FIELD OF THE INVENTION

This invention relates to a method of constructing a compacted stone or granular column. In particular, the invention relates to a method of constructing a stone or granular column in a soil mass for purposes of, including without limitation, increasing soil stability, providing vertical drainage and generally increasing the load bearing capacity of the soil.

Any reference to soil in this specification shall be sufficiently broadly interpreted to include rock, clay, sand, alluvial soil and any body consisting of layers of mineral constituents of variable thicknesses.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided a method for constructing a compacted column in a mass of soil, the method including the steps of: Compacting particulate material into the soil by imparting vertical force onto a driving formation in a number of controlled, rapidly successive blows while the driving formation remains in constant contact with the particulate and whereby a generally elongate cavity is defined in the soil above the compacted particulate material; back-filling the elongate cavity with particulate material; repeating the steps of back-filling and compacting the particulate material into the soil until the particulate material has been driven to a predetermined depth thereby defining a column within the soil.

In a preferred embodiment of the invention, an initial elongate cavity may be formed in the soil which may be back-filled with particulate material to be compacted into the soil.

The depth of the cavity may be in the region of 1 metre.

The driving formation may comprise a solid, substantially elongate foot member.

The foot member preferably has a diameter of approximately 1 meter.

Vertical force may be applied to the driving formation by causing a predetermined weight to be dropped repeatedly onto the driving formation from a predetermined height.

The predetermined weight may be in the order of between 2 to 30 tons and may be dropped from a predetermined height of approximately 1.5 meters. The driving formation and weight may be mounted onto a front end of a track- mounted rapid impact compactor machine and wherein the weight is vertically displaceable relative to the driving formation.

In a preferred embodiment of the invention, displacement of the weight may be controlled by means of a hydraulic system for raising and dropping the weight relative to the driving formation.

The blows may be imparted to the driving formation at a rate of succession in the range of 40 to 60 blows per minute.

The particulate may be driven to a maximum depth in the order of 6 metres.

The steps of back-filling and compacting may be repeated between 8 and 10 times before the maximum depth is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention are described hereinafter by way of a non-limiting example of the invention, with reference to and as illustrated in the accompanying schematic drawings. In the drawings:-

Figures 1 to 3 are sectional side views showing the first step of the method of the invention of creating a cavity in soil by repeatedly dropping a weight onto a driving formation wherein the weight and driving formation are included on a rapid impact compactor machine;

Figure 4 is a sectional side view showing the cavity of the first step back-filled with particulate; Figure 5 is a fragmentary sectional side view showing the compacting of the particulate in accordance with the method of the invention;

Figure 6 is a sectional side view showing the redefinition of the cavity after the compacting of the particulate;

Figure 7 is a fragmentary sectional side view showing the steps of back-filling and compacting the particulate into the redefined cavity using a rapid impact compactor machine;

Figure 8 is a sectional side view showing the increase in depth of the cavity as a result of repetitive back-filling and compacting in accordance with the method of the invention; and

Figure 9 is a sectional side view of a column formed in the soil in

accordance with the method of the invention.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

With reference to the drawings, the invention provides for a method of constructing a stone or granular column in a deposit of soil.

Stone or granular columns may be installed in soil for a number of reasons, including without limitation the stabilisation of soil having insufficient load bearing capacity to withstand the load of a structure or super structure, the prevention of liquefaction of the soil by improving drainage and for the purpose of improving the equivalent stiffness of a soil mass. Soil deposits having insufficient load bearing capacity would include soil having high water content, loose soil or those comprised of different soil types which causes differential settlement of the soil over time. The method of the invention includes the general steps of compacting particulate material 10 into the soil 12 by imparting vertical force onto a driving formation 14 in a number of controlled, rapidly successive blows and whereby a generally elongate cavity 16 is defined in the soil above the compacted particulate material; back-filling the elongate cavity with particulate material; and repeating the steps of back-filling and compacting the particulate material into the soil until the particulate material has been driven to a predetermined depth thereby defining a column 18 within the soil.

In circumstances where the soil composition is unstable, for example in soft or wet sands or clay, it may be necessary to create an initial elongate cavity 16 in the upper layer of the soil which may be comprised of rock. The cavity is back-filled with particulate material which is supported within the cavity to define a plug formation (as shown in Figure 4). The plug formation serves as a seat with which the driving formation 14 can make contact in order to prevent it from becoming lodged in the weaker soil.

