Geotechnical Properties of Soft Improved Ground from insitu Time-Settlement Plots

Padmavathi M^1*, Divya J² and Madhav MR^3
*Correspondence: Padmavathi M, Department of Civil Engineering, JNTU, Hyderabad, Telangana, India, Tel: +919866814305, Email:

Keywords

Coefficient of consolidation • Sand drains • Asaoka plots

Introduction

Settlement of soft clay consists of immediate, consolidation and creep or secondary settlements. Largest proportion of the settlement of soft clay is attributed to the consolidation process. Because of the very low permeability, the primary consolidation settlement takes a long time to complete [1-3]. Structures constructed on soft soils face problems of stability and serviceability if measures are not taken to improve them. Ground improvement techniques are required to overcome these problems. Ground improvement techniques essentially increase the shear strength and reduce the compressibility of the soil to a desired extent. Pre-fabricated vertical drain technique is one of the most suitable methods to accelerate the dissipation of excess pore pressures. Application of ground improvement method using prefabricated vertical drains (PVD) coupled with surcharge or preloading can significantly shorten the period of primary settlement. PVDs or sand drains greatly shorten the drainage path. A method proposed in the paper involves estimation of the coefficient of consolidation for radial flow from in situ time – settlement plots. Queyroi et al. [4] presented the solution for the degree of consolidation, U_r, for radial flow and time-dependent loading for the equal strain case. The degree of consolidation, U_rt0, at the end of construction period for no smear (s becomes unity) case is:

(1)

Where - the time factor at the end of construction

(2)

(3)

(4)

n is the ratio of diameter of unit cell, d_e, to the diameter of the drain, d_w, d_e = 1.128S and 1.05S for drains installed in square and triangular patterns respectively, S – spacing between drains. Smear factor, s is the ratio of radius of smear zone to radius of drain and c_vr- the coefficient of consolidation for radial flow.

The degree of consolidation, U_r(t0) at the end of construction period is plotted with time factor, T_R0, for different values of ‘n (the ratio of unit cell, d_e, to that of drain, d_w) (Figure 1).

Degree of consolidation, U_r(t0) increases (Figure 1) with time factor, T_R0, corresponding to end of construction but decreases with n. i.e., with larger spacing of the PVDs or sand drains.

Application of Proposed Method

From a given time-settlement plot, the final/ultimate settlement, Sult, is estimated using Asaoka method [2]. The settlement, S_t0, at the end of construction time (t₀) is noted from the in situ time – settlement data. The degree of consolidation, U_r(t0), is

(5)

The time factor, T_R0, corresponding to the end of construction, is obtained from Figure 1 from the estimated U_r(t0) and the known value of ‘n’. The coefficient of consolidation with radial flow, c_vr is then estimated from the known values of t₀, T_R0 and de, as

(6)

This method is applied to few case studies reported in literature.

Mud flats, South of Iraq

Mud flats, an extension of the deltas of the rivers Tigris and Euphrates exist at the Arabian Gulf in the South of Iraq. The site is underlain by approximately 10 m of soft, sensitive, normally consolidated clay to salty clay containing seems of silt and/or sand. Unit weight of soil is 1.8 g/cm³ and plasticity index varies from 20 to 28. Initial void ratio is 0.81 and the compression index is 0.346. Three tanks, A, B and C with diameter of 76.2 m were built.

Sand drains of dia. 0.3 m were installed in triangular pattern with spacing of 2.0 m, 2.25 m and 2.5 m beneath Tanks A, B and C respectively. Ratios, n = d_e/d_w, of diameter of unit cell to diameter of well are 7.0, 7.875 and 8.75 respectively. Observed time versus settlement plots for Tanks-A, B and C are shown in Figures 2, 3 and 4 respectively.

