GB2198765A - A tool for in-situ subsurface stabilisation of soil - Google Patents

Abstract

A tool comprising a mixing means (1) fixedly secured to an auger (2). The auger (2) has a hollow drill rod stem (3) with a continuous helical flight (4). The mixing means (1) is in the form of a screw having a root diameter the same as that of the drill rod stem (3), and having four helical flight portions (5a to 5d). The flight portions (5a, 5d) at the upper end of the head portion (1) of the tool are interrupted by respective cut-outs (11, 12) to form indentations in the helical flight surface. In use, the tool is screwed into the ground, held at a given depth and rotated. A stabilising agent is introduced into the ground. The movement of the soil over and through the interrupted helical flight portions causes a homogenous mixture to be obtained. <IMAGE>

Description

A TOOL FOR IN-SITU SUBSUKFACE STABtLISATION OF SOIL This invention relates to a tool for in-situ subsurface stabilisation of soil.

Cement stabilisation of friable low plasticity soils has been used extensively in some parts of the world for road pavement and bases where scarifying surface soils and mixing in-situ has proved effective.

Typically, such stablisation takes place at a depth of about 200mm. It has not hitherto been possible to apply a cement stabilisation technique to much greater depths than this because of the difficulties of effecting mechanical mixing deep in the ground without removing large quantities of soil.

It is hence desirable to provide a tool capable of achieving a substantially uniform distribution of a stabilising agent and soil without removing a large quantity of soil from the ground.

According to the present invention there is provided a tool for stabilising soil using a stabilising agent, the tool comprising: a helically flighted auger; and a helically flighted mixing means carried by the auger so as to be introduced into the ground before the auger, the mixing means comprising a helical flight portion having a tip to tip helical diameter greater than the tip to tip helical diameter of the helical flight portions of the auger and, being interrupted so that, when the auger is rotated with the mixing means in the ground, the mixing means causes a stabilising agent beneath the surface of the ground to form a substantially homogenous mixture with the soil thereby to stablise the soil.

The tip to tip helical diameter is the diameter of the cylindrical envelope defined by the edge of the helical flight portions.

In a preferred embodiment, the combination of the mixing means, with its interrupted helical flight, and the auger, with its continuous flight, act together during rotation at a constant depth to mix the soil and the stabilising agent in-situ by moving them along vectors with simultaneous horizontal and vertical components. As will become apparent from the description of the mixing action below, a mixing effect is also achieved, albeit reduced, by rotating the auger as it moves slowly upwards or downwards, provided that the rate of vertical movement is less than that which would be obtained as a result of a screw action at the same rate of rotation. Preferably the tool includes means for introducing a stablising agent into the ground. For example, the auger may be hollow to receive a stablising agent or to receive a pipe through which the stabilising agent can be pumped.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a perspective view of a tool in accordance with one embodiment of the present invention; and Figure 2 is a cross-section through the auger, but not showing the helical flight.

The tool comprises a mixing means in the form of a head portion 1 fixedly secured to an auger 2. The auger 2 has a hollow drill rod stem 3 with a continuous helical flight 4. The stem may have an outer diameter D of 140mm and a wall thickness t of 20mm (see Figure 2). The tip to tip helical diameter D of the flight 4 might be 300mm. The head portion 1 is in the form of a screw having a root diameter the same as that of the drill rod stem 3, and having helical flight portions four of which 5a to 5d are illustrated.The tip to tip helical diameter of the flight portions might be 750mm, and the length 1 of the head portion might be l095m. The flight portions 5a to 5d of the head portion 1 can form a double or single start screw (a double start screw being illustrated). A double start screw is preferred for stability while the tool is being screwed into the ground, and to enhance the mixing action as described below. A pipe 6 is arranged in the drill rod stem 3 and the head portion 1 for the injection of a stabilising agent. The pipe may be rigid or it may be a flexible rubber hose. Alternatively it can be dispensed with and stabilising agent introduced directly into the drill rod stem.The pipe 6 also serves to permit water to be introduced into the ground while the tool is being screwed into the ground and before mixing takes place.

This may be necessary for frictional soils, e.g. very sandy soil. The stabilising agent could be cement grout, cement clay grouts, or quick or slake lime suspensions with or without admixtures to improve the strength or rheological properties of the soil and stabilising agent mixture. If quick lime is used it could be suspended in compressed air. The head portion 1 has an opening 7 in the side wall at its lower end communicating with the pipe 6 through which the stabilising agent can emerge into the ground. A rubber petal valve, not shown, is arranged at the opening 7. At the lower end of the tool there are provided two cutting portions 8, 9 formed by terminating respectively the lowermost flight portions 5c, 5d. Teeth 10 of the cutting portions serve to cut into the ground on introduction of the tool in a manner known per se with conventional augers.Reference numeral 13 denotes a cap for the hollow stem 3, of which the length may vary to provide a centering point for initial penetration of the tool, as is known.

