WO2000044994A1

WO2000044994A1 - Soil mixing process

Info

Publication number: WO2000044994A1
Application number: PCT/AU2000/000045
Authority: WO
Inventors: Paul Lawrence Mcbarron; Derek Brian Edwards
Original assignee: Austress Freyssinet Pty. Limited
Priority date: 1999-01-28
Filing date: 2000-01-28
Publication date: 2000-08-03
Also published as: AUPP836299A0

Abstract

A method and apparatus for stabilising soil including the steps of: providing a bore hole in the soil to a desired depth to be stabilised by way of a drilling stem (3); agitating the soil in a region radially adjacent the bore hole by use of mechanical agitation means (10) whilst simultaneously mixing and fluidifying the agitated soil with a cementitious slurry; and continuing the supply and mixing of the slurry with the agitated soil during retraction of the agitation means (10).

Description

TITLE: SOIL MIXING PROCESS

FIELD OF THE INVENTION

The present invention relates to a method of stabilising soil.

The invention has been developed primarily as a method of improving or

stabilising soil and will be described hereinafter with reference to that application.

However, it will be appreciated that the invention is not limited to that particular field of

use.

The invention may be used to form piles for building foundations, to create

retaining walls of interlocking columns as described in this document, or to demobilise

toxic substances by decreasing permeability or solidifying the soil.

BACKGROUND OF THE INVENTION

Other present forms of pile construction include driven steel or precast concrete

columns, however in urban environments the use of these techniques is limited due to

noise and vibration restrictions. Bored piles are formed by the excavation of an open

bore which is subsequently filled with concrete, however for small schemes these

techniques are uneconomic. For smaller piles, systems such as continuous flight augers

have been developed where an auger is drilled into the ground to the required depth and

then concrete slurry is injected through the drill rods as the auger is withdrawn.

Where access of large equipment is difficult and a pile diameter greater than the

diameter of the drill rod is required systems such as Solicrete ® have been developed.

This system is developed from a pilot hole into which is inserted a grouting pipe with a

number of jet nozzles on the bottom. This is connected to a supply of high pressure fluid

- usually water sheathed in compressed air. These jets are used to erode the existing structure of the soil surrounding the pipe. A second series of nozzles then supplies

cementitious slurry to be mixed in the loosened soil. This process continues as the

grouting pipe is extracted from the pilot hole until a column of cement reinforced soil of

the required depth remains, it will be appreciated that the energy requirements necessary

to loosen soil with small jet nozzles is very high. Further, due to varying soil

compositions, the predicability of the diameter of the stabilising column is largely

uncertain. Also the amount of water used to agitate the soil can lead to a diluted slurry

which may result in decreased strength of the resulting column and/or increased curing

times.

It is an object of the present invention to overcome or ameliorate at least one of the

disadvantages of the prior art, or to provide a useful alternative.

DISCLOSURE OF THE INVENTION

According to a first aspect of the invention there is provided a method of

stabilising soil including the steps of:

providing a bore hole in the soil to a desired depth to be stabilised;

agitating the soil in a region radially adjacent the bore hole by use of mechanical

agitation means whilst simultaneously mixing and fluidifying the agitated soil with a

cementitious slurry; and

continuing the supply and mixing of the slurry with the agitated soil during

retraction of the agitation means.

Preferably, simultaneously with the agitation of the soil cementitious slurry is

injected into the soil to fluidify it and make the drilling easier. Also, as the agitation

means progresses downwardly, the slurry is mixed with the agitated soil. Preferably, the cementitious slurry is provided by one or more nozzles operably

associated with the agitation means. The slurry may be expressed from the nozzles at

sufficient pressure to mix with an unagitated region of soil of selected radius adjacent the

agitated region thereby enhancing the radius of the treated region.

The agitation means may provide the bore hole whilst simultaneously agitating the

adjacent radial region of soil. Even more preferably the agitated region is further

agitated during retraction of the agitation means. The radius and efficiency of the

treatment can be controlled by varying the speed of rotation of the auger and the flow

rate of grout through the nozzles.

This method may be used to prepare a vertical contiguous retaining wall or

horizontal membrane of stabilised soil columns.

According to a second aspect of the invention there is provided an apparatus for

stabilising soil, the apparatus including:

an elongate drilling stem;

drive means adapted for driving engagement with one end of the drilling stem for

driving the drilling stem into and retracting the stem out of the soil;

mechanical soil agitation means operably associated with the drilling stem for

agitating soil in a region radially adjacent the drilling stem; and

supply means operably associated with a lower portion of the stem and

connectable to a supply of cementitious slurry to mix the slurry with an unagitated

region of soil of selected radius adjacent the agitated region.

Preferably the agitation means is a longitudinally extending helical blade or flight

adapted for mounting to an elongate stem or shaft. More preferably, the drilling stem and the agitation means are integrally formed as a drilling means. In other forms the

drilling means is a conventional auger however, in a highly preferred form, the flight

only extends along a lower portion of the drilling stem.

Preferably, the mechanical agitation means acts in a rotary manner. In other forms

vibration mechanisms may be used.

