A Research on Underground Water Control Technology for Deep

A Research on Underground Water Control Technology for Deep
Foundation Pit Construction
LIU Jing
Civil engineering department Wuhan Polytechnic University, P.R.China
ABSTRACT: In deep foundation pit construction process, it is necessary to control underground water
effectively. Starting from the damage pattern and formation mechanism of underground water, based
on the underground water flow theory, this paper discusses several underground water control method’s
application condition and practice key point, e.g., light well point water lowering deep well water
lowering waterproof curtain, etc., analyses the impact on surrounding environment due to water
lowering and prevention method, and suggest that the comprehensive application of waterproof and
water lowering is the best practical method for underground water control.
Keyword: well point water lowering, waterproof curtain,
the comprehensive application of
waterproof and water lowering
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1 Introduction
With the continuous development of urban construction, more and more buildings and structures
adopt deep foundation pit method. In the construction process of deep foundation pit, in order to keep
the stability of the surrounding buildings, roads and municipal pipelines, and at the same time for the
stability of the deep foundation pit themselves, it is necessary to choose appropriate supporting system
for deep foundation pit and underground water control method. Amongst everything, underground
water poses a big damage role for the overall safety of deep foundation pit. If underground water is not
controlled properly, it can lead to the collapse of the pit slope, water burst from pit bottom, make it
impossible to form a relatively dry construction environment in foundation pit, and lead to the
foundation construction process to be impossible. This paper mainly discusses how to control
underground water in deep foundation pit construction process in a basic way.
2 The underground water damage types and formation mechanism of deep
foundation pit construction
The underground water damage of deep foundation pit construction means that in the
construction process, the underground water lead to the instability of the soil, severely changed shape
etc., and cause damage to the construction process and surrounding environment. In the treatment of
underground water, no matter waterproof or water lowering is adopted, it is always necessary to make it
to be in a basically dry state in foundation pit construction process. And under the condition of not to
cause underground water damage, not to influence the construction development, not to cause damage to
the surrounding environment, it is also desirable to reduce prevention expense and shorten construction
time.
2.1 Underground water damage types
2.1.1 Instability of deep foundation pit boundary slope. This case is most likely seen in saturated soft
earth places, the main phenomenon is that in the top of a deep foundation pit boundary slope, the soil
body drops sharply in a certain range, which lead to the appearance of one or several cracks of arc shape
parallel to the slope, with the crack thickness of 0.5 30mm, even thicker in some part. The foot of
the slope slides toward the inner part of the pit, some rises upward, and in severe cases lead to single
direction inclination of the pile in the bottom of the pit. In exceptional cases it can lead to the breakage
of the cantilever pile, and cause severe damage.
2.1.2 Uneven drop of the surrounding earth surface of pit. With the expulsion of underground water,
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the surrounding earth of pit accelerates its consolidation, making the buildings and structures within
10m of pit to have cracks in some parts, and lead to the breakage of underground pipes in severe cases.
2.1.3 Water output and sand output from the pit slope and pit bottom. Underground water usually
outputs from pit slope and pit bottom in a tube shape, the output water from miscellaneous fill usually
does not contain sand, but the output water from sand layer usually contains large amount of powder
sand, they can influence construction process and surrounding environment.
2.1.4 The regular earth surface drop of the relatively large area around pit. This case usually happens,
when reducing the level of confined water, within 1~2 times of reduced depth of the level of confined
water, if the muck layer of the soil body of the field is thick (over 5m), or the transition layer of the top
part of confined water is very thick (over 8m), the buildings and underground pipes within this range
will be influenced.
2.2 The formation mechanism of underground water damage
2.2.1 The underground water reduces the shear strength of the soil of pit boundary
The stability of the soil body of pit slope during the excavation process of deep foundation pit is
supported by its shear strength. After the excavation of pit starts, underground water, under the action
of flowing pressure, streams into pit slope and cause the increase of the water content, then the shear
strength of the soil decreases sharply. The foundation pit support system designed under the original
soil body shear strength cannot bear the increased active earth pressure and water pressure due to
underground water’s force, therefore lead to the damage of foundation pit.
