EP1609914A1

EP1609914A1 - Method and structure for ground improvement

Info

Publication number: EP1609914A1
Application number: EP05253909A
Authority: EP
Inventors: Alan Lyness Bell
Original assignee: Keller Ground Engineering
Current assignee: Keller Ground Engineering
Priority date: 2004-06-25
Filing date: 2005-06-23
Publication date: 2005-12-28
Also published as: GB0414300D0

Abstract

There is disclosed a ground improvement structure for improving the load bearing capabilities of ground comprising a column of particulate material and at least one reinforcement element, inserted through at least part of the column in the direction of the axis of the column, wherein the reinforcement element is in the form of one or more of the group comprising a rod, a flat strip, a cable, a hollow cylinder, a cage.

Also disclosed is a method for improving the load bearing capabilities of ground comprising the steps of a) forming a hole in the ground; b) filling the hole with particulate material to form a column; c) inserting a reinforcement element into the hole or column before, during or after step a) or b), wherein the reinforcement element is in the form of one or more of the group comprising a rod, a flat strip, a cable, a hollow cylinder, a cage.

Description

The present invention relates to a method and structure for ground improvement to improve the load bearing capability of the ground.
When constructing buildings, it is critical to provide good foundations for the building. Ideally, the foundations need to be formed in firm underlying ground or strata. Firm strata will resist the load without failing or undergoing excessive compression. With suitable building ground becoming more and more scarce, there is often a need to form buildings on weak or unsuitable strata such as soft clays, peat and even domestic refuse. Weak or unsuitable strata is not capable of withstanding the load from the foundations.
Various methods and devices for improving load-bearing capability of ground are known. For example, it is known to drive concrete or steel columns or bars in the form of piles into weak or unsuitable strata in order to reach between the weak or unsuitable strata down into ground which is strong enough to support the load of a new building. However, bars of concrete or steel of the required dimensions for pile driving are expensive and the equipment needed to drive these piles into the ground are also costly and time consuming to use. Alternatively, it is known to excavate holes in ground on which it is considered unsuitable to build and to fill the holes with concrete. However, this method is unsuitable where the soil is lacking in structural integrity such that it is not possible to prevent the holes from collapsing before the concrete is inserted.
The present invention seeks to address the above problems, and, in a first aspect, provides a ground improvement structure for improving the load bearing capabilities of ground comprising a column of compacted particulate material; and at least one reinforcement element, inserted through at least part of the column in the direction of the axis of the column, wherein the reinforcement element is in the form of one or more of the group comprising a rod, a flat strip, a cable, a hollow cylinder, a cage.
The structure of the invention can improve the load bearing capability, in particular the compressive or shear load bearing capability, of ground by providing columns of particulate material which are resistant to shear or compression. It has been found that by placing a reinforcing member in or around the particulate material, sufficient resistive capacity can be obtained to prevent substantial movement of the particulate material.
The particulate material contained in the ground improvement structure may be one or more of the group comprising stone, gravel, sand, soil or crushed stone. More preferably, the particulate material will be stone. The particles of the particulate material will preferably have an average diameter of between 0.1 mm and 150 mm, more preferably between 20 and 80 mm.
The particulate material may preferably be compacted. Methods of compacting particulate material are known to the person skilled in the art and include for example vibrating methods. By "compacted" it is meant that, if the sides of the column are completely restrained, the material in the column will not compress under a vertical load by more than 2% of its length. This helps to prevent settling after a building is constructed on the ground.
The column itself will comprise a longitudinally extending formation, extending along its axial direction. Normally, the column will have a substantially circular cross section, but any suitable cross section may be used.
The column of compacted particulate material will preferably have a length of between 1 m and 40 m, more preferably between 2 and 8 m. However, this dimension is usually dictated by the ground into which the ground improvement device is to be put and the intended end result in a manner which can be determined by the person skilled in the art. The diameter of the column is preferably in the range of 300 to 1200 mm. However, the diameter of the column may also be smaller or larger than this range, as required by the particular application.
The reinforcement element may have, over substantially its whole length, a relatively small cross sectional area in a plane normal to the axis of the hole in which the column is formed compared to the cross sectional area in the same plane of the hole. For example, it may be less than 10% preferably less than 5% and most preferably less than 3% of the area of the hole. It has been found that even with such small relative areas, large quantities of particulate material can be held in place thus giving a strong structure.
There may be more than one reinforcement element. The reinforcement element is in the form of a rod, a flat strip, a cable, a hollow cylinder or a cage or more than one of the aforementioned forms may be used. The term "rod", as used herein, takes its ordinary meaning of a slender bar which is small or narrow in circumference, width or cross-section in proportion to its length or height. Preferably, the ratio of the maximum width of the reinforcement element in a direction normal to its axis to the length of the reinforcement element is less than 1 to 6, preferably less than 1 to 10.
A suitable material for the reinforcement element may be steel, synthetic material, composite material, iron, galvanised steel or galvanised iron.
