Bearing Capacity of Shallow Foundations
Recall the following terms: i. Foundation and Footing ii. Gross pressure intensity iii. Net pressure intensity iv. Ultimate bearing capacity v. Net ultimate bearing capacity vi. Net safe bearing capacity vii. Safe bearing capacity 1
Recall the following terms: i. Foundation and Footing ii. Gross pressure intensity iii. Net pressure intensity iv. Ultimate bearing capacity v. Net ultimate bearing capacity vi. Net safe bearing capacity vii. Safe bearing capacity
- Foundation: A foundation is the lower part of a structure that provides support to the building and transfers the loads from the structure to the underlying soil or rock.
- Footing: A footing is a type of foundation that spreads the loads of the structure over a larger area to reduce the pressure on the soil or rock.
- Gross pressure intensity: Gross pressure intensity refers to the total pressure per unit area that is exerted on the soil or rock by the structure.
- Net pressure intensity: Net pressure intensity refers to the pressure per unit area after deducting the soil or rock pressure.
- Ultimate bearing capacity: Ultimate bearing capacity refers to the maximum load that the soil or rock can support before failure occurs.
- Net ultimate bearing capacity: Net ultimate bearing capacity refers to the ultimate bearing capacity after deducting the soil or rock pressure.
- Net safe bearing capacity: Net safe bearing capacity refers to the net ultimate bearing capacity minus a safety factor to account for uncertainties in the soil or rock conditions.
- Safe bearing capacity: Safe bearing capacity refers to the maximum load that can be supported by the soil or rock without failure, taking into account the safety factor. It is used as a design criteria for the foundation and footing design to ensure the stability and longevity of the structure.
Depending on the stiffness of foundation soil and depth of foundation, the following are the modes of shear failure experienced by the foundation soil.
General Shear Failure
This type of failure is seen in dense and stiff soil. The following are some characteristics of general shear failure.
- Continuous, well defined and distinct failure surface develops between the edge of footing and ground surface.
- Dense or stiff soil that undergoes low compressibility experiences this failure.
- Continuous bulging of shear mass adjacent to footing is visible.
- Failure is accompanied by tilting of footing.
- Failure is sudden and catastrophic with pronounced peak in curve.
- The length of disturbance beyond the edge of footing is large.
- State of plastic equilibrium is reached initially at the footing edge and spreads gradually downwards and outwards.
- General shear failure is accompanied by low strain (<5%) in a soil with considerable (>36o) and large N (N > 30) having high relative density (ID> 70%).
Local Shear Failure
This type of failure is seen in relatively loose and soft soil. The following are some characteristics of general shear failure.
- A significant compression of soil below the footing and partial development of plastic equilibrium is observed.
- Failure is not sudden and there is no tilting of footing.
- Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed.
- Failure surface is not well defined.
- Failure is characterised by considerable settlement.\
- Well defined peak is absent in curve.
- Local shear failure is accompanied by large strain (> 10 to 20%) in a soil with considerably low (<28o) and low N (N < 5) having low relative density (ID> 20%).
Punching Shear Failure of foundation soils
This type of failure is seen in loose and soft soil and at deeper elevations. The following are some characteristics of general shear failure.
- This type of failure occurs in a soil of very high compressibility.
- Failure pattern is not observed.
- Bulging of soil around the footing is absent.
- Failure is characterised by very large settlements.
- Continuous settlement with no increase in P is observed in curve.
- 1. The soil is semi-infinite, homogeneous, and isotropic.
- 2. The problem is studied in two-dimensional.
- 3. The base of the footing is rough.
- 4. The ground surface is horizontal.
- 5. The failure is studied as a general shear failure.
- 6. The load acting on the footing is vertical and symmetrical.
- 7. The overburden pressure at the foundation level is equivalent to the surcharge load calculated as qo = γDf, Where γ is the effective unit weight of the soil, and Df is the depth of the foundation less than the width of the foundation.
- 8. The principle of superposition can be applied.
- Coulomb law, i.e. σ = C + σ tan ф; is employed.
Terzaghi’s Theory of Bearing Capacity: Terzaghi’s Theory of Bearing Capacity is a widely accepted theoretical framework for determining the bearing capacity of soil or rock beneath a foundation. The theory was developed by Karl Terzaghi in the 1920s and is based on the principles of soil mechanics.
