Soil Testing

‘Boring’ is done as per IS: 1892, making use of mechanically operated equipments. Boreholes of nominal diameter 150mm minimum will be sunk employing shell and auger equipment or any other duly approved method or technique. The size of the drill holes will be normally NX (73mm), but may be reduced to BX (60mm) depending on soil surface layers or strata conditions.

Drilling in rock and collection of rock core samples are done in accordance with IS: 6962, IS: 4464 and IS: 10208. The observations for Core drilling are undertaken in accordance with IS: 5313.

Boreholes are taken down to a layer at which N value is greater than 50. However, if hard rock is encountered on a continuing basis during the process of boring, drilling is continued for a depth of one meter into hard rock before terminating the boring process.

‘Standard penetration test’ is an in situ test done to evaluate the engineering properties of sub-surface soil strata. It is a common, simple, and inexpensive test conducted to determine the shear strength parameters and the relative densities of soils. The tests are generally carried out at intervals of 1.5 meters or at every change of strata, whichever is met earlier. The tests are conducted by following IS: 2131. The intervals of penetration are increased to 3m if in between vane shear or cone penetration test is performed.

‘Cyclic Plate Load'[ test is carried out in the field to assess or evaluate the allowable or maximum bearing capacity of soil for design of foundation structures. The test is done as per IS: 5249.The plate will be located at a depth of 3.0 m in a pit excavated as mentioned in IS: 1888.

The application of the load on the test plate is done gradually taking care to avoid impact, fluctuation, tilting of plate or eccentricity, so as to apply a uniformly distributed static load on the soil. The load is maintained constant throughout for a particular time period till no further settlement occurs or the rate of settlement becomes almost negligible.

The loading is increased in increments of 5 tons per square meter until the total settlement of the plate is 25mm or the soil under the plate fails, whichever occurs earlier. Settlement of the plate is recorded by dial gauges of minimum accuracy of 0.02mm, fixed at diametrically opposite ends, and the tests are done as per IS: 1888.

The entire load is made to remove quickly but gradually and the plate is allowed to rebound; when no further rebound occurs or the rate of rebound becomes negligible the readings of the dial gauges are again noted.

The cycles of loading, unloading are continued till the estimated ultimate load has been reached; the final values of readings of dial gauges being noted each time. The value of the load increment will be such that the ultimate or final load is reached within 5 to 6 increments.

From the data obtained during this test, the elastic rebound of the plate corresponding to each intensity of loading is plotted. Finally, the dynamic properties of soil are furnished along-with detailed calculations and plots.

Vane Shear test is a simple test done to estimate the sensitivity of a soil sample. It is a useful test method for the determination of shear strength of clay. Vane Shear test is carried out in accordance with IS: 4434 (if the clayey layer is found in borehole/trial pits).

‘Standard penetration test’ is an in situ test done to evaluate the engineering properties of sub-surface soil strata. It is a common, simple, and inexpensive test conducted to determine the shear strength parameters and the relative densities of soils. The tests are generally carried out at intervals of 1.5 meters or at every change of strata, whichever is met earlier. The tests are conducted by following IS: 2131. The intervals of penetration are increased to 3m if in between vane shear or cone penetration test is performed.

• This test is simple, cheap and can be done quickly.
• This test can be conducted on soft clays which are situated at inaccessible depths.
• This test can determine the un-drained shear strength of non-fissured fully saturated clay.

Trial pits are done as per IS: 4453 and IS: 1892 by open excavations in order to collect samples and determine the nature and characteristics of soil.

The base of pit has a size of 1.5 mx1.5 m (minimum) and 4.0 m depth for all types of soils. After completion of all the tests the trial pits are backfilled and compacted as per the requirements of the client.

In-situ tests are conducted to determine the CBR values of existing materials at different locations along the proposed road alignment, as per different clauses of IS: 2720 (part 31). The tentative locations of CBR tests are indicated in the ‘Drawing Result’ of the tests; in addition the suitable recommendations need to be included in the investigation report as well.

All test data, charts, curves etc along with calculations for determination of in-situ CBR values have to be finally reported.

Electrical resistivity of soil is a measure of the resistance that a soil type (of a specified volume) offers to the flow of electricity through it. An estimate of soil resistivity and its variation with depth in the soil is critical in the designing of the grounding system in an electrical substation. It is also needed for designing earthing electrodes for high-voltage direct current transmission systems. The soil resistivity value varies substantially with changes in moisture content, chemical c content (salts), temperature etc. It also governs the corrosiveness of soil. It is measured in Ohm-meter (Om). The Wenner 4-point method is the most common & preferred test method for measuring the electrical resistivity of soil. The test result should contain the general description of the soil tested like type of soil, grain size distribution of the soil at the test location, etc.

Testing of soil (for engineering purposes) is usually done to assess its physical properties which affect or control its behavior in relation to various construction purposes. Although testing activities include on-site/in-situ and laboratory testing, the latter one is assumed to give a better and accurate estimate of its properties. Each of the different properties of soil gives an estimate of a particular characteristic of it; like the moisture content of a soil specimen provide a guide to the compaction process, besides governing the Atterberg limits (which characterize the soil’s plastic and the liquid limits), consistency, shrinkage characteristics, and a host of other strength properties. Size-grading of soil helps in the optimum designing for preparing concrete mixes and porosity along-with permeability tests are conducted on a soil sample to determine the suitability of semi-pervious materials for use in the core of fill dams. Laboratory testing of soil also aids in excavation activities and constructions of earthworks (cut & fill).