4.3 Materials

NOTE For guidance on specific formations (e.g. London Clay, Lambeth group, glacial soils and tills, problematic soils, chalk and Mercia mudstone group), see Annex C.

4.3.1 Soils

4.3.1.1 General

COMMENTARY ON 4.3.1.1

The design of foundations usually involves effective stress analysis, although, in some circumstances, total stress analysis might be appropriate or necessary for the design of foundations in fine soils. Soil properties are determined as part of the site investigation process but might be supplemented by data from back analysis of comparable foundations in similar ground conditions.

4.3.1.1.1 The identification and description of soil should conform to BS EN ISO 14688-1.

4.3.1.1.2 The classification of soil should conform to BS EN ISO 14688-2.

4.3.1.1.3 Soil properties should be determined in accordance with BS EN 1997-2 and BS 5930.

4.3.1.1.4 Characteristic soil parameters should be selected in accordance with BS EN 1997-1, based on the results of field and laboratory tests, complemented by well-established experience.

NOTE 1 Guidance on soil description can be found in Soil and rock description in engineering practice [19].

NOTE 2 Information about the behaviour of soils as particulate materials can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 14 [2].

NOTE 3 Information about the strength and deformation behaviour of soils can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 17 [2].

4.3.1.2 Very coarse soils (cobbles and boulders)

COMMENTARY ON 4.3.1.2

Very coarse soils contain a majority (by weight) of particles >63 mm in size. Cobbles are between 63 mm and 200 mm in size; boulders are greater than 200 mm.

Very coarse soils should be identified and classified in accordance with BS EN ISO 14688 and BS 5930.

4.3.1.3 Coarse soils (sands and gravels)

COMMENTARY ON 4.3.1.3

Coarse soils contain a majority (by weight) of particles ≤ 63 mm in size and do not stick together when wet. Sand particles are between 0.063 mm and 2 mm in size; gravels are between 2 mm and 63 mm. Coarse soils cannot be remoulded.

4.3.1.3.1 When establishing the values of parameters for coarse soils, the following should be considered:

• the items listed in BS EN 1997-1:2004+A1:2013, 2.4.3(5);
• the weakening of collapsible soils above the groundwater table, owing to percolation or a rise in groundwater levels;
• disturbance of dense deposits owing to unsuitable construction methods; and
• the presence of weaker material.

4.3.1.3.2 For coarse soils above the groundwater table, the suggested values for characteristic weight density given in Figure 1 may be used in the absence of reliable test results.

4.3.1.3.3 For coarse soils below the groundwater table, the suggested values for characteristic weight density given in Figure 2 may be used in the absence of reliable test results.

4.3.1.3.4 A superior value of weight density should be selected when a high value is unfavourable; an inferior value should be selected when a low value is unfavourable.

Source: Data from Bond [20].

Source: Data from Bond [20].

4.3.1.3.5 For siliceous sands and gravels, the characteristic constant volume (also known as critical state) effective angle of shearing resistance (φ'_cv,k) may, in the absence of reliable test results, be estimated from:

where:

φ'_ang is contribution to φ'_cv,k from the angularity of the particles; and

φ'_PSD is contribution to φ'_cv,k from the soil's particle size distribution.

Values of φ' ang and φ'_PSD are given in Table 1.

4.3.1.3.6 For siliceous sands and gravels with a fines content less than 15%, the characteristic peak effective angle of shearing resistance (φ'_pk,k) may be estimated from:

where:

φ'_dil is contribution to φ'_pk,k from soil dilatancy.

Values of φ'_dil are given in Table 1.

4.3.1.3.7 The value of φ'_dil may alternatively be estimated from:

where:

n is 3 for triaxial strain or 5 for plane strain;

I_R is the soil's relative dilatancy index;

I_D is the soil's density index (defined in BS EN ISO 14688-2);

σ_C is the aggregate crushing stress; and

σ'_f is mean effective stress in the soil at peak strength.

NOTE 1 Bolton [21] defines the relative dilatancy index as I_R = I_D(Q – l_n p') – 1, where Q = l_n σ_c and p' = σ'_f.

NOTE 2 Many geotechnical problems can be simplified into a two-dimensional form where the foundation or structure is significantly long in one direction in comparison with other dimensions. Hence, a large number of stability problems involving embankments and cuttings, retaining walls and strip footings are commonly analysed by assuming a plane strain condition in which no deformation occurs in the direction of the long dimension of the foundation or structure.

