Stabilization of Soft Clay

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Stabilisation of soft clays – A review


Abstract— Soft clays are soils having a poor bearing capacity and are highly compressible. Soft clays can be stabilised by the following methods: lime-soil columns, stone columns, electro-osmosis and preloading and vertical sand drains. This paper mainly deals with description of the above techniques and some case studies related to them.


Soft clays are those clays the natural water content (w%) of which would be near liquid limits. Hence, they are poor in bearing capacity and are highly compressible (Broms, 1979). If the existing soft clay sites are to be used as construction sites they must be first treated or subjected to various modification techniques (which are called ground improvement techniques) so that their engineering behavior can be improved in a manner suitable to render them fit for use as construction sites. While some of these ground improvement techniques are based as well developed theories, some are empirical and semi-empirical in nature (Broms, 1979; Mitchell, 1981). This paper reviews some of the foremost of ground improvement techniques employed in the case of soft clays. The paper also furnishes some interesting case studies.

2. Lime Columns
The method of ground improvement using lime columns consists of placing columns of specially prepared quick lime into the soft soils without mixing. Lime soil columns are installed with a special tool called dough mixing tool. The inclined cutting blades of the tool cut the soil and lime can be injected through a provision provided for it as shown in Fig. 1. As the cutting tool is withdrawn the mix of soft clay and lime get compacted, resulting in a lime-soil column or lime column. The resulting column has the same diameter as that of the dough mixing tool. The diameter of the column must not be greater than 0.8m (Broms, 1985). With time, lime can radially move into the soft clay layer and stabilise the surrounding soil by the methods explained below:

a. Consolidation / dewatering effect:
Quick lime, CaO, absorbs water from the surrounding ground, causing the lime to swell and form slaked lime (Ca(OH)2)

a 3rd year Civil Engineering Student, VIT University, Vellore 632014 ( b 3rd year Civil Engineering Student, VIT University, Vellore 632014 ( c Professor of Civil Engineering, VIT University, Vellore 632014 b. Ion exchange effect:

As the surface of fine particles of clay is negatively charged, calcium ions (Ca++) from the slaked lime are absorbed by the surface of clay particles. As a result, clay particles are bonded with each other and the weak clay is improved with a resultant increase in shear strength. c. Pozzolanic effect:

Calcium ions continue to react with SiO2 and Al2O3 in the clay for a long time thereafter, forming compounds that cause the clay strength to be improved. This reaction is termed a Pozzolanic reaction. The lime piles themselves have considerable strength and therefore act to reinforce the soil as well as alter its properties.


Fig. 1 Installing lime-soil columns

2.1 Case Study:
A full scale test is described below where a small residential building was built on a foundation of lime columns. The experimental house was one among the 190 built at Glomsta in the middle of Sweden. The soil at the site consisted from the ground surface of 1.2m dry crust, 4.5m to 5.2m of soft clay, sand and gravel. The ground water level was located at the lower boundary of the dry crust. The properties of the soft clay at Glomsta are summarized in Table I. Table I

Properties of clay
|Undrained shear strength |10kpa |
|Water content |65% |
|Liquid limit |70% |
|Unit weight...
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