You measure soil capacity, primarily its bearing capacity, using a variety of field and laboratory tests that assess its ability to support loads. These tests help determine the soil's strength and suitability for construction.
Field Tests for Measuring Soil Bearing Capacity
Field tests are crucial for directly assessing the soil's behavior under in-situ conditions. Some common methods include:
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Standard Penetration Test (SPT): This is a widely used in-situ test.
- A split-spoon sampler is driven into the ground by repeated blows of a hammer.
- The number of blows required to drive the sampler a specific distance (usually 300 mm or 12 inches) is recorded as the N-value.
- The N-value is correlated with the soil's relative density and bearing capacity.
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Cone Penetration Test (CPT): The CPT involves pushing a cone-shaped penetrometer into the soil at a constant rate.
- Sensors measure the cone resistance (qc) and sleeve friction (fs), providing a continuous profile of soil properties.
- CPT data can be used to estimate soil type, strength, and bearing capacity.
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Plate Load Test: This direct method involves applying a load to a steel plate placed on the soil surface.
- The load is gradually increased, and the corresponding settlement is measured.
- The bearing capacity is determined from the load-settlement curve.
- This test directly evaluates the load-bearing characteristics of the soil at the testing location.
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Vane Shear Test (VST): This test is primarily used for cohesive soils (clays).
- A four-bladed vane is inserted into the soil, and the torque required to rotate the vane is measured.
- The torque is correlated with the undrained shear strength of the soil, which is a key parameter for determining bearing capacity.
Laboratory Tests for Measuring Soil Properties
While field tests provide in-situ data, laboratory tests allow for more controlled analysis of soil samples. These tests determine key soil parameters used in bearing capacity calculations.
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Unconfined Compression Test (UCT): This test applies axial compression to a cylindrical soil sample until it fails.
- The unconfined compressive strength (qu) is determined, which is half the unconfined compressive strength (qu = 2cu) under undrained conditions. This is primarily used for cohesive soils.
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Triaxial Test: A more versatile test that allows for control of confining pressure.
- Different types of triaxial tests (e.g., Consolidated Drained (CD), Consolidated Undrained (CU), Unconsolidated Undrained (UU)) can be performed to simulate different loading conditions.
- The test provides information about the soil's shear strength parameters (cohesion and angle of internal friction).
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Direct Shear Test: A simple test that involves shearing a soil sample along a horizontal plane.
- The shear strength parameters (cohesion and angle of internal friction) can be determined.
- This test is commonly used for granular soils.
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Consolidation Test: Used to determine the compressibility of the soil.
- Important for estimating settlement of structures.
- Key parameters derived are the compression index (Cc) and the preconsolidation pressure.
Estimating Bearing Capacity
The data obtained from field and laboratory tests is used in conjunction with bearing capacity equations to estimate the allowable bearing pressure for a given soil. These equations typically consider factors such as:
- Soil shear strength parameters (cohesion, angle of internal friction)
- Soil unit weight
- Depth of foundation
- Width of foundation
- Shape factors
- Depth factors
- Inclination factors
Several different bearing capacity theories exist, such as Terzaghi's bearing capacity theory and Meyerhof's bearing capacity theory. Each theory makes different assumptions and is applicable to different soil conditions.
In summary, measuring soil capacity involves a combination of field tests to assess in-situ behavior and laboratory tests to determine key soil properties. The data from these tests is then used in bearing capacity equations to estimate the allowable bearing pressure for a specific foundation design.
