SAND DRAINSFOR CONSOLIDATION SETTELEMENT

REPORT BY:-

KABRUTI SUBHAM NAYAK

1101110022

CONTENTS INTRODUCTION BREIF HISTORY CONSOLIDATION APPROCH APPLICATION ADVANTAGES DISADVANTAGES CONCLUSION REFERENCE

INTRODUCTION

Soils are deposited or formed under different environmental conditions.

Man does not have any control on the process of soil formation.

So, we need to accept as they are and any construction has to be adapted to suit the subsoil consideration.

The existing soil at a given site maynot be suitable for supporting structures like buildings, bridges, dams etc. because of safe bearing capacity of soil may not be adequate to support the provided load.

Although pile foundations may be adapted in some situation, which often became too high for low to medium rise buildings.

IN THAT CASE AN ENGINEER WILL GO FOR THE FOLLOWING STEPS……

i. Change the site.

ii. Change the structural design.

iii. Increase expenses.

iv. Go for ground improvement technique.

GROUND IMPROVEMENT TECHNIQUES

•Ground improvement in its broadest sense is the alteration of any property of a soil to improve its engineering performance.

•Ground improvement for the purpose of foundation essentially means increasing the shear strength of the soil and also density of soil and to decrease the compressibility, permeability and the settlement, which makes the soil more water resistance, durable, and stable.

HISTORYIn the 1930s the first reasonable application of vertical sand drains was made in California. In Sweden, during the same decade, Kjellman introduced the first prototype.

(Jamiolkowski et al., 1983). Subsequently, several types of vertical drains developed which are basically consisting longitudinal channel wick functioning as a drain.

CONSOLIDATION OF SOILConsolidation is a process by which soils decrease in volume.

According to ”Karl von Terzaghi” consolidation is any process which involves a decrease in water content of saturated soil without replacement of water by air.

In general it is the process in which reduction in volume takes place by expulsion of water under long term static loads.

It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume.

When this occurs in a soil that is saturated with water, water will be squeezed out of the soil. 

When stress is removed from a consolidated soil, the soil will rebound, regaining some of the volume it had lost in the consolidation process.

If the stress is reapplied, the soil will consolidate again along a recompression curve, defined by the recompression index.

The highest stress that it has been subjected to is termed the pre consolidation stress.

The over consolidation ratio or OCR is defined as the highest stress experienced divided by the current stress.

A soil which is currently experiencing its highest stress is said to be normally consolidated and to have an OCR of one.

APPROCH TOWARDS SAND DRAIN…..

Because of its low permeability, the consolidation settlement of soft clays takes a long time to complete.

To shorten the consolidation time, vertical drains are installed together with preloading either by an embankment or by means of vacuum pressure.

Vertical drains are artificially-created drainage paths which are inserted into the soft clay subsoil.

Thus, the pore water squeezed out during consolidation of the clay due to the hydraulic gradients created by the preloading, can flow faster in the horizontal direction towards the vertical drains.

It is taken advantage of the fact, that most clay deposits exhibit a higher horizontal permeability compared to the vertical.

Subsequently, these pore water can flow freely along the vertical drains vertically towards the permeable layers.

Therefore, the vertical drain installation reduces the length of the drainage path and, consequently, accelerates the consolidation process and allows the clay to gain rapid strength increase to carry the new load by its own.

IN ORDER TO ACCELERATE THE PROCESS OF CONSOLIDATION SETTLEMENT FOR THE CONSTRUCTION OF SOME STRUCTURE, THE USEFUL TECHNIQUE OF BUILDING SAND DRAINS CAN BE USED.

SAND DRAINS ARE CONSTRUCTED BY DRIVING DOWN CASINGS OR HOLLOW MANDRELS INTO THE SOIL.

THE HOLES ARE THEN FILLED WITH SAND, AFTER WHICH THE CASINGS ARE PARALLEL OUT.

WHEN A SURCHARGE IS APPLIED AT GROUND SURFACE, THE PORE WATER PRESSURE IN THE CLAY WILL INCREASE, AND THERE WILL BE DRAINAGE IN THE VERTICAL AND HORIZONTAL DIRECTIONS.

THE HORIZONTAL DRAINAGE IS INDUCED BY THE SAND DRAINS. HENCE, THE PROCESS OF DISSIPATION OF EXCESS PORE WATER PRESSURE CREATED BY THE LOADING (AND HENCE THE SETTLEMENT) IS ACCELERATED.

SAND DRAIN….

APPLICATIONSAirport Runways Golf Courses Mine Tailings ConsolidationTailing Ponds Swampland/Wetland Development Building Foundations Retaining Walls Parking Lots Landfills

ADVANTAGES…1.INCREASED RATE OF GAIN OF SHEAR STRENGTH OF CLAY

Enable the load to be applied more rapidly, thus better use of construction plant. In case of embankments, steeper slopes and provision of berms can be avoided.Lower amount of fill required.Increased rate of consolidation. Consequent savings in construction cost.

2.INCREASED RATE OF CONSOLIDATION Reduction in time required for primary settlement. Structure or embankment s can put into commission and use far earlier. Reduction in cost of maintenance. 3.STABILITYT OF EMBANKMENTS Many soft clay strata containing band, or parting of sand or silt.Excess horizontal spread of pore pressure along these partings take place.Vertical drains installed can relieve these excess pore pressure.

CONCLUSIONS…1. Sand drains effectively reduce the time

taken for consolidation of saturated clay for both stiff and soft clays.

2. This reduction is much more pronounced in case of stiff clays where the time taken reduces by about 6 to 11 times.

3. For both stiff and soft clays, settlement steadily increases with applied load.

4. The settlement of soft clay was found to be 3 to 5 times more than that for stiff clay.

5. The final settlement does not vary with the diameter of sand drain.

6. And the time of consolidation steadily decreases with increase in the diameter of drains.

REFERENCES….Asaoka, A. 1978. Observational Procedure of Settlement Prediction. Soils and Foundations, Vol. 18, No. 4, Dec. 1978. Japanese Society of Soil Mechanics and Foundation Engineering. pp. 87- 101. Barron, R. A. 1948. Consolidation of fine-grained soils by drain wells. Transactions ASCE, Vol. 113, paper 2346, pp. 718-724. Bergado, D. T., Anderson, L. R., Miura, N., Balasubramaniam, A. S. 1996. Soft ground improvement in lowland and other environments. New York: ASCE Press. Bergado, D. T., Manivannan, R., Balasubramaniam, A. S. 1996. Proposed criteria for discharge capacity of prefabricated vertical drains. Geotextiles and Geomembranes 14 (1996), 481-505. Chai, J.-C., Carter, J. P., Hayashi, S. 2005. Ground Deformation induced by Vacuum Consolidation. Journal of geotechnical and geoenvironmental engineering, Vol. 131, No. 12, Dec. 2005, 1552-1561.

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