Stability of Strutted Excavations in Clay-Bjerrum

STABILITY OF STRUTTED EXCAVATIONS IN CLAY
bY
L. BJERRUM and 0.
EIDE
SYNOPSIS
The present Paper describes a procedure for
estimating the danger of a base failure in a strutted
excavation in clay. Terzaghi and Tschebotarioff
have previously given formulae for such calculations,
but these are limited to shallow excavations.
On the basis of the familiar bearing capacity
calculations for foundations on clay a new approach
to the problem is developed, in which the critical
depth of an excavation is given by :
DC=
NC.2
L’article suivant donne une description de la
methode theorique pour la calculation du danger
d’un soulevement du fond d’une fouille cadragee
dans l’argile.
Terzaghi et Tschebotarioff ont deja
donne des formules pour un calcul pareil. mais ces
formules ne sont valables que pour des fouilles de
petite profondeur
BasCe sur les formules de la capacite portante des
fondations dans l’argile, une nouvelle solution du
probleme de soulevement du fond a 6th developpee.
La profondeur critique d’une fouille est don&e
par :
D, = NC. :
Y
where s and y are the undrained shear strength
and density of the clay, respectively. NC is a
dimensionless coefficient depending on the form of
the excavation, i.e., the width/length ratio and the
depth/width ratio. The values of NC are identical
to those given by Skempton (1951) for bearing
capacity formulae.
The reliability of the formula is indicated by comparisons with fourteen excavations. In seven of
these excavations complete failure occurred, the
theoretical safety factor having an average of 0.96.
In four excavations a partial failure was obtained,
with an average theoretical safety factor of 1.11.
The last three excavations were successfully performed, but site observation indicated that, with
further depth, danger of failure would be serious.
These excavations had an average theoretical safety
factor of 1.08.
These investigations
danger of base failure
be estimated from the
with sufficient accuracy
therefore indicate that the
in excavations in clay may
above theoretical approach
for most practical purposes.
Y
oh s et y sont respectivement 1: resistance au cisaillement et le poids d’unite de I’argile. Ne Ctant un
coefficient sans dimensions et dependant de la forme
de la fouille, c’est-a-dire de la proportion largeurlongueur et de la proportion profondeur-largeur.
Les valeurs de N,, identiques aux valeurs donnees
par Skempton (1951) pour la capacite portante,
sont montrees graphiquement dans la Figure 2.
La validite de ces formules a Cti: examinee par
comparaison de quatorze fouilles.
Dans sept de
ces fouilles, un soulevement total du fond a eu lieu,
les coefficients theoriques de securite donnant comme
valeur moyenne de 0.96.
Quatre fouilles, oh on
constatait un soulevement partiel du fond, ont
fourni une valeur moyenne des coefficients thcoriques
de securite de 1,ll.
Les trois dernieres fouilles ont
Cti: accomplies sans Bvenements speciaux, mais des
observations sur place indiquaient que, avec une
profondeur agrandissante, on aurait dh craindre un
soulevement du fond, la valeur moyenne des coefficients theoriques de securite &ant de 1,OS.
Les resultats obtenus ci-dessus montrent, ainsi,
que la methode d&rite nous permet d’estimer avec
une prCcision suffisante le danger d’un soulevement
du fond d’une fouille dans I’argile, et que la methode
suffit pour la plupart des usages pratiques.
INTRODUCTION
While excavating in soft Norwegian clays base failure sometimes occurs, accompanied by
Such failures have occurred in excavations
a consequent sinking of the surrounding ground.
for basements, in trenches for water and disposal pipes, and in a number of shafts for piers
for the foundations of buildings.
For shallow excavations of great width compared to the depth the danger of base failure
For deep excavations and shafts,
may be estimated by a conventional stability analysis.
however, experience shows that a conventional
stability analysis will lead to unreliable
estimates.
The problem of calculating the stability of strutted excavations in clay has become of
The areas available for new
increasing importance in Norway during the last few years.
buildings are limited and, consequently, there is a big demand for basement floors in new
32
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STABILITY
OF STRUTTED
EXCAVATIONS
IN
CLAY
33
structures. Again, the increasing use of floating foundations for buildings on soft clay has
posed the problem of estimating the danger of failure in the excavations for the basements.
Finally, the stability problem has become of particular importance in Oslo, in connexion with
the design of a subway through the town centre.
The Norwegian Geotechnical Institute commenced work on this problem shortly after its
establishment in 1952. This work has consisted mainly of collecting case records of base
failures, together with a theoretical treatment of the problem. The results of this work are
presented in the present Paper, and a comparison is made between field experience and
theoretical analysis.
METHOD
OF CALCULATION
The problem to be treated is confined to the stability of excavations which are sufficiently
strutted to prevent horizontal displacements of the walls. Furthermore, only temporary
excavations are considered ; that is, no attention is directed to changes in shear strength of
the clay which, in permanent excavations, may occur with time.
During removal of the clay shear stresses are set up underneath an excavation, since the
weight of the surrounding soil masses will tend to press the clay towards, and up into, the
excavation.
If the shear strength of the clay is sufficient to withstand the imposed shear
stresses, then the excavation is stable. If, however, the shear strength of the clay is smaller
than the applied shear stresses, the bottom of the excavations will be forced upwards and the
soil masses on one or several sides of the excavation will sink vertically. The problem is thus
one of stability, the solution of which requires a consideration of the direction of movement
and the extent of the failure zone.
In the literature, this stability problem was discussed by Terzaghi (1943), later by Terzaghi
and Peck (1948), and by Tschebotarioff (1951).
