GB2135361A - Ground treatment - Google Patents

Abstract

A method of increasing the load- bearing capacity of ground is disclosed. The method involves the formation of a generally columnar shaft in the ground which is filled, or substantially filled, with stone backfill to form a stone column. This column is then compacted, and rigid elongate element is driven into the upper part thereof. Conveniently, the stone backfill material is compacted with a poker vibrator. It is generally advantageous for there to be a delay of at least 24 hours between the compacting step and the insertion of the rigid elongate element. <IMAGE>

Description

SPECIFICATION Ground treatment This invention relates to ground treatment, and more particularly is concerned with a technique for strengthening ground of otherwise inadequate load bearing capacity.

A well known technique for improving ground strength is the vibroflotation method. This involves inserting a large poker vibrator (usually, but not essentially, hydraulically operated) into the ground at appropriate locations, where the vibratory action of the poker vibrator causes the surrounding ground to be compacted, resulting in increased density which in turn enables the ground to support greater loads. The vibroflotation technique was initially used for compacting granular or cohesionless soils. Subsequently, the technique was developed for use in cohesive soils, the development consisting of feeding a stone backfill into the ground around the vibrator to form so-called stone columns. Typically, a columnar cavity is bored in the ground and then the stone backfill is deposited in the bore. The poker vibrator is then inserted into the stone backfill material to effect compaction.The compacted stone columns thus formed interiock with the original ground material, stiffening it, and thus preventing shear failure and reducing differential settlement.

We have found that, in practice, it is more difficult to compact the stone in the region of the column head (close to the ground surface).

Further, the soil confining pressures which contribute to the strength of the column are often least in this same region. In extreme cases, it is possible for the upper part of the stone column to fail under load, even though no deep failure occurs.

In order to obviate this problem, we have devised a technique which aims to strengthen such a stone column particularly in its upper region. More particularly, the present invention provides a method of increasing the load-bearing capacity of ground, which comprises (a) forming a generally columnar shaft or hole in the ground; (b) filling all or a substantial part of the shaft with stone backfill to form a stone column; (c) compacting the stone column; and (d) driving into the upper part of the compacted stone column a rigid elongate element.

The rigid elongate element expands the upper part of the stone column and tends to distribute any surface load applied to the top of the stone column to a sub-surface region of the stone column.

Conveniently, the stone backfill material is compacted using a iarge poker vibrator -- e.g. one about 6 metres in length -- similar to, but larger than, those used in vibroflotation techniques.

Preferably, there is a delay between steps (c) and (d) of the above method; in general, insertion of the rigid elongate element into the compacted stone column will take place at least 24 hours after the compaction of the stone column This enables pore water pressures in the ground around the stone column to dissipate before the rigid elongate element is driven into place.

The rigid elongate element can take the form of a lightly-reinforced concrete column whose dimensions are considerably smaller than those of the stone column into which it is inserted. In general, such a concrete column will be of a length and diameter (or width) not greater than half the corresponding dimensions for the stone column into which it is to be inserted. Conveniently, the reinforced concrete column can have a diameter about one third that of the stone column (measured at the ground surface).

The rigid elongate element is preferably tapered so as to be of maximum diameter or width at the ground surface. In general, the length of the rigid elongate element will amount to from 10 to 60% that of the stone column.

As an example, a compacted stone column may be 0.6 metres in diameter at the ground surface and from 3 to 6 metres deep: and the rigid element to be inserted into such a stone column may be 200 mm in diameter at the head end and 1 50 mm diameter at the toe, having a length of 1.5 metres.

The delay between compaction of the stone column and insertion of the rigid element into the compacted stone column will vary according to the nature of the ground which is being reinforced.

Where the stone column is formed in highly cohesive soils such as soft clays, the delay is preferably from 2 to 5 days; in made ground (or fill" soils), the delay need not be so great, and the rigid element can be inserted into the stone column a matter of hours after compaction has taken place.

The head of the rigid elongate element may be driven to a level above the stone column head and cast directly into any subsequently placed foundation. Alternatively, it may be driven slightly below the head of the stone column, allowing the column to be trimmed and its head prepared to receive an applied foundation as economically as possible.

We have found that the load-bearing capacity of ground treated in accordance with the present invention greatly exceeds that of otherwise identical ground subjected to a standard vibroflotation technique.

The accompanying drawing illustrates a compacted stone column incorporating a rigid elongate element in accordance with the present invention.

Referring to the drawing, there is shown a compacted stone column 1 of generally cylindrical shaped formed in a clay soil 2. The stone column is 3.5 metres in depth and 0.6 metres in diameter.

Three days after the formation of the compacted stone column 1, a rigid elongate element 3 was driven into the centre of the column as.shown. The rigid element 3 was a cylindrical obelisk 1.5 metres long, with a diameter of 1 50 mm at the toe 4 and 200 mm at the head 5, formed of lightly reinforced concrete.

Claims (9)

1. A method of increasing the load-bearing capacity of ground, which comprises (a) forming a generally columnar shaft or hole in the ground; (b) filling all or a substantial part of the shaft with stone backfill to form a stone column; (c) compacting the stone column; and (d) driving into the upper part of the compacted stone column a rigid elongate element.

2. A method according to claim 1, wherein the stone backfill material is compacted using a poker vibrator.

3. A method according to claim 1 or 2, wherein there is a delay between steps (c) and (d).

4. A method according to claim 3, wherein step (d) is carried out at least 24 hours after step (c).

5. A method according to any preceding claim, wherein said rigid elongate element is a lightlyreinforced concrete column.

6. A method according to claim 5, wherein said concrete column is dimensioned such that its length and diameter (or width) are not greater than half of the corresponding dimensions of the stone column into which it is inserted.

7. A method according to any preceding claim, wherein said rigid elongate element is tapered, being of maximum diameter or width at the ground surface.

8. A method according to any preceding claim, wherein the length of the rigid elongate element is from 10 to 60% that of the stone column.

9. A method of increasing the load-bearing capacity of ground, substantially as hereinbefore described with reference to the accompanying drawing.

1 0. A foundation incorporating a stone column produced by a method as claimed in any preceding claim.