The reinforcement element may extend the whole length of the column or only part of the length of the column. For example, where the structure is formed in ground comprising weak strata, the reinforcement element suitably extends through at least that part of the column which lies within the weak strata.
Preferably the reinforcement element terminates in an end portion which is of substantially the same width normal to the axis of the reinforcement element as the adjacent length of the reinforcement element. This makes the structure very easy to assemble, as no special steps need be taken to include a large diameter stop member at the bottom. However, surprisingly, excellent interaction is obtained between the reinforcement element and the particulate material.
An important feature of the present invention is that the reinforcement element need only be small in size when compared to the size of the column in which it is inserted to have the desired strengthening effect on the column.
The ground improvement structure of the invention is obtainable by inserting the reinforcement element through at least part of the column, and preferred methods of construction will be described below.
Typically, a hole will be excavated through the weak strata down to the level of suitable strata. Suitable strata can increase in strength with depth. Therefore, it may be preferable for the excavated holes to penetrate into the firm strata, rather than meeting the firm strata and ending there. Alternatively, the column may be formed as a partially penetrating column such that it does not rest on a firm bearing strata. This can be helpful for lightly loaded structures to reduce settlement at low cost.
In a second aspect, the present invention provides a method of improving the load bearing capabilities of ground comprising the steps of:
a) forming a hole in the ground;
b) filling the ground with particulate material to form a column;
c) inserting a reinforcement element into the hole or column before, during or after step a) or b)
The columns may extend through both weak and firm ground and the reinforcement element may extend over the full length or a part of the length of the column in order to bridge the weak layers or strata. Preferably, the method may also include the step of compacting the particulate material. The ground improvement structure of the present invention therefore enables buildings to be built on ground which would otherwise not be possible.
Several embodiments of the present invention will now be described, by way of example only, with reference to the following Figures in which:
Figure 1 shows a schematic cross-section of a ground improvement structure according to a first embodiment of the present invention;
Figure 2 shows a schematic cross-section of a ground improvement structure according to a second embodiment of the present invention;
Figure 3 shows a schematic cross-section of a ground improvement structure according to a third embodiment of the present invention;
Figure 4 shows a schematic diagram of the conventional Top Feed Process comprising steps 1 to 4;
Figure 5 shows a schematic diagram of the conventional Bottom Feed Process comprising steps 1 to 5; and
Figure 6 shows a schematic diagram of the conventional Wet Process comprising steps 1 to 3.
Throughout the description, the same reference numerals are used for like items in the drawings.
In Figures 1, 2 and 3, the ground directly underneath the ground improvement structure 1 comprises firm underlying strata 6 which are capable of bearing the load of, say, a new building. However, the strata 7 overlying the firm strata 6 are weak and unsuitable for bearing substantial load. The ground improvement device 1 bridges the weak strata 7 so that the load of the building or structure eventually built on the ground surface 8 is transferred to the firm strata 6 which are capable of withstanding the load.
In a first embodiment of the present invention, the ground improvement device 1 comprises a column 5 of particulate material 9, such as stones, gravel, sand, soil or crushed stone, which has been compacted. As can be seen in Figure 1, a reinforcement element in the form of a rod 2 is located through the central axis of the column.
Although preferred, the rod need not be located centrally within the column. Preferably, the rod is made of steel.
In an alternative embodiment, several rods are inserted into the column and, although preferred, it is not essential that a rod is located in the centre of the column.
It has been found that stone columns, which are more economic to use than concrete or metal columns, are not suitable for improving many types of weak strata or unsuitable ground. However, the addition of a reinforcement element in accordance with the present invention produces a ground improvement structure which is suitable for use in the majority of types of weak strata.
As can be seen in Figure 2, the reinforcement element in a second embodiment of the present invention is in the form of a reinforcement cage 3 which is located within the column 1. The cage 3 may be a three or four bar cage.
As can be seen in Figure 3, the reinforcement element in a third embodiment of the present invention is in the form of a reinforcement cylinder 4, which is located within the column 5. The cylindrical reinforcement element 4 would be particularly useful if the ground improvement structure 1 were to be used in weak strata into which the particulate material 9 would usually permeate. The cylindrical reinforcement element 4 would retain a large portion of the compacted particulate material 9 within the column 5, thereby enhancing the longevity of the ground improvement structure 1.
The particulate material 9 may be of a single type, for example all crushed stone. Alternatively, materials which all have a similar particle size may be used in combination, for example crushed stone and gravel. Alternatively, materials having a dissimilar particle size may be used, for example stone and sand. An advantage of using particulate materials over, say, a solid column is that the individual particles react to and interact with the strata and ground in which it is placed.
There are three principal conventional methods for constructing ground improvement structures which comprise particulate material, namely the top feed process, the bottom feed process or the wet process. The method chosen will depend in each case on the type of ground and strata being built on. These three processes are described below.

