The strength of soil is typically defined as the resistance to shear stress in terms of the effective internal friction angle and effective cohesion (c’). This technical note is intended to provide a basis for estimating a soil’s unit weight in the absence of specific testing results.
Factors Affecting Soil Shear Strength
The shear strength of a soil is achieved by interaction between the solid, liquid, and gas particles in its make-up. So the shear strength of a soil depends on the composition of the soil’s particles, the amount of water in the soil, and how well compacted the soil is. The contributing factors include, but are not limited to:
- Mineralogy of the soil particles (e.g. silica, quartz, feldspar, etc.).
- The range of sizes of the soil particles, also known as the particle size distribution.
- The angularity of the soil particles (most relevant to coarse sands and gravels)
- The moisture content of the soil – whether the voids between the soil particles are completely filled with water (fully saturated) or mostly air – and the capillary forces created by the interaction of the solid particles, water and air.
- Degree of compaction of the soil.
Measuring Soil Shear Strength
The shear strength of a soil is measured directly in a laboratory or estimated from correlations with testing undertaken on site. In a laboratory, shear strength is measured by shear box or triaxial testing in accordance with BS 1377-7:1990 or BS 1377-8.:1990 respectively.
Shear box testing of a soil / geotextile interface (provided courtesy of Geospec Ltd)
On site the most common technique is the correlation between shear strength and SPT (Standard Penetration Test) results.
However, there are a wide variety of correlations that can be used to estimate soil strength parameters such as CPT (Cone Penetration Test), DCP (Dynamic Cone Penetrometer), HSV (Hand Shear Vane), Mexe Probe, DMT (Flat Dilatometer Test), etc.
Typical Soil Shear Strength Parameters
Typical shear strength parameters for various soils have been estimated by several sources depending on the amount of information available. Broadly speaking, information about the soil, and the method of estimating shear strength falls into one of two categories:
- Information limited to soil type – in the absence of any in-situ testing, such as when a site investigation has not yet been conducted, ABG recommends the correlations from Oritz (Oritz et al., 1986). This information has been reproduced in Table 1, below.
- Limited Testing Information – a typical site investigation will provide a lot of useful information but may not include any direct measurements of soil strength information. In this case ABG recommends using the advice in BS 8002:2015, and BS EN 1997-2. This information has been reproduced below in (Tables 2, 3 & 4). Alternatively, the ‘Handbook of geotechnical investigation and design tables‘ (Look, 2007) contains a wide variety of useful correlations with a wide range of common geotechnical tests.
Typical general arrangement of shearbox apparatus (BS 1377-7:1990)
Table 1: Typical soil shear strength based on soil type
Table 2: Sands and Gravels – Determination of soil shear strength parameters (BS 8002:2015)
Notes for Table 2:
- Terms for defining particle shape can be found in BS EN ISO 14688-1.
- The uniformity coefficient Cu is defined in BS EN ISO 14688-2
- The density index /D is defined in BS EN ISO 14688-2. Density terms may be estimated from the results of field tests (e.g. Standard Penetration Test, Cone Penetration Test) using correlations given in BS EN 1997-2:2007.
- Values of “Fines” refers to that fraction of the soil whose particle size is less than 0.063 mm.
Table 3: Sands and Gravels – Correlation between normalised blow count and density index (BS EN 1997-2:2007)
Notes for Table 3:
For fine sands, the N-values should be reduced in the ratio 55:60 and for coarse sands increased in the ratio 65:60
Table 4: Silts and Clays – Determination of soil shear strength parameters (BS 8002:2015)
Notes for Table 4:
The characteristic constant volume effective cohesion (c’cvk) should be taken as zero
British Standards Institution, BS 1377-7:1990 – Methods of test for soils for civil engineering purposes. Shear strength tests (total stress)
British Standards Institution, BS 1377-8:1990 – Methods of test for soils for civil engineering purposes. Shear strength tests (effective stress)
British Standards Institution, BS 8002:2015 – Code of Practice for Earth Retaining Structures
British Standards Institution, BS EN 1997-2:2007 – Eurocode 7 – Geotechnical design Part 2: Ground investigation and testing
Look, B. (2007). Handbook of geotechnical investigation and design tables. London: Taylor & Francis. Ortiz et al. (1986).