Key components of Terzaghi’s Theory: The key components of Terzaghi’s Theory of Bearing Capacity include the following:
a. Ultimate bearing capacity: The ultimate bearing capacity is the maximum load that the soil or rock can support without failing. The ultimate bearing capacity is determined by the type and strength of the soil, the size and shape of the foundation, and the depth of the foundation.
b. Gross pressure intensity: The gross pressure intensity is the total load per unit area on the foundation and is calculated as the total load divided by the area of the foundation.
c. Net pressure intensity: The net pressure intensity is the difference between the gross pressure intensity and the overburden pressure, which is the pressure exerted by the soil above the foundation. The net pressure intensity is a measure of the effective stress on the soil and is used to determine the bearing capacity.
- Factor of safety: The factor of safety is a measure of the margin of safety in the design and is calculated as the ratio of the ultimate bearing capacity to the applied load. The factor of safety is used to ensure that the foundation will not fail under expected loads.
Applications of Terzaghi’s Theory: Terzaghi’s Theory of Bearing Capacity is widely used in foundation design and is applied in the following ways:
a. Shallow foundations: The theory is commonly used to design shallow foundations, such as spread footings and mat foundations.
b. Deep foundations: The theory is also used to design deep foundations, such as piles and drilled shafts, by accounting for the effects of soil or rock strength and the depth of the foundation.
- Slope stability: The theory is used to evaluate the stability of slopes and embankments, by considering the soil strength, the slope angle, and the loads applied to the slope.
- Importance of Terzaghi’s Theory: Terzaghi’s Theory of Bearing Capacity is an important framework for understanding the bearing capacity of soil or rock and is widely used in foundation design. The theory provides a basic understanding of the soil behaviour and the loads that it can support, which is essential for designing safe and stable foundations. The theory is also a useful tool for evaluating the stability of slopes and embankments, which are common features in surveying and civil engineering projects.
The Terzaghi’s bearing capacity equation is given by:
qu = CNc + γ1DfNq + 0.5Bγ2Nγ
In the above equation,
- qu= Ultimate Bearing Capacity of the soil
- C= Cohesion
- γ1,γ2= Unit weight of the soil above and below the footing level
- Nc,Nq, Nγ= Bearing capacity factors that are a function of friction angle
- Df= Depth of the foundation below the ground level
In the above equation:
- CNc = Contribution of cohesion
- γ1DfNq = Frictional contribution of overburden pressure or surcharge
- 0.5Bγ2Nγ = Frictional contribution of self-weight of soil in the failure zone
Shallow Foundation: A shallow foundation is a type of foundation that transfers loads from a structure to the soil or rock layer located near the surface. Shallow foundations are typically used when the soil or rock layer is capable of supporting the loads imposed by the structure.
Types of Shallow Foundation: The most common types of shallow foundation are:
- Spread Footing: A spread footing is a type of shallow foundation that consists of a rectangular or circular concrete pad that is supported by one or more columns. The load from the structure is transferred to the soil through the footing.
- Mat Foundation: A mat foundation is a type of shallow foundation that consists of a large, thick concrete slab that spreads the loads from the structure over a large area. Mat foundations are used in high-rise buildings and other structures that require a large, stable foundation.
- Strap Footing: A strap footing is a type of shallow foundation that consists of two separate footings connected by a strap or beam. Strap footings are used in structures that have load-bearing walls or columns that cannot be supported by a single footing.
- Isolated Footing: Also called single-column footing, it is a square, rectangular, or circular slab that supports the structural members individually. Generally, each of its columns gets its footing to transmit and distribute the load of the structure towards the soil underneath. Sometimes, an isolated footing can be sloped or stepped at the base to spread greater loads. This type of footing is used when the structural load is relatively low, columns are widely spaced, and the soil’s bearing capacity is adequate at a shallow depth.
Factors affecting Bearing Capacity: The bearing capacity of a shallow foundation is determined by several factors, including:
a. Soil type and strength: The soil type and strength, such as sand, clay, or rock, affects the bearing capacity of the foundation. Soils with high strength and stiffness, such as rock, have higher bearing capacities than soils with low strength and stiffness, such as clay.
b. Foundation size and shape: The size and shape of the foundation, such as circular or rectangular, also affect the bearing capacity. A larger foundation has a higher bearing capacity than a smaller one.
c. Depth of foundation: The depth of the foundation affects the bearing capacity by influencing the net pressure intensity, which is a measure of the effective stress on the soil. A deeper foundation has a higher bearing capacity than a shallow one.