The flight portions 5a, 5d at the upper end of the head portion 1 of the tool are interrupted by respective cut-outs 11, 12 to form indentations in the helical flight surface. As illustrated, the cut-outs take the form of complete sectors of the flight surface, the sectors having an included angle of 900. In the illustrated embodiment, the lowest cut-out is centred half the flight pitch from the start, and the next on the 0 same flight is centred 180 around the circumferential surface, half a pitch higher. On the second flight, the cut-outs are spaced circumferentially 1800 and half a pitch apart. Hence, the lower cut-out on the second flight is at three quarters pitch from the start. An alternative would be to reduce the width of each flight over part of its surface.A further alternative would be to form a "window" in the helical flight surface, retaining a full helical flight circumference.

In use, the tool is screwed into the-ground using a conventional auger driving machine. When the tool has reached the lowermost depth to be treated, a stabilising agent is introduced into the pipe 6 at a point near the upper end (not shown) of the auger 2. The stabilising agent emerges from the opening 7 into the ground at the depth to be treated. The auger is held at that depth and rotated intermittently clockwise and anticlockwise, a few revolutions in each direction. Soil is continuously carried upwardly (or downwardly, depending on the rotational direction of the screw) and circumferentially on the helical flight portions 5a, 5b of the head portion 1 and pasts through the interrupted parts 11, 12 of the helical flight portions. This causes the soil to "bulk", i.e. to increase its volume per unit weight. The extra volume of soil is accommodated above the head portion 1 by the action of the auger 2. The auger action creates a small borehole by soil being carried continuously upwardly along its helical flight during penetration of the tool and, ultimately, to the surface of the ground where it is removed. The action of the auger 2 as the tool rotates hence serves to "pump" the soil up and down while the head portion 1 breaks up the soil by moving it radially and circumferentially to mix the stabilising agent with the soil to give a substantially homogenous mixture. A uniform distribution of the stabilising agent within the soil can hence be achieved without removing the tool from the ground and without removing a large quantity of soil from the ground.A similar but reduced mixing effect may be achieved by rotating the auger portion 2 while slowly moving it upwardly or downwardly in the ground. With the double start head portion 1, the helical flight portions 5a, 5b overlap, which enhances the mixing effect. When that depth of soil has been treated, the auger is screwed back out of the ground until the head portion 1 is located at the next depth to be treated. This is repeated until the whole column of soil has been stabilised.

The tool permits relatively economical penetration to depths in the order of 25m with augers up to 750mm diameter. Cement or lime grouts introduced down the stem of the auger and mixed in-situ as described above form stable hardened columns of soil and cementitious material. By overlapping groups of such columns and treating only those depths of relevance to a specific project, blocks of treatment can be formed of any shape and depth- limits. - The strength of the product can be adjusted to some degree by controlling the content of the stabilising agent. For example, 6-12% of dry soil weight of cement is used to yield unconfined crushing strengths 20MN/m2 on hardening of the order of 1 to 20MN/m2 according to the natural clay or organic content of the soil.The above tool can bore in a single rapid pass without significant soil displacement, and allows grout injection whilst using the auger intermittently reversed as an in-situ mixer. While it is convenient to introduce the stabilising agent down the stem 3, it would be possible to use a hose externally of the tool.

The tool is especially valuable for treatment of waterlogged silty sands which are generally unstable when excavated for basements or trenches. The benefits of a comparatively firm seepage resistant and stable soil of the consistency of weak sandstone are immense when unexpected instabilities are encountered in construction of sewers and excavations.

The tool enables the stabilising process to be quick (requiring only about 1 minute per metre for treatment of a column) and, given adequate surface access, it is considerably more economical than conventional injected or jet grouting systems.

Claims (6)

CLAIMS:

1. A tool for stabilising soil using a stabilising agent, the tool comprising: a helically flighted auger; and a helically flighted mixing means carried by the auger so as to be introduced into the ground before the auger, the mixing means comprising a helical flight portion having a tip to tip helical diameter greater than the tip to tip helical diameter of the helical flight portions of the auger and being interrupted so that, when the auger is rotated with the mixing means in the ground, the mixing means causes a stabilising agent beneath the surface of the ground to form a substantially homogenous mixture with the soil thereby to statlise the soil.

2. A tool as claimed in claim 1, which includes means for introducing a stabilising agent into the ground.

3. A tool as claimed in claim 2, in which the auger is hollow, and in which said stabilising agent introducing means is arranged internally of the auger.

4. A tool as claimed in any preceding claim, in which the flight portion of the mixing means is interrupted by a cut-out in the form of a sector of the flight portion.

5. A tool as claimed in any of claims 1 to 3 in which the flight portion of the mixing means is interrupted by an aperture in the surface of the flight portion.

6. A tool substantially as hereinbefore described with reference to, or as shown in, the accompanying drawing/.