THE METHOD OF INFORMATION CONSTRUCTION FOR FOUNDATION PIT SUPPORT STRUCTURE

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Nov 26, 2013 (3 years and 8 months ago)

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127

THE METHOD OF INFORMATION CONSTRUCTION FOR FOUNDATION
PIT SUPPORT STRUCTURE



Liu, Xiaoli

Dept. of civil Engineering, North China Institute of Astronautics, Lang Fang, P.R.
China

Dou, Yuanming

School of Civil Engineering and Architecture, Hebei University
of Technology,
Tianjin, P.R. China

Yan, Chi

School of Civil Engineering, Tian Jin university, Tianjin, P.R. China



Introduction


There are many foundation pit excavation engineering every year in China, but now the design method is
still 1/3 based on it’s

theory, 1/3 based on it’s experience, 1/3 based on it’s information feedback
controlled. So there are two questions which based on design method be caused:


(1) the result of design is too safety to supporting cost higher;

(2) according to the theory meth
od ,the structures is safety, but may is dangerous.


It is reported that the foundation pit support structures failure or partly failure lead to the safety questions
and surrounds questions ,and these questions are about 10%~15% in total engineering projec
ts ,even in
soft areas they about 20%.


People always hope to find the balance between safety and economy for the pit
-
retaining structure which
regard it as a temporary structure, that is, achieving the best result at least cost. The result of making a
com
parison the pit
-
retaining structure, pile foundation and ground treatment is: the pit
-
retaining structure
being difficult, the designers can’t determine whether the design is really reasonable, can’t authentic
appraise the safety and economy when the soil
is digging after the pit
-
retaining construction is finished.
So, one of the most effective ways is developing a computer controlling system at all time and on line,
and the designers can immediately sort out and feedback the information data, thus gain a c
orrect
understand of the foundation pit about its developing trend and the regular of mutual relations. Then the
features and state of foundation pit structure can be analyzed in a coming short time, the monitoring fully
directs construction process. The s
afety of foundation pit structure can be pledged in the whole process.



The Method of Information Construction for the Foundation Pit of a Office Building in Tianjin


Engineering general situation


The building locate in the Red Bridge District, Tianjin.
It lies in the West of Dafeng Road, Stone Bridge
Street is in the south of it. There is a large space in the north and west.25
-
storeyed up surface of the
building and 3
-
storeyed in subsurface, the depth of the pit is 8.9m. The foundation bottom sheet size
is
52m
×
37.6m.


Design plan


Plan 1: The bored piles make up the retaining wall; roundness ring supporting system and a row of rock

Bolting is used in this structure.


The bored pile are set up along the circumference of the foundation pit, the diameter of

piles is 700mm,
the distance between the piles is 750mm, the average length of the piles is 16m.One concrete roundness
ring beam is set up on the top of the bored piles, the inter diameter of ring beam is 52m, the section size is
2000mm
×
600mm.


The sectio
n size of the cap beam which locate on four corners of the foundation pit is 1000mm
×
600mm,
the diameter of anchor rod is
φ
40, the horizontal distance between the rods is 1.5m, locate at 6m beneath
the ring beam. There are two rows cement
-
soil piles interlo
cking each other are set outside the pit
-
retaining piles. The cement
-
soil piles can being sealing installation to prevent the groundwater beyond the
128

pit. The effective length of cement
-
soil piles is 14.5m. This plan is designed through the data which come
from investigation reports.


Plan 2: Based on practical experience for a long time, the technical staff think: first, this pit
-
retaining
structure is a temporary structure, it’s foundation is retaining soil; second, the parameters of soil
mechanics, such a
s
γ,φ
,
с
are used in lower value. There parameters can be increased in the practical
engineering. So all anchor rods can be canceled according to Plan 1, see Figure 2.1.


For Plan 1 and Plan 2, the contract decides to adopt Plan 2 finally. Then the monitorin
g work seems much
more important.


