Soil biology

How to Measure Soil Strength and the Factors Affecting It

Soil or shear strength refers to the soil’s capacity to accommodate and withstand a structural load without shearing or deformation. It alludes to the soil’s mechanical properties — whether it could remain cohesive while being subjected to an external force.[1] 

It is one of the essential mechanical properties of the soil. In geology and civil engineering, soil strength relays the soil stability, which affects any building or infrastructure constructed on it. In agriculture, soil strength impacts the expansion of plant roots, which plays a major role in plant productivity.[1,2] 

Factors Affecting Soil Strength

Soil strength is the interplay between material failure and effective stress. Material failure refers to soil particles deforming due to an external force, while effective stress is the tension and pore water pressure between the soil particles.

Thus, on the one hand, soil strength is determined by the structural load that exerts stress and friction onto the soil surface, pushing for failure in the form of soil deformation. On the other hand, soil strength is influenced by the effective stress that acts as the cohesive force which holds the soil particles together against the opposing forces.

Several factors influencing both effective stress and material failure include[1,3]

  • Soil composition refers to soil forming components such as minerals and organic matter. The species and ratios of minerals are the major influence on the shear strength of cohesionless soils such as gravel and sand.
  • Soil gradation includes the size and shape of the soil particles, which contribute to the soil structure and
  • Water content, moisture, and drainage condition relate to the amount of water between the pore of the soil particles.
    • Water content and soil moisture refer to water between the pore spaces. The soil is termed saturated when the pore spaces are filled with water.
    • Drainage condition describes the water movement and change in the soil moisture. Typically, soils are in the drained condition where water can be loaded and drained out of the soil. Undrained soil condition refers to when water cannot be drained out of the soil or when the water drainage rate is much slower than water entering the soil.
    • In cohesive soils such as silts and clay, water content, soil moisture, and drainage condition significantly influence pore water pressure, which makes up for the effective stress.
  • The void ratio and dry density relate to the soil mass without the water content. The shear strength of cohesionless soils is directly proportional to the dry density. In cohesive soils, dry density is less influential to soil strength when compared to the water content.   

These factors are involved in soil formation events and dynamics, which are reflected in the soil components and profile. To learn more, check out our article on Soil biology and how it influences soil formation.

How to Measure Soil Strength

1. Laboratory Measurement of Soil Strength

Laboratory measurement methods are designed to measure the shear strength of the sampling soil when material failure occurs. These methods are:[1]

Simple shear tests

Simple shear tests encompass three tests used to determine the shear strength of saturated cohesive soils:

  • The unconfined compression test determines the shear strength of the soil by axially compressing the cylindrical-shaped soil sample. The soil strength is taken from one-half of the compression strength at material failure.
  • The cone test determines the soil strength from the force used to push a cone into the soil sample and the penetration depth at a specified angle.
  • Vane shear test uses a vane to penetrate the cylindrical-shaped soil sample. Soil strength is determined by the applied torque that causes material failure.

The results of these tests can be combined with other parameters to calculate soil stability. However, the setup of these tests restricts the applicability of these tests only to saturated cohesive soils under undrained conditions.

The triaxial test

The triaxial test uses a triaxial cell to stimulate the desired saturation and drainage condition and determine the soil strength.

Here, the soil sample is packed in a membrane-sealed cylinder container inside the triaxial cell, filled with fluid to the desired level. Then, pressure is applied to the fluid-containing cell, subjecting the cell to hydrostatic stress until material failure.

Soil drainage conditions can be simulated by letting the fluid flow through porous stones at the bottom of the cell while pressure is applied. Soil strength is calculated from the change in the drainage volume and the applied pressure. In the case of an undrained soil condition, fluid is trapped inside the triaxial cell, and pore water pressure is measured to use in place of the volume change.

Direct shear test or box shear test

Direct shear test or box shear test determines the soil strength by repeatedly and incrementally applying mechanical force onto the horizontal plane of the soil sample.

The soil specimen is packed in a shear box, consisting of two halves connected by two clamping screws. The upper half contains porous stones and a loading cap on the top where a shearing device applies mechanical force. The lower half is packed with porous stones at the bottom, where water can be drained from the sample, allowing measurement under drained conditions.