Top Feed Process

In this process, a hole is formed by a crane suspended vibrator and the hole remains open whilst the stone infill is added and compacted by the vibrator in stages, assisted by compressed air. It can be seen in Figure 4 that the process includes the following steps:
a) The vibrator penetrates under the action of vibrations and compressed air jetting to form an open shaft to the design depth, which is usually a competent bearing stratum.
b) After being held at the required depth for a short time, the vibrator is withdrawn and the charge of stone or other particulate material is placed into the hole.
c) The vibrator is reintroduced into the hole, the stone is compacted, forced outwards and tightly interlocked with the surrounding ground.
d) By adding successive charges of particulate material and compacting each one, a column of very compact particulate material is built up to ground level.
This method is only suitable if the ground is able to sustain a hole for long enough that the vibrator is able to be withdrawn and a charge of stone or other particulate material placed in the hole before the hole collapses. It is also a problem that the particulate material may move into the surrounding ground and not be constrained by it, providing a weak structure. By inserting at least one reinforcement element according to the present invention during or after construction of the open shaft, it has been found that the hole can be maintained while the particulate material is filled.
The reinforcement element helps to retain the particulate material, in a surprising manner.

Bottom Feed Process

This is a "dry" process used to treat unstable soils and/or soils with a high ground water level. With regard to Figure 5, the method includes the following steps:
a) With a Vibrocat (TM) stabilised on hydraulic outriggers, the leaders are elevated to the vertical and the vibrator located on the ground at the stone column position. The skip is charged with stone or other suitable particulate material.
b) The skip travels up the leaders and automatically discharges the particulate material into the reception chamber at the top of the vibrator.
c) The vibrator penetrates the weak soils to the design depth under the action of the vibrations, compressed air and pull-down winch facility.
d) At the required depth, the particulate material is released and compacted by small upward and downward movements of the vibrator, the pull-down winch being employed on the downward compacting action.
e) With particulate material being added to the system as necessary at any stage of the construction procedure, a column of very high integrity, tightly interlocked with the surrounding soil, is built up to ground level.

The Wet Process

This process is used in fully saturated and very weak soils. Water jetting removes soft materials, stabilises the hole and allows the stone backfill to reach the bottom of the vibrator. This is then compacted and interlocked with the surrounding soil.
With regard to Figure 6, the method comprises the following stages:
a) At full water pressure, the vibrator penetrates to the design depth and is surged up and down as necessary to agitate sand, remove fines and form an angular gap around the vibrator.
b) Once at depth, the water pressure is reduced and with the vibrator remaining in the ground, sand infill is added from ground level and compacted at the base of the vibrator.
c) When the required compaction resistance is achieved, the vibrator is raised and more sand infill added and compacted as before. This procedure is repeated until compaction point up to ground level.
With regard to the device according to the present invention, the ground improvement device may be formed adapting any of the above known methods or alternative known methods with the additional step of inserting a reinforcement element into the hole or column either before, during or after the known column production method. The method may include the compacting step but this is not essential.
With conventional methods as described above, it is known to use particulate material for example stone or sand. Such materials have an advantage over concrete or metal piles, in that the material interacts with the ground and is far cheaper. The present invention provides a significant improvement in that the particulate material can be retained by the reinforcement element.
The ground improvement device of the present invention provides a low cost and time saving solution to the problem of stabilising weak strata and unsuitable ground.
The ground improvement device of the present invention can be used to bridge layers of strata which include layers of soft strata, e.g. peat up to about 600 mm in thickness and also may be used to provide increased shear resistance to increase stability for slopes, excavations and embankments.
In many applications, steel reinforcement elements will be appropriate. However, where corrosion is of concern, galvanised metal reinforcement elements are preferred. Alternatively, the reinforcement elements could be made from synthetic material, for example HDPE, or composites such as glass fibre, KEVLAR™, or carbon fibre.