Terzaghi, as shown in Fig. 1, considers a clay mass alongside the excavation as exerting a
certain pressure on the clay on a plane level with the bottom of the excavation. The bearing
Fig. 1. Calculation
of excavation
Terzaghi (1943)
of the stability
according
to
capacity qf is calculated by the usual methods, and for an excavation of great length compared
to the width the following equation is applicable :
qf =
5.7s
where s is the undrained shear strength pf the clay (assumed to be saturated).
When calculating the pressure applied by the surrounding mass to a plane level with the
bottom of the excavation, Terzaghi considers a section of clay of width i 1/s, this width being
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34
L.
BJERRUM
AND
0.
EIDE
determined by the position of the failure zone. The weight of the clay block is reduced by the
shear strength D . s on the plane of section. Thus the vertical stress is :
where y is the density of the clay.
When the excavation reaches the depth where failure occurs, then
:
Pv = 9f
Thus the critical depth D, of the excavation is given by :
D, =
5.7s
.
.
.
.
.
.
.
.
(1)
Tschebotarioff’s considerations are in principle similar to those of Terzaghi, assuming
only a different shape of the failure zone. He also considers the ratio,of the length L to the
breadth B of the excavation, and his expression for the critical depth is :’
(2)
On the basis of observations from excavations in Oslo where bottom heave has occurred,
it can be stated that Terzaghi’s equation (1) gives a reliable estimate of the stability of an
excavation, provided that (a) the width of the excavation is large compared with the depth,
and (b) the clay is comparatively homogeneous with no stiff upper layer (weathering crust).
In cases of deep excavations, where the ratio depth/width is great, or in cases where the
surface clay has a stiff drying crust, equation (1) gives unreliable results.
In other words, failure occurs at considerably smaller depths than indicated by equation
(1) ; and corresponding divergencies between theory and field observations are found with
Tschebotarioff’s equation (2).
These divergencies can be explained by shortcomings of the theoretical assumptions
regarding the shape of the failure zone. Both equations (1) and (2) assume a circular sliding
surface underneath the bottom of the excavation, and that this surface extends vertically up
to the ground surface. The formulae thus assume that the shear strength of the clay is fully
mobilized right up to the surface. For very deep excavations this assumption will not hold
true. Underneath a deep excavation, such as a shaft., for a pier, failure will occur without
fully mobilizing the shear strength of all the upper clay layers.
In order to obtain a reliable estimate of the stability of deep excavations it is thus necesAn exact
sary to consider the danger of local failure below the bottom of the excavation.
treatment of this problem is not possible at present since the necessary plasticity theory is
not available.
The calculation of the bearing capacity of foundations, however, presents a
corresponding problem, for which a satisfactory approximate solution has recently been
found. The solution of this problem may be applied to the calculation of the stability of
excavations.
The bearing capacity of a footing on clay is expressed by the equation :
qf = yD + IV,. s .
.
.
.
.
.
.
.
.
(3)
where qf = the bearing capacity of the footing,
.
Y = the density of the clay,
undrained shear strength of the clay,
S
D z depth of footing,
N, = coefficient depending on the shape and depth of the footing.
For foundations of great width compared to the depth, NC may be calculated by assuming
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STABILITY
OF STRUTTED
EXCAVATIONS
IN
35
CLAY
that the sliding surface is extended up to the ground level. For shallow footings NE will
thus increase approximately linearly with depth. If the depth of the foundation is increased
NC will, however, not increase correspondingly.
When the depth exceeds about three times
This is because, when a deep footing is loaded to
the breadth NC will remain constant.
failure, a local failure zone will be produced underneath the footing and the surrounding soil
masses will be pressed aside without mobilizing the full shear strength of clay in the upper
layers. The value of NC will thus remain constant for depths greater than about three times
the width of the footing.
A complete discussion of the problem of calculating the bearing capacity of deep foundations was given by Skempton (1951). In this Paper he mentions the investigations carried
out by Meyerhof and Wilson, but his main treatment is based on a calculation method
developed by Bishop, Hill, and Mott for metals and later extended by Gibson to include clays.
According to this treatment, failure is caused under a deep circular footing by the formation
of a spherical plastic zone underneath the footing whilst the surrounding clay remains in an
elastic state.
For this case NC, Gibson derives the equation :
,=$log;+l]
+l
where E is the elastic modulus of the clay for undrained deformation.
E may be obtained
from the inclination of the stress/strain curve of the clay, determined by unconfined compression tests. For ordinary clays, the ratio E/s varies between 50 and 200. This corresponds
to a variation of N, from 7.6 to 9.4. In the sensitive Norwegian clays, E/s will vary between
100 and 200 and N, will consequently vary between 8.5 and 9.4.
On the basis of all existing theoretical determinations and a number of model tests,
Skempton finally sums up the present knowledge of this problem by giving a diagram showing
NC as a function of the depth and the shape of the foundation. This diagram is shown in
Fig. 2.
A comparison between calculated and observed bearing capacities has confirmed the
reliability of this diagram, although loading tests seem to indicate that the values for NCmay
be slightly conservative.for sensitive Norwegian clays (Bjerrum, 1955).
Professor Skempton pointed out to the Authors in 1952, that it was probable that the
above calculation method for the bearing capacity of foundations may, without any adaptations, be used to calculate the stability of excavations ; in fact, the two problems are analogous.