2.2.2 Abnormal drop and displacement of the surrounding earth surface of pit due to seep of ground
water and surface water
After the start of excavation, seep state of ground water and surface water will change, when
ground water seep in pit through silty or sand layer, because of ground water’s large hydraulic slope,
flowing sand usually will be generated. Flowing sand will devoid the foot of the pit slope, in mild
cases this will lead to the uneven drop of the surrounding earth surface, and in severe cases cause the
instability of the boundary slope.
2.2.3 The pressure of confined water surpasses the earth body pressure of upper confining bed in the pit
bottom, causes water and sand output
With the increase of the depth of pit excavation, the pressure of the confined water to the upper
confining bed increases gradually, but the thickness of the upper confining bed in the pit bottom will
then become thinner, and the self weight pressure of the earth body will decrease gradually, when the
pressure of the confined water surpasses the self weight pressure of the upper confining bed, or if the
upper confining bed is dug through, then the water output and sand output will be generated, leading to
the underground water damage.
3 Basic theory of water lowering
In the foundation pit excavation process, if the underground water level is relatively high, and the
earth quality is not very good, it is usually necessary to adopt well point water lowering method. The
well point water lowering system’s design and computation is rather complicated, it is necessary to take
into consideration the construction field’s planar diagram, data on hydrology and geological prospecting,
foundation pit design document etc., in order to appropriately decide the number, space and equipment
of well. At present, theory of computation still adopts the model of confined and unconfined
completely penetrated well.
3.1 Completely penetrated well of ground water
Completely penetrated well of ground water is as shown in Fig. 1, the well is distributed in the
ground water compartment that has free water surface, its water output computation formula is as
following:
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Q = 1 . 366 K
(2 H − S ) S
lg R − lg r
（m /d）
K ---permeability coefficient of the earth（m/d），which can be determined by laboratory or
where Q ---Output volume of the well point system
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pumping water at site
H ---Thickness of water stratum(m)
S ---decrease value of the water level (m)
R ---Influence radius of water pumping (m)
r ---Well radius (m)
3.2 Completely penetrated well of confined water
Completely penetrated well of confined water is as shown in Fig. 2, the well is distributed in the
confined water stratum, its water output computation formula is as following:
MS
Q = 2.73K
lg R − lg r
where M---Thickness of confined water stratum,
Q , K , R , r , S --- Same as those in formula above
Fig. 1 water level descending graph of ground water Fig. 2 water level descending graph of confined water
4 Underground water control methods
4.1 Light well point water lowering
When the earth in foundation pit is mainly sand type earth or silty, and the pit excavation depth is
not very deep (usually less than 6m), it is ok to consider light well point water lowering method. The
so called light well point water lowering is a system that, by using the parallel laid main pipe to
transport vacuum to start a group of vertically laid well pipes to pump underground water, so that the pit
is kept at a relatively dry state which makes construction possible. This method has very good result
for preventing flowing sand and increasing the stability of boundary slope and pit bottom.
Well points distribution depends on the construction’s planar size and digging depth etc., and
should make the water lowering curve after stabilization less than the digging depth at each point by at
least 0.5 m. Closed circle distributed well points’ inner region curve’s hydraulic slope can be
computed using 1/10, outer region curve’s hydraulic slope depends on geological conditions, it can be
determined by test or recent experience. When the pumping height is relatively high, it can be solved
by multi-series well point distribution, and during construction period well points must operate
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continuously to stabilize water lowering curve by dynamic equilibrium.
When laying well point pipes within well point hole, besides the filtering pipe surrounding must be
buried into sand filter, it is necessary to continuously fill up completely the hole with sand to form sand
well. In this way, in one hand it can ensure the atmospheric pressure to pass through the sand well and
directly act on the water level in the well, causing the sand well to be able to absorb water streaming
towards the well point from each earth layer within the range of the length of the well pipe. On the
other hand, the sand in the completely filled up sand well can prevent the shrinking hole when the well
point starts pumping water and when removing the well point, reducing the unfavorable influence on the
surrounding buildings.
Because after underground water pass through the filter pipe and enter the vertical pipe, it is forced
to move under pressure difference, as long as there is no air leakage for all the pipes in the well point
system, it is not necessary to connect the pump at the center of the main pipe and lay the main pipe at a
certain slope. Relatively thin main pipe is not only easier to move and install, but also make the
pressure difference that act between the top and bottom of vertical pipe for each well point to be
essentially the same, so that the survival rate of the well point is very high which can almost reach 100%.
