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Work Plan far Refined Plume Definition and Management ofFloating Product-7500 W J 1 th St., Tracy, CA. Page 50 -- he at Ieast several feet deeper than the depth to the water table, be restricted to somewhat less than the preliminary design value. This might be the case if the presence of the sandy soils below the surficial stratum of clayey soil that covers the site causes instability in the floor of the trench excavation due to the development of flowing sand conditions or if the stability of the excavation might otherwise be compromised. Flowing sand is often encountered in the alluvial fan that covers the deeper basement soils in the Tracy area, particularly in strata of i"' the type present beneath the 7500 West EIeventh Street (The San Joaquin Company Inc. 1994, 2002c). 'W To complete the excavation, it will be necessary to remove the existing 18-in. diameter corrugated metal pipe culvert Iocated along a portion of the proposed trench's length (see Figure I I). It will not be necessary to replace the culvert because its original purpose was to permit vehicular entry across the ditch on the north side of West Eleventh Street, which vehicular entry is no longer extant. The scrap steel from the culvert will be shipped off-site for recycling in beneficial use. 9.4.1.2 Trenching Technique The cut-off trench will be excavated using a Komatsu PC200-LC-5, or similar heavy excavator, equipped with a 1-112-cu. yd. bucket. A Case 821 front loader with a 4-cu. yd. bucket will be used to manage excavated spoil. This array of equipment is well suited to the work and will permit obstructions such as abandoned culvert pipes and concrete slabs to be removed easily from the surface and managed in the appropriate manner. When the cut-off trench is being constructed, the excavator will be operated by a senior SJC engineer who has extensive practical experience in the management of floating product and application of the observational method as it is employed in environmental engineering. There is no substitute for this arrangement, because commonly qualified heavy equipment operators, however well skilled in use of the machine, 'do not have the necessary technical .- background to interpret the engineering implications of observations that can only be made by the machine operator as soil is being removed from the ground. As Ralph Peck has stated: }. An instrument too often overlooked in our technical world is a human eye connected to the brain of an intelligent human being. It can detect most of what we need to know about subsurface construction. Only when the eye AM cannot directly obtain the necessary data is there a need to supplement it by more specialized instruments. The excavation work will be organized so that surficial soils that are not affected by hydrocarbons will be segregated from those close to the depth of the water table and in deeper zones that are so affected. Clean surficial soil will be stockpiled separately from contaminated soil. Based on the site characterization work that has been completed to date, it is expected that some 90 cu. yd. of clean soil, as measured in situ, will be excavated, and a maximum of 135 cu. yd. of affected soil will be set aside for later off-site disposal. At no time, either when the excavation work is in progress or at any other time, will any SJC