Bearing Capacity of Soil
A foundation is the part of a structure that transmits the weight of the structure to the ground. All structures constructed on land are supported on foundations. A foundation is a connecting link between the structure proper and the ground which supports it. The bearing strength characteristics of foundation soil are major design criteria for civil engineering structures. In nontechnical engineering, bearing capacity is the capacity of soil to support the loads applied to the ground. The bearing capacity of soil is the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil.
The bearing capacity of your soil will help you determine if you need a shallow foundation or deep foundation. Soil strength directly under the footing, where loads are concentrated, is crucial to foundation performance.
Standard Penetration Test (SPT)
The standard penetration test is an in-situ test that is coming under the category of penetrometer tests. The standard penetration tests are carried out in the borehole. The test will measure the resistance of the soil strata to the penetration undergone. A penetration empirical correlation is derived between the soil properties and the penetration resistance. The test is extremely useful for determining the relative density and the angle of shearing resistance of cohesion-less soils. It can also be used to determine the unconfined compressive strength of cohesive soils.
Tools for Standard Penetration Test
The requirements to conduct SPT are:
- Standard Split Spoon Sampler
- Drop Hammer weighing 63.5kg
- Guiding rod
- Drilling Rig.
- Driving head (anvil).
Procedure
The test uses a thick-walled sample tube, with an outside diameter of 50.8 mm and an inside diameter of 35 mm, and a length of around 650 mm. This is driven into the ground at the bottom of a borehole by blows from a slide hammer with a mass of 63.5 kg (140 lb) falling through a distance of 760 mm (30 in). The sample tube is driven 150 mm into the ground and then the number of blows needed for the tube to penetrate each 150 mm (6 in) up to a depth of 450 mm (18 in) is recorded. The sum of the number of blows required for the second and third 6 inches of penetration is termed the “standard penetration resistance” or the “N-value”. In cases where 50 blows are insufficient to advance it through a 150 mm (6 in) interval the penetration after 50 blows is recorded. The blow count provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae.
Writer,
Md. Nur Alam
Instructor
Daffodil Polytechnic Institute