Necessity of site exploration: 1. To know stratification below ground surface. 2. To determining index properties of soil like bulk density, voids ratio, water content, permeability, bearing capacity, compressibility etc. 3. To determine safe bearing capacity for design of foundation of proposed structure. 4. To control the seepage and rise of ground water below surface 5. To decide size, depth and type of foundation for the proposed structure. 6. To know grain size distribution by sampling undistributed soil sample and classify soil accordingly. 7. To decide suitability of soil for proposed structure.
Criterion for deciding the number of test-pits for site investigation: 1. According to IS: 1892-1979, one test pit at center of plot is sufficient for smaller and less important area. 2. For larger area about 0.4 hectare, four test pit at corners of plot and one at center should be taken. 3. The sounding tests or cone penetration tests should be taken at a spacing 50-100 m in the grid pattern. 4. The test pits should be taken two at edges and one at center for alignment of roadway, runway and railway etc. 5. When the roads are of lesser width, one bore hole at central alignment is sufficient. The spacing varies according to ground variation about 100 m. 6. For dams. The test pits should be taken along top line of upstream side across the abutments with 50 m spacing.