2875
Technical Committee 212 /
Comité technique 212
segments is about 0.2, similar as the law of soil tests.
As the
reloading ratio is still low, the recompression deformation is
obvious. As the reloading ratio of soil at the bottom of
excavation increases to 0.8, the recompression deformation is
roughly equal to the rebound deformation.
3 THE CALCULATION METHOD OF REBOUND AND
RECOMPRESSION DEFORMATION
3.1The Calculation Method of Rebound Deformation
The systematic analyzing method of rebound deformation is as
follow
[1]
:
i) get no less than 6 samples for each solum by investigation,
and then obtain the basic data of soil by consolidation-rebound-
recompression test;
ii) analyz the data by graphing curves of unloading ratio-
rebound proportion and curves of unloading ratio- modulus of
resilience, determine the value of
R
cr
;
iii) determine the calculation depth of rebound deformation
according to
R
cr
, divide the calculation depth to several layers,
and calculate both unloading ratio
R
and modulus of resilience
E
c
of every layer;
iv) calculate rebound deformation of every layer and add them
together to get the total rebound deformation.
The rebound deformation can be calculated as formula 1:
1
1
(
)
n
c
c
i i
i
i
ci
p
s
z
z
E
i
(1)
Where
：
c
s
- rebound deformation of foundation soil,
mm
;
c
p
- gravity of soil upon foundation base,
kPa
, buoyancy
should be deducted underneath ground water;
ci
- modulus of resilience, it can be determined from soil test
directly or calculated from the relation curves of
R-E
c
and
R-
lgE
c
;
E
n
– number of layers which the calculation depth is divided;
z
i
,
z
i-1
–
distance between foundation base and the bottom of No.
i
, No.
i-1
layer
；
i
,
1
i
–
additional stress coefficient of No.
i
, No.
i-1
layer.
In this method, both
E
c
and
R
cr
must be determined by
corresponding stress state of soil and stress history. This method
is not only simplifying the process of calculation but also
improving accuracy.
3.2 The Calculation Method of Recompression Deformation
According to the law of recompression deformation, the
recompression deformation can be calculated as formula 2
[7]
:
'
'
0
0
'
0
'
'
'
'
1.0 0
0
0
0
'
0
[
(
)]
1
c
c
c
c
R
c
c
c
p r s
p R p
p R
s
r
r p
c
s r
R R p
R p
p p
(2)
Where:
c
s
- recompression deformation of foundation soil,
mm
;
c
s
- rebound deformation of foundation soil,
mm
;
0
- the critical recompression ratio. The curves of reloading
ratio-recompression ratio of soil can be divided into two line
segments. The recompression ratio of intersection of the two
line segments is critical recompression ratio, and its value can
be determined in the curves of reloading ratio-recompression
ratio from soil test;
r
R
- the critical reloading ratio. Similar to
0
r
, the reloading
ratio of intersection of the two line segments is critical reloading
ratio, and its value also can be determined in the curves;
1.0
R
r
- The value of recompression ratio, when
1.0
R
. it is
equal with the increase proportion of recompression
deformation.
4 ENGINEERING APPLICATION
In Beijing there is a 20-storeyed building with 4-storeyed
basement. The foundation area is about 6000 m
2
, and the
excavation depth is 19.1m. In the process of the building’s
construction, research on rebound and recompression
deformation was carried out by Institute of Foundation
Engineering China Academy of Building Research. Rebound
deformation was monitored during the excavation, and
settlement was also observed in the process of construction.
4.1 Calculation of Rebound Deformation
According to the data of soil tests, the expression of
R-E
c
and
R-
lgE
c
, and the value of
R
cr
,
R
u
of soil can be determined. For
example, for sample 1-3:
R
cr
=0.33
,
R
u
=0.92, and the expression
of
R-E
c
is listed in table.1.
Table 1 The expression of
R-E
c
of sample 1-3
Condition of R
expression
0
0.
R
33
257533.3 386300
c
E
R
0.33
0.92
R
181009.2 165111.8
c
E
R
0.92
1.0
R
219419.9 203110.2
c
E
R
The rebound deformation can be calculated as formula 1. The
calculated deformation contour of the foundation pit shows as
figure 7.
The rebound deformation of the foundation pit is not
the same, the maximum of rebound deformation is 35 mm in the
center of foundation base, and it reduces gradually to 6 mm at
the edge of foundation base. The measured rebound
deformation is about 32 mm.
0 10 20 30 40 50 60 70 80 90 10
Length of foundation pit�m
0
�
0
10
20
30
40
50
60
70
Width of foundation pit (m)
Figure. 7 the deformation contour in foundation pit of calculating
Figure. 8 the rebound deformation distribution along depth
The rebound deformation distribution along depth is shown as
figure 8. The curve of calculating deformation by this method is
approximate to the curve of measured deformation. The
rebound deformation beneath 12.0m depth under foundation
base is less than 30% of the total deformation.