For the mud and sand that is brought into the pipes during the first day after the well point starts
pumping water, because the well point pipe is relatively thin, and the velocity is relatively large, it won’t
deposit in the pipes.
The effectiveness of well point water lowering is not to measure how much water is pumped out,
because this depends on the local hydrology and geological compensating water amount, but to measure
whether the underground water level is reduced to the design requirement.
4.2 Deep well water lowering
With the increase of the depth of deep foundation pit excavation, the water pressure of the confined
water to the upper confining bed increases gradually, but the thickness of the upper confining bed in the
pit bottom will then become thinner, and the self weight pressure of the earth body will decrease
gradually, when the water pressure of the confined water surpasses the self weight pressure of the upper
confining bed, or if the upper confining bed is dug through, then the water output and sand output will
be generated, leading to the underground water damage.
In order to reduce the water pressure of confined water, before excavation, it is usually to dig
several deep wells within the range of foundation pit, install deep well pumps inside the wells, reducing
the pressure through water pumping by the deep well pumps, so that the water lowering curve can
satisfy the requirement of the digging depth. Deep well points can reduce water level by over 15m, but
before digging it is necessary to do water pumping test, to control the sand content in the pumped
underground water strictly, in order to prevent the unfavorable influence on the surrounding
environment due to the massive amount of water pumping in the foundation construction process.
4.3 Set up waterproof curtain
Besides purely considering the water level lowering method for underground water control, it is
also necessary to consider waterproof method. Purely waterproof method is not only costly but also is
hard to guarantee its effectiveness; purely water level lowering on the other hand is hard to satisfy
environmental protection requirement. Practice shows that, adopting the combination of waterproof
and water level lowering methods can avoids the shortcomings of both methods to the maximum extent
and achieve a relatively ideal technological and economical result. The frequently used waterproof
curtain are as following:
4.3.1 Deep mixing pile
Deep mixing pile is to adopt the construction method that make the piles lap one another, after the
mixture of cement and earth, chemical reaction occurs, the mixture’s diffuse coefficient is less than
10-7cm/s forming a continuous waterproof wall, which can be distributed behind the steel sheet pile, or
can be directly used as support structure for screening water from the wall.
4.3.2
Cement mortar water-tight screen
First put an array of H-section steel with pipe of grouting into the position where waterproof
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curtain is needed, then pull out the steel pile and pour in cement mortar, to form a rim of cement mortar
waterproof curtain. It is suggested to use 20-30# H-section steel, the key for construction quality is to
ensure the verticality of the H-section steel and the compact contact between the piles.
4.3.3 Root pile waterproof curtain
Adopt root pile of diameter Φ100 Φ200, do not use steel, pressure pure cement into hole, which
can then form a waterproof curtain. In construction, it is ok to adopt a common geological drilling rig,
drive piles at a distance to prevent perforation, the key for construction quality is to ensure the good
verticality of the pile, it is not acceptable to have cave-in and necking down etc. phenomena, when
necessary case pipe can be put in, and pull out the case pipe and pour in cement at the same time.
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5 The environmental impact of water lowering and the relevant prevention
methods
When water pumping starts after the well pipes have been laid out, the water level in the well will
decrease, the water in the surrounding water containing layer continuously flow toward the filter pipe,
after some time, a cone shaped curved water surface will form around the well point, this cone shaped
water surface will stabilize gradually, usually this will take a few days. After the underground water
level within this water level reducing cone decreases, it will lead to the drop of the surrounding earth
surface. This drop is usually uneven. In practice, due to the bad quality of filter net and sand filter
layer, it is very common that clay and sand in the earth layer can be brought into the earth surface, this
phenomena can cause the surrounding earth surface to drop unevenly, which lead to various degrees of
damage to the buildings above the ground and the underground pipes. For this phenomenon, its
influence can be reduced to minimum by raising the effectiveness of the water lowering method.
5.1 Estimation of well point water lowering influence range and drop
In order to estimate well point water lowering method’s influence on surrounding environment and
the resulting earth surface drop, it can be done through similar construction project’s result, it can also
be done through some simple estimation methods.
