An Effort Towards Construction of Structures on Heterogeneous Soil of Backfilled Opencast Mines

Abstract

In this age of rapid urbanisation, due to the scarcity of space in prime locations especially in major cities, constructions are proposed even on refilled areas, either natural or engineering fill. In the present scenario, the major problem is the geotechnical issues acting as barriers to the construction of buildings in such areas. Basically, in engineering refill of opencast mines, proper precautions often cannot be taken for the selection of the refill material and care taken for the compaction is not so seriously dealt with. In the past, requirement of making such constructions on backfilled earth has not been envisaged. Such backfilled mines are often extremely heterogeneous which throws two different challenges. First one is to arrive at a safe bearing capacity which can be uniformly used for construction at the backfilled site. The other one is to decide the nature of foundation and structure which can be built safely with such low bearing capacity expected in such backfilled soil. The present paper is a very humble effort to issues with the living examples of two backfilled sites at Talcher, Odisha, India.

Appendix: Sample Calculations

SPT Value (Test 1)

The SPT number from the borehole data obtained from SPT Test 1 can be calculated as N_avg = (8 + 8+15 + 22 + 13 + 14 + 23 + 19)/8 = 15.25.

It is known that the range of the corrected N value will lie between ± 50% of the average N value as mentioned in the code [7]. Range of the corrected N value = 22.875 to 7.625. Hence, N value of 23 may be neglected. Thus, N_corrected = (8 + 8+15 + 22 + 13 + 14 + 19)/7 = 14.14 ≈ 14.

SPT Value (Test 2)

The SPT number from the borehole data obtained from SPT Test 2 can be calculated as N_avg = (25 + 30 + 33 + 25 + 20 + 31 + 24 + 35)/8 = 27.88.

It is known that the corrected N value will lie between ± 50% of the average N value as mentioned in the code [7]. Hence, range of the corrected N value = 41.81 to 13.94. Thus, N_corrected = 27.

Bearing Capacity Calculation from Terzaghi’s Equation

The ultimate bearing capacity is given by the following equation

$$Q_{\text{ult}} = cN_{c} + \gamma D_{f} N_{q} + 0.5\gamma BN_{\gamma }$$

The values of N_c, N_q and N_γ can be calculated from the following equations as provided by the literature [10] are obtained as 3.29, 10.76 and 1.7, respectively.

Putting the corresponding values in Terzaghi’s bearing capacity equation, the ultimate bearing capacity is obtained as

$$Q_{\text{ult}} = \left( {10.36 \times 3.29} \right) + \left( {2.61 \times 9.81 \times 3 \times 10.76} \right) + \left( {0.5 \times 2.61 \times 9.81 \times 10 \times 1.7} \right) = 1094.05\;{\text{kN}}/{\text{m}}^{2}$$

The net ultimate bearing capacity is given by

$$\begin{aligned} Q_{\text{netult}} & = Q_{\text{ult}} {-} \gamma D_{f} \\ & = 1094.05{-}\left( {2.61 \times 9.81 \times 3} \right) = 1016.05\;{\text{kN}}/{\text{m}}^{2} \\ \end{aligned}$$

The net safe bearing capacity is given by

$$\begin{aligned} Q_{\text{netsafe}} & = \frac{{Q_{\text{netult}} }}{F.O.S } \\ & = 338.68\;{\text{kN}}/{\text{m}}^{2} \\ \end{aligned}$$

where a factor of safety of 3 has been considered.

Finally, the safe bearing capacity is obtained as follows.

$$\begin{aligned} Q_{\text{safe}} & = Q_{\text{netsafe}} + \gamma D_{f} \\ & = 338.68 + \left( {2.61 \times 9.81 \times 3} \right) = 416.68\;{\text{kN}}/{\text{m}}^{2} = 41.668\;{\text{t}}/{\text{m}}^{2} . \\ \end{aligned}$$