4.3.1.3.8 The value of σ_C may be taken as 20 MPa for quartz sands, but can be substantially larger for quartz silts, and substantially smaller for carbonate sands (see The strength and dilatancy of sands [21]).

4.3.1.3.9 If the fines content of the coarse soil exceeds 25%, then φ'_dil should be assumed to be zero, unless testing demonstrates otherwise. For coarse soils with fines content between 15% and 25%, φ'_dil may be determined by linear interpolation.

4.3.1.3.10 If shearing is matrix (i.e. fines) controlled, then φ'_ang should be assumed to be zero, unless testing demonstrates otherwise. For coarse soils with fines content between 15% and 25%, φ'_ang may be determined by linear interpolation.

4.3.1.3.11 The characteristic angle of shearing resistance (φ'_k) for coarse soils with fines content exceeding 25% should be determined as for fine soils (see 4.3.1.4).

4.3.1.4 Fine soils (silts and clays)

COMMENTARY ON 4.3.1.4

Fine soils contain a majority (by weight) of particles ≤63 mm in size and stick together when wet. Silt particles are between 0.002 mm and 0.063 mm in size; clay particles are smaller than 0.002 mm. Fine soils can be remoulded.

Clay soils with plasticity indices greater than about 20% might exhibit considerably lower angles of shearing resistance than observed at the critical state, if their particles become fully aligned with one another. This phenomenon is termed «sliding shear» to distinguish it from «rolling shear» observed in other soils (including coarse soils and fine soils with plasticity indices less than 20%). The angle of shearing resistance exhibited during sliding shear is called the «residual angle of shearing resistance».

4.3.1.4.1 When establishing the values of parameters for fine soils, the following should be considered, as a minimum:

• the items listed in BS EN 1997-1:2004+A1:2013, 2.4.3(5);
• pre-existing slip surfaces;
• dessication; and
• any changes in stress state either induced by construction or resulting from the final design condition.

4.3.1.4.2 For fine soils above the groundwater table, the suggested values for characteristic weight density given in Figure 1 may be used in the absence of reliable test results.

4.3.1.4.3 For fine soils below the groundwater table, the suggested values for characteristic weight density given in Figure 2 may be used in the absence of reliable test results.

4.3.1.4.4 A superior value of weight density should be selected when a high value is unfavourable; an inferior value should be selected when a low value is unfavourable.

4.3.1.4.5 In the absence of reliable test data, the characteristic undrained shear strength of a fine soil (c_u,k) may be estimated from:

(7)

where:

p'_v is the effective overburden pressure;

p'_v,max is the maximum effective overburden pressure that the soil has previously been subjected to;

R_O is the soil's overconsolidation ratio; and

k₁ and k₂ are constants.

NOTE The ratio c_u/p'_v normally varies with depth (i.e. it is not a constant).

4.3.1.4.6 In the absence of reliable test data, the values of k₁ and k₂ in equation (7) may be taken as 0.23 ±0.04 and 0.8 respectively, following New developments in field and laboratory testing of soils [22].

4.3.1.4.7 When determining the characteristic undrained strength of high strength fine soils, due allowance should be made for:

• the detrimental effect of any sand or silt partings containing free groundwater;
• the influence of sampling;
• the influence of the method of testing; and
• likely softening on excavation.

4.3.1.4.8 For fine soils, the characteristic constant volume (also known as critical state) effective angle of shearing resistance (φ'_cv,k) may, in the absence of reliable test results, be estimated from:

where:

I_P is the soil's plasticity index (entered as a %).

NOTE 1 Equation (8) is based on an expression proposed by Santamarina and Díaz-Rodriguez [23], which fits data presented by Terzaghi, Peck, and Mesri [24].

NOTE 2 Values of φ'_cv,k based on this expression are given in Table 2.

4.3.1.4.9 The characteristic constant volume effective cohesion (c'_cv,k) should be taken as zero.

4.3.1.4.10 The peak effective angle of shearing resistance (φ'_pk) may be related to the constant volume effective angle of shearing resistance (φ'_cv) by:

where:

φ'_cv is the soil's constant-volume angle of shearing resistance; and

φ'_dil is the contribution to φ'_pk from soil dilatancy.

NOTE 1 The value of φ'_dil for fine soils is not the same as that for coarse soils. For fine soils, it is typically in the range of 0°–4°. No specific guidance is given in this standard for values of φ'_dil for fine soils.