Claims

A ground improvement structure for improving the load bearing capabilities of ground comprising:

a column of particulate material; and

at least one reinforcement element, inserted through at least part of the

column in the direction of the axis of the column,

wherein the reinforcement element is in the form of one or more of the group comprising a rod, a flat strip, a cable, a hollow cylinder, a cage.
A ground improvement structure as claimed in Claim 1, wherein the particulate material is one or more of the group comprising stone, gravel, sand, soil, crushed stone.
A ground improvement structure as claimed in Claim 1 or Claim 2, wherein the particulate material comprises particles having an average diameter of between 0.1 mm and 150 mm.
A ground improvement structure as claimed in Claim 3, wherein the average diameter is between 20 and 80 mm.
A ground improvement structure as claimed in any of the preceding claims,
wherein the column is 1 m to 40 m long.
A ground improvement structure as claimed in Claim 5, wherein the column is 2 m to 8 m long.
A ground improvement structure as claimed in any of the preceding claims,
wherein the column has an average diameter of 300 mm to 1200 mm.
A ground improvement structure as claimed in any of the preceding claims,
wherein the cross-sectional area of the reinforcement element is less than 10% of the surface area of the hole in which it is formed.
A ground improvement structure as claimed in any of the preceding claims,
wherein the reinforcement element is made of one or more of the group comprising steel, synthetic polymeric material, composite material, iron, galvanised steel, galvanised iron.
A ground improvement structure as claimed in any of the preceding claims,
wherein the reinforcement element extends through the entire length of the column.
A ground improvement device as claimed in any of the preceding claims, comprising more than one reinforcement element.
A ground improvement device as claimed in any of the preceding claims,
wherein the column extends through ground comprising weak strata.
A ground improvement structure as claimed in any of Claims 1 to 11, wherein the column extends through ground comprising weak strata and firm strata, an end of the column resting on the firm strata.
A method for improving the load bearing capabilities of ground comprising the steps of:

a) forming a hole in the ground;

b) filling the hole with particulate material to form a column;

c) inserting a reinforcement element into the hole or column before, during or after step a) or b), wherein the reinforcement element is in the form of one or more of the group comprising a rod, a flat strip, a cable, a hollow cylinder, a cage.
A method for improving the load bearing capability of ground as claimed in Claim 14, wherein the column is formed in ground comprising weak strata.
A method for improving the load bearing capabilities of ground as claimed in Claim 14, wherein the column is formed in ground comprising weak strata and firm strata and an end of the column is resting on firm strata.
A method for improving the load bearing capabilities of ground as claimed in Claim 14, Claim 15 or Claim 16, wherein more than one reinforcement element is inserted.
A method for improving the load bearing capabilities of ground as claimed in any of Claims 14 to 17, wherein the cross-sectional area of the reinforcement element is less than 10% of the surface area of the hole in which it is formed.
A method for improving the load bearing capabilities of ground as claimed in any of Claims 14 to 18, wherein the reinforcement element is made of one or more of a group comprising steel, synthetic material, composite material, iron, galvanised steel, galvanised iron.
A method for improving the load bearing capabilities of ground as claimed in any of Claims 14 to 19, wherein the particulate material comprises particles having an average diameter of between 0.1 mm and 150 mm.
A method for improving the load bearing capabilities of ground as claimed in any of Claims 14 to 20, wherein the column is 1 m to 40 m long.
A method for improving the load bearing capabilities of ground as claimed in any of Claims 14 to 21, wherein the column has an average diameter of 300 mm to 1200 mm.
A method for improving the load bearing capabilities of ground as claimed in any of Claims 14 to 22, wherein the reinforcement element extends through the entire length of the column.
A method for improving the load bearing capabilities of ground as claimed in any of Claims 14 to 23, further comprising the step of compacting the particulate material.
Ground improved by the structure of any of Claims 1 to 13 or the method of any of Claims 14 to 24.