Case Studies

GROUND IMPROVEMENT USING PRE-LOADING WITH PREFABRICATED VERTICAL DRAINS: A CASE STUDY

A. Gadhiya Shadab, (Manager – Design, Techfab (India) Industries Ltd., shadabg@gmail.com)
D. Vyas Saurabh, (Head – Technical Services, Techfab (India) Industries Ltd., sdvyas18@gmail.com)

ABSTRACT: A Stock yard was constructed for handling Filter Cake, Coke Breeze, Limestone/Dolomite, Product Pellet, etc at 8MTPA Integrated Iron Ore Pellet Plant – Paradeep, Orissa. The Investigation revealed the presence of sub-soil that included a layer of soft silty clay at depths of 0 to 1.5 m below grade. Thicknesses of the soft stratum varied from 5 m to 7 m. Ground improvement using pre-loading with prefabricated vertical drains was undertaken to pre-consolidate the compressible sub-soils.

A ground improvement work was designed and carried out to pre-consolidate the soft subsoil before construction of the Stockyard so that the settlements after stacking the material are minimized during the service life. Temporary surcharge fill height was designed to reduce long term creep settlement. Construction commenced in September 2011 and was successfully completed in April 2012.
Though Instrumentation and Monitoring was not in scope, this paper presents the analytical aspects of the both stability and settlement performance with prefabricated vertical drains.

Keywords

    • Ground improvement
    • consolidation
    • prefabricated vertical drain
    • settlement
    • slope stability
    • limit equilibrium method
    • numerical analysis

Introduction

Essar Steel Orissa Ltd. has proposed to set up an 8MTPA Integrated Iron Ore Pellet/Steel Plant at Paradeep, which comprises of various facilities & stock pile areas where the movement of stacker cum reclaimer was planned. Heavy loading in terms of metric tons were expected and the subsoil beneath consist of soft soil in nature. At some places the area was already filled with dredged sand for 2-3 mt. A ground improvement work was designed and carried out to pre-consolidate the soft subsoil before construction of the yard so that the settlements of the yard are minimized during the service life.

Based upon the soil stratification, the entire area was divided in few zones and an average profile was suggested by Investigation agency for further design of foundation system..

Problem Description

Looking to the soil stratification and properties, the top few mt. of soil was too weak to resist the load of stacking. And actual height of Stacking was undetermined at the time of proposal. So it was decided to stack 1.5 mt. ht. of preload with locally available soil. Considering 90 % of degree of consolidation to be achieved, the design of Ground Improvement with PVD was done. This paper reveals the futuristic analysis viz. Stability and Settlement of the embankment with 8 mt. of stacking over the ground where PVD was inserted and which was preloaded. The Stability and Settlement Analysis was carried out using Rocscience Slide v6.009 & Settle3D v2.015.

Solution Proposed

Execution On Site

CONCLUSIONS

Based upon the field data, it was revealed that the existing ground had inadequate bearing capacity and will undergo large settlements on application of surcharge load. A large settlement of such magnitude may damage the structures constructed on such soils. So to improve the Load carrying capacity of soil, Ground Improvement in terms of Prefabricated Vertical Drain (PVD) was proposed with a preload of 1.5 mt. Installation of the vertical drains reduced pre-consolidation time significantly

(from about 3 to 5 years without vertical drain to about 31 days with PVDs).
For product pellet stacking, stability and settlement analysis was carried out. Man made slope stability analysis was done with Limit equilibrium approach using software “Slide”. Anticipating the construction sequence and slope stability, stacking in three stages viz. 3mt., 3mt., and 2mt., was found safe. The factor of safety for all the stages was ensured more than intended in codal provision. Rigorous settlement analysis was computed using software Settle3D.

Related Products

TechDrain PVD