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S O M E A S P E C T S OF C L A Y M I N E R A L S CIVIL E N G I N E E R I N G * IN By E. H. STEGER Central Laboratory, George Wimpey & Co. Ltd., Hayes, Middlesex [Read 15th April, 1959.] ABSTRACT The utilization of clay minerals in grouting is discussed. Very generally, the design of an economic foundation for a given structure resolves itself into considerations of bearing capacity, settlements and permeability. When dealing with cohesive soils-clays--methods have been developed in the last thirty to forty years which enable the determination of mechanical properties like shear strength, compressibility, plasticity, permeability, etc., by laboratory tests. There is no doubt that all these properties depend basically on the nature of the clay mineral present in any particular soil, but, for practical purposes, the vast majority of problems can be solved mathematically or experimentally without any detailed knowledge of these clay minerals. It may well be that the understanding of mechanical properties of soils can be made easier by knowing the minerals which are present, but methods of identification like differential thermal analysis or X-ray diffraction are currently far too costly. A promising development in this connection seems to have been made by work on dye-staining methods. In all problems involving work in clay soils, the foundation engineer has to accept their properties as they are and make the best of them. Compared to the structural engineer, who can determine the strength of his steel or his concrete, this is rather an unenviable position. This passive dependence on the natural subsoil can be changed in the case of some non-cohesive soils, i.e., sands and gravels. Here it is often possible to influence their properties--mainly permeability and strength--by grouting, a process which might be defined as the forcing of one or more chemicals, suspensions, or emulsions into the voids of the soil; only suspensions will be considered here.t *This paper was read at a discussion on "Clay Minerals and Industry" and the main emphasis is on engineering aspects. Nevertheless, the particular industrial use of clay minerals described may interest a wider circle of scientists. ?Similar suspensions are also used for surface work, such as waterproofing reservoirs and canals. 106 CLAY MINERALS IN CIVIL ENGINEERING 107 The suspensions which are used to reduce permeability are almost always clay slurries, with or without the addition of a chemical or cement, depending on the specific problem. This type of grout will penetrate gravel and coarse and medium sands, rendering the soil impermeable without significantly increasing its strength. It has been used frequently in alluvial deposits to prevent the flow of water through and underneath the foundations of large dams, to enable cut-off trenches, and other excavations, to be carried out without sheet piling or lowering of the ground-water table, and for the driving of tunnels by means of compressed air through sands and gravels. The most recent applications in this country are the extensive grouting carried out during the construction of tunnels under the Thames at Dartford, Kent, and under the Clyde at Whiteinch, Glasgow. The purpose of injecting a clay grout is the formation of a gel in the voids. This gel formation is made possible by the thixotropy exhibited by certain clay slurries. Thixotropy is shown most strongly by some bentonites--Na- and K-montmorillonites. When agitated, the suspension is fluid and can be forced into voids, but as soon as agitation ceases, a gel begins to form. Owing to its ability to absorb water up to 700 times its dry weight, the clay filling the voids expands and thus effectively prevents the flow of water. With a suitable concentration - - usually between 10 per cent. and 15 per cent.--no d'.cantation takes place except if the stability of the grout is reduced by the electrolyte content of the soil. In some instances the stability is purposely reduced or neutralized by means of the addition of chemicaAs in order to delay the gelling time. This may be done to ensure a greater i:adius of penetration when it is desired to force the grout a given distance from the injection pipes. It may be of interest to describe the technique of grouting. One method which is widely used, and which was developed in France,* is briefly as follows: a 3 in. diameter casing is sunk into the ground to the full depth which has to be grouted. Inside this casing the injection pipe is lowered. This consists of a 1~ in. diameter hollow tube with groups of four 5 mm holes at 12 in. centres. Each group of holes is covered by a tightly fitting rubber sleeve. The grout is applied through an inner 1 in. pipe, choosing the depth of application by using packers above and below the appropriate group of four holes. When the 1~ in. pipe is in position a weak cement-clay grout is pumped through the bottom of this pipe so that it completely fills *Most of the clay grouting techniques originated in France where some injections were done ill the early 19th century, 108 E . H . STEGER the space between it and the 3 in. casing. This is then withdrawn. Now grout is forced through the 189 in. pipe, through the cementclay lining and into the ground. The rubber sheath ensures that there can be no back flow and it also makes it possible to keep the 11- in. tube permanently clear by flushing it through with water under pressure. Generally, grout is injected until refusal, which means a gradual building-up of pressures up to between 200 and 301) lb/in. 2 This type of installation permits various types of grouts, like pure clay slurries, clay-cement mixes or chemicals, to be injected at various times and depths, according to the nature of the ground and the purpose of the treatment. It will be obvious that, in order to achieve success, it is essential to design a suitable grout. This can only be done when all the relevant properties of the grout can be defined and controlled. The ideal is to make this process similar to the design of a concrete mix. After a study of the grain-size and permeability characteristics of the material to be grouted, and of the properties of the groundwater, it should be possible to design an injection liquid of known viscosity. Depending on the local conditions, it is then possible to calculate the radius of penetration from each pipe and so determine the distances at which injections will have to be made in order to ensure a continuous grout curtain. This presupposes a reliable knowledge of the thixotropic behaviour of the grout and the ability to delay its natural gelling time to a calculated amount. It should further be possible to reduce site testing and control to a minimum --something like the Marsh funnel used to determine a suitable viscosity for drilling fluids. These are, of course, the ideal conditions which are still very much a matter of the future. A knowledge of the lattice-structure and colloid-chemical properties of clay minerals makes it possible, in the first place, to choose a suitable raw material. Na-montmorillonite has already been mentioned as the ideal substance. It has been shown, for example, that a Na-montmorillonite has a liquid limit of 700 per cent. compared to 160 per cent. for a Ca-montmoriUonite. The free swell of the latter, in water, is one tenth that of the Na-saturated mineral. Consequently, there is considerable difference even within montmorillonites. Neither Na- or K-montmorillonite may be readily available at a particular site, but, from a knowledge of the cationexchange properties of clay minerals it is often possible to use a local clay and, by chemical treatment, effect cation-exchange by replacing the natural ions with Na + and K +. Usually this is done CLAY MINERALS IN CIVIL ENGINEERING 109 in suspension and the product is then dried and powdered. A local clay which has a large proportion of colloidal particles and which has been activated in this way will have the required properties of thixotropy and will, of course, be much more economical to use than :an imported material. Assuming that a thixotropic grout is available, its viscosity is of great importance for the actual injection process. It must be possible to pump it easily so that no pressure is wasted above ground or in the pipes. Gelling should generally not take place the moment the rate of flow is reduced as this might prevent the grout from penetrating the smallest voids. Once in place, however, swelling and thixotropic gelling must take place at the required distance and time. It might be noted here that thixotropic suspensions behave as nonNewtonian liquids and the rheological laws governing their flow are not yet fully explained. Although many clay (and clay-cement or clay-chemical) permeability reducing operations have been carried out successfully all over the world and on very large scales, the methods and processes used still depend to a large extent on practical experience, if not on trial-and-error methods. Engineers are stilt some way from being able to design mixes with the degree of confidence one would wish. This is largely due to the fact that the knowledge of certain aspects of the theory of clay suspensions is still in its infancy. It seems also that new laboratory methods are needed to explain the thixotropic behaviour of clay suspensions and to establish a natural relationship between this and the viscosity of such fluids.