GB2253869A - Forming strengthened ground columns - Google Patents

Abstract

Strengthened ground columns are formed by breaking up the ground with a high pressure jet of water followed by the introduction of sand or fine gravel, and grout into the broken ground. A boring-injection rod (2) is used to effect the above breaking up and introduction steps. <IMAGE>

Description

2253869 APPARATUS AND METHOD FOR CREATING STRENGTHENED GROUND COLUMNS

This invention relates to systems for the forced introduction of sand or granular material in general, as well as the equipment for the implementation of such a system.

In the field of engineering works it is becoming more and more indispensable to exploit terrains of poor mechanical capacities, thus making it necessary to improve, even at great depths, the characteristics of the ground itself. on the other hand, the technique of mixing chemical agents with the ground in order to obtain its strengthening and stabilization has been known since remote times.

one of the techniques for obtaining the above which is used, with several variations, during our century, is "jet-grouting", a technique which, having originated in Japan, has rapidly spread to Europe and America also, with continuous evolutions of the application methods.

The "jet-grouting" technique permits obtaining columns of strengthened ground by means of the injection into the ground of high-speed chemical mixtures; the effect of these jetting injections is the breaking up of the soil and its mixing with the agents thus introduced.

The technologies currently in use foresee the creation of deep strengthened columns according to the following operational phases: 1) Rotation boring of the ground with destruction of the core, with the use of a special duct with 2 one or more openings, at the bottom end of which there are one or more orifices which permit the outflow of the injected fluids and mixtures; 2) Treatment of the soil obtained by making the duct rotate during its ascent according to pre-set times and speeds and with simultaneous injection.

The main systems used differ in their second phases, i.e.:

a) C.C.P. System. The conduct has one opening and the cement grout jetting is horizontal. This system is therefore of the single-fluid type (Cf. ATTI delle Conferenze di Geotecnica di Torino. XII ciclo, "Recenti sviluppi nel campo dei pali di fondazione" [Records of the Geotechnical Conferences of Turin.

12th cycle, "Recent developments in the field of foundation piles"] - G. Botto, "Progressi nelle tecniche di jet-grouting" [Advances in jet-grouting techniques] - published by Edizioni Dipartimento di Ingegneria Strutturale del Politecnico di Torino, page 12).

b) C.J.G. - Kajima System. The duct has three openings with orifices one above the other. Through the upper orifice is issued, through horizontal jetting, water surrounded by compressed air, and through the lower orifice pressurized cement grout. This is therefore a three-fluid system.

(Cited document, page 15).

3 c) J. S. P. System. The duct has two openings and the orifices of the two openings are concentric; from the central orifice is issued horizontally the jet of cement grout surrounded by compressed air coming out of the peripheral orifice. This is therefore a two-fluid system.

(Same above-mentioned document, page 3).

The drawback in the abovesaid technologies, differently structured but with the single common component of high-speed cement grout jetting, is that wherever the ground is characterized by a lack of structure, high organic substance content, a considerable degree of porosity, or is the site of circulating groundwaters, the process of hydration of the binder may be very slow or even completely ineffective.

It is therefore also necessary to furnish the soil with additives such as monogranular siliceous sands, calcium carbonates, calcium chlorides, or other granular and/or filiform additives, so that the treatment of the soil will produce effective results for the improvement of its mechanical characteristics.

In order to make up for the abovementioned shortcomings of the system and to permit the introduction of additive aggregates, in Italy the following variations have been developed:

a) The formation of jet-grouting columns in which, during the soil conglomeration phase, a certain quantity of gravel 1 4 is introduced (PRESSPALI gravel-grouting system).

b) The introduction into the ground, prior to the cement mixture jetting phase, of a suitable quantity of granular material with the use of water jetting action. This technique thus precedes the grouting phase and its purpose is to replace the soil in situ with aggregate (Fondedile Hydroreplacement System).

The purpose of this invention is to obtain columns of strengthened ground through the forced introduction of strengthening elements, such as cement grout and aggregates, mainly sand or fine gravel, introduced or mixed into the soil in a single phase.

