2002-01-28_REVISION - M1994093 - Laserfiche WebLink

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Mr. Andrew Legg -2- November 16, 2001 encasement at the slurry wall penetration was placed directly against the steel sheet piles of the temporary excavation support system, before the sheet piles were removed. Because the sheet piles are relatively thin, it is our opinion that potential voids left by the sheet piles will be filled by lateral movement of the cohesionless soils outside of the sheet piles as the sheet piles are removed. Soil-bentonite backfill was used to repair the portions of the existing North Dahlia slurry wall above the cement flow-fill pipeline encasements. An approximate 4- to 8-foot-wide trench was excavated along the alignment of the existing slurry wall (perpendicular to interconnection pipeline) at the locations where the interconnection pipelines intersected the slurry wall. Each trench extended at least 2 feet into the undisturbed portions of the existing slurry wall, and to the top of the cement flow-fill pipeline encasements. Soil-bentonite backfill was prepazed by mixing water, soil, and powdered bentonite in the trench with the bucket on a hydraulic excavator. The soil-bentonite backfill was prepared in successive 3- to 5-foot-thick lifts and was thoroughly mixed in the trench until the desired consistency (slump) was achieved. The soil-bentonite backfill was a relatively homogeneous mixture upon completion. Based on two laboratory permeability tests with similar results, the soil-bentonite backfill placed has a permeability of about 1.4 X ] 0-' centimeters per second (cm/s), which did not meet the project specifications of less than I.0 x 10 -' cm/s. However, the reconstructed portions of the slurry wall were between 4 and 8 feet wide, which is about 1 to 5 feet wider than the existing 3-foot-wide slurry wall. Although the permeability test results were slightly greater than the specified permeability, it is our opinion that the reconstructed portion of the slurry wall will perform as intended by the design and that the soil-bentonite backfill placed will not significantly affect the performance of the existing slurry wall as a seepage barrier because a longer seepage/Flow path is created by the wider slurry wall. In terms of "effective permeability," (permeability/width of wall), the longer seepage path will tend to reduce seepage because relatively thinner portions of the existing slurry wall will become the path of least resistance for water flow. It is our opinion that the existing Dahlia North slurry wall near the northeast and southwest comers of the pit was repaired in general conformance with the intent of the design, construction drawings, technical specifications, and Technical Revision-02 and Amendment-02 to Permit No. M-1994-093, approved by the DMG on February 2, and March 6, 2001, respectively. Dewaterin2 and Physical Damage to Nearby Private Wells During Construction The unlined groundwater pond north of East 88`" Avenue was temporarily dewatered during construction of the Dahlia North and Dahlia South Interconnection Pipeline. To evaluate if dewatering activities caused damage to nearby private wells, water levels in four private wells were measured before, during, and after dewatering the pond. The four wells that were monitored are within about 500 feet of the pond, and are relatively shallow wells (less than about 50 feet) that draw water from the alluvial aquifer above the top of bedrock. A fifth well, which was within 200 feet ofthe pond, was not monitored because it was a relatively deep well (about 530 feet) that does not draw water from the alluvial aquifer. Table 1 is a summary of the four shallow wells that were monitored. 01-~ 1-Ifi 001 ]] Dahlia Nonh Ralamaiion Com0laion hr