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7 FOUNDATION PREPARATION 7.1 PIPE TRENCHES 7.1.1 Trenches to be used for installation of solid wall pipelines shall be excavated to a depth 6 inches below the invert of the pipeline if the excavation is in fine soils or 8 inches below the invert of the pipeline if the soils are coarse, sharp, and angular, as shown on the Drawings. If the foundation material is suitable at that depth, the bedding layer may be installed. 7.1.2 If the foundation soils at the base of the bedding layer are unsuit- able because of over-wet condition, presence of peat, muck, or tailing slimes, or apparent lack of consolidation for significant depth, the trench shall be over-excavated by at least 12 inches and the over-excavation shall be filled with select fill if the excavation is dry or embedment material if wet before the bedding layer is constructed. 7.1.3 If bedrock is encountered less than 4 inches below the final invert grade of the pipeline, the bedrock shall be closely examined for sharp outcroppings that may damage the pipe. Any such outcrop- pings shall be removed, or alternately the trench alignment shall be moved to avoid such material. Smooth bedrock surfaces will be allowed, provided however that a minimum bedding depth of at least 3 inches will result. 7.2 FOUNDATIONS FOR MANHOLES, VAULTS AND OTHER PRECAST STRUCTURES 7.2.1 For manholes, utility vaults, and other precast concrete structure foundations; excavation shall proceed to a minimum depth of 6 inches below the bottom of the structure, or as specified on the Drawings. One or more 6-inch thick layers of foundation bedding material shall be placed and consolidated with two passes of a plate type vibrator until the foundation is brought to grade and level. 7.3 FOUNDATIONS FOR CONCRETE STRUCTURES 7.3.1 For cast-in-place concrete structure foundations; excavation shall proceed to a minimum depth of 12 inches below the bottom of the structure, or as specified on the Drawings. The Contractor shall proof roll the foundation material to determine its competency. If the foundation material is suitable at that depth, two or more 6- SPEC. A -EARTHWORK PAGE l0