In the first case we have to determine the additional load which, at a certain depth, will
cause failure in the clay. In the second case, the question is to what limit the clay, by an
excavation, may be unloaded before failure occurs. As the shear stresses in the two cases
simply have opposite signs, one can directly derive from equation (3) the following expression
for the critical depth of excavation :
D,=N,s
Y
.
.
.
.
.
.
.
.
.
.
More generally the safety factor against base failure may be expressed by the equation :
F=N$rD+q
’
’
’
*
*
*
’
*
.
(5)
where F = the safety factor,
D = the depth of the excavation,
y = the density of the clay,
=
the undrained shear strength of the clay in a zone underneath and immediately
S
around the bottom of the excavation,
p = the surface surcharge,
NC = a coefficient depending upon the dimensions of the excavation.
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L.
36
BJERRUM
AND
0.
EIDE
D
B
1
p
= depth of excavation
= width of excavation
= length of excavation
= surcharge
=
undrained shear strength
S
y = density of clay
NC = coefficient
F = safety factor
circular
or
square,
of clay
B/l - I.0
9
8
7
6
5
4
I
I
I
I
I
I
2
3
4
5
rectanyu~ar
-
D/B
Nc
Fig.
6
(0.84+0.16
s/l).Ncyua~
2. The critical depth of excavations
calculated on the basis of the bearing
Values
of NC from
capacity
theory.
Skempton (1951).
As a working hypothesis the above-mentioned analogy is assumed to be so complete that
the NC-values for the bearing capacity of footings on clay may be directly used in equations
(4) and (5). But the accuracy of this theory must be investigated by field observations.
Indeed, one cannot ignore the fact that the NC-values for an excavation may differ from those
in Fig. 2, the latter being valid for the calculation of the bearing capacity of a rigid
foundation with uniform deformations, whereas the deformations at the bottom of an
excavation may take place freely and therefore will be greater in the middle of the excavation
than along the edges.
The reliability of the above theory is investigated by field observation from fourteen deep
excavations, given in the following examples.
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STABILITY
EXAMPLES
OF STRUTTED
OF
COMPLETE
EXCAVATIONS
FAILURES
IN
IN
37
CLAY
EXCAVATIONS
Pumping station, Fmnebu, Oslo
A typical example of base failure occurred in October 1954 during the excavations for a
sewage tank in OxenGvegen, Fornebu, Oslo.
The dimensions of the excavation were 5 x 5 m and it was designed to be excavated to
a depth of 3.20 m below ground level. The excavations were made within heavy wooden
sheet piling, with tongue and groove connexions. The sheet piling itself was securely strutted.
As the work proceeded the excavated soil was removed from the vicinity of the excavation
so that no surface surcharge loads were acting.
When the excavation reached a depth of 3.0 m (Fig. 3) a total failure occurred. The bottom rose about O-5 m and there was a simultaneous sinking of the surrounding ground of
,- G.I. be&e Fohre
piling
De th OF excovotion
Per Mure
OF
Depth OF excovobon
betbre &lure
-%%t-
Fig. 3.
-*-,
Rock *-%a.
Pumping
Station, Fornebu,
Oslo
about 1 m on that side where the ground was at its highest. Furthermore, cracks appeared
in the ground around the excavation. Apart from small displacements the sheet piling
remained intact during the heave.
Shortly after the failure field investigations were carried out by the Norwegian Geotechnical
Institute. Four vane borings were made around the excavation, and one in the middle.
Furthermore, in one boring undisturbed samples were taken for detailed laboratory investigations.
Underneath a fill of thickness 0.4-0.7 m there was a relatively stiff and dry clay crust
down to about 1.4 m below ground level. Vane borings and samples showed that below this
crust there was a layer of very soft and very quick clay which went down to rock, which was
found at a depth of 3-9 m below ground level. The shear strength of the quick clay was
nearly constant throughout the depth and equalled 0.75 ton/sq. m. A vane boring performed in the excavation showed that the clay was completely disturbed to a depth of 2-2.5 m
below the bottom of the excavation.
A stability calculation indicated an NC-value of 7.0 corresponding to a safety factor
of 1-O. The theoretical value corresponding to a depth/width ratio of the excavation equal
to 060 is 7.2. When the failure occurred the theoretical safety factor was thus 1.03.
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38
L.
BJERRUM
AND
0.
EIDE
Store house, Drammen
A failure occurred during the excavation for a store house in Drammen in August 1954,
just before the required depth of excavation of 2.5 m had been reached. Further work
proved to be fruitless and the basement floor was abandoned since, with further excavation,
clay masses were forced up in the bottom of the excavation.
The work was performed by a mechanical excavator, leaving the sides of the excavation
vertical and not strutted. In spite of the bottom heave the unstrutted sides remained intact
and vertical. Furthermore, there was no appreciable sinking of the ground immediately
around the excavation, but at a distance of 4-5 m from the work marked cracks and settlements were observed. The continued excavation after the base failure had occurred led to
considerable sinking of a nearby road fill about 1.5-2.0 m high.
According to earlier field investigation of this area the ground may be described as having
a firm clay crust down to a depth of 2 m. Underneath this crust was a layer of very soft and
very sensitive clay. Rock was found to a depth of 6-7 m. The shear strength of the clay
was, to a depth of 3-4 m, 1.0-1.6 ton/sq. m. The average value of the shear strength was
estimated to be 1.2 ton/sq. m. The weight of the excavator together with other loads
amounted to a surcharge of approximately 1.5 ton/sq. m. As the density of the clay was 1.9
ton/cu. m, the coefficient N, will be 5.1 for a critical depth of excavation of 2.4 m. The
depth/width ratio was approximately 0.5, leading to a theoretical NC-value of 5.9. The
calculated safety factor was thus 1.16. Such a high figure may possibly be explained by the
load from the nearby road filling, but it is difficult to consider this factor in the calculation.
