basal instability
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Document Basal Stability of Excavations in soft to firm clays
Author M.M. Hamzeh, P.Eng.
Number Crunchers (www.iosphere.net/~mhamzeh)
Method Base on stabi!it" number charts presente b" #anbu in $%&'
Metho escribe in the Canaian ounatin Engineering Manua! ection *+..$
Equations
a-et" actor against basa! instabi!it"
#anbu stabi!it" -actor -rom chart
Correcte unraine shear strength o- soi!
ota! o0erburen pressure at the base o- the e1ca0ation
-o!!owing trans-ormation
B 2 3ith o- rectangu!ar e1ca0ation
4 2 4ength o- rectangu!ar e1ca0ation
or c!a"s o- moerate to high p!asticit", the e-inition o- the unraine shear
can !ea to an o0erestimate o- the shear strength. 5 correction o- the
shear strength ca!cu!ate b" con0entiona! -ie! 0ane tests is suggeste b" B6errum
(CEM r Eition, ection '.&.7, page &&)
Correcte unraine shear strength o- soi!
B6errum ($%+*) 0ane correction -actor (po!"n. cur0e -itte -rom chart)
ie! measure shear strength
References Canaian ounation Engineering Manua! (CEM), $%%*.
ection *+..$, Page '$
#anbu, N. ($%&'). 85pp!ication o- Composite !ip ur-aces -or tabi!it" 5na!"sis89
European Con-erence on oi! Mechanics an ounation Engineering,
3iesbaen, :o!. $ an :o!. *.
B6errum, 4. ($%+*). 8Emban;ments on o-t <roun89 Proceeings 5CE pecia!t"
Con-erence on Per-ormance o- Earth an Earthupporte tructures, 4a-a"ette
:o!. *, pp. $&'.
sb
2 N b τ
u / σ
z2H
sb
2
N b 2
τu 2
σz2h
2
he spreasheet appro1imates #anbu=s N b -or a strip shape using a po!"nomia! -it
o- the pub!ishe chart cur0e. N b -or a rectangu!ar shape is ca!cu!ate using the
N b rectang!e
2 N b strip
( $> ?.* B/4)
τu 2 µ C
u
τu 2
µ 2
Cu 2
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*%+?@&7+.1!s1
C54CA45N B554 5B4D
Spreadsheet by MMH, 95
Pro6ect No. 1011210002
ate 2Dec1!
Ca!cu!ation B" eso
ria! No. 1
Trench Excavation
epth o- rench, H + m
4ength o- rench, 4 &? m
3ith o- rench, B $& m
Stress Relief Excavation
epth, ? m
3ith, 3 ? m
4ength, 4= ? m
Soil Information
oi! Anit 3eight $+.&
P!asticit" ne1 *' F
(H>)/B ?.&
B/4 ?.
&.@
7.*
?.%& B6errum Cu correction -actor enab!eG "es
ota! stress $* Pa
tress re!ie- ?Pa (ue to stress re!ie- e1ca0ation)
Cu
$? ?.'@
$& ?.+*
*? ?.%7
*& $.*?
? $.''
& $.7@
'? $.%*
'& *.$7&? *.'?
&& *.7'
7? *.@@
N/m
N b strip
N b
µ = - (P)
& $? $& *? *& ? & '? '& &? && 7? 7&
?.??
?.*?
?.'?
?.7?
?.@?
$.??
$.*?
$.'?
$.7?
$.@?
*.??
*.*?
*.'?
*.7?
*.@?
.??
.*?
Cu (Pa)
.. against Basa! nstabi!it"
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*%+?@&7+.1!s1
C54CA45N B554 5B4D
Spreadsheet by MMH, 95
Pro6ect No. 1011210002
ate 2Dec1!
Ca!cu!ation B" eso
ria! No. 1
Trench Excavation
epth o- rench, H + m
4ength o- rench, 4 &? m
3ith o- rench, B $& m
Stress Relief Excavation
epth, ? m
3ith, 3 ? m
4ength, 4= ? m
Soil Information
oi! Anit 3eight *?.&
P!asticit" ne1 *' F
(H>)/B ?.&
B/4 ?.
&.@
7.*
?.%& B6errum Cu correction -actor enab!eG "esota! stress $'' Pa
tress re!ie- ? Pa (ue to stress re!ie- e1ca0ation)
Cu
$? ?.'$
$& ?.7*
*? ?.@*
*& $.?
? $.*
& $.''
'? $.7'
'& $.@&
&? *.?&
&& *.*7
7? *.'7
N/m
N b strip
N b
µ = - (P)
& $? $& *? *& ? & '? '& &? && 7? 7&
?.??
?.*?
?.'?
?.7?
?.@?
$.??
$.*?
$.'?
$.7?
$.@?
*.??
*.*?
*.'?
*.7?
.. against Basa! nstabi!it"
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*%+?@&7+.1!s1
Cu (Pa)