Shear Strength Testing
Shear strength testing is an essential element in the assessment of the stability of soil and rock masses. Soil shear strength refers to the capacity of soil to resist deformation caused by applied stresses. Different types of shear tests can be used to determine soil shear strength. Here are some of the most commonly used tests:
This is an in-situ test used to determine the undrained shear strength of cohesive soils such as clay-based soils. The test is carried out using a vane shear tip inserted into a bore hole on-site. The tip is then rotated to measure the torsional force required to cause shearing. This test can only be carried out on samples with a soil shear strength of up to 0.5 kgf/cm2.
This in-situ test is also used to determine the shear strength of soft clays. A device is inserted into a bore hole and filled with compressed air to expand into the soil. As the soil shears, the shear strength can be estimated.
This is a simple lab-based shear strength test carried out in a ‘shear box’. A soil sample is positioned in a shear box on a predefined horizontal plane. The specimen is placed under different normal stresses, and the tangential (shear) stress is determined in each case during the consolidation stage. During the shearing stage, a shear stress is applied along the predetermined horizontal plane to determine the shear strength of the sample.
These shear strength tests can be carried out on all soil types regardless of drainage condition. The term ‘triaxial’ refers to the fact that the soil sample is subjected to pressure in all directions during this type of test. This test is widely used in geotechnical engineering and soil mechanics due to its reliability and accuracy.
Unconfined Compression (UCC) Test
A specific type of triaxial test that can only be conducted on saturated cohesive soils. In this case, the cell in which the test takes place has no confining pressure.
The torsional ring shear test is a method used to determine the shear strength and nonlinear strength envelope of cohesive soils under drained conditions. This test involves subjecting a soil sample to torsional stress, which is gradually increased until failure occurs. The test results provide information on the fully softened shear strength of the soil, as well as its nonlinear strength envelope.
Undrained shear strength refers to the maximum shear stress that soil can withstand without undergoing any volume change in undrained conditions. In simpler terms, it is the highest shear stress that a soil sample can bear without permitting drainage, thereby maintaining constant pore water pressure.
What is shear strength?
The shear strength of soil refers to its ability to resist deformation when subjected to tangential stress. This property is determined by the cohesion and frictional forces present between soil particles. Higher shear strength indicates greater cohesion and interlocking between particles, resulting in better resistance to sliding. The shearing strength of soil plays a crucial role in geotechnical engineering projects, as it is used to calculate the bearing capacity of soil and to design retaining walls, slopes, and embankments. Therefore, understanding the shear strength of soil is essential for ensuring the stability and safety of any construction project that involves soil as a foundation or support.
Why is shear strength important?
The shear strength of soil is a critical factor in geotechnical engineering and plays a significant role in the design and construction of various structures such as building foundations, pavements, and retaining walls. Shear strength refers to the soil's ability to resist deformation under tangential or shear stress, which depends on the cohesive and frictional forces between soil particles.
Engineers need to understand the shear strength of soil to predict the behavior of geotechnical projects under different loading and shear stress conditions. Without proper knowledge of soil shear strength, it is impossible to ensure the stability and safety of structures. Therefore, determining the shear strength of soil is an essential step in geotechnical design and construction.
To increase soil shear strength, engineers can use mechanical or chemical processes, as well as reinforcing materials. They can modify their design by incorporating suitable geogrids or wall and slope systems based on the soil's shear strength. By improving soil shear strength, engineers can optimize the design of structures and ensure their long-term stability and safety.
What impacts shear strength?
Soil shear strength is influenced by various factors, with one of the key considerations being the type of soil. In general, soil particles are non-compressible, and soil masses do not possess tensile strength. Soil failure occurs when one block of soil moves relative to another block, causing the soil particles at the failure plane to move over each other, a phenomenon known as shear.
The resisting or shear force in this case is friction. Soil shear strength is dependent on the physical characteristics of the soil, such as particle size, shape, distribution, and orientation, as well as the stresses acting on the soil at that specific location. These factors all play a significant role in determining the soil's shear strength and are critical considerations in geotechnical engineering design projects.
Friction
Friction is a fundamental factor that contributes to soil shear strength. When two objects are in contact, the force of friction along a plane varies with the pressure acting perpendicular to the plane, which is known as normal stress. As the normal stress increases, the frictional resistance, or shear stress, also increases. This highlights that the shear strength of soil is not a fixed value but depends on the stresses that are acting on the soil. In granular soils, the relationship between shear stress and normal stress is a straight line, which is defined by an angle known as the friction angle (ø). The friction angle is an essential parameter in understanding soil behavior and is determined through shear strength tests.
Friction Angle
The friction angle is a crucial factor in determining the shear strength of soil, along with the stresses acting upon it. It is a fundamental property that defines strength when comparing different types of granular soil. Engineers must know the friction angle of soil when selecting materials for reinforced soil structures, working platforms, and road foundations. It is important to note that any alteration in the friction angle may require a redesign of the structure.
Clay Soil Shear Strength
Clay soils are composed of fine particles that have electrostatic charges between them, along with surface tension from pore water that holds them together, giving them some shear strength even without external confining forces. This additional strength is called apparent cohesion, but it is not a fundamental soil property. However, when determining the shear strength of granular soils that contain some clay, cohesion can mostly be disregarded, as the friction angle is the crucial factor. The friction angle determines the strength of the soil in resisting shear stresses.
Ground water
The shear strength of soil can be influenced by the presence of groundwater. When soil is fully saturated with water, the pore pressure increases as the water cannot be compressed. In granular soils, the water can drain away slowly, allowing the load to be transferred to the soil skeleton. As a result, the pore water effects can be ignored. However, in clay soils, the void spaces are very small and poorly connected, so water can only move at low rates and drainage is slow. Therefore, when a load is applied to clay soil, the pore water pressure cannot dissipate, and the shear stress is a constant value known as the undrained stress. The undrained shear strength, denoted by cu or su, is critical when considering the bearing capacity of clay soils, especially in regions like the UK where high clay content is common. While the strength of clay soils can increase over time as water drains, this is a slow and long-term effect.
Soil State
Soil state is another crucial factor that affects the shear strength of granular soil. When loose and uncompacted soil is subjected to a load, the particles will move closer together, resulting in soil contraction. As the soil compacts, shearing occurs as particles move over each other, causing the soil shear strength to increase. The soil's shear strength will remain constant at a constant density or volume after compaction.
On the other hand, when granular soils are already densely compacted, there is little or no contraction, and the particles are interlocked. As the load increases, particles must move apart along the shear plane, which unlocks the interlock before they can shear over one another. This is called dilation. The shear force required to overcome dilation is known as the peak strength (øpeak). After dilation, particles can move over each other more easily, requiring a lower shear force than at peak, known as strength at constant volume (øcv).
When designing structures where soil shear is not expected, such as in a reinforced soil wall's structural fill, peak strength should be used. However, in situations where high deformation is expected, the soil's shear strength at constant volume is preferred. It is crucial to ensure that the materials report provides the appropriate strength for the design purpose.