Figures 1 to 3 illustrate the first step of forming the initial cavity using dynamic compaction equipment. In forming the initial cavity, a weight 20 of about 12 tons is suspended from a crane 22 and is allowed to free-fall onto the targeted area. Through this procedure, a cavity of approximately 1 metre in depth is formed.

The particulate is then back-filled with particulate material to define the plug formation (shown in Figure 4). It will not always be necessary to commence the installation process by forming an initial cavity. In circumstances where the soil is sufficiently dense or stable, the particulate material will simply be placed on the targeted position of the intended column on top of the soil from where it will be driven directly into the soil.

The particulate generally comprises crushed stone having a crushing strength of around 100 Megapascals (MPa). Using a rapid impact compaction machine 24, the particulate material is driven into the soil by applying vertical force thereto. The rapid impact compaction machine comprises a track-mounted portion to which is mounted a driving formation 14 and a weight 26. The driving formation comprises a substantially solid, elongate foot of about 1 metre in diameter which remains in constant contact with the particulate material during the installation process. The weight, which may typically be around 12 tons but not necessarily so, is suspended from a height of approximately 1.5 metres and is allowed to free fall onto the foot after which it is pulled back to its suspended position by a hydraulic mechanism. The weight is dropped onto the foot in a series of controlled, rapidly successive blows which are delivered at a rate of about 40 to 60 blows per minute. The relatively short vertical path of travel of the weight between an uppermost point from which it is dropped and a lowermost point where it impacts the foot has the effect of increasing the accuracy and consistency with which the weight strikes the foot. The relatively high degree of accuracy presents an improvement over prior art methods which generally involve hoisting a similar weight using a cable into the air to a height of about 18 metre and allowing the weight to free fall directly onto the particulate material to be driven into the soil. As a result of the extended path of travel of the weight, factors like wind and cable stretching or twisting can affect the accuracy and consistency with which the weight is able to impact its target. Furthermore, as a result of the height from which the weight is dropped in prior art methods, it is the view of the inventors that there is limited control over the speed and eventual contact point of the falling weight. Accordingly, these weights frequently strike the particulate material in such a manner as to propel pieces thereof into the air causing danger to workers and property.

In accordance with the invention, the particulate material is driven to a depth of in the region of 1.35 metres, as illustrated in Figures 5 and 6. The process of driving the particulate material into the soil results in the definition of a cavity above the compacted particulate material. The cavity is again filled with particulate material and again compacted and driven into the soil as deeply as possible in order to partially redefine the cavity. The process of compacting and driving the particulate material is repeated until a point of refusal is achieved. The point of refusal is the maximum depth to which the particulate material can be driven before the soil refuses to accept further penetration. According to the invention, the maximum depth achievable is in the region of 6 metres after the process of back-filling and compacting has been repeated roughly 6 to 8 times, wherein in each repetition is generally referred to as a pass. Once the point of refusal is achieved, the particulate that has been compacted into the soil within the cavity defines a granular column 18 extending through the soil, as shown in Figure 9 of the drawings.

The invention provides for an efficient method of installing columns in soil utilizing rapid impact compaction technology and it is envisaged by the inventors that the time taken to construct a single column ranges between 10 and 15 minutes which has the result that approximately 60 columns may be constructed per average day. Prior art dynamic compaction methods of installation which may involve dropping a weight from a crane or similar other structure are slower as a result of the necessity to safely anchor the machinery and the fact that the weight must be repeatedly hoisted a substantial distance into the air between successive blows. The rapid impact compaction machine with its track-mounted carrier is highly mobile and is, furthermore, able to operate in built up areas between buildings relatively safely as a result of the control and accuracy which may be exercised in the hoisting and dropping of the weight onto the foot from a substantially lower height with and with consistent accuracy.

Furthermore, prior art methods which involve penetrating a tube into the soil through which to introduce particulate causes mixing of the native soil into the particulate and may have the effect of contaminating and, thus, weakening the resulting column. The method of the invention eliminates the need for the use of tubes or other casings to be penetrated into the soil and, accordingly, reduces the risk of contamination of the soil.