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Straight Wall (Flush Base) Manhole:
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(Following the methodology from Design Data 41 published by the ACPA, 1996)
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Problem:
Determine if the manhole installation shown below in Fig. 7 is stable with respect to buoyancy, and has a minimum factor of safety of 2.0 as required by the project engineer.
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In this straight wall manhole example, it is the weight of the structure itself combined with the downward frictional resistance of the soil surrounding the manhole which resists the upward buoyant force.

Shear strength as referred to in soils mechanics is the resistance to sliding of one soil mass against another in a uniform system.

Certain analyses (such as the present problem) require the determination of shear strength between dissimilar substances - soil and concrete in this case. This shear strength is an apparant rather than true shear resistance and is more accurately referred to as sliding resistance.
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1. Find Weight of Structure where:
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gc=150
unit weight of concrete - lb/ft3
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Di=5
inside diameter of structure - ft
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tw=0.5
wall thickness - ft
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Bd=Di + (2∙tw)=6
outside diameter of structure - ft
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H=23
height of fill - ft
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tb=1
thickness of bottom slab - ft
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ts=0.67
thickness of top slab - ft
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Dc=3
diameter of cover - ft
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Wcover=500
weight of cover - lb
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Wt=Wwalls + Wbase + Wtop + Wcover
total structure weight - lb
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Wt=
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Wt=34,514
total structure weight - lb
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2. Sliding Resistance
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Because a high groundwater condition is being analyzed, the effective unit weight if the saturated soil must be determined.
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gs=120
unit weight dry soil - lb/ft3
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SG=2.75
specific gravity of soil - dimensionless
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gsub==76.36363636
effective weight of submerged soil - lb/ft3
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Normal Pressure

In order to quantify sliding resistance, it is necessary to determine the lateral pressure on the walls of the manhole - Fig. 3 below. As shown in Fig. 7 above, the top of the manhole is at the ground surface and, since we are concerned about flotation, the case will be considered where the ground water elevation is at the top of the manhole.
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