PE-16-Braced-Cuts

# PE-16-Braced-Cuts - 16 - Braced Cuts *01: *02: *03: *04:...

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16 - Braced Cuts *01: Determine the type of lateral pressure diagram. *02: Factor of safety against heaving failure of an excavation. *03: Forces and moments in the struts of a shored trench. *04: A 5 m deep excavation with two struts for support. *05: Four-struts bracing a 12 m excavation in soft clay. 401

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*Braced-cuts-01: Determine the type of lateral pressure diagram. (Revision: Feb-2009) Which pressure distribution would you select from the ones shown below if you must excavate a deep trench for a utility line, and the geotechnical report identifies the retained soil as having zero cohesive strength? Diagram A Diagram B Diagram C Solution. The answer is “ Diagram A ”. Ralph Peck (1943) suggested these apparent-pressures for most deep excavations: Cuts in sand: σ a = 0.65 γ H K a Cuts in soft clays: The clay is soft if 4 4 10 which ever is larger. a H c c Ho r H . 3 H γ σ γγ > ⎡⎤ ⎛⎞ =− = ⎢⎥ ⎜⎟ ⎝⎠ ⎣⎦ Cuts in stiff clays: The clay is stiff if 4 0.2 0.4 a H c Hto H σγ =
*Braced-cuts-02: Factor of safety against heaving failure of an excavation. (Revision: Feb-2009) Is the factor of safety FS 2 against a heaving failure of the bottom of the 20-foot deep excavation shown below? The surcharge q = 2 ksf, γ = 115 pcf, c = 500 psf and T = 5 feet. Solution. The ultimate bearing capacity at the base of the soil column with the width ' is given by, where 5.7 for a perfectly rough foundation. The vertical load along the line " " is, uc c o o Bq c N N qf i cH qqH γ == =+ () ( ) ( ) ( ) where ' if or ' if ' 222 Therefore, the factor of safety is, 0.5 5.7 2.85 0.5 20 2.30 20 . 1 1 5 2 1 0 ' .24 2 5 o BBB BTT B T B FS ksf qc N FS cH ksf ft NG q qH ksf kcf ft B ft =≤ = > = = +− < =

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*Braced-cuts-03: Forces and moments in the struts of a shored trench. (Revision: Feb-2009) You have been asked by a contractor to design the internal supports (struts) of a temporary utility trench, as shown below. In order to design the steel horizontal strut shown, you must first find the force and moment on one of them, if they are spaced every 4 m horizontally. Two triaxial laboratory tests were performed on samples of the clayey sand. The first sample had a confining pressure of 0 kN/m 2 , and the sample reached failure with a deviator stress of 90 kN/m 2 . (N.B.: the deviator stress is the additional vertical stress required to reach failure, i.e. s-
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## This note was uploaded on 09/11/2011 for the course CEG 4011 taught by Professor Staff during the Summer '10 term at FIU.

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PE-16-Braced-Cuts - 16 - Braced Cuts *01: *02: *03: *04:...

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