What is stone columns? Why it is required?
It is a technique adopted for the soil which didn’t have the sufficient characteristics to permit construction. The stone column technique, also known as vibro-replacement or vibro-displacement, is a ground improvement process where vertical columns of compacted aggregate are formed through the soils to be improved.
Stone columns, also known as granular piles, consist of stone aggregates compacted into a vertical hole. Generally, the size of aggregate used is 20mm to 75mm and the depth of hole is about 15m to 20m with the diameter of 0.6m to 1.0m. The compaction is achieved by a vibrating probe or by ramming.
Uses Of Stone Columns:
- Increases the shear strength.
- Lowers the compressibility of the composite soil consisting of the stone column and the surrounding soil
Priebe’s Method of Stone Columns Design:
The design of stone columns by Priebe’s method, mainly involves the following steps
- Determination of basic improvement factor.
- Determination of the improvement factors by considering, column compressibility and overburden pressure.
- Finding the compatibility controls.
- Determination of the shear values of improved ground.
Determination of Basic Improvement Factor
- The column is based on a rigid layer
- The column material is uncompressible
- The bulk density of column and soil is neglected
Form the above assumptions, the column cannot fail in end bearing and any settlement of the load area results in a bulging of the column which remains constant all over its length. These assumptions are just for finding basic improvement factor only. After that these are eliminated one by one.
Basic improvement factor is denoted by “” and it is determined by the formula given below:
Where, , and
Here Ac = unit cell area (area affected by single stone column)
A = single stone column area
= Poisson’s ratio
= coefficient of active earth pressure
= angle of friction
Consideration of the Column Compressibility
In this case we are considering column compressibility, so we are eliminating one of the assumptions.by this we can find out reduced improvement factor denoted by “”. Which results from the formula developed for the basic improvement factor n0 when the given reciprocal area ratio A/AC is increased by an additional amount of .
Consideration of the Overburden
The neglect of the bulk densities of columns and soil means that the initial pressure difference between the columns and the soil which creates bulging, depends solely on the distribution of the foundation load p on columns and soil, and that it is constant all over the column length. As a matter of fact, to the external loads the weights of the columns WC and of the soil WS which possibly exceed the external loads considerably, has to be added. Under consideration of these additional loads the initial pressure difference decreases asymptotically and the bulging is reduced correspondingly. In other words, with increasing overburden the columns are better supported laterally and therefore, can provide more bearing capacity. Since the pressure difference is a linear parameter in the derivations of the improvement factor, the ratio of the initial pressure difference and the one depending on depth – expressed as depth factor fd – delivers a value by which the improvement factor n1 increases to the final improvement factor n2
Therefore, n2 = fd. n1
The first compatibility control limits the depth factor and thereby the load assigned to the columns so that the settlement of the columns resulting from their inherent compressibility does not exceed the settlement of the composite system. In the first place this control applies when the existing soil is considered pretty dense or stiff.
If fd < 1
Go for second compatibility condition, soil is considered pretty loose or soft.
Shear Values of Improved Ground
The shear performance of ground improved by vibro replacement is out most favourable. The stone columns receive an increased portion of the total load m thereby which depends on the area ratio Ac/A und the improvement factor n or nmax.