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Evaluation of elementary filtration properties of a cement grout injected in a sand Zied Saada, Jean Canou, Luc Dormieux, and Jean-Claude Dupla
Abstract: This paper presents the results of an experimental study aimed at evaluating the elementary filtration properties of a fine cement grout injected in a sand. In a first step, the experimental setup or filtration cell, specially developed for injecting thin samples of sand put under stress, is presented. Next, the results of an experimental programme carried out with this cell are presented, allowing for the study of the influence of basic parameters (density index, consolidation stress, cement concentration in the grout, and injection flow rate) on the filtration properties of a typical grout composed of fine cement. A filtration coefficient is then defined, allowing for characterization of the elementary filtration properties of the tested grout by the sand matrix. Finally, the respective influence of tested parameters on the value of this coefficient is presented and discussed. Key words: cement grout, suspension, filtration, flow, sand, injection. Résumé : On présente, dans cet article, les résultats d’une étude expérimentale destinée à étudier les propriétés de filtration élémentaires d’un coulis de ciment fin injecté dans une matrice sableuse. On présente, tout d’abord, un dispositif expérimental développé de manière spécifique pour ces travaux, appelé cellule de filtration, permettant d’injecter de fines éprouvettes de sable placées sous contrainte. On étudie ensuite, à partir de ce dispositif, l’influence relative de paramètres de base tels que l’état initial de la matrice sableuse (indice de densité et niveau de consolidation appliqué), la concentration en ciment du coulis et le débit d’injection sur les résultats obtenus en mettant en évidence l’influence significative de certains de ces paramètres. A partir des essais réalisés, on introduit un coefficient de filtration élémentaire permettant de caractériser les propriétés de filtration pour des caractéristiques d’injection données (matrice, coulis, débit d’injection). Finalement, on présente et l’on discute l’influence des paramètres étudiés dans le cadre de ce travail sur l’évolution du coefficient de filtration. Mots clés : coulis de ciment, suspension, filtration, écoulement, sable, injection. Saada et al.
Introduction Permeation grouting has been widely used in civil engineering with the aim of improving soil properties in terms of increase in mechanical characteristics (soil improvement) and (or) in terms of reduction of permeability (creation of low permeability zones). Among others, we may in particular refer to reference documents published by Cambefort (1967), Kutzner (1996), or Gouvenot (1998). Because of different problems related to the use of chemical grouts (resins, silicates, etc.), such as stability in time (long term deformations) and possible pollution of water tables, in recent years Received 13 May 2004. Accepted 11 July 2006. Published on the NRC Research Press Web site at http://cgj.nrc.ca on 9 January 2007. Z. Saada, J. Canou,1 and J.-C. Dupla. CERMES–Navier Institute, Ecole Nationale des Ponts et Chaussées – Laboratoire Central des Ponts et Chaussées, 6 et 8, av. Blaise Pascal, Cité Descartes, Champs-sur-Marne, 77455 Marne-laVallée, CEDEX 2, France. L. Dormieux. LMSGC–Navier Institute, Ecole Nationale des Ponts et Chaussées – Laboratoire Central des Ponts et Chaussées, 6 et 8, av. Blaise Pascal, Cité Descartes, Champssur-Marne, 77455 Marne-la-Vallée, CEDEX 2, France. 1
Corresponding author (e-mail:
[email protected]).