Pier shaft, Nordenskioldsgatan,
Giiteborg
In 1930 a block of flats was built in Nordenskioldsgatan in Gdteborg. The building was
founded on steel piles driven down to rock. The structure was built on a slope with a comparatively steep gradient, and that may be the reason for the slow creep which has been
observed in the slope. Shortly after the building was completed horizontal movements were
observed, causing cracks and other damage in the building.
In 1954 the damage had become so comprehensive that the building had to be pulled down.
A new building on the same site was designed, but this time the foundations were to be piers
down to rock. Circular pier shafts were excavated to rock at a depth of 25 m. Sheet piling,
strutted by concrete rings, was successively .driven down as excavating proceeded. At the
surface the shaft had a diameter of 1.30 m, this diameter decreasing gradually to 0.90 m at
the rock.
The foundation work was, on the whole, successfully performed in the above manner.
Only one pier shaft had to be abandoned because of base failure. In this shaft the excavation
had almost reached the rock at a depth of 25 m. At this point the rock sloped steeply.
Just before the failure occurred the distance down to the rock was approximately O-3 m on
one side of the shaft and 1.3 m on the other. The workman who was in the excavation
relates that the bottom rose so quickly that he only just managed to climb up before the clay
masses reached him. The bottom rose about 5 m, and repeated attempts to complete the
excavation failed since, when further clay was removed, new clay masses continued to flow
into the shaft.
Field investigations carried out in connexion with the planning of the building showed
the existence of clay layers from ground level down to rock. The top layer was a post-glacial
clay containing some organic matter, whereas the bottom layer was a marine clay of high
sensitivity (St = 20-50).
The water content of the clay decreased from about 80% in the upper layer to about 55%
at a depth of 20 m. The average density of the clay was 1.54 ton/cu. m. The undrained
shear strength increased linearly with depth. Underneath a weathering crust 3-4 m thick
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STABILITY
OF
STRUTTED
EXCAVATIONS
IN
CLAY
39
the shear strength was 1.5 ton/sq. m, increasing to approximately
3.5 tons/sq. m at a depth of
20-25 m.
As the depth of excavation was 25 m when failure occurred, the total overburden at the
bottom of the excavation
was 38.5 tons/sq. m. The shear strength of the clay being 3.5
tons/sq. m, N, will in this case be 11. According to the theory for the D/B ratio of 28, an
NC-value of 9.0 is indicated.
The high actual value of NC may be partly due to the close
proximity of the rock surface to the bottom of the excavation.
Sewage tank, Drammen
During the construction of a nursing-home in Drammen in August 1952, a failure occurred
in the excavation for a sewage tank.
The dimensions of the excavation were 5.5 x 8.0 m,
with a designed depth of 4.5 m. Down to a depth of 3.5 m, a stiff clay was encountered, but
as soon as the underlying soft, quick clay was reached the bottom of the excavation started
to rise, and continued during the subsequent attempt to complete the excavation.
The
excavation was not braced, and in spite of the bottom failure the walls of the upper stiff clay
remained intact and vertical.
Several attempts to complete the excavation
within sheet
piles driven well below the final depth proved to be unsuccessful, and only after the removal of
the upper soil layers surrounding the excavation was the intended depth reached.
Soil investigations
carried out after the failure showed that below an upper drying crust
there was a soft, quick clay of shear strength approximately
1 ton/sq. m, with a density of
1.8 tons/cu. m.
Based on the collected data an NC-value is calculated for the conditions
immediately before the first failure, which took place at a depth of 3.5 m. A surcharge of 1
ton/sq. m on the surface around the excavation was taken into account.
The result of this
calculation is that NC should be 7.2 to obtain the safety factor 1.0.
The theoretical value is
6.7, but it is emphasized that this excellent agreement between observation and theory may
be somewhat fortuitous since there are some uncertainties in the estimate of the shear strength
and the surcharge.
Test shaft (N), EnsjBveien,
Oslo
In connexion with the design of a new subway in Oslo, three trial shafts were excavated
in 1953for the purpose of determining the reliability of the theoretical analysis of the stability
of excavations.
For all three shafts thorough soil investigations were made including sampling, vane tests,
and pore pressure measurements.
During the excavation
large samples were cut out at
different depths for a detailed laboratory investigation.
At different stages of the excavation
vane tests were made from the bottom in order to investigate possible changes in shear strength
of the clay.
The excavation was started by digging a lG1.5
m-deep circular hole with a diameter
just exceeding 1.5 m. A number of 1.5-m C$rings, made by welding 30-mm C$ steel bars,
were placed in the bottom of the hole and 2-in. timber planks were driven down as sheet
piles outside the rings.
The sheet piles were driven successively as the clay was excavated,
splicing each plank to the preceding one as they were driven into the ground.
The rings were
lowered in steps each time leaving the top ring behind.
In this way the rings were distributed
along the shaft at a distance of 0.5 m from each other, thus forming a regular support for the
sheet piles (Fig. 4).
The northern shaft (N) was excavated through 1.5 m of fill and then through 2 m of stiff
drying crust.
Below the drying crust a soft-and
in parts a very soft-quick
clay was enThe quick clay continued down to a depth of lo-11 m, and from this depth,
countered.
continuing down to rock at a depth of 15 m, was a sandy clay.
The shaft (N) was terminated at a depth of 7 m, at which point failure was expected.
