Updated March 6th, 2018
We all know, it is essential to investigate the soil of the selected plot on which a structure will be constructed.
Based on soil investigation a soil report is prepared for the purpose of designing the building foundation. When an engineer designs building foundation he/she must carefully read the report and designs the foundation based on the data provided in the report.
Why soil investigation is required for designing foundation?
Soil investigation is required for the following purposes –
- To know the allowable bearing capacity of foundation for proposed building.
- To know the depth and type of foundation for the proposed building.
- To know the allowable passive resistance for the foundation of proposed building.
- To know the type, grading and nature of soil.
- To know the ground water level.
Methods of soil investigation
The common methods of soil investigation are –
- Test pits
- Probing, and
Inspection: In some places you don’t have to investigate much. You’ll get enough data to design the foundation of the proposed building by just inspecting the plot. This method of soil investigation includes know the geological condition of the plot, getting data about neighbour buildings, their foundation type and depth, etc.
Test pits: This is done to collect soil samples for detail analysis. In this method several pits are dug by hand or excavator. The depth of pit is below 5 feet so that one can have visual inspection. Several samples are collected from the pit of both disturbed and undisturbed soil.
Probing: In this method a 25 mm or 40 mm diameter steel bar is driven into the ground till solid soil strata is found. It is normally driven by hammer. The penetration and withdrawl of the steel rod is closely observed to know the nature of soil layer.
Boring: In this method several bore holes are made for the purpose of collecting soil sample from below the ground. Then the collected sample is analysed for preparing the soil report.
Typical steps of soil investigation
Soil investigation involves following steps –
- Details planning for the sequence of operations.
- Collecting the samples of soil from the plot.
- Determining the soil characteristics by conducting field tests.
- Study the condition of ground water level.
- Collecting ground water sample for chemical analysis.
- Soil exploration.
- Testing all collected samples in the laboratory.
- Analysis the test results.
- Preparing report.
Based on this prepared report a structural engineer designs the foundation of structure for the proposed plot.