2. On-site Soil Strength Measurement

On-site soil strength measurement relies on the empirical correlation between the value obtained from portable devices and parameters factored into soil strength.

In other words, the field devices do not directly measure the variables used to calculate the shear strength. Instead, they measure one or two variables that have a statistical relationship with one of the calculatable soil strength parameters.

Notable testing procedures and field testing devices are:

Standard penetration test (SPT)

Standard penetration test (SPT) uses a split-spoon sampler consisting of a sample tube at one end of a drilling rod and a slide hammer at the other end.

The measurement is performed by placing the sample tube at the bottom of a borehole and letting the slide hammer fall from a specified distance, repeated until the sample tube is buried in the soil at a certain depth. The depth of the punctured sample tube and the number of blows required are parameters for calculating soil density and ultimately converted to soil strength.

SPT is a simple and inexpensive test. However, it is unreliable for determining cohesive soils’ shear strength.[1]  

Cone penetration test (CPT)

Cone penetration test (CPT) uses a cone penetrometer with a cone attached to one end of a drilling rod and a gauge attached to the other.

This method involves drilling a cone into the soil at a constant and controlled rate. As the cone penetrates the soil, the cone index (CI) values recorded at specified intervals reflects the force per area unit required for the cone to press through the soil profile.[1,4]   

CPT is the most widely used on-site soil testing method because it applies to cohesive and cohesionless soils. Cone penetrometer is available as hand-held devices and in mobile measuring stations.

Dynamic cone penetrometer (DCP)

Dynamic cone penetrometer (DCP) is a field-testing device consisting of a hammer at one end of a long driving rod and a cone at the other.

The measurement is performed by placing the cone on the soil surface and dropping the hammer from a standard angle and distance. The impact of the free-falling hammer bores through the soil. The bearing capacity and shear strength are calculated from the distance the cone pierces through the surface after each drop.[4] 

Clegg Impact Hammer

Clegg Impact Hammer comprises a compaction hammer, guiding tube, and accelerometer mounted to the compact hammer.

The compaction hammer is raised and dropped through the guiding tube from a specified distance to measure soil strength. As the hammer hits the surface, the accelerometer displays the peak deceleration in gravities (g) or the Clegg Impact (CIV or IV) unit, which reflects the bearing capacity of impacted the surface. The measurement is repeated three to four times on one spot to obtain the average value, mathematically converted into soil strength.[4-5]

The Clegg Impact Hammer can be used on cohesive and cohesionless soil and is the only on-site non-destructive soil strength measuring device.[4-5] 

In Conclusion

The shear strength of the soil reflects its ability to maintain its rigid form while being subjected to external forces. It is influenced by several soil-formation factors and measured by testing soil samples in laboratories.

Alternatively, several portable devices can infer soil strength from variables that correlate with soil-strength determining parameters. Cone penetrometers are the widely-used device to measure soil strength on-site, while Clegg Impact Hammer is the only non-invasive soil strength measurement equipment available to date.

Check out our Clegg impact tester if you need an automated device that measures soil strength while providing accurate results!


  1. Wu, Tien H. “Chapter 12 Soil Strength Properties And Their Measurement” Landslides: Investigation and Mitigation, edited by A. Keith Turner and Robert L. Schuster, National Academy Press, 1996, pp.316-336
  2. O’Sullivan, M.F. and Ball, B.C. “A comparison of five instruments for measuring soil strength in cultivated and uncultivated cereal seedbeds” Journal of Soil Science, 1982, 33,  pp. 597-608
  3. Langfelder, L.J. and Nivargikar, V.R. “Some Factors Influencing Shear Strength and Compressibility of Compacted Soils” The 46th symposium on compaction of earthwork and granular bases, 1967
  4. Wieder, W., Shoop, S., Barna, L., et al. “Comparison of soil strength measurements of agricultural soils in Nebraska” Journal of Terramechanics, 2018, 77, pp. 31-48
  5. Garrick, N.W. and Scholer, C.F. “Rapid Determination of Base Course Strength using the Clegg Impact Tester” Transportation Research Board, 1986, 1022,  pp. 115-119
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