The
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40
L.
BJERRUM
Depth
in metres
Fig. 4.
AND
fi
0.
EIDE
Sheur shwgth
Test shaft (N), Ensjiiveien, Oslo.
Cross-section after failure
failure did not occur, however, until two weeks after the excavation had been completed.
The bottom rose about 2 m and, as shown in Fig. 5, a sinkhole appeared in the ground close
to the sheet piling. Two vane tests carried out after the failure showed that the clay was
remoulded to 19-1.5 m below the original shaft bottom.
The clay below the bottom of the shaft had a shear strength of 1.2 ton/sq. m, and the
average density of the soil was 135 ton/cu. m. A calculation based upon these values
shows that NC should be 10.8 in order to obtain the safety factor 1.0. This is 20% in excess
of the theoretical value.
Excavation,
Grey Wedels Plass, Oslo
In 1913 a total failure occurred in an excavation in Oslo. The excavation was 5.8 x 8.1 m
in area and was designed to reach a depth of 5 m, but a base failure took place at a depth at 4.5
m. Even after the driving of heavy sheet piling the bottom continued to rise as the clay was
removed. The work was finally completed by excavating small sections which were covered
with concrete and then loaded.
Investigations carried out in 1954 by the Norwegian State Railways showed the soil to
consist of a soft clay. The shear strength of the clay layer from 4.5 m-at which depth
the failure took place-down
to 7.5 m was determined, by vane tests, to be 1.4 ton/sq. m.
A calculation of the stability of the excavation immediately before failure led to a safety
factor of 1.08. A surcharge loading of approximately 1 ton/sq. m due to building materials
was included in the calculation.
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STABILITY
OF STRUTTED
EXCAVATIONS
IN CLAY
41
“ Kronibus ” shaft, Tyholt
During the last war the German occupation authorities started the construction of a
railway tunnel in clay near Trondheim (Hartmark, 1955). A German contractor, who was
supposed to be a specialist, made an attempt to sink a 20-m-deep shaft midway on the tunnel
length. At this site the soil consists of a very sensitive clay. When the shaft had reached
a depth of 19.7 m a failure occurred and the shaft was partly filled with liquid clay. The work
was thereupon abandoned and the tunnel was not completed until after the war, when it
was built by the shield-driving method.
The dimensions of the shaft were 2.7 x 4.4 m. The shear strength of the clay at the
bottom of the shaft was approximately 3.5 tons/sq. m, as determined by vane tests carried
out by the Norwegian State Railways. The density of the clay was 1.8 ton/cu. m.
A calculation of the stability indicates a theoretical safety factor of 034. The N,-value
which gives a safety factor of 1.0 is 10.1, compared with the theoretical value &5.*
EXAMPLES
OF PARTIAL
FAILURES
IN EXCAVATIONS
Pumping station, Jernbanetorget, Oslo
In 1943 a deep excavation was made for a pumping station in Oslo. During the work
a movement in the surrounding ground was experienced, and Mr S. Skaven-Haug, civil
engineer of the Norwegian State Railways, was consulted. His report, and additional information required for the calculations, was placed at the disposal of the Norwegian Geotechnical
Institute.
The dimensions of the excavation were 12.2 x 8.5 m with an intended depth of about
9 m below ground level. The top 2.5 m of the excavation was strutted, and from this depth
slender and unsatisfactory sheet piles about 10 m long were driven.
When the excavation reached a depth of 6.3 m below ground level cracks appeared around
the hole at a distance of about 8-10 m, i.e., at a distance relatively large compared with the
depth. Mr Skaven-Haug explained the cracks as being caused by movements in the underlying clay masses, following slight base failure in the excavation. A complete failure did
not occur.
The existing struts were strengthened, and the work was completed by the excavation of
small sections which were then loaded by casting the base of the footing.
Site investigations were made in advance by a private firm, and vane tests to a depth of
19 m were carried out by the Norwegian Geotechnical Institute in 1952. Under 2 m of fill,
soft to medium-firm clay was encountered to great depth. The shear strength of the clay as
determined by vane tests was about 2-3 tons/sq. m, the lowest values being experienced at a
depth of about 12 m below ground level. The shear strength near the bottom of the excavation, when the cracks occurred, was about 2.2 tons/sq. m. The average density of the clay
was 1.9 ton/cu. m. For a failure at a depth of 6.3 m an NC-value of 5.5 may be expected.
A calculation according to the above theory indicates an NC-value of 6.9. This means that
deformations occurred at a depth at which the calculated safety factor is 1.25. It is probable
that the excavation could have been deepened without total failure occurring. The critical
depth according to the theory is about 8 m.
Store house and o&e block, A/S Freia, Oslo
During the summer and autumn of 1954, excavations were dug for a new store house in
Oslo. The building was supported partly by steel piles to rock, and partly it rested directly
The present Authors suggest that the effect of
* The theoretical NC-value 8.5 is Skempton’s value.
For shafts
the width/length ratio on the NC-value may be questioned as far as deep shafts are concerned.
with depth/width as well as depth/length ratios greater than 4 (see Fig. 2) it may be that the theoretical
NC-value should be 9 instead of 8.5 as used in this case.
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42
L. BJERRUM
AND
0. EIDE
on the rock, which was encountered at depths varying from 5-20 m below ground level. The
depth to rock, of interest in the present Paper, was about 9 m at a side wall, where the rock
sloped somewhat towards the centre of the building. The depth of excavation was about 5 m,
as shown in Fig. 6.
II
@npmry excovotion
5
Fig. 6. Store house, A/S Freia, Oslo.