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much emphasis has been put on the use of cement grouts for injection in soils. They have the advantage of forming relatively stable “composite” materials with soil and do not present the toxicity of chemical grouts (Zebovitz et al. 1989; Ribay-Delfosse 2001). The main problem associated with cement grouts is that they form more or less stable suspensions. These are susceptible to progressively clogging up the sand matrix during the process, which may result in a blocking of the injection process. To improve the penetrability of cement grout, research efforts in recent years have focused on the use of special cements composed of very fine particles, like fine and ultrafine cements (Shimoda and Ohmori 1982; Arenzana 1987; Arenzana et al. 1989 ; Zebovitz et al. 1989 ; Bouchelaghem 1994; Schwarz 1997; Dupla et al. 2001; Henn et al. 2001; Bouchelaghem 2001, 2002; Warner 2003). The prediction of the penetrability of a given cement grout into a given soil and of the efficiency of an injection treatment, is an important issue that may be addressed, based on a better understanding of the grouting process and the mechanisms controlling this process. Such an understanding may help to develop appropriate flow models, allowing simulation of the transport of a cement suspension through a soil with application to the evaluation of the extension of the treated zone. Different types of models have already been developed and published in the literature, rang-
doi:10.1139/T06-082
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ing from macroscopic models to models based on discrete approaches, taking into account individual grains within the suspension (Bortal-Nafaa 2002). In particular, models of the macroscopic type, as developed by Bouchelaghem and Vulliet (2001), Saada (2003), and Saada et al. (2005), need the identification of macroscopic parameters (e.g., the elementary rate of filtration per volume unit, which appears in the mass conservation equations) in order to be used. It appears that most experimental work carried out to date on cement grout injection has dealt with testing of long or intermediate length columns, which do not allow elementary filtration (or clogging) parameters to be obtained. In this respect, we refer to experimental work carried out by Arenzana (1987), Zebovitz et al. (1989), Bouchelaghem (1994), Schwarz and Krizek (1994), Dupla et al. (2001), Bouchelaghem (2002), and Dupla et al. (2004). Therefore, the objective of the work presented in this paper is to evaluate the elementary filtration properties of sands to determine filtration parameters that are required in macroscopic transport models describing the flow of a cement grout through a sand. As a complement to more classical column tests, the idea was to develop a specific setup allowing injection into thin samples of soil for the purpose of obtaining elementary filtration properties. After the description of the experimental setup and testing procedure, typical results are presented, followed by an analysis of the influence of basic parameters on the behaviours observed during injection and corresponding filtration parameters.
Experimental setup and testing procedure Experimental setup The experimental setup, known as the filtration cell, was developed to evaluate the elementary filtration properties of a soil matrix injected with a cement grout. The idea here is to use thin cylindrical samples of soil, which allows the elementary filtration properties to be obtained. Another important feature of the setup is that it provides good control of the initial state of the soil to be tested with respect to density (or void ratio) and the initial state of applied stress, which characterizes a given state of the soil at a given depth. The option chosen was therefore to reconstitute a sample in a manner similar to a triaxial sample placed in a confining cell, allowing application of an initial isotropic consolidation stress, the grout then being injected through the sample. Figure 1 shows a schematic cross section of the filtration cell. The configuration shown in the figure is adapted for testing granular materials (e.g., sands) but could be easily adapted for other types of soils. The sample has a diameter of 80 mm and a height of 40 mm. The height may be varied between 20 mm and 80 mm. The sample is laterally contained in a latex membrane and fashioned at both ends with special end plates equipped with holes (3 mm in diameter, giving a plate porosity of 0.5) on which fine sieves (80 µm) are stuck, allowing for the retention of sand grains and the passage of the grout. The end plates are fitted on special parts at both ends, forming conical cavities to produce a regular flow through the sample. These parts are equipped with a hole allowing for grout inflow and outflow.
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The cement grout was prepared with a blender having an adjustable rotation speed between 300 and 3300 revolutions per minute. Different characteristics of the grout were determined before injection: viscosity with a Marsh cone and (or) a FAN viscosimeter; density with a Baroïd densimeter; and sedimentation properties with a normalized tube. A laser granulometer was also used to determine the grain size distribution curve of the cement within the grout. Injection was carried out with a membrane pump at a constant flow rate, which was adjustable between 0 cm3/s and 10 cm3/s, The following measurements were taken during the injection test. The cell was placed on a precision balance (8.2 kg capacity with a precision of 0.01 g), which allows measurement of the mass variations of the cell during successive operations. The grout reservoir was also placed on a precision balance (32 kg capacity with a precision of 1 g), which allows measurement of the mass variation of the