5.1.1 Estimation of water lowering to environmental influence range
The radius of water lowering cone is usually estimated by the following formula
R = 2S Kh0
where
R --- radius of water lowering cone(m);
S ---depth of water lowering cone(m);
K ---earth layer diffuse coefficient (m/day);
h0 ---initial underground water depth (m);
Because the earth body is usually distributed in layers, the influence range is strongly influenced by
earth layers, it is necessary to adopt water pumping test to determine the influence radius of water
lowering method before hand for important construction project.
5.1.2 Estimation of earth surface drop due to water lowering method
Under the condition that no large amount of fine grains be brought away along with the
underground water in the well point water lowering method, the drop of the surrounding earth surface
can be computed through layers summation method:
n
αi
S =∑
∆pi ∆hi
1
+
eoi
i =1
where
S ---the ultimate drop of the earth surface (cm)
α i ---the compression coefficient of the ith earth layer（KPa-1）
eoi ---the initial void factor of the ith earth layer
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（ ）
∆pi ---the additional stress due to water lowering for the ith earth layer KPa
∆hi ---the thickness of the ith earth layer (cm)
n ---the number of earth layers within the compression layer range
During the water lowering process, the earth layers below the water lowering surface can not form
rather apparent amount of consolidation drop, but the earth layers between the water lowering surface
and the original underground water surface, due to good water expulsion condition, will rapidly drop
because of their increased self weight stress, the earth surface drop due to water lowering is mainly due
to this process.
5.2 Methods to prevent the unfavorable environment influence due to well point water lowering
5.2.1 Carefully study the surrounding environment before implementing well point water lowering
Before implementing well point water lowering, it is necessary to have complete geological survey
document, including earth layer distribution, variation of underground water, diffuse coefficients of
earth layers, earth body void factor and compression coefficients etc.. At the same time, it is also
necessary to know the water supply and drainage pipes, natural gas pipes, electricity, telephone and
cable etc. pipes’ distribution; to know the surrounding above ground buildings and underground
buildings, including the foundation pattern of these buildings, upper level structure patterns and their
resistance to differential earth surface drop etc.. And it is also necessary to consider whether
preventive solidification measures are needed.
5.2.2 Use well point water lowering appropriately, reduce its influence on the surrounding environment
to the most extent
In the water level reducing process it is always necessary to see whether the pumped out
underground water is cloudy or not. If the pumped out water contains fine grains, it will not only
increase the drop of the surrounding but also cause the congestion of the well pipes, leading to the
failure of the well point water lowering. So firstly it is necessary to choose appropriate filtering net
according to the surrounding earth layer condition, and at the same time put emphasis on the quality of
refilling sand filter.
If the water lowering depth is the same, the smoother the slope of the water lowering cone is, the
larger water lowering method’s influence range is, the less uneven the resulting earth surface drop is, the
less damage the water lowering method has on the buildings and underground pipes within the influence
range.
For earth layer of sand quality, the drop due to water level reducing is rather small, but if
implementing water lowering intermittently and repeatedly, on site and laboratory tests all show that the
earth surface will drop for each water lowering process. The depth of the drop for each water lowering
process decreases with the increasing number of the repetition, and approaches zero gradually, but the
overall drop can accumulate to a relatively high extent. It is therefore recommended to avoid pumping
water repeatedly.
6 Conclusion
In deep foundation pit excavation process, underground water control is a problem that all must
face. Through well point water lowering, setting up waterproof curtain etc. methods, the water damage
problem can be handled rather well. Because these methods are to create working conditions for the
foundation construction, as long as the construction conditions are satisfied, it is for the best interest to
consider them economically. At present the most popular method is half screening half lowering,
which proves to be rather ideal by practice. But at present there is not a mature computation method
for seep analysis under the condition of three dimensional uneven earth quality, it is not possible to
reliably forecast the influence on the environment due to water lowering, so that project design and
construction still lack more accurate standard. These are the problems that require further research and
analysis.
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References:
［1］Gu Xiaolu.Qian Hongjin.Liu Huisan.Wang Shimin,Ground and Foundation,China Architecture and
Building Press,1995:62～65
［2］Jiang Jinbo.Zao Zhijin,Building Construction,Tongji University Press,1993:27～35
［3］Huang Qiang,Design Method of Deep Foundation Pit Support,China Building Material Industry
Publishing House,1995:168～177
［4］Huang Xilin,Underground Structure and Foundation Pit Support of Tall Building,China Astronautic
Publishing House,1994:179～182
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