Cross-section through excavation
The work was carried out with a side slope of 1 : 1 to a depth of 0.5 m above the required
depth. At this stage a small slide occurred in the side slope, but this had no effect on the
underlying clay. The excavation was widened to vertical walls in small sections, which were
strutted. After the sides were strutted and the piles driven, the foundations for the outside
walls and columns were cast. Excavation of the remaining 0.5 for the basement, and the
casting of the basement floor, was carried out after completion of the walls, columns, and
ground floor of the building.
The placing of the foundation and walls up to ground-floor level, and also a little higher,
was completed by May 1955, and then work on the basement commenced. Before the casting
of the basement floor the remaining 05 m was excavated and at that time a settlement of the
ground outside the wall was noticed.
The gable wall of a single-storey brick workshop without a basement was situated about
3 m from the side wall. The gable wall had settled slightly during the first part of the excavation, but the settlements increased noticeably with the excavations for the basement, to an
extent of about 10 cm. Cracks were noticed on the ground, and the settlement near the side
wall was less than that near the gable wall. Those settlements, and the cracks, indicated
that the safety factor against base failure in the basement was very small, and the basement
slab was cast as soon as possible.
The Norwegian Geotechnical Institute carried out two vane borings and took samples
before the excavation commenced. The top 3 m consisted of a hard weathering crust and,
under that, the shear strength of the clay decreased with depth to a minimum value of 1.4
ton/sq. m at a depth of 6 m, and then increased to 2.5 ton/sq. m at a depth of 10 m.
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STABILITY
OF STRUTTED
EXCAVATIONS
IN
CLAY
43
At a depth of 5 m, with a density of clay of 1.9 ton/cu. m, together with a surcharge load
of 0.5 ton/sq. m due to the workshop, the pressure was 10 tons/sq. m. The average shear
strength of the clay was 1.6 ton/sq. m, which gave an NC-value of 6.25 at that depth.
The excavation was very long compared to the width, and since the depth to rock was
limited the depth/breadth ratio was approximately equal to 1.0 which gave a theoretical NCvalue of 6.4 with a safety factor of only 1.02.
Shaft, Shellhaven,
England
In connexion with the construction of a pumping station at Shellhaven, in the Thames
Estuary clays, a shaft of 55 m dia. was excavated and, at a depth of 5.2 m, a slight failure
occurred. Messrs John Mowlem kindly supplied the relevant facts and gave permission for
their publication in this Paper. The excavation was supported by cast-iron rings. A total
failure was not experienced and it was thought that the work could possibly have proceeded
a further O-5 m. But a movement of the surrounding ground had been noticed and, for
safety, the work was stopped for a while, and then completed under air pressure.
Site investigations have been described by Skempton and Ward (1952) and Skempton and
Henkel (1953) in connexion with adjacent works at Shellhaven. Under a hard brown clay a
soft-to-fairly-soft sensitive clay is found, with occasional layers of peat and silt. The shear
strength of the clay in the vicinity of the bottom of the shaft at the depth of 5.2 m is about
1.6 ton/sq. m.
The pressure due to the clay mass, together with a surcharge load of 1 ton/sq. m, gives a
pressure of about 9 tons/sq. m at the base of the excavation. The average shear strength
of the clay being 1.6 ton/sq. m, NC-values of 5.6 and 6.1 are obtained, corresponding to depths
of 5.2 and 5.7 m.
According to theory, an NC-value of 7.5 and a critical depth of 7.6 m are indicated. A comparison between the theoretical values and the field experience indicates that a partial failure
occurred when the safety factor was 1.34. Part of this divergence between theory and practice may be explained by the decrease in shear strength due to remoulding at the bottom of the
shaft.
Test shaft, Siirligaten, Oslo
In a similar manner to the test shaft at Ensjoveien a test shaft was excavated near the
tunnel at Sorligaten, Oslo. Here also, extensive site investigations, including vane borings
and samplings, were performed, and several blocks of the clay were obtained for laboratory
investigations. The shaft is shown in Fig. 7.
The ground conditions may be described as follows. Under a 1*5-m-thick top layer of
fill, silt is encountered to a depth of 3 m, and then a fairly sensitive clay is found to a depth
of 10 m, the shear strength of which decreases from 4 tons/sq. m to l-5-2 tons/sq. m. At a
depth of 9-11 m the clay contains some sand and gravel, and below this-to rock at 17 mis a quick clay of shear strength about 2 tons/sq. m.
Excavation proceeded to a depth of 13 m without failure. Tests performed during the
work showed a decrease in the shear strength of the clay underneath the bottom of the shaft
when a depth of 12 m was reached. That may be considered as the critical depth, as the
decrease in the shearing strength was due to large shear deformations. The decrease in shear
strength could be seen very clearly from a vane boring carried out, after the excavation,
about 0.5 m away. That local failure did not, however, lead to a complete failure.
At a depth of 12 m the shear strength of the clay is 2.1 tons/sq. m and, with an average
density of 1.85 ton/cu. m, an NC-value of 10.6 is obtained. The depth/diameter ratio for the
shaft is 8.0 and the theoretical NC-value is 9, giving a safety factor of 0.85.
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44
L.
BJERRUM
AND
0.
EIDE
Depth
in metres
P’wood
piles -
5
3Omm q
steel rinys
- Sheor drenglh
be/&e cxcadan
L?.--_
- Sheur a/&y%
uFhzr CXCOVOtIM
Test shaft, Siirligaten,
Fig. 7.
