This preview shows pages 1–5. Sign up to view the full content.
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:
Fourstruts bracing a 12 m excavation in soft clay.
401
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document *Bracedcuts01: Determine the type of lateral pressure diagram.
(Revision: Feb2009)
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 apparentpressures 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
≤
=
*Bracedcuts02: Factor of safety against heaving failure of an excavation.
(Revision: Feb2009)
Is the factor of safety
FS
≥
2
against a heaving failure of the bottom of the 20foot 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
=≤
=
>
=
=
+−
<
=
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document *Bracedcuts03: Forces and moments in the struts of a shored trench.
(Revision: Feb2009)
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
This is the end of the preview. Sign up
to
access the rest of the document.
This note was uploaded on 09/11/2011 for the course CEG 4011 taught by Professor Staff during the Summer '10 term at FIU.
 Summer '10
 STAFF

Click to edit the document details