In preferred forms, the nozzles may be formed on the mechanical agitation means

or the drilling stem. In a particularly preferred form, the nozzles direct the slurry radially

outward towards the outer perimeter of the agitated region of soil.

The term soil as used herein is not intended to be limiting on the present apparatus

and method which are also suitable for stabilising sand, clay, gravel or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way of

example only, with reference to the accompanying drawings in which:

Figure 1 shows an apparatus according to the invention in use.

Figure 2 shows a front elevation of a portion of an apparatus for stabilising soil

according to the invention;

Figure 3 shows a cross-sectional view of the apparatus of figure 2; and

Figure 4a shows a first step of the method of stabilising soil according to the

invention;

Figure 4b shows a second step in the process according to the invention;

Figure 4c shows a third step in the process according to the invention;

Figure 4d shows a fourth optional step in the method according to the invention;

Figure 4e shows a fifth step of the method according to the invention; Figure 5 shows a plan view of a retaining wall formed by the process according to

the invention;

Figure 6 shows a side elevation of a modified auger for use in the process

according to the invention; and

Figure 7 shows a plan view of the auger of Figure 6.

PREFERRED EMBODIMENTS OF THE INVENTION

Referring to Figure 1 the apparatus for stabilising the soil includes an elongate

drilling stem 1. A drive means 2, in this case in the form of a crane, preferably supports

and positions the drilling stem over the desired area of soil to be stabilised. In an

alternate form, an excavator with a vertical mast or a drilling rig may be used. The drive

means is adapted for driving engagement with one end of the drilling stem 1 for driving

the stem into the soil to form a bore hole 11. It may also be used to extract the stem

from the bore hole.

As shown in Figure 2 the mechanical soil agitation means 3 is provided in the form

of a helical flight or blade attached to the drilling stem to enable the agitation of soil in a

radial region adjacent the drilling stem. It will be appreciated, however, from the

following description that other mechanical agitation means may be used such as

vibration mechanisms or the like. In a preferred form the drilling stem 1 and the soil

agitation means 3 are provided in the form of a conventional auger 10. In a particularly

preferred form the auger 10 includes a helical flight or screw that extends only along a

lower portion of the stem 3 as shown in Figure 6.

A pair of nozzles 4 are fitted preferably to the head region 20 of the auger 10 and

are connectable to a supply of cementitious slurry. Alternatively, three or more nozzles may be used. The nozzles 4 may be positioned on either the stem, as shown in figure 3,

or on the helical flight. At least some of these nozzles 4 are preferably disposed to

express the slurry radially outward and or downward as shown in Figures 3 and 6.

One preferred form of auger 10 is shown in Figures 6 and 7. The auger also

includes a three toothed drilling bit 25 at its end. Four nozzles 4 extend from the stem

outwardly and downwardly towards the drilling bit. Each of the nozzles are in fluid

communication with an internal through bore 26 within the stem which is able to be

connected to a source of cementitious slurry via the hollow stem 1.

The method by which the apparatus is used will now be described with reference to

figures 4a through 4e. Figure 4a shows an auger 10 made up of drilling stem 1 and

helical flight 3, at the full depth to which stabilisation treatment is required. The auger

10 may be used to widen an already pre-drilled pilot bore hole or alternatively may drill

the main bore hole 11 itself. It should be noted that helical flight 3 of the auger 10 is not

necessarily of constant diameter or pitch along the length of drilling stem 1. As shown

in figures 4a-4e, the diameter of the helical flight is greatest in the head region 20. The

drill stem 1 may extend to the start of the head region 20.

As the auger 10 is drilled into the soil some of the soil in the bore hole is removed

and the region of soil between the outer diameter of the helical flight 2 and the indicated

side walls 12 of the bore hole 11 will have been agitated and loosened by rotary

movement of the auger 10 through the soil. To make this drilling stage easier,

cementitious slurry can be injected during the drilling phase to fluidify the soil. By

injecting slurry in this fashion, the amount of soil removed by the auger can be reduced

and the soil agitated and loosened and treated. As indicated in figure 4b, once the auger 10 has reached its full depth it may be

held in this position and rotation continued to further agitate and loosen the soil in the

region radially adjacent the drilling stem. This further rotation may be in the original or

opposite drilling directions or may alternate.

When the operator determines that the soil is sufficiently agitated, the auger is

gradually retracted from the bore hole while cementitious slurry continues to be forced

from nozzles 4 into the agitated soil as shown in figure 4c. Not only does the injected

slurry mix with the agitated soil, the slurry penetrates further into the unagitated region

of the soil 30 surrounding the agitated soil in the bore hole. The degree to which this

penetration into region 30 takes place can be controlled by varying the pressure of the

slurry leaving nozzles 4 and varying the rate of retraction of the drilling stem from the

bore hole.

This aspect of the method provides a substantial benefit over the prior art.

Previous methods e.g. Soilcrete® use high pressure air and water jets to agitate the soil.

The penetration of such high pressure air and water jets into the soil is difficult to

control.