The content and procedure of the pit
-
retaining structure monitoring


There are four monitoring works according to the practical engineering required:

(1)

monitoring the inter forces and deformation of the ring beam

(2)

monitoring

the deformation along the bored piles length

(3)

monitoring the active earth pressure of bored piles

(4)

monitoring the settlement of ground surface around the foundation pit


Monitoring points see in Figure 2.1.



M

S

M

S

M

S

T

P

M

S

T

P

1

2

3

8

4

5

6

9

10

11

12

13

14

15

16

7

M

S

M

S

M

S

M

S

M

S

M

S

M

S

T

P

M

S

T

P

M

S

T

P

M

S

T

P

M

S

T

P

M

S

T

P

the depth of pit

8.9m


Figure 2.1

The
shape of the pit and the arranging of monitoring points


Procedure of the pit
-
retaining structure monitoring see in Figure 2.2.


The result of monitoring and analysing


There are two steps for the foundation pit excavation. First, excavate 4.1m depth from
ground surface, the

Directions is the from southern to the northern pit. Then, excavate 4.8m depth to the bottom of the pit.
The direction is from western to the eastern of the pit.


The monitoring of the ring beam bending moment


Though the monitoring res
ults, some regularities show as:


(1)

During excavating the pit, there is a little bending in the cap beam which at the top of bored piles
locate on four corners. Comparatively, there is a higher bending moment in the ring beam. These
shows that the ring beam
mainly bears the loads which act on the top of the bored piles. The result
accords with theory analysis.

(2)

On the ring beam, the monitoring points No.1 and No.9 all bear a little bending moment, see Figure
2.3. The reason is: first, the arranging shape o
f the bored piles is unanimous with the plane shape of
the ring beam. They are all circular arc shape and are densely arranged, the forces in a circular arc
shape is little than in a line shape; second, there is a partly soil in the pit still, and is set a

slope along


129

the inside of the ring beam. At the same place, a retaining wall is built. So, a little forces act on the
bored piles, and then a little bending moment act on the ring beam.



Excavating 4.1m depth

Excavating 7m depth

Finishing excavation soil

Finishing cushion



Figure 2.3

The deformations of th
e ring beam


(3)

The southern and the northern of the piles are densely arranged in plane around the pit, and these
piles are joined by the cap beam and the ring beam. The active soil pressures act on the ring beam
though the cap beam, then the negative be
nding moment is created at the points of No.5 and No.13.

(4)

The largest positive bending moment is: M=270tm, act on the point No.8. And the largest negative
bending moment is: M=
-
331tm, act on the point No.5. The reason is that there is a 0.9m depth in th
e
southern of the pit than in the northern, the more soil loads act on the southern piles than the
northern piles.

(5)

After finished the bottom plate, there is a decreased bending moment in the ring beam. Though the
decreased value isn’t large, the result

shows that the bottom plate begins to play the supporting role.

(5)

When the ring beam is cut off at No.10 and No.16, the ring beam bending moment changed: the
bending moments of No.11, No.12 and no.15 are rapidly increased. Especially, the bending moment
of
No.11 is 2196tm. At that time, the constructors heard a loudly voice which came from No.11, and
then the ring beam is cut off. The ring beam axial forces are set free to the cap beam though No.11,
the cap beam bears too large force to creating the concrete

failure. But the bottom plate and
basement wall play the supporting role, the pit
-
retaining structure is still safe.


The monitoring of the ring beam deformation


During the whole construction process, the ring beam is monitored every each day. Its horizo
ntal
deformations see Figure.2.4. The largest value is 7mm at No.5.


130

The exploration survey on-site
Construction plan
Deciding the contents of monitoring
Choosing the monitoring equipments
Arranging the monitoring points and equipments
constructing/monitoring
Information form
Feedback to interrelated department
Whether has a warn
Doing the plan
Whether the dangerous
be controlled
Analyzing the reason,
deciding the treatment plan
Whether finished the construction
Whether the design
is optimation
Optimation
design
end
Forecasting dangerous and
preparing for dangerors
conditions

Figure 2.2

The process of the information construction monitoring





Figure 2.4

The horizontal deformations of the ring beam




131

-16
-12
-8
-4
0
0
20
40
60
80
deformation (mm)
pile length (m)
excavation -4.1m depth
excavation -7.0m depth
excavation -8.9m depth
placing concrete cushion
placing concrete bottom plate


