Soil Tests for Shallow and Raft Foundations
This passage discusses the soil tests required to determine the safe bearing capacity of shallow and raft foundations, in accordance with the IS 6403 – 1981 standard. The tests include determining the highest groundwater table level, classifying the soil based on grain size analysis and index properties, and conducting the standard penetration test to evaluate shear strength for both coarse and fine grained soils, as well as semi-pervious clayey soils.
The context provided describes two types of soil testing methods. The first method is the determination of the liquid limit and plastic limit of soil, which involves measuring the moisture content of the soil to determine its plasticity. This test is conducted using a standard procedure outlined in the Indian Standard code IS 2720 (Part 5) – 1985.
The second method described is the direct shear test, which is used to determine the shear strength of cohesive soils with a clay content of up to 30%. This test is performed according to the procedure specified in IS 2720 (Part 13) – 1986, using a controlled strain approach.
Both of these testing methods are important in geotechnical engineering and are used to assess the characteristics and properties of soils. The results obtained from these tests can be used to inform decisions related to the design and construction of various structures such as buildings, roads, and bridges.
The determination of soil properties is an important aspect of geotechnical engineering. One method commonly used for measuring the strength of cohesionless soils is the consolidated drained test. However, when dealing with semi-pervious soils containing clay between 15% and 30%, this test may not be entirely accurate due to partial drainage caused by the escape of pore water during box shear. In such cases, triaxial tests are needed to determine shear strength accurately.
Another method used to assess foundations on non-stiff clayey soils, specifically fine-grained soils, is the static cone penetration test as per IS 4968 (Part -3) – 1976. This test is also carried out on fine and medium coarse sands to correlate with standard penetration test results and indicate soil profiles. By comparing the results of the cone penetration test with those of the standard penetration test, engineers can obtain valuable information about the soil properties and determine the strength of the soil.
Three different types of tests are commonly conducted to determine the strength properties of different types of soils. The first test is the unconfined compressive strength test, which is conducted according to the IS 2720 (Part-10) – 1973 standard for highly cohesive clays, excluding soft or sensitive clays. The second test is the vane shear test, which is conducted for impervious clayey soils, excluding stiff or fissured clays. Finally, the third test is the triaxial shear test, which is conducted for predominantly cohesive soils, when the shear strength is likely to be critical.
Soil Tests for Shallow Foundations
When determining the allowable bearing pressure for shallow foundations, it is important to consider settlement. To do so, several tests may be required. The first test is the standard penetration test, which involves driving a split-spoon sampler into the soil to determine the resistance to penetration.
If the soil is clayey and the settlement calculated based on liquid limit and in-situ void ratio indicates that settlement may be critical, a consolidation test may be necessary. This test is specified by the IS 2720 (Part-15) standard. However, if the superimposed load on the foundation soil is likely to be less than the pre-consolidation pressure, which can be assessed from liquidity index and sensitivity or from un-confined compressive strength and plasticity index, then a consolidation test may not be needed.
For cohesionless soils, plate load tests are recommended as per the IS 1888 – 1982 standard. These tests involve applying a load to a plate placed on the soil surface and measuring the settlement. Overall, these tests are necessary to ensure that the foundation soil can support the intended load without excessive settlement, which could cause damage to the structure.
In certain soil conditions, it may not be possible to utilize the standard penetration test or consolidation test as reliable methods for soil analysis. For instance, soils that contain fissured clay or rock, or clay with boulders, present unique challenges to these testing techniques. In such situations, alternative methods must be employed to gather accurate data about the soil’s characteristics and properties. It is important to use appropriate testing methods to ensure that the soil’s behavior and load-bearing capacity can be properly understood and accounted for in engineering and construction projects.
Soil Tests Required for Raft Foundations
(As per Para 3 of IS 2950 (Part-1) – 1981.
Raft foundations are a type of shallow foundation that distribute the load of a structure over a large area of soil. To ensure the stability and safety of a building on a raft foundation, it is important to perform specific soil tests.
In addition to other tests commonly used for shallow foundations, such as soil boring and plate load tests, two tests are particularly important for raft foundations. These tests are the static cone penetration test and the standard penetration test.
The static cone penetration test, as per IS 4968 (Part-3) – 1976, is used for cohesionless soils to determine the modulus of elasticity according to IS 1888 – 1982. This test measures the resistance of the soil to penetration by a cone-shaped probe that is pushed into the ground at a constant rate. By measuring the depth and resistance of the cone, engineers can determine the soil’s stiffness and strength.
The standard penetration test, as per IS 2131 – 1981, is used for cohesionless soils as well. This test measures the resistance of the soil to penetration by a standard cone-shaped probe that is driven into the ground by a standard weight falling a standard distance. By counting the number of blows required to drive the probe a certain distance, engineers can determine the soil’s density and stiffness.
These two tests, in addition to other commonly used soil tests, provide engineers with critical information about the soil’s properties and behavior. With this information, they can design a safe and stable raft foundation that will support the weight of a building and ensure its longevity.
Several methods can be employed to determine the modulus of sub-grade reaction for a particular soil. One of the methods is the California Bearing Ratio (CBR) test, which is specified in IS 2720 (Part -16) – 1987. This test involves measuring the pressure required to penetrate a soil sample with a piston at a standard rate and comparing it to the pressure needed to penetrate a standard material. Another method is the plate load test as specified in IS 2950 (Part -1) – 1981. This involves measuring the deflection of a plate placed on the soil under a known load and comparing it to the load-deflection curve of a standard material.
For cohesive soils that are saturated but not pre-consolidated, the unconfined compressive strength test as per 2720 (Part -10) – 1973 can be used to determine the modulus of sub-grade reaction. This test involves measuring the compressive strength of a cylindrical sample of the soil under unconfined conditions.
In cases where the CBR test, plate load test, or unconfined compressive strength test is not appropriate, such as for fissured clays or clay boulders, the plate load test as per IS 1888 – 1982 can be used as specified in IS 2950 (Part -1) – 1981.
For deep basements in pervious soils, the permeability of the soil is determined from a pumping test. This information is necessary for analyzing the stability of the excavation and designing an appropriate dewatering system.