Oslo
In spite of the partial failure at a depth of 12 m it was possible to excavate to a depth of
13 m where the safety factor is 0.75.
It is possible that total failure was retarded by the relatively hard top layer preventing
the development of deformations.
EXAMPLES
Basement excavation,
Storgaten
OF SUCCESSFUL
EXCAVATIONS
5, Oslo
In the summer of 1952 a 6-7-m-deep excavation for two basement floors was made in the
central part of Oslo. The excavation was carried out in sections within sheet piles, and as
soon as a section was excavated concrete was placed and loaded with sand.
Careful soil investigations were made by the Norwegian Geotechnical Institute. The
shear strength of the soft and partly quick clay was 1.75 ton/sq. m and the density was 1.9
ton/cu. m. The depth to rock was 13-15 m.
The above data are valid for the place where the most critical section was excavated.
The dimensions of that section were 8 x 4 m and the depth was 6.7 m. Corresponding to the
depth/width ratio of 1.7, the theoretical NC-value is 8.0. The safety factor is thus only 1.10.
During the excavation careful observations were made, including an attempt to measure
the rise of the bottom of the excavation. Those measurements proved to be very difficult,
and were only partly successful. From the results it was, however, concluded that the
bottom of the critical section rose about 2 cm. The excavation was situated close to neighbouring buildings and some settlements and cracks of secondary importance were observed.
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STABILITY
Subway
OF STRUTTED
EXCAVATIONS
IN
CLAY
45
in Chicago
One of the examples used by Tschebotarioff
(1951) in his treatment of the problem was a
deep excavation for the Chicago subway.
A detailed description of the work and of the site
conditions is given by Peck (1943), but it must be remarked that Peck does not direct attention
to the question of stability.
At one particular point in the subway the excavation was 16.0
m wide and 11.3 m deep.
The excavation was strutted, but a maximum horizontal deformation of 6 cm occurred.
During excavation a maximum heave of 10 cm was observed together with a sinking of the
adjacent ground of about 3-15 cm, the largest values being nearest the excavation.
Although
a complete failure did not occur it is clear that the safety factor was low.
The clay underneath the excavation had, according to Tschebotarioff,
a shear-strength
of
3.5 tons/sq. m. A calculation of the stability of the excavation shows a theoretical N,-value
of 6.1 with a safety factor of 1.0.
Test shafts
(S), Ens$ueien,
Oslo
,4s described earlier, two test shafts were excavated at Ensjoveien, Oslo, in the summer of
1953.
Whilst a total failure was obtained in the northern shaft, the southern shaft was
excavated to a depth of 11 m, which was not sufficient to produce failure.
The dimensions
and method of excavation of the test shafts have already been described.
The shear strength of the clay at the bottom of the shaft was 2.7 tons,sq. m, the average
density of the clay was 1.93 tonjcu. m, the depth/diameter ratio was 7.3, and a theoretical
S,-value
of 9.0 was obtained.
The theoretical safety factor was 1.11.
The test shaft was
kept open for 12 months without any sign of failure.
The data of the investigated
excavations
are summarized in Table 1, which also gives
the factors of safety against base failure calculated by the proposed method.
From the
data presented in Table 1 the following conclusions may be drawn :
(1) The theoretical safety factor for the se\-en cases in which complete failure occurred
varies between 032 and 1.16.
The average value is 0.96.
The field observations
therefore confirm the reliability of the theoretical
analysis, and the accuracy
with which the observed total failures could have been predicted by this method
is considered to be satisfactory.
(2) Also the four excavations
in which pavtial failures
were observed support the
theoretical approach, since the calculated safety factors vary between 035 and
1.25 with an average of 1.11.
(3) The three successful excavations are selected cases in which the safety factors against
base failure are judged to be small.
The theoretical safety factors of these cases
vary between 1.0 and 1.14 with an average value of 1.08.
If compared with the
field observations made during the excavations,
the calculated safety factors are
believed to give a correct evaluation of the stability.
It is very likely that a
further excavation of the two cases which show the lowest safety factors would
have led to failure.
(4) Included among the excavations with a total or partial base failure are four shafts
It is immediately seen from Table 1 that
great depth compared to the width.
the safety factors of these four shafts are remarkably
low compared with the
The safety factors of the four shafts vary between 0.82 and 0.85
other cases.
with an average of 0.84.
These results may be explained by a too low theoretical
W,-value for deep excavations,
and it is thus possible that Skempton’s IV,-values,
which are confirmed by field observations of foundation failures or loading tests
of
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46
L. BJERRUM
Table
Data concerning
1. Pumping station,
Fornebu,
0.
EIDE
1.
instability
of excavations
1
Oslo
2. Storehouse, Drammen
.4KD
5.0 Y 5.0 ~ 3.0
0.0
1.75
0.60
!
