Optimum design of a soil improvement system by preloading with wick-drains

Abstract

The design of a soil improvement system, by preloading with wick drains, is formulated as a constrained optimization problem. The drain spacing, length, embankment height, and time required to achieve a specified consolidation settlement are selected as design variables whereas, the total cost of the system is adopted as an objective function. The cost function includes excavation, sand blanket, engineering fill, surcharge, wick drains, instrumentations and observation cost components. For a given site dimensions, soil profile, targeted settlement, and maximum allowed time to achieve, a computer program is coded in Fortran-90 to solve the problem of consolidation in vertical and radial directions based on Hansbo's (1981) and Olson (1977) methods, with different options to include the effects of smear, well resistance, ramp loading, and wick drains characteristics. In conjunction with the modified Hooke and Jeeves optimization method, the program is applied to a real project under construction in Basra province-Iraq. The real site dimensions, soil profile, and soil characteristics, as obtained from the site investigation program, are adopted. For the unit prices assigned, the results support the capability of the optimization method in manipulating such a decisionmaking problem. They also revealed failure of decision taken of canceling the inclusion of wick drains and adopting preloading only as a technique to improve the site soil. Studying the effects of the values of coefficients of consolidation in two directions on the behavior highlighted the vital importance of conducting a preliminary site investigation to evaluate foundation proposals. After arriving to a decision regarding soil improvement, the detailed phase of site investigation should be oriented towards finding the first order soil parameters associated with the selected soil improvement method, instead of wasting the efforts and money through conducting an exaggerated number of traditional less important tests. It is concluded that increasing the coefficient of consolidation in the vertical direction and the ratio of its radial value to the vertical one will increase the optimum drain spacing and decrease the drain length, the time required to achieve a specified settlement, and the required cost of the system. Embankment height, time of consolidation and total cost are proportional to the required settlement.

Optimum geometric design in spacing and depth for prefabricated vertical drains (PVDs) to accelerate the consolidation rate with validation by finite element modelling (PLAXIS 3D)

“…According to a case study on a design and build project in Salt Lake Valley, the field tests indicated that prefabricated vertical drains (PVDs) with a spacing of less than 1.75 m do not give significant changes to the settlement rate (Rollins, 2009) [27] . While the spacing between the drains should be more than the equivalent drain diameter (Jabir, 2013) [2] . The drain spacing may be varied to optimize the design, and when the drains are installed closer to each other, the consolidation time will be reduced, but the installation costs will increase (Kishore, 2004) [13] .…”