Asaoka plots with 10-day time step are given in Figures 5, 6 and 7 respectively for Tanks A, B and C. Ultimate settlements, S_ult for the tanks A, B and C are obtained as 2.7 m, 3.4 m and 2.9 m respectively and the slopes of the lines fitted are 0.934, 0.956 and 0.93. Settlements, S_t0, and the times, t₀, at the end of construction are 1.14 m and 110 days, 1.18 m & 150 day and 1.2 m & 100 days. Degrees of consolidation at end of construction are 0.42, 0.35 and .0.41 and the corresponding time factors, T_R0 are 0.18, 0.16 and 0.2 from (Figure 1). The coefficients of consolidation, cr for radial flow are = 2.63, 2.17 and 5.05 m²/year (Equation 6).

Embankment on soft clay treated with sand drains

A highway embankment was constructed with a height of 6 m and length of 900 m. Strata is very plastic organic clay with a crust approximately 1.0 m thick. The organic compressible soft clay is 10 m thick Profiles PL1 and PL4 represent different types of foundation soils. Sand drains with outer and inner diameters of 0.3 m and 0.1 m respectively were installed in a triangular mesh of 2.0 m spacing at both the sites.

Profile PL1

PL1 profile was instrumented with settlement gauges and piezometers. The ratio, n, of diameter of unit cell, d_e, to diameter, d_w, of well is 7.0. Observed time-settlements at points T1, T2 and T3 are shown in Figure 8. Ultimate settlements estimated from Asaoka plots shown in Figures 9, 10 and 11.

Settlements, S_t0, and time t₀, for end of construction, final settlements, S_ult, degrees, U_r(t0), of consolidation and time factors, T_R0, corresponding to end of construction are determined and listed in Table 1 for the three points for Profile PL1.

Profile PL4

At PL4, five surface settlement plates are installed at different points on ground. In situ time-settlement plots at points T1, T2, T3 and T4 for PL4 profile are shown in Figure 12. Ultimate settlements, Sult estimated using the Asaoka plots for the time-settlement curves for various points T1, T2, T3 and T4 are presented in Figures 13-16.

Diameter of unit cell (d_e) =2.1 m, diameter of well (d_w) is 0.3 m. ratio, n, of diameter of drain to diameter of well) is 7.0, F (n) is 1.598, time interval, Δt, to draw S_i Vs S_i-1 plot, is 50 days. Parameters for Profile PL4are tabulated in Table 2. Coefficients of consolidation for radial flow, c_vr are estimated by substituting the appropriate parameters in Equation 6, and listed in Table 3 for the two profiles PL1 and PL4.

Results and Discussions

For the site of Mud flats the range of coefficients of consolidation for radial flow (c_vr) from the laboratory test results is from 0.95 to 9.47 m²/year [3] while the range of in situ coefficients of consolidation for radial flow (c_vr) estimated using the proposed degree of consolidation at end of construction is between 2.63 to 5.05 m²/year, very different from the laboratory determined values.

For the site of treated ground at profiles PL1 and PL4 the range of laboratory coefficients of consolidation is between 4.73 to 37.87 m²/year [4] while the estimated in situ coefficients of consolidation for radial flow by the proposed new approach, i.e., based on degree of consolidation at the end of construction time, is in the range 2.19 to 4.02 m²/year for profile PL1 and 5.15 to 6.60 m²/year for profile PL4. Once again the in situ coefficients are very different from the laboratory determined values. The behaviour of the ground is very different from that determined in the laboratory based on small size samples.

Conclusion

PVDs or Sand drains with surcharge helps in accelerating the consolidation process in situ. The proposed method estimates the coefficient of consolidation for radial flow from in-situ time-settlement plots at early times. The results estimated from in situ time – settlement plot are different from laboratory determined values.

References

1. Schiffman, Robert L. "Consolidation of soil under time-dependant loading and varying permeability." In Highway Research Board Proceedings 37 (1958).
2. Asaoka, Akira. "Observational procedure of settlement prediction." Soils Found 18 (1978): 87-101.
3. Al, Alusi, H. R. Al Alusi, and A. F. Al Alusi. “Prediction of Settlements of soft clays caused by Sand drains and Preloading.” International Symposium on Soft Clay, Bangkok, Thailand (1977):195 – 210.
4. Queyroi, D, Pilot, G, and Magman, J. P. “Construction of an embankment on a soft clay foundation treated with sand drains.” International Symposium on Soft Clay, Bangkok, Thailand (1977): 245 -259.