NOTE 2 Values of φ'_dil are known to increase with a fine soil's overconsolidation ratio and are greater than or equal to zero.

4.3.1.4.11 When a clay soil is able to undergo «sliding shear», normally only where pre-existing slip surfaces exist in the ground, then the operational angle of shearing resistance is the clay's residual value (φ'_res):

where:

φ'_cv is the soil's constant-volume angle of shearing resistance; and

φ'_pk is the soil's peak angle of shearing resistance.

NOTE Guidance on the undrained strength and the residual shear strength of clay soils can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 17 [2].

4.3.1.5 Mixed soils

COMMENTARY ON 4.3.1.5

Some deposits (especially glacial deposits) can comprise a mixture of fine and coarse soils, e.g. sandy clays and clayey sands. The behaviour of mixed soils is typically intermediate between that of coarse and fine soils.

For mixed soils with clay fraction less than 50% (plasticity index less than 30% or liquid limit less than 60%), φ'_cv,k may, in the absence of reliable test data, be estimated from Drained shear strength parameters for analysis of landslides [25].

4.3.1.6 Soil stiffness

COMMENTARY ON 4.3.1.6

Young's modulus of elasticity of an isotropic soil (E) is related to its shear modulus (G) by:

where:

ν is the soil's Poisson's ratio.

Stiffness parameters for a soil depend on the level of strain (axial ε or engineering shear strain γ) applied to the soil, as illustrated in Figure 3:

• at very small strains (ε or γ ≤ ~ 0.001% ), the soil's moduli of elasticity (E and G) reach their maximum values (E_max and G_max), also known as «very-small-strain» values; E_max and G_max are normally measured using dynamic methods, such as laboratory tests using bender-elements or field tests using seismic methods;
• at small strains (~ 0.001% ≤ ε or γ ≤ ~ 0.1% ), E and G decrease rapidly with increasing strain, as shown in Figure 3; their values are measured using advanced methods, such as laboratory tests with local gauges;
• tangent values of small-strain stiffness (E_tan and G_tan) are commonly used in numerical methods of design;
• at large strains (ε or γ >~ 0.1% ), E and G decrease less rapidly with increasing strain and are commonly taken to be constant in value; those values are measured using conventional laboratory testing;
• secant values of large-strain stiffness (E_sec and G_sec) are commonly used in routine methods of design;

• soil stiffness parameters are not normally isotropic. Simple isotropic models are to be used with caution, especially if their adequacy has not been demonstrated by back analysis. Depending on the soil's deposition history, different values of stiffness might be appropriate for vertical strains (E_v) compared to horizontal strains (E_h) and for shearing in different directions (G_hh, G_hv).

a) Definition of tangent and secant moduli

b) Definition of very small, small and large strain stiffnesses

c) Typical strain ranges for common geotechnical constructions (solid arrows) and laboratory tests (dashed arrows)

NOTE Based on Non-linear soil stiffness in routine design [26] and the ICE manual of geotechnical engineering (2012), Volume II, Chapter 52 [1].

4.3.1.6.1 The difference between the direction of loading and the direction of measurement of soil stiffness should be taken into account in the assessment of soil stiffness.

4.3.1.6.2 In the absence of reliable test results, the secant shear modulus of a soil, G_sec, may be estimated from (see also Stiffness of sands through a laboratory database [27]):

(12)

where:

G_max is the soil's very-small-strain shear modulus;

γ is the engineering shear strain in the soil;

γ_e is the elastic threshold strain beyond which shear modulus falls below its maximum value;

γ_ref is a reference value of engineering shear strain (at which G_sec/G_max = 0.5); and

m is a coefficient that depends on soil type.

4.3.1.6.3 In the absence of reliable test results, the values of the parameters in equation (12) may be taken from Table 3.

4.3.1.6.4 In the absence of reliable test results, the very-small-strain shear modulus of a soil, G_max, may be estimated from (see Non-linear soil stillness in routine design [26] and Stiffness at small strain: research and practice [29]):

(13)

where:

e is the soil's voids ratio;

p' is the mean effective stress in the soil;

p_ref is 100 kPa; and

k₁, k₂, and k₃ are coefficients that depend on soil type.

4.3.1.6.5 In the absence of reliable test results, the values of the parameters in equation (13) may be taken from Table 4.