Subsurface explorations should generally be carried out in two stages, that is, preliminary and detailed.
3.2.1.2 Detailed exploration
Detailed investigation follow preliminary investigation and should be planned on the basis of data obtained during review as the investigations progress. The scope of detailed exploration is generally to determine shear strength and compressibility of all types of soils, density, density index, natural moisture content and permeability. It is also necessary to determine the preconsolidation pressure of the strata from oedometer tests and consolidation characteristics beyond the preconsolidation pressure. The detailed investigation includes boring, detailed sampling and laboratory tests to determine the physical, chemical and engineering
properties of soil and rock. Field tests which may be performed are in-situ vane shear tests (suitable in case of soft cohesive soil) and plate load tests.
In a situation of liquefaction possibility, exploratory boreholes with SPT and sampling at very close depth intervals should be carried out, while static cone penetration tests with or without pore water pressure measurement are very useful. The cross hole shear tests and laboratory cyclic shear tests are useful in assessing the liquefaction potential.
NOTE — A method for assessment of liquefaction potential of a site is given in Annex J of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.
3.2.1.3 The common methods of subsoil exploration are given below:
a) The most commonly used exploration procedure is boring in which different field testing procedures, like, standard penetration tests (SPT), field vane shear tests (VST), collection of undisturbed and disturbed sampling (UDS and DS), ground water table observations and field classification are employed.
b) Open trial pits — The method consists of excavating trial pits and thereby exposing the subsoil surface thoroughly, enabling undisturbed samples to be taken from the sides and bottom of the trial pits. This is suitable for all types of formations, but should be used for small depths (up to 3 m). In the case of cuts which cannot stand below water table, proper bracing should be given. The method may be used for examining the suitability of borrow pits.
Auger boring — The auger is either power or hand operated with periodic removal of the cuttings. Auger boring can be adopted in soft to stiff cohesive soils above water table. Augers shall be of helical or post hole type which may be manually or power operated. While boring, care shall be taken to minimize the disturbance to the deposits below the bottom of the borehole. The cuttings brought up by the auger shall be carefully examined in the field and the description of all the strata shall be duly recorded. No water shall be introduced from the top while conducting auger boring.
d) Shell and auger boring — Both manual and mechanized rig can be used for vertical borings. The tool normally consists of augers for soft to stiff clays, shells for very stiff and hard clays, and shells or sand pumps for sandy strata attached to sectional boring rods.
e) Wash boring — In wash boring, the soil is loosened and removed from the borehole by a stream of water or drilling mud is worked up and down or rotated in the borehole. The water or mud flow carries the soil up the annular space between the wash pipe and the casing and it overflows at ground level where the soil in suspension is allowed to settle in a pond or tank and the fluid is re-circulated as required. Samples of the settled out soil can be retained for identification purposes but this process is often unreliable. However, accurate identification can be obtained if frequent sampling is resorted to using undisturbed sample tubes.
f) Geophysical methods like electrical resistivity method and seismic method
g) Percussion boring
h) Rotary boring — In this system, boring is effected by the cutting action of a rotating bit and a mud-laden fluid or grout is pumped continuously down hollow drill rods and side of the hole.
j) Core drilling — To obtain the core samples, core drilling using double table barrel, preferably using hydraulic feed to avoid disturbance to the rock core, is used.
k) Pressure meter test (PMT) — In this, a uniform radial stress is applied to the borehole and consequent deformation is measured from which limiting pressures and modulus are obtained. This test can also be used where the sampling in hard strata becomes difficult or where there is no UDS collected. For industrial structures, 80 bars capacity energy will suffice. However for nuclear projects, higher capacity up to 200 bars may be required.
m) Plate load test — This test is conducted at shallow depths with an assumption that it directly determines the bearing capacity. Small size steel plates or relatively large RCC pads are used for conducting this test.
NOTE — While this procedure may be adequate for light or less important structures under normal conditions, relevant laboratory tests or field tests are essential in the case of unusual soil types and for all heavy and important structures. Plate load test, though useful in obtaining the necessary information about the soil with particular reference to design of foundation has some limitations. The test results reflect only the character of the soil located within a depth of less than twice the width of the bearing plate. Since the foundations are generally larger than the test plates, the settlement and shear resistance will depend on the properties of a much thicker stratum. Moreover, this method does not give the ultimate settlements particularly in case
of cohesive soils. Thus the results of the test are likely to be misleading, if the character of the soil changes at shallow depths, which is not uncommon. A satisfactory load test should, therefore, include adequate soil exploration {see good practice [6-2(2)]} with due attention being paid to any weaker stratum below the level of the footing. Another limitation is the concerning of the effect of size of foundation. For clayey soils the bearing capacity (from shear consideration) for a larger foundation is almost the same as that for the smaller test plate. But in dense sandy soils the bearing capacity increases with the size of the foundation. Thus tests with smaller size plate tend to give conservative values in dense sandy soils. It may, therefore, be necessary to test with plates of at least three sizes and the bearing capacity results extrapolated for the size of the actual foundation (minimum dimensions in the case of rectangular footings).
n) Laboratory investigation — This gives numerical values for strength, stiffness and consolidation parameters apart from the description and classification of different soil layers.
p) In-situ permeability test — In case of high water table and construction extending below the water table, prior lowering of water table would be necessary. In such situations, this test may be required.
q) Dynamic cone penetration test (DCPT)
r) Static cone penetration test (SCPT) — Apart from the above methods there are few other sounding procedures like Light Cone Penetration Tests (LCPT) and Dynamic Probing Tests (DPT), in which similar sounding techniques as in the case of DCPT are used, probably more effectively. Hydraulic feed of 20 t capacity helps to obtain data even from hard soils. For small projects 10 t hydraulic rig may suffice.