AU2005291642B2 - Method for de- watering a slurry - Google Patents

Abstract

A method of de-watering a slurry mixture produced from a jet grouting operation, comprising the addition to the mixture of an anionic polyacrylamide in such manner that it is uniformly distributed therein, the anionic polyacrylamide having a high molecular weight and a low-medium charge density. Dewatering is achieved readily and economically.

Description

WO 2006/037403 PCT/EP2005/009112 METHOD FOR DE-WATERING A SLURRY This invention relates to jet grouting and to chemical compositions for use in jet grouting. Jet grouting is a soil consolidating teclinique, widely used for many civil engineering applications. For exarnple, it can be used to provide foundations for structures to be erected, to underpin existing structures, to create low permeability barriers, to create retaining or supporting structures, to reinforce a soil mass, and to complement other geotechnical structures. The jet grouting process consists of the disaggregation of the soil or weak rock and its mixing with, and 10 partial replacement by, a cementing agent, normally a cement-based grout; the disaggregation is achieved by means of a high energy jet of a fluid which can be water or the grout itself, depending on the system employed. First of all, a bore of typically 90-150 mm diameter is drilled in the soil to the depth to which the 15 soil is to be consolidated. When the appropriate depth is reached, the rod is rotated in place and a fluid ( water or the cementing grout itself) is injected at high pressure through nozzles placed at the end of the rod. This has the effect of cutting away the existing soil and replacing it with a soil-grout mixture which will harden. The rod is gradually withdrawn while injection through the rotating rod is continued. The result is a cylindrical column of consolidated soil which will 20 harden and form a consolidated soil element. During the operation, the movement of the rod can be manipulated to form soil mass of different geometry, such as columns and panels. While much of the soil is used in the consolidation, a large amount is carried away in the form of slurry which is a mixture of soil, cement and water. The slurry is normally pumped away 25 from the spot where the jet grouting operation is taking place, so, if the slurry is not fluid enough for pumping using a hydraulic conventional pump, additional water is normally added. The result is a slurry which contains a very large proportion of water, freqCuently in excess of 80%. Typical contents of a slurry are (by weight) 50-85% water and 15-50% each of both cement and soil (the latter generally clay or silty sand). However, it is not unknOwn for slurries to have 30 compositions lying outside these ranges. In addition, such slurries can include other ingredients, such as placticisers, superplasticisers, accelerators and retarders, added to modify the properties WO 2006/037403 PCT/EP2005/009112 2 of the grout. Because of the presence of cement, such slurries often have high pH, sometimes in excess of 12. The amount of slurry produced varies, depending on the type of soil treated. In clay soil 5 where the jet routing operation is typically much slower, the amount of slurry produced can be considerable. While such slurries can sometimes be utilized in, for example, backfills, large amounts usually have to be disposed of in landfills, which is expensive for jet grouting contractors. 10 It has now been found that it is possible to de-water such a slurry mixture by a simple, effective treatment. This invention provides a method of de-watering a slurry mixture produced from a jet grouting operation, comprising the addition to the mixture of an anionic polyacrylamide in such maner that it is uniformly distributed therein, the anionic polyacrylamide having a high molecular weight and a low-medium charge density. 15 It is surprising to find that an anionic polyacrylamide of low-medium charge density has such a good dewatering effect in a slurry with high pH, because the accepted practice is to use high charge density polyacrylamides. 20 An anionic polyacrylamide for use in this invention has a high molecular weight, that is, a weight-average molecular weight of at least 4,000,000. The molecular weight is preferably between 5,000,000 and 20,000,000. In addition, the polyacrylamide has a low-medium charge density. Charge density, which is an indication of the number of charged species for a given molecular weight, is a parameter well known to the art and to which there are frequent references 25 in the commercial literature. It is quoted as the percentage of the sodium acrylate content of the polymer. For the purposes of this invention, a low-medium charge density is one which lies within the range of from 5-70%. Those high MW anionic polyacrylamides having a charge density of from 10%-50% are the most effective and are the preferred materials for the purposes of this invention. 30 WO 2006/037403 PCT/EP2005/009112 3 Among the commercially-available materials which comply with the abovementioned requirements are materials in the "Sedipur" (trade mark) range of BASF. Typical examples of these include "Sedipur" AT 203 , TF2-TR and AF 404. 5 The dosage used is fiom 150-500, preferably from 200-350, ppm of the slury. In the process according to the invention, the polyacrylamide is first dissolved in water, to give a typically 0.5- 1% weight solution, and this solution is added to the slurry with stirring. This has the effect of separating water out of the soil-cement slurry, and changing the consistency of 1o the slurry from fluid to mortar-like. The separation normally takes less than 5 minutes when some agitation is employed, and the separated water can be simply drained out, or some pressure may be employed to press it out. The water which comes out is nonally clear, and after appropriate neutralization can be simply discharged into the sewage system, or where allowed, the open environment. As a result, the disposal cost such as in a landfill for the abovementioned 15 slurry would be drastically reduced, because of the lower volume of slurry due to de-waterin . and the lower price for disposing of dry material as opposed to fluid material. For the optimum working of the process according to the invention, the solids content by weight of a slurry to be treated should generally be 50% maximum, preferably 45% maximum, as 20 measured by drying a sample in an oven at 105'C. These contents figures are based on a relatively uniform distribution of particle sizes of clay, silt, cement, etc.in the slurry. However, when such a slurry contains a minor proportion of materials of larger particle size, such as sand and lumps of clay (as can happen in practice), the measured solids content of a given sample can be considerably higher. 1n such a case, the performance of the process of the invention is 25 unaffected, and, for the purposes of this invention, such a slurry is considered to fall within the solids content limits given hereinabove. The invention is further described with reference to the following non-limiting examples. 30 A series of anionic polyacrylamides are tested for dewatering activity. The materials tested are as follows: WO 2006/037403 PCT/EP2005/009112 4 Example No. Polvacrylamide Averaee MW Charge density 1 SEDIPUR AF100 10,000,000 10% 2 SEDIPUR AF203 5,000,000 20% 3 SEDIPUR AF204 12,000,000 20% 5 4 SEDIPUR AF205 19,000,000 20% 5 SEDEPUR TF2-TR 11,000,000 20% 6 SEDIPUR AF404 15,000,000 40% 7 SEDTPUR AF701 13,000,000 70% 8 SEDTPUR AF900 15,000,000 90% 10 The materials are tested as follows. A slurry containing 25% fine soil (silt and clay), 15% cement and 60% water is prepared. Each of the polyacrylamides hereinabove mentioned is added with gentle stilTing to a sample of this slurry at a rate of 280ppm. The effectiveness of each polyacrylamide is assessed by how quickly water is removed, how much Water is removed and 15 how quickly the remaining solid forms a stiff mass as a result of this dewatering. The results obtained by using the various materials are assessed and the results are shown in the following table, where +++ represents excellent, ++ average (but acceptable), + some effect (bit unacceptable) and - no effect. 20 Example No. Result 2 25 4.. 5.. 6 ++ 7 + 8 30

Claims (1)

1. A method of de-watering a slurry mixture produced from a jet grouting operation, comprising the addition to the mixture of an anionic polyacrylamide in such manner 5 that it is uniformly distributed therein, the anionic polyacrylamide having a high molecular weight and a low-medium charge density.