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  17 Grout Curtains 17.1 INTRODUCTION Grout curtains, also called grouted cutoffs, are barriers to groundwater flowcreated by grouting a volume of soil or rock of large extent normal to theflow direction and generally of limited thickness in the flow direction.Typically, a grout curtain could be used alongside or underneath a dam toprevent drainage of the impounded water. Curtains may also be placedaround construction sites or shafts to prevent water inflow. Where therequired service life of a cutoff is of limited duration, well points or otherconstruction methods often prove more practical. For long-term shutoffs,where the zone to be impermeabilized is close to ground surface, slurrywalls, jet grouting, and deep soil mixing have in the past decade become verycompetitive with grout curtains. Where the treatment is deep or below astructure which cannot be breached, grout curtains remain the mostpractical and sometimes the only solution. 17.2 SELECTION OF GROUT All the common, commercially available grouts, if applied properly, willreduce the permeability of a granular formation to values similar to those of clays. (See Chap. 11 for a discussion of the permanence of silicates under Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  high hydraulic heads.) This is more than adequate for water cutoff. Strengthis not a critical factor, since the formations to be grouted are generally stablein their natural conditions. Resistance of the grout to extrusion from the soilvoids is of course of importance. However, even the so-called ‘‘weak’’ groutssuch as polyphenols can resist hydraulic heads of several hundred poundsper square inch for every foot of curtain thickness (in sands and silts; incoarse sand and gravels extrusion resistance is lower). Since the minimumpractical thickness of a grout curtain is 5ft or even more, adequateresistance to extrusion will always exist. Consolidation of the grout issometimes thought to be a possible problem. However, grout gels, like clays,will consolidate only under mechanical pressure. The erection of a groutcurtain causes no significant change in mechanical pressure on the gel. Thecurtain, if it functions properly, does create a hydraulic gradient from oneside to the other. However, hydraulic pressures will not cause consolidation,and as previously noted, adequate resistance to extrusion is always present.In view of the foregoing discussion, the only factor of importance inselecting the grout is its ability to penetrate the formations to be grouted(Secs. 15.2, 15.3, and 15.4). If more than one grout meet this criterion, theneconomic factors enter into the selection.Grout curtains are generally large jobs in terms of time and materialinvolved. For large jobs of any kind, there is often economic merit in the useof more than one grout, using a less expensive material for the firsttreatment (cement, clay, and bentonite should be considered if they willpenetrate coarser zones) and following with a (generally) more expensiveand less viscous material to handle residual permeability.An interesting discussion of the grout selection process appears in theparagraphs quoted below in reference to grouting for a dam in Cypress [1].Before it was possible to select a proposed hole layout it wasnecessary to choose a chemical grout with properties acceptable tothe Engineer. In relation to gel permeability, permanence and groutviscosity, several grout systems could be considered, namely chromelignin/dichromate (e.g., TDM and Sumisoil), resorcinol formalde-hydes (e.g., MQ14), acrylamides (e.g., AM-9), phenol formalde-hydes (e.g., MQ4) and silicate-based grouts. When consideringpotential hazards from the materials, however, the choice narrowsfurther, since the acrylamide AM-9 is neurotoxic and the chromelignin/dichromate systems, whilst less toxic, are dermatitic. Thephenol fomaldehydes by comparison are non-toxic, and tests todate on resorcinol formaldehydes show that there is little danger if the materials are ingested. Silicate based systems are also non-toxicand have been used successfully on other dams and are less Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  expensive than any of the other chemical grouts mentioned above.It was on this basis that sodium aluminate/sodium silicate groutwas proposed to the Engineer.Extensive tests were carried out on a sodium aluminate/sodium silicategrout which had been successfully used at Mattmark Dam in Switzerland[1a] to establish the suitability of this type of grout for Asprokremmos Dam.The mix tested comprised: Solution (1) Sodium aluminate 16 parts/wtWater 320 parts/wt(i.e., 4.8% Sodium aluminate)Solution (2) Sodium silicate (M75) 522 parts/wtWater 284 parts/wt and from the results obtained, the following conclusions were drawn:The sodium aluminate/sodium silicate mix tested had an initialviscosity of 3.6cP and gel time of approximately 33 minutes at20 8 C when tested under laboratory conditions. The gel time of thismix could be extended to approximately 80minutes by reducing thesodium aluminate content of solution (1). A reduction in sodiumaluminate content, however, resulted in a weaker gel and atconcentrations less than 3.6 %  in solution (1), the gel strength was judged to be too weak to be of any value where ‘‘permanence’’ isrequired.The gel time could also be extended by increasing the total watercontent of the mix. However, for gel times greater than 60minutes,the degree of dilution required is excessive.The viscosity of a sodium aluminate/sodium silicate grout solutionincreased gradually with time and a mix with an initial viscosity of 3.6cP and gel times 73 minutes at 20 8 C remains below 5.0cP for 50 % of its life and below 10cP for 75 %  of its liquid life.The sodium aluminate/sodium silicate reaction is temperature sensitiveand a rise of approximately 8 8 C will halve the gelation time.Further tests carried out by an independent body, as directed by theEngineer, also showed that the chemical grout proposed was satisfactorywith regard to permanence and that there was very little likelihood of anyleached material acting as a dispersive agent on the clay core. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  17.3 GROUT CURTAIN PATTERNS The pattern for a grout curtain is a planview of the location of each groutline or row, and every hole in each row. The sequence of grouting each holeshould be noted on the pattern. It has been shown previously (Sec. 13.8) thatin order to approach total cutoff a grout curtain must contain at least threerows of grout holes and that the inner rows should be grouted last. Thedistance between rows, and the distance between holes in each row, isselected by balancing the cost of placing grout holes against the cost of thevolume of grout required. For any selected distance, the spread of grouthorizontally must be at least half the spacing. For any specific job the actualcosts of drilling and grout can be readily computed for several differentspacings to determine the specific spacing for minimum cost. This generallyturns out to be in the 2 to 5ft range. The process is graphically illustrated inFig. 17.1.Determination of optimum spacing is a mathematical process suitablefor computer solution. A program (written in BASIC) to achieve this endappears in Appendix B. This program was written in 1985 by Keith Foglia,then a student at Rutgers University. Slight revisions were made in 1987. F IGURE  17.1  Optimum spacing of grout holes. (Courtesy of Keith Foglia,Rutgers—The State University of New Jersey, New Brunswick, New Jersey.) Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.
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