Bearing Capacity
Bearing capacity is the maximum load that a soil or rock mass can support without undergoing excessive settlement, shear failure, or other detrimental deformations. It is a crucial consideration in geotechnical engineering and foundation design, as it determines the safe load carrying capacity of the ground upon which structures are built.
Bearing Capacity Types
Ultimate Bearing Capacity (\(q_u\)) - This is the maximum load per unit area that the soil can support without failure. It is often determined through laboratory tests or field tests and is used as a design parameter in foundation engineering.
Allowable Bearing Capacity (Safe Bearing Capacity) (\(q_a\)) - It is the maximum load per unit area that is deemed safe for the soil without causing excessive settlement or shear failure. It is typically a fraction of the ultimate bearing capacity and is used in design calculations to ensure the stability and safety of foundations.
Net Bearing Capacity (\(q_n\)) - This refers to the effective bearing capacity of the soil after considering factors such as the weight of the footing or foundation and the influence of groundwater.
The determination of bearing capacity involves various factors, including soil type, density, moisture content, loading conditions, and the geometry of the foundation or footing. Engineers use methods such as plate load tests, standard penetration tests, cone penetration tests, and empirical correlations to estimate bearing capacity and ensure that foundations are designed to safely support the intended loads without causing settlement or instability issues.
Key Points about Bearing Capacity
Bearing capacity factors are coefficients used in the calculation of the ultimate bearing capacity of soil. These factors are applied to various soil parameters and geometrical factors to estimate the maximum load that a foundation or footing can safely support without causing failure or excessive settlement. The bearing capacity factors are typically denoted by symbols and represent the influence of different factors on the bearing capacity of the soil.
Shape Factor (\(N_c\)) - This factor accounts for the shape of the foundation. It depends on the shape of the footing and the depth at which the foundation is located. For example, for a square footing, the shape factor is different than for a circular or strip footing.Depth Factor (\(N_q\)) - This factor considers the depth of the foundation relative to the ground surface. It takes into account the influence of the depth on the bearing capacity of the soil. It is typically higher for shallow foundations and decreases with increasing depth.
Inclination Factor (\(N_{\gamma}\)) - This factor accounts for the inclination of the failure plane with respect to the horizontal. It is relevant for sloping ground conditions where the failure plane may not be vertical.
Soil Friction Coefficient (\(tan(\theta)\;\)) - The angle of internal friction (\(tan(\theta)\;\)) of the soil is a crucial parameter in bearing capacity calculations. It represents the resistance of the soil to shear deformation. A higher value of (\(tan(\theta)\;\)) indicates a stronger soil, resulting in higher bearing capacity.
Overburden Pressure Factor (\(\gamma\)) - This factor considers the effect of the weight of the soil above the failure plane on the bearing capacity. It depends on the unit weight of the soil and the depth of the foundation.
These factors are typically used in bearing capacity equations, such as Terzaghi's bearing capacity equation or Meyerhof's bearing capacity equation, to estimate the ultimate bearing capacity of the soil and ensure the safe design of foundations. The values of these factors depend on soil properties, foundation geometry, and loading conditions and are determined through empirical correlations, laboratory tests, or field test.
Ultimate Bearing Capacity formula |
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| \( q_u = ( c \; N_c) + (\gamma \; D_f \; N_q) + (0.5 \; \gamma \; W_f \; N_{\gamma}) \) | ||
| Symbol | English | Metric |
| \( q_u \) = Ultimate Bearing Capacity | \(lbf\;/\;in^2\) | \(Pa\) |
| \( c \) = Cohesion (Internal Molecular Attraction) | \(lbf\;/\;in^2\) | \(Pa\) |
| \( N_c \) = Shape Factor | \(dimensionless\) | \(dimensionless\) |
| \( D_f \) = Foundation Depth | \(ft\) | \(m\) |
| \( N_q \) = Depth Factor | \(dimensionless\) | \(dimensionless\) |
| \( \gamma \) (Greek symbol gamma) = Unit Weight of Soil | \(lbf\) | \(N\) |
| \( W_f \) = Foundation Width | \(ft\) | \(m\) |
| \( N_{\gamma} \) = Inclination Factor | \(dimensionless\) | \(dimensionless\) |
Allowable Bearing Capacity formula |
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| \( q_a = q_u \;/\; FS \) | ||
| Symbol | English | Metric |
| \( q_a \) = Allowable Bearing Capacity | \(lbf\;/\;in^2\) | \(Pa\) |
| \( q_u \) = Ultimate Bearing Capacity | \(lbf\;/\;in^2\) | \(Pa\) |
| \( FS \) = Factor of Safety | \(dimensionless\) | \(dimensionless\) |
Methods of Bearing Capacity Testing
There are several methods for conducting bearing capacity testing to assess the load bearing capacity of soil. These methods help engineers and geotechnical professionals in designing foundations and structures that are safe and stable.
Plate Load Test (PLT)
- In this method, a steel plate of known size and shape is loaded incrementally with a hydraulic jack.
- The settlement of the plate and the applied load are recorded at each increment.
- The test provides direct measurement of the soil's bearing capacity at the specific location of the test.
Standard Penetration Test (SPT)
- The SPT is a widely used in-situ test for determining the soil's resistance to penetration.
- It involves driving a split-spoon sampler into the soil at the bottom of a borehole using a standard weight and falling distance.
- The number of blows required for penetration in each soil layer is recorded, which provides an indication of soil strength.
- Empirical correlations are then used to estimate bearing capacity based on the SPT results.
Cone Penetration Test (CPT)
- The CPT involves pushing a cone-shaped penetrometer into the soil at a constant rate and measuring the resistance encountered.
- The cone measures both tip resistance and sleeve friction along the length of the probe.
- The data collected from the test can be used to determine soil properties, including bearing capacity, shear strength, and soil stratigraphy.
Pressuremeter Test
- In this method, a cylindrical probe is inserted into a borehole and pressurized to measure the radial deformation of the surrounding soil.
- The test provides information on the stress-strain behavior of the soil and can be used to estimate the ultimate bearing capacity.
Static Cone Penetrometer Test (SCPT)
- Similar to the CPT, the SCPT involves pushing a cone penetrometer into the soil at a constant rate.
- However, in this method, the penetration is performed at a slower rate, allowing for continuous measurements of cone resistance and sleeve friction.
- The test provides more detailed information on soil properties compared to the standard CPT.
Plate Bearing Test (PBT)
- In this method, a large plate (usually larger than in PLT) is placed on the ground surface, and a load is applied to the plate through a reaction frame.
- The settlement of the plate and the applied load are measured, and bearing capacity is calculated based on the results.
- This test is suitable for evaluating the bearing capacity of cohesive soils.
These methods can be used alone or in combination, depending on the project requirements, soil conditions, and available resources. They provide valuable data for assessing the soil's load bearing capacity and are essential for the safe and economical design of foundations and structures.
Tags: Structural Steel Soil