reservoir, and, therefore, calculation of the flow rate. A pressure transducer was placed between the pump and the cell, allowing measurement of the injection pressure, before penetration into the sample. The confining pressure applied to the cell was also recorded during the injection test to verify that it remained constant. The different transducers were connected to a computer through a data acquisition card, allowing for automatic data acquisition and display of data in real time during the test. Figure 2 presents a functional scheme of the experiment. Figure 3 shows a general view of the experimental setup. Test procedure The test procedure involved fabrication, consolidation, and saturation of the sample, preparation and characterization of the grout, and finally, injection of the grout. The procedure used to make the sample is similar to the procedure used to fabricate a triaxial sample. A forming mould is used for sands, and the sand may be placed by light compaction of successive layers or by pluviation. For the 4 cm high samples, the sand is placed in four successive layers 1 cm thick. The end plates are then adjusted and a vacuum is applied to the sample to take the mould out. An initial low level confining pressure is then applied to the sample for subsequent saturation. The saturation is obtained by initial flow of CO2 through the sample followed by a flow of de-aired water to obtain a good level of saturation. The sample is finally consolidated to the desired isotropic stress and is then ready for grout injection. The saturation phase allows one to check that the whole instrumentation is working properly before the injection phase. Grout is prepared by progressively adding cement powder to water maintained at a high rotation speed (1500 revolutions per minute) until the selected value of the water mass over cement mass (W/C) ratio has been reached. Different products may also be added to the grout, such as stabilizing agents (bentonite) and (or) dispersing agents to prevent formation of cement aggregates within the grout. The characterization of the grout, including measurement of density, viscosity, and identification of grain size distribution of the suspension is done just before injection. After preparation and characterization, the grout is poured into the injection reservoir, resting on the 32 kg balance. Ag© 2006 NRC Canada
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Fig. 1. Schematic cross-section of the filtration cell.
Fig. 2. Functional scheme of the experimental setup.
itation is maintained in the grout with a magnetic stirrer. The circuits of the injection pump are saturated with grout and connected to the base of the sample. The injection is then started at the preselected flow rate. The procedure used for
the injection phase consists of letting the grout flow through the sample until one of the following two criteria is verified: either the volume of the injected grout is equal to a predefined volume (8 L) or the inflow injection pressure reaches © 2006 NRC Canada
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Fig. 3. View of the filtration cell.
the value of the cell confining pressure. In both cases, the injection process is then stopped.
Fig. 4. Grain size distribution curve of Fontainebleau sand.
Typical test results The results of a typical test are presented and discussed in this section, followed by a study of the repeatability of the experiment. Materials used The sand used for this test is a reference sand widely used in France, called Fontainebleau sand. It is a fine uniform silica sand (D50 = 0.19 mm, CU = 1.9) with subrounded grains. Figure 4 presents the grain size distribution curve of Fontainebleau sand. The cement used for this experiment is a Spinor A12 cement, which is a fine cement powder with maximum grain size equal to 12 µm (D50 = 3.5 µm). Table 1 gives the composition and main physical characteristics of Spinor A12. Figure 5 presents the grain size distribution curve of Spinor A12 within the grout. To obtain optimum injectability properties, Spinor A12 requires the addition of a superplasticizer to avoid cement particle flocculation within the grout. It is important to note here that, as shown by Henn et al. (2001) and Warner (2003), the behaviour of a grout during injection may be very dependant (for a given grain size distribution curve of the cement) on the type of cement (composition, adjuvants, etc.). Hence, the results presented in this paper for our specific cement grout are relevant for this grout only and could be different for other types of ultrafine cement products.
Test characteristics The test presented here corresponds to a dense sand matrix characterized by a density index of 0.95 (ρm = 1.7 Mg/m3). The consolidation stress applied is equal to 200 kPa. The grout is characterized by a cement over water (W/C) ratio equal to 5.0 (C/W = 0.20) and a superplasticizer over cement ratio equal to 0.05. The injection flow rate was taken to be equal to 2.42 cm3/s, corresponding to a flow rate per unit surface area of 5×10–4 m3/s/m2 (1.8 m3/h·m2). Saturation phase Figures 6a and 6b present the results of the saturation phase of the sample using de-aired water in terms of the injection pressure and mass variation of the cell during saturation. In terms of pressure response (Fig. 6a), a linear increase in pressure is clearly observed during the initial filling up of the tubings and sample until a clear stabilization is reached © 2006 NRC Canada
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Specific surface; Blained method (cm2/g) 10 000
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c
b
a
3.0 2.0 1.5 1.0
Water reference: 27 s. Measured on 1 L graduate cylinder after suspension made. Association Française pour les Travaux en Souterrain. d Measured on injected sand (grain size distribution: 0.1–0.3 mm).
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