Methods for Settlement of Cohesionless Soils | Soil Engineering

There is no practicable laboratory procedure for the determination of compressibility characteristics of cohesionless soils because of the difficulty of sampling of these soils. Therefore, settlement of cohesionless soils may be estimated by a semi-empirical method, based on the results of SCPT, DCPT, or PLT.

1. Schmertmann’s Method for Settlement of Cohesionless Soils:

Schmertmann (1970) presented a method to compute settlements using static cone test data. The soil, over the depth 2B below the base of the foundation, is divided into a convenient number of layers and the average static cone resistance of each layer is determined. Schmertmann gave the following equation for the calculation of settlements –

where C₁ is the depth embedment factor given by Eq. (19.3), σ’₀ the effective overburden pressure at foundation level, Δσ ‘ the net increase in pressure at foundation level, C₂ the empirical creep factor given by Eq. (19.4), t the time period in years for which the settlement is calculated, I_z the average strain influence factor, Δz the thickness of each layer, E the deformation modulus of each layer given by Eqs. (19.5) and (19.6), and q_c the average value of cone resistance in each layer. They are given by –

Tables 19.1 and 19.2 give the vertical strain influence factor for square and rectangular footings for different values of z/B as recommended by Schmertmann. For intermediate values of z/B, the value of I_z is obtained by linear inter­polation. Figure 19.7 shows the graphical representation of Tables 19.1 and 19.2.

2. IS(8009) Code Method for Settlement of Cohesionless Soils from SCPT:

IS – 8009 (Part I)-1976 recommends a method for the estimation of settlement for cohesionless soils from SCPT data. The SCPT shall be conducted as per IS – 4968 (Part III)-1971. The results of SCPT are presented in the form of a graph with cone resistance (C_kd) on the x-axis and depth on the y-axis. It is divided into several layers in such a way that each layer has approximately same value of C_kd. The average cone resistance C_kdi for each layer is also calculated. The settlement is calculated for each layer using the average value of C_kdi, and total settlement is obtained by adding the settlement computed for all layers.

Total settlement is, thus, as follows –

where S_t is the total settlement, σ’_0i the initial effective stress at mid-height of ith layer in kgf/cm², Δσ_i‘ the pressure increment at mid-height of ith layer in kgf/cm², C_kdi the average static cone resistance for the ith layer in kgf/cm², as obtained from Fig. 19.9, and n the number of layers of cohesionless soil having different values of C_kd.

3. IS(8009) Code Method for Settlement of Cohesionless Soils from Plate Load Test:

IS – 8009 (Part I)—1976 recommends a method for the estimation of settlement for cohesionless soils from PLT data. PLT shall be conducted as per IS – 1888-1971 using a square test plate of size 30 cm. The settlement of the soil is computed using –

where S_f is the settlement of foundation in meter, S_p is the settlement of square test plate of size 30 cm under the pressure of σ’₀, σ’₀ is the effective pressure from the foundation for which settlement is to be computed, and B is the width of foundation in meter.

If PLT is conducted with a square test plate of size other than 30 cm, the settlement of the soil is computed by –

where B_P is the width of the test plate in meter.

4. IS(8009) Code Method for Settlement of Cohesionless Soils from SPT:

IS – 8009 (Part I)-1976 recommends a method for the estimation of settlement for cohesionless soils from SPT data. Figure 19.10 shows the settlement per unit pressure plotted on the y-axis with the width of footing on the x-axis as recommended by the code with several curves for different values of N, the standard penetration resistance. For the given value of width of footing and SPT N value, the settlement in meter per unit pressure in kgf/cm² can be read from the chart in Fig. 19.10. This value shall be multiplied with the actual effective pressure from the foundation in kgf/cm² to obtain the total settlement.

The SPT N value is to be corrected for overburden pressure and dilatancy as per IS – 2131-1981 before using Fig. 19.10 for the computation of settlement.

If the water table is at a shallow depth, the settlement obtained from Fig. 19.10 shall be multiplied by the water table correction factor, W’. The value of W is obtained from Fig. 19.11 (b) based on d/B ratio for the footing shown in Fig. 19.11(a).