Question

What are the engineering properties needed for the design of deep foundations? Suggest the laboratory test program for the evaluation of these properties.

Answer 1

Deep foundations transfer some of a structure’s entire load to deeper soils. Deep foundations are considerably more expensive and complex than shallow foundation systems.

Because the bearing capacity of soil often increases with depth, deep foundations generally possess higher structural load capacities relative to shallow foundations. Hence, deep foundations typically are used when adverse near-surface soil conditions are encountered (i.e., soft clays, collapsible soils, and/or expansive soils) and structural load requirements are sufficiently large to warrant the additional cost of deep foundations, or when a high lateral load capacity is needed.

REQUIREMENT FOR DEEP FOUNDATIONS
Generally for structures with load >10 ton per sqm , we go for deep foundations. Deep foundations are used in the following cases:

• Huge vertical load with respect to soil capacity.
• Very weak soil or problematic soil.
• Huge lateral loads eg. Tower, chimneys.
• Scour depth criteria.
• For fills having very large depth.
• Uplift situations (expansive zones)
• Urban areas for future large and huge construction near the existing building.

LABORATORY TESTS AND RESULTS

1. Characteristics of soil considered in Pile foundation design:

• Liquid limit
• Plastic limit
• Optimum moisture content
• Co-efficient of cohesion

Some tests of the soil.

Liquid Limit:

• We have conducted Casagrande’s liquid limit test for a sample of soil.We have obtained a liquid limit of 25%from the test.

Plastic Limit:

• Plastic limit is the water content below which the soil stops behaving as a plastic material.The plastic limit of soil obtained is 20%.

Compaction factor:

• From compaction factor test we can obtain the values of Optimum moisture content, Bulk density, Dry densityof the soil.

For Example:-

The values obtained from compaction factor test are:

• OMC = 8%
• Bulk density=26.22kN/cubic mD
• Dry density = 24.28 kN/cubic m.