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kKLEINFELDER V- 2) 5 Manhole Foundations Prior to placement of manholes the exposed soils at the bottom of excavations should be proof rolled to detect soft, phant, loose, or otherwise disturbed soils If detected, these soils should be scarified to a depth of at least 8-inches or to the depth of unstable soils whichever is greater, uniformly moisture-conditioned or dried to between 1 and 4 percentage points above the optimum moisture content, and compacted to at least 90 percent of the maximum dry density Alternatively, the unstable soils should be removed and replaced with bedding material conforming to the specifications presented in Section 5 3 All excavations resulting from removal activities should be cleaned of loose or disturbed material and dish-shaped with sides sloped 3(h) l(v) or flatter to permit access for compaction equipment Where the excavations extend below groundwater into wet clays and sands, the bedding material at the foundation level should be wrapped entirely with geotextile fabnc, such as Mirafi 140N or equivalent substitute Foundations for manhole structures bearing on undisturbed native stiff to hard clays and loose to dense sands may be designed using a net allowable bearing pressure of 1,500 pounds per square foot (ps fl for dead-plus-sustained live loading The values may be increased by one- third for transient loading such as seismic forces Total settlement of manhole structures proportioned in accordance with the above bearing capacities should be less than 3/4 inch In order to account for differential settlement between the manhole structures and pipeline, flexible pipe points should be considered • 56 Horizontal Thrust Blocks Resistance to lateral pressures (including those due seismic forces) may be provided by frictional resistance between the bottom of concrete thrust blocks and the underlying soils or by passive soil pressure against the sides of the thrust blocks These two modes of resistance can be combined However, since horizontal movement is required to mobilize passive resistance, the allowable at-rest factional resistance should be reduced by 50 percent A coefficient of friction of 0 2 and 0 4 may be used between pre-cast and cast-in-place concrete thrust blocks, respectively, and the underlying soil Passive pressure available in engineered fill or undisturbed native soil above the water table may be taken as the equivalent fluid pressures presented in the following table Passive pressures available in engineered fill or undisturbed native soils below the water table should be one-half of the values shown below I Subsurface Condition Earth Pressure, pef Stiff Clays and Silts 340 Very-Stiff to Hard Clays and Silts 400 Loose Sands 350 Medium-Dense Sanas 400 Dense to Very-Dense Sands .150 • i_629 GQ 1/20028369 -kprd 19,2002 2002 Kleinfeider Inc Page i 2 of 16