The invention is hereinafter described by way of example only, with reference to the attached drawings, in which:

Fig. 1 shows an installation including an embodiment of apparatus according to the invention; Figs. 2 and 3 show in axial section the boring- injection rod, cut according to section B-B, and the swivel of the Fig. 1 embodiment; Fig. 4 shows a diagrammatical representation of the operation of the Fig. 1 embodiment.

The mechanical arm (1) shown in Fig. 4 carries the boringinjection rod (2). This rod (2) begins to send out, from the nozzles (3), water under strong pressure to break up the ground up to the predetermined depth. At the same time, in order to polarize the jet of water, compressed air comes out of the annular chamber (4) to be discharged into (5). When the rod (2) has arrived at the predetermined depth, a column has been obtained in which the soil is 1 broken up and soaked with water is ready to receive the materials to strengthen the ground. To close the emission of the water from the hole (7), a ball (6) is inserted. The boringinjection rod (2) is formed of batteries of concentric rods between which is obtained an annular chamber (4) in which a jet of sand and air passes through the entrance (26) where a funnel (27) protects the upper part of the duct (8) from the entry of the sand under pressure.

At the center of the rod (2) the central duct or internal rod (8), of rubber or steel, is connected at end (11) with the upper swivel (17) which permits the grout which has penetrated from the hole (12) to enter the central rod (8), and at end (10) is connected to the monitor device or distribution element. From hole (12) the grout penetrates into orifice (13), and comes out from two or more radially placed rotating nozzles (14).

i The boring-injection rod (2) performs the breaking operation during the descent and the strengthening with the pressurized introduction of aggregates, f luids, and strengtheners duripg the return upwards. The installation is essentially composed of silos (29) containing sand or f ine gravel which, through an ascending screw conveyor (28), is sent to the propulsors which, through duct (16), send it to the swivel (17) to be discharged from the nozzles (5).

6 The equipment for the pumping of the sand consists of a plenum chamber (18), a drier (19), a cooler (20), and a motordriven compressor (21).

The other branch for the introduction of the grout through pipe (22) has a high-pressure pump (23) which takes the grout from the mixers (24) fed from cement silos (25).

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Claims (7)

1) - System and installation for the forced introduction of dry sand or fine gravel with the addition of strengthening agents in fluid form for the creation of columns of strengthened ground, comprising a double rod for boring-injection in the ground with a swivel, characterized by the fact that the boring- injection rod performs during the descent the breaking up of the ground by means of a high-pressure water jet, and during the return upwards sends out through nozzles a jet of sand or fine gravel with highpressure air and from the center pipe emits grout through the nozzles, all while rotating, thus permitting a radial and homogeneous distribution of the strengthening elements.

2) - system as in claim 1, characterized by the fact that in the annular space of the boringinjection rod is introduced sand or fine gravel with pressurized air.

3) - Installation as in the preceding claims, characterized by the succession of the following phases:-, a) storage in water- and airtight silos of granular and/or filiform additive materials; b) transport of the above materials by hopper screw conveyor of a predosage of the quantity of material necessary to obtain a strengthened column, equipped with scale and equipment for measuring and recording the amounts carried and the doses of the material itself.

8 c) Transport and introduction by screw conveyor of the abovesaid materials into pressurized cylindrical or inverted semi-conical f orm propulsors, equipped with electropneumatic head and foot sealing valves and valve system for the decompression inside the propulsor at the moment of discharge of the material. d) Transport of the abovesaid materials through pipes to the peripheral annular outlet orifice pre-set on the monitor device, and going through the swivel device and the battery of boringinjection rods in the annular-peripheral section with the aid of compressed air.

4 A method of creating a column of strengthened ground, comprising the steps of: breaking up the ground surrounding a bore by means of a high pressure water jet; and subsequently spraying the broken ground with one or more strengthening elements by means of at least one jet, the jet(s) rotating about the axis of the bore in order to achieve a homogeneous distribution of the strengthening elements.

Apparatus for carrying out the method of Claim 4.

6 Apparatus substantially as hereinbefore described, with reference to and as shown in the drawings.

7 A method substantially as hereinbefore described.