1 7.2
1 1.8 Y u3 / 2.4 ~ I.5 ~1%
3. Pier shaft, GGteborg
4 04
4. Sewage tank,
Drammen
5.5 x 8.0
, 25.0
0.0
3.5
1.0
1.03
Total
failure
1.16
11.54 3.5
i 20~~50~1.0 ~28.0 ~ 9.0
I
~1.80 ~1.0
20
~0.69 10.64
6.7
0+X2
0.93
5. Test shaft (S),
EnsjGveien, Oslo
6. Excavation,
Grev
Vedels pl., Oslo
___~
7. “ Kronibus shaft,”
Tyholt, Trondheim
--_
8. Pumping station,
Jernbanetorget, Oslo
-__
9. Storehouse, Freia,
Oslo
i
j
I 5.8 x s.1
~12.2 x 8.5
5.0 Y cc
--____
11. Test shaft, Sijrligaten
Oslo
:
14. Test shaft (S),
Ensjdveien, Cslo
19.7
0.0
1 ,SO
1.4
5510
0.72 ~0.78 1 7.0
~
10.61 ~7.3
1.80
3.5
40
6.3
0.0
1.6
12
0.0
11.00 I 6.4
1.02
6
1.o
: 095 ! 7.5
1.34
60
1.0
~f+()(J1
~ 9.0
10-20
0.5
1.40
I
i
10. Shaft, Shellhaven
13. Subway, Chicago
1.0
I
-
12. Basement excavation,
Storgaten Oslo
I
4.5
‘-_-
2.7 X 4.4
I
.$5.5
41.5
;
~8.0 x 4.0
/
i 16~~
5.0
0.0
j.2
14
1.90
i 1.54
1.61
I
I
12.0 ~ 04 ~1.85 2.1
6.7
i 0.0 ~I.90 1.75
3.5
I
’ do.
do.
1
j do.
~ 0+?5 ~
No
~ ~_
1.70
s.0
i 1.10
failure
0.0 ~0.70
6.1
1.00
j do.
1.14
d(,.
I
I
4 1.5
11.0
0~0 1 1.93
2.7
1 200 / 1.0
7.30
/ 9.0
on English clays, are somewhat smaller than the values valid for the sensitive
Norwegian clays with a high value of E/s. A better agreement would result if
NC was assumed to be 10 for deep shaft excavations instead of 9.
The observations made during excavation of the test shaft in Sijrligaten indicate,
however, another explanation of the low safety factors of the deep shafts.
This
test shaft proved that under certain conditions it is possible to excavate a shaft
to a greater depth than that calculated theoretically
without obtaining a total
failure.
If a shaft is dug through a stiff clay down to soft clay, the upper stiffclay layer will prevent, at least for a time, a local overstressing of the soft clay
below the bottom of the shaft from extending to a total failure.
This effect of
an upper stiff-clay layer was observed in the test shaft in Siirligaten which was
excavated down to almost 2 m below the calculated critical depth.
Moreover,
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STABILITY
OF STRUTTED
EXCAVATIONS
IN
CLAY
47
this effect may explain why the Iota1 failure in the test shaft (N) at Ensjiiveien
occurred a certain time after the excavation was completed.
(5) The scattering of the calculated safety factors for some of the excavations
seems
to indicate that the danger of failure will increase if the excavation is not sufficiently braced to prevent excess horizontal deformations.
If the upper clay layers
are stiff, and the depth of the excavation is small, the danger of a base failure
seems to be less affected by the bracing.
(6) Experience
cannot yet indicate exactly how much the danger of a base failure is
The stabilizreduced if sheet piles are driven below the bottom of an excavation.
ing effect of sheet piling seems, however, to be small for the cases where the shear
strength of the clay does not increase with depth.
XCKXOWLEDGEMENTS
Four of the described fourteen case records were collected by Mr Skaven-Haug,
chief
engineer of the Norwegian State Railways.
The Authors are indebted to Mr Skaven-Haug
for permission to use and publish his field observations.
The Authors are, moreover, indebted to the Planning Office for Suburban Lines and Subways, Oslo, whose support made
it possible to excavate the three test shafts.
The Authors also wish to express their particular
gratitude to Professor A. W. Skempton and to Dr A. W. Bishop for valuable discussions on
the problem, and to the Directors of John Mowlem & Company for permission to use and
publish the Shellhaven case records.
Finally, the Authors acknowledge the assistance of
their colleagues, especially the work done by Mr I. Johannessen,
during the laboratory and
field investigations.
REFERENCES
BJERRUM, L., 1955. Stability of Natural Slopes in Quick Clay. Gtotechnique, 5 : 101.
Proc. 1st Int. Conf. Soil Mrch. (Cambridge),
CUEVAS, I. A., 1936. Foundation Conditions in Mexico City.
3 : 233.
HARTMARK, H., 1955. Geotekniske Observasjoner fra utfdrelsen av Tyholdt Jordtunnel.
Teknisk Ukeblad,
27 : 565.
JANBU, N., 1952. Stabilitet av skjaeringer i apen sjakt.
Norges Geotekniske Institutf. Int. r@ort
F 13,
OSLO.
PECK, R. B., 1943. Earth pressure measurements in open cuts, Chicago (Ill.) subway.
Trans. Amer. Sot.
Civ. Eqrs, 108 : 1008.
Sldg
Res.
Cong.,
London,
1951.
Papers preSKEMPTON, A. W., 1951. The Bearing Capacity of Clays.
sented in Div. 1. DD. 180-189.
SKEMPTON,A. W., and WARD, W. H., 1952. Investigations Concerning a Deep Cofferdam in the Thames
Estuary Clay at Shellhaven.
Giotechnique, 3 : 119.
SKEMPTON. A. W., and HENKEL, D. J., 1953. The Post-glacial Clays of the Thames Estuary at Tilbury
and Shellhaven.
Proc. 3rd Int. Conf. Soil Mech. (Zurich), 1 : 302.
TERZAGHI, I<., 1943. Theoretical soil mechanics.
W&y, Niw York.
TERZAGHI, I<., and PECK, R. B., 1948. Soil mechanics in engineering practice.
Wiley, New York.
TSCHEBOTARIOFF,G. P., 1951. Soil mechanics, foundations, and earth structures.
McGraw-Hill, New
York.
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