Sub-soil exploration is done for the following purposes:
(a) For New Structures
1. The selcction of type and depth of foundation.
2. The determination of bearing capacity of the selected foundation.
3. The prediction of settlement of the selected foundation.
4. The determination of the ground water level.
5. The evaluation of the earth pressure against walls, basements, abutments etc.
6. The provision agains constructional difficltties.
7. Thte suitability of soil and degree of compaction of soil.
(b) For Existing Structures
1. The investigation of the safety of the structure.
2. The prediction of settlement.
3. The determination of remedial measures if the structure is unsafe or will suffcr detrimental settlement.
An inspection of the site and study of topographical features is often helpful in setting useful information about the soil and ground water conditions and in deciding the future programme of exploration. On going over the site, a study of the following features may be useful : local topography, excavations, cuttings, quarries, escarpments evidence of erosion or land slides, fills, water level in wells and drainage pattern for the building site. If there has been an earlier use of the site, information should be gathered, if any particular about the underground workings, if anv, and about the location of fills and excavations.
The object of the site exploration is to provide reliable, specific and detailed information about the soil and ground water conditions of the site which may be required for a safe and economic design of foundations.
For this purpose, an exploration of the region likely to be affected by the proposed works should yield precise information about the following
(i) the order of occurrence and extent of soil and rock strata.
(ii) the nature and engineering properties of the soil and rock formation, and
(iii) the location of ground water and its variation.
Depth of exploration
Exploration, in general, should be carried out to a depth upto which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of foundations. Such a deph, known as significant depth, depends upon the type of structure, its weight, size, shape and disposition of the loaded areas, and the soil profile and its properties. The significant depth may be assumed to be equal to one-and-a-half to two times the width (smaller of the lateral dimension) of the loaded area.
The depth of exploration at the start of the work may be decided according to the following guide rules, which may need modification as exploration proceeds:
1. Isolated spread footing or raft: One and a half times ihe width.
2. Adjacent footings with clear spacing less than twice the width: One and a half times the length.
3. Pile foundation: 10 to 30 metres, or more, or at least one and a half times the width of the structure.
4. Base of the retaining wall: One and a half times the base width or one and a half times the exposed height of face of wall, whichever is greater.
5. Floating basement: Depth of construction.
6. Weathering considertions: 1.5 m in general and 3.5 m in black cotton soils.