- Soil Type and Strength: The type and strength of the soil that the shallow foundation is placed on is a critical factor affecting the bearing capacity. Soils with high strength and stiffness, such as rock, have higher bearing capacities than soils with low strength and stiffness, such as clay.
- Foundation Size and Shape: The size and shape of the foundation also play a role in determining the bearing capacity. A larger foundation has a higher bearing capacity than a smaller one. The shape of the foundation also affects the bearing capacity, with circular footings typically having a higher bearing capacity than rectangular footings.
- Depth of Foundation: The depth of the foundation affects the bearing capacity by influencing the net pressure intensity, which is a measure of the effective stress on the soil. A deeper foundation has a higher bearing capacity than a shallow one, as the net pressure intensity decreases with increasing depth.
- Loads: The loads imposed on the shallow foundation, such as dead load, live load, and wind load, also affect the bearing capacity. The loads are important factors in determining the size and design of the foundation, as a foundation with higher loads will require a higher bearing capacity.
- Soil Structure: The soil structure, such as the presence of soil layers, fractures, and voids, can also affect the bearing capacity. Soil structures that are not uniform, such as those with multiple soil layers, can cause uneven stress distributions and result in lower bearing capacities.
- Groundwater Level: The groundwater level can also affect the bearing capacity, as higher groundwater levels can result in increased buoyancy and decreased effective stress on the soil. This can result in a lower bearing capacity for shallow foundations.
- Overloading: Overloading a shallow foundation beyond its capacity can result in failure. It is important to ensure that the loads imposed on the foundation are within its capacity to avoid overloading and failure.
- Climate and Environmental Factors: Climate and environmental factors, such as temperature changes and soil moisture, can also affect the bearing capacity of shallow foundations. Changes in temperature and soil moisture can result in changes in the soil structure and soil strength, which can affect the bearing capacity.
- Equipment and Materials: Plate load tests require a plate bearing test apparatus, load cell, and control system. A steel plate of known size and weight is used to apply load to the soil.
- Site Preparation: A test location is selected and the soil is cleared and levelled. A reference mark is established to measure the deflection of the plate.
- Loading and Measurement: The plate is placed on the soil surface and a load is applied to it using a hydraulic jack. The load is increased incrementally and the deflection of the plate is measured after each increment.
- Data Collection: Data is collected and recorded, including the load applied and the deflection of the plate. The load-deflection curve is plotted and used to determine the bearing capacity of the soil.
- Load-Deflection Analysis: The load-deflection curve is analyzed to determine the ultimate bearing capacity of the soil, which is the maximum load the soil can support before failure. The analysis includes the determination of the maximum load, maximum deflection, and slope of the load-deflection curve.
- Repeat Testing: The plate load test is typically repeated several times at the same location to obtain a consistent and reliable estimate of the soil bearing capacity.
- Interpretation of Results: The results of the plate load test are interpreted and used to design the shallow foundation. The results provide information on the soil strength and bearing capacity, which can be used to determine the size and type of foundation required for a given structure.
- Limitations: Plate load tests are limited by the size of the plate and the depth of the soil layer tested. The results of a plate load test may not be representative of the entire soil layer and may only be applicable to the specific location tested. The test is also limited by the size and type of soil layer present and may not be applicable to all soil types.
The Limitations of the Plate Load Test are as follows:
- Limited depth of investigation: Plate load tests can only measure the bearing capacity of shallow soil layers, typically no deeper than 4 metres.
- Limited size of the test area: The size of the test area is limited to the size of the plate used, which is typically between 0.3 to 1.0 square metres.
- Environmental conditions: Plate load tests can be affected by environmental conditions such as temperature, moisture content, and wind, which can alter the soil properties and affect the test results.
- Unrepresentative soil samples: Plate load tests rely on obtaining an adequate soil sample to represent the soil conditions at the test site. However, obtaining a representative soil sample can be challenging and if not done properly, the results may not accurately reflect the soil conditions at the site.
- Load distribution: Plate load tests can only provide information on the bearing capacity of the soil at the point of load application. The load distribution beyond this point is not well understood, and this can limit the accuracy of the test results.
- Dynamic effects: Plate load tests are static tests and do not take into account dynamic effects such as soil deformation, soil-structure interaction, and soil-water interaction, which can affect the soil bearing capacity.
- Load application rate: Plate load tests are sensitive to the rate at which the load is applied, and changes in load application rate can affect the test results.