs. 

Plate Load Test 

It is one of the earliest'tests to be performed to determine the bearing capacity of soil. The test consists of making a pit, usually at foundation level and measuring the settlements of a circular or square mild steel plate not less than 25 mm in thickness and 300 to 750 mm in size subject to gradual load increments. From the test results, load vs settlement curve is plotted and the ultimate bearing capacity of the soil is arrived at. This test has the following limitations : 

• The results are indicative of the character of boil within a depth of less than twice the width of the bearing plate which is usually 300 to 750 mm. Since the actual dimensions of foundations are larger, the settlements and shear resistance will depend on a much thicker stratum of the sub-soil. 
• It is a short duration test and does not give an idea about consolidation settlement particularly in cohesive soils. Thus, the results of the test could be misleading if the character of soil changes at shallow depths, which is not uncommon. 
• While for clayey soils, the bearing capacity (from shear consideration) for a larger foundation is almost the same as for the test plate; while in the  case of dense sandy soils, the bearing capacity increases with the size of foundation and hence, in such cases, the plate load test gives a conservative value. 

Penetration Tests 

These tests involve the measurement of the resistance to penetration of a sampling spoon, a cone or other shaped tools under dynamic or static loading. The commonly used tests are the standard penetration test and the static and dynamic cone penetration tests. From these tests, the soil strata at various depths along with their properties can be obtained. From these penetration values, bearing capacity can be assessed. 

Soil exploration should be able to provide necessary data to evaluate the following parameters : 

• safe bearing capacity of the soil, 
• subsoil water level, and 
• probable settlement of the structure under the design load. 

The test report generally gives recommendations regarding the depth of foundation and safe hearing capacity.