Figure 2.5

The curve of the bored pile deformation


The monitoring of the deformation along the bored piles length


The deformations along the bored pile length are see Figure 2.5. Some regularities show as:


(1)

The shape of every bored pile deformat
ion is same during the excavation stage, that is, the shape is
bulgy belly, the middle is bigger, the both ends are smaller. The largest deformation value locate on
-
8.2m~
-
9.5m from ground surface; and all less than 35mm. The result tally with the theory
design
regularity.

(2)

It takes 7d~15d that placing concrete cushion. During the 7d~15d, due to soil can creep, the bored
piles have a 15mm~25mm deformation again. The time is longer, the deformation is larger.

(3)

It takes 16d that placing concrete bottom

plate. There is a 8mm deformation during 16d. The result
shows that the bottom plate play a very important role for the supporting of the bored piles.

(4)

After finished the bottom plate, the bottom plate play a horizontal supporting role at
-
8.3m depth ,

then the bored piles deformation tend to become stability. At that time, the largest deformation is
less than 60mm.

(5)

After cutting off the ring beam, the bored piles deformation is increasingly. But the displacement
increment at the top of the bored pi
les is only 15mm~25mm.The largest deformation is still at
-
8.2m~
-
9.5m depth, and the deformation shape doesn’t be changed.


The monitoring of the active earth pressure of bored piles


The monitoring point is set at every 2 meters along the bored pile len
gth. The active earth pressure at the
back of pit
-
retaining wall can’t reach the limited state, due to the bored pile deformation is small. So the
active earth pressure is less than the Rankine active earth pressure, see Figure 2.6, this value compare
with

E
0
, the reduced volume at the top of bored piles is more than at the bottom of bored piles. The result
is similar to the examples in the reference
1
.


-16
-12
-8
-4
0
0
40
80
120
160
earth pressure(KPa)
pile length(m)
excavating -4.1m depth
excavating -7.0m depth
excavating -8.9m depth
placing concrete cushion
placing concrete bottom plate


Figure 2.6

The curve of active soil pressure distribution


The monitoring o
f the ground surface settlement around the foundation pit


The result of the monitoring, see Figure.2.7. The settlements of points are between 30mm to 40mm.

-16
-12
-8
-4
0
0
40
80
120
160
earth pressure(KPa)
pile length(m)
excavating -4.1m depth
excavating -7.0m depth
excavating -8.9m depth
placing concrete cushion
placing concrete bottom plate

132


-40
-30
-20
0
2
4
6
8
10
12
14
distance beside the pit
(m)
settlement
(mm)
survey No.9
survey No.13
survey No.5

Figure 2.7

The ground surface settlement around the foundation pit



Conclusion


(1)

The stress and strain in the pit
-
retaining structure are monitored during the whole construction
process. The pit is enough safe, Plan 2 is reasonable. The anchor rods cost more than a half million
RenMin Yuan, and it is saved. The economy
effect is notable.

(2)

The concrete cushion can prevent the soil creep. So placing concrete cushion must early set up when
excavating to the bottom of the pit.

(3)

The inflection point of the bored pile placement doesn’t appear. The soil pressure before th
e bored
piles is passive, and the soil pressure back of the bored piles is still active.

(4)

After finished the construction of the basement, the basement becomes a horizontal supporting for
the bored piles. When the ring beam is cut off, the structure is
still safe.



Suggestion


(1)

The construction method around No.1 and No.9 is a good way. The monitoring results show that
there is a little force in the bored piles. When the condition is allowable, some plan can be design
like this. It can reduce the cos
t. The result should be pay attention.

(2)

The ring beam is a reasonable supporting structure, it make the pit
-
retaining wall safety and
trustiness.



Reference


Dezhang Feng. The monitoring result of clayey soil pressure on the pit
-
retaining pile. The thi
rd
Conference of Soil Mechanics & Foundation Engineering Paper Collection. Beijing, China, 1981.