On many occasions, where geo-fabric has been used adjacent the bore hole, such

fabric is damaged by such high pressure water jets. By using mechanical means to

agitate the soil an operator has greater control over both the depth of soil that is agitated

or loosened and also over the penetration of the slurry through the agitated soil to the

unagitated region. Since penetration can be more accurately predicted than previously

proposed methods, the present process can be carried out in close proximity to geo-

fabric, other delicate materials, structures or indeed other bore holes. Figure 4d shows an additional step that may be included in the present process

whereby once a given portion of bore hole has been agitated and mixed with injected

slurry, the auger 10 may be reintroduced into this portion to provide further mixing of

the agitated soil with the slurry to provide a more homogenous mixture. After such a re-

mixing stage, retraction of the auger may be continued until a further portion of the bore

hole has been agitated and mixed with injected slurry, at which stage the newly provided

portion can undergo the re-mixing step.

This step is continued until the entire depth of the bore hole has been treated as

shown in figure 4e. The supply of slurry is then stopped and the stem is completely

removed from the bore hole resulting in a column 50 of loosened and agitated soil mixed

with cementitious slurry. The resulting column is then allowed to cure.

As with conventional techniques, several such columns may be provided side by

side or in a spaced apart array to stabilise an area of soil. Examples of such situations

may include the provision of large grouted pads or rafts, the binding of toxic materials or

the like.

A number of stabilised columns can be provided such that they overlap to form a

contiguous impermeable retaining wall or membrane. An example of such a wall 60 is

shown in Fig 5. In this example the retaining wall is being formed to produce an

impermeable membrane behind the facing block of a reinforced earth structure.

Reinforcing straps or ties are placed in the reinforced earth to hold the facing blocks in

position. In this example it is imperative to envelop these straps adjacent to the wall to

protect them from corrosion. The wall includes a number of regularly spaced,

reinforcing straps 61 or ties. The construction of the wall takes place in two stages. First, a pair of primary columns 62 are prepared either side of each strap. As is shown in

Fig 5 these columns overlap and in so doing bind the strap into the wall. The next stage

is to provide one or more intermediate columns 63 between adjacent primary pairs.

To avoid excessive wear of the apparatus, and especially the nozzles, new columns

should be added whilst adjacent columns are still soft.

To aid in locating each column, a precast drilling guide or template may be used

wherein the template includes drilling holes having a predetermined spacing.

It will be clear to persons skilled in the art that the above mentioned

embodiment(s) of the present invention provides clear and specific advantages over the

prior art. The use of the mechanical agitation means provides a more reliable and

controllable agitation of the soil in and around the bore hole. This in turn allows an

operator to use a low pressure slurry injection system with subsequent advantages in

terms of cost and complexity. Such a low pressure system also reduces ground

disturbance outside the desired diameter of influence ie. the diameter of the resultant

columns.

Additionally, previous systems which use water/air mixes to agitate the soil

increase the water content in the resulting slurry thereby possibly reducing the strength

of the resultant column and increasing curing time. Since the present process uses

mechanical agitation, water is only necessary for maintaining fluidity of the cementitious

slurry. This not only increases the density of the slurry and resultant column but also

may reduce curing times as compared with conventional techniques. Although the invention has been described with reference to specific examples, it

will be appreciated by those skilled in the art, that the invention may be embodied in

many other forms.

Claims

CLAIMS:

1. A method of stabilising soil including the steps of:

providing a bore hole in the soil to a desired depth to be stabilised;

cementitious slurry; and

continuing the supply and mixing of the slurry with the agitated soil during

retraction of the agitation means.

2. A method according to claim 1 wherein the cementitious slurry is provided by one

or more nozzles operably associated with the agitation means.

3. A method according to claim 2 wherein the slurry is expressed from the nozzles at

agitated region.

4. A method according to any one of claims 1 to 3 wherein the agitation means

provides the bore hole and simultaneously agitates the adjacent radial region of soil.

5. A method according to any one of claims 1 to 4 wherein the agitation is continued

during retraction of the agitation means.

6. A contiguous retaining wall or membrane of stabilised soil columns formed by the

method of any one of the preceding claims.

7. An apparatus for stabilising soil, the apparatus including:

an elongate drilling stem;

driving the drilling stem into and retracting the stem out of the soil; mechanical soil agitation means operably associated with the drilling stem for

agitating soil in a region radially adjacent the drilling stem; and

supply means operably associated with a lower portion of the stem and

region of soil of selected radius adjacent the agitated region.

8. A method according to claim 7 wherein the agitation means is a longitudinally

extending helical blade or flight adapted for mounting to an elongate stem or shaft.

9. A method according to claims 7 or 8 wherein the drilling stem and the agitation

means are integrally formed as a drilling means.

10. A method according to any one of claims 7 to 9 wherein the drilling means is a

conventional auger.

11. A method according to claims 7 to 10 wherein the mechanical agitation means acts

in a rotary manner.

12. A method according to claim 7 wherein the mechanical agitation means includes a

vibration mechanism.

13. A method according to any one of claim 7 to 12 wherein said supply means is a

nozzle.

14. A method according to claim 13 wherein the nozzles are formed on the mechanical

agitation means or the drilling stem.

15. A method according to claim 13 or claim 14 wherein the nozzles direct the slurry

radially outward towards the outer perimeter of the agitated region of soil.