Settlement Analysis for Cohesive Soils | Foundations | Soil Engineering

The settlement of foundations located in cohesive soils (clays) can be computed from the equation, which is –

S_f = S_{i +} S_c …(19.17)

where S_i is the immediate settlement and S_c is the consolidation settlement.

The settlement analysis in cohesive soils is considered for the following cases:

1. Thin clay layer sandwiched between thick sand layers.

2. Clay layer resting on cohesionless soil or rock.

1. Thin Clay Layer Sandwiched between Thick Sand Layers:

Figure 19.18 shows a thin clay layer sandwiched between thick sand layers. The settlement of the clay layer consists of (a) immediate settlement and (b) consolidation settlement.

(a) Immediate Settlement:

For a soil profile consisting of a relatively thin clay layer sandwiched between thick sand layers or between cohesionless soil and rock, we get immediate settlement, S_i = 0.

(b) Consolidation Settlement:

Consolidation settlement for normally consolidated clay is given by –

For pre-compressed or over-consolidated clay, settlement is computed from –

S_c = m_vHΔ σ’ …(19.19)

where m_v is the field coefficient of volume compressibility.

IS – 8009 (Part I)-1976 recommends the method for determining the field value of m_v from the lab consolidation curve. Figure 19.19 shows the laboratory pressure-void ratio curve (k_u line). The line cd represents the rebound curve.

The following are the steps for the determination of field value of m_v:

i. Identify the location of point b with coordinates (σ’₀, e₀).

ii. From b, draw a line parallel to the line cd to intersect the vertical line having x-coordinate equal to p_c‘ at point a’, where p’_c is the pre-consolidation pressure.

iii. Extend the steep straight portion of k_u line in the downward direction to intersect the horizontal line with y-coordinate equal to 0.4 e₀ in point f.

iv. Complete the line ba’f by a smooth curve at the top with the initial top portion approximately parallel to k_u line.

v. The curve ba’f, known as k line, is the field consolidation curve.

vi. The field value of m_v is obtained from the slope of k line at a’.

In preliminary investigations, the compression index may be estimated from the following relation –

C_c = 0.009(ω_L – 10) …(19.20)

C_c = 0.30(e₀ – 0.27) …(19.21)

where w_L is the liquid limit and e₀ is the initial void ratio.

2. Clay Layer Resting on Cohesionless Soil or Rock:

The settlement of clay layer for this case consists of (a) immediate settlement and (b) consolidation settlement which are determined separately as explained in the following subsections.

i. Immediate Settlement:

Immediate settlement may be computed from the theory of elasticity for saturated clays.

where q is the net foundation pressure, B the width of foundation, µ. the Poisson’s ratio, which is equal to 0.5 for clays; E is Young’s modulus of the soil obtained from stress-strain curve from triaxial compression test in consol­idated undrained condition, and I the influence factor obtained from Fig. 19.20 for clay layers of finite extent and Table 19.9 for clay layers of semi-infinite extent.

ii. Consolidation Settlement:

The settlement of clay layer resting on cohesionless soil layer (Figs. 19.21 and 19.22) is –

where λ is called the settlement coefficient, read from Fig. 19.23 for the given values of h/b and pore pressure coef­ficient A.

If the value of pore pressure coefficient is not known, A may be obtained from Table 19.10.

iii. Soil Profile with Several Regular Soil Layers:

If the soil deposit consists of several regular soil layers (Fig. 19.24), the settlement of each layer is computed individ­ually and the total settlement is obtained by adding the settlement of all layers. This method may be used depending upon the type of soil layer, that is, cohesionless or cohesive soil and its relative thickness.

iv. Erratic Soil Deposits:

In variable erratic soil deposits (Fig. 19.26), the settlement analysis should be based on the worst and the best soil conditions if the variation occurs over distances greater than half the foundation width. Worst properties are considered for heavily loaded regions, and the best soil conditions are considered below lightly loaded regions.

In case the variation in soil properties occurs over short distances less than half the foundation width, the settlement analysis is based on the worst and average conditions. Worst properties are considered for heavily loaded regions and the average soil conditions are considered below lightly loaded regions.