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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />trench may need to be deepened, or a different cutoff type may need to be implemented. Obviously, soil <br />testing is an advisable precaution to minimize changes and avoid failures. <br /> <br />e <br /> <br />2.8.2 Baffle Chute Construction. There are numerous steps necessary in the construction of a baffle <br />chute, but a contractor usually easily controls them. For quality control and inspection there are <br />consistent, measurable, and repeatable standards to apply. <br /> <br />Potential problems include foundations, riprap quality control, water control, and the finish work with <br />regard to architectural and landscape treatments. Formwork, form ties, and seal coatings can leave a <br />poor appearance if not done properly. <br /> <br />Baffle chutes are highly successful as far as hydraulic performance is concerned and are straightforward <br />to construct. Steel, formwork, concrete placement and finish, and backfill generally require periodic <br />inspection. <br /> <br />2.8.3 Vertical Hard Basin Construction. Foundation and seepage concems are critical with regard to <br />the vertical wall. Poor construction and seepage control can result in sudden failure. The use of caissons <br />or piles can mitigate this effect. Put in comparative terms with the baffle chute, seepage problems can <br />result in displacement of the vertical wall with no warning, where the box-like structure of the baffle chute <br />may experience some movement or cracking, but not total failure, and thus allow time for repairs. <br /> <br />e <br /> <br />The quality control concerns and measures for reinforced concrete are the same as baffle chutes. The <br />subsoil condition beneath the basin is important to insure that the basin concrete or grouted rock is stable <br />against uplift pressures. <br /> <br />2.8.4 SloDina Grouted Boulder Construction. The sloping grouted boulder drops require significant <br />construction control efforts. Seepage analysis is required to determine a compatible combination of cutoff <br />depth, location of weep and toe drains, and the thickness of grouted rock layer. The greatest danger lies <br />with a "sugar-coated" grout job, where the grout does not penetrate the voids fully between the rock and <br />the subgrade and leaves yoids below the grout that act as a direct piping route for water. <br /> <br />Individual boulders should be used that are all larger in diameter than the grout layer so that the <br /> <br />contractor and the inspector can verify the grout depth and have grout placed directly to the subgrade. <br /> <br />The best balance appears to have rock 50% higher in size than the grout thickness and with overall mass <br /> <br />sufficient to offset uplift with a safety factor. This procedure also improves the overall appearance of the <br /> <br />structure. <br /> <br />The condition of the subgrade, adequate seepage control, and sub-drainage of the seepage flow are all <br />critical. There is a tendency to disturb the subgrade during rock placement, leaving a potential piping <br />route. This should be controlled by good subgrade preparation, careful rock placement, and removal of <br />loose materials. Absolutely no granular bedding or subgrade fill using granular materials should be used <br /> <br />e <br /> <br />HS-38 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />