CN110387884B - Construction method for preventing inrush of ultra-deep foundation pit - Google Patents

Construction method for preventing inrush of ultra-deep foundation pit Download PDF

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CN110387884B
CN110387884B CN201910526968.0A CN201910526968A CN110387884B CN 110387884 B CN110387884 B CN 110387884B CN 201910526968 A CN201910526968 A CN 201910526968A CN 110387884 B CN110387884 B CN 110387884B
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reinforcing layer
foundation pit
water
layer
underground continuous
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CN110387884A (en
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潘伟强
曹骥
王建秀
龙冠宏
朱雁飞
裴烈烽
陈思慧
王彦杰
诸颖
魏林春
李�昊
严振祥
陈怡�
郭彦
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Shanghai Tunnel Engineering Co Ltd
Shanghai Tunnel Engineering and Rail Transit Design and Research Institute
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    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
    • E02D17/00—Excavations; Bordering of excavations; Making embankments
    • E02D17/02—Foundation pits
    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
    • E02D17/00—Excavations; Bordering of excavations; Making embankments
    • E02D17/02—Foundation pits
    • E02D17/04—Bordering surfacing or stiffening the sides of foundation pits
    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
    • E02D19/00—Keeping dry foundation sites or other areas in the ground
    • E02D19/06—Restraining of underground water
    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
    • E02D19/00—Keeping dry foundation sites or other areas in the ground
    • E02D19/06—Restraining of underground water
    • E02D19/10—Restraining of underground water by lowering level of ground water
    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
    • E02D19/00—Keeping dry foundation sites or other areas in the ground
    • E02D19/06—Restraining of underground water
    • E02D19/12—Restraining of underground water by damming or interrupting the passage of underground water
    • E02D19/18—Restraining of underground water by damming or interrupting the passage of underground water by making use of sealing aprons, e.g. diaphragms made from bituminous or clay material

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  • Life Sciences & Earth Sciences (AREA)
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  • General Engineering & Computer Science (AREA)
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  • Environmental & Geological Engineering (AREA)
  • Investigation Of Foundation Soil And Reinforcement Of Foundation Soil By Compacting Or Drainage (AREA)
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Abstract

The invention relates to a construction method for preventing a surge of an ultra-deep foundation pit, wherein the bottom of the foundation pit to be excavated is close to a pressure-bearing water layer, and the construction method comprises the following steps: arranging an underground continuous wall around an area of a foundation pit to be excavated; pouring a waterproof reinforcing layer at the bottom of the underground continuous wall by using MJS, wherein the periphery of the waterproof reinforcing layer is attached to the underground continuous wall; pouring a depressurization reinforcing layer at the bottom of a foundation pit to be excavated above the waterproof reinforcing layer by using MJS, wherein the periphery of the depressurization reinforcing layer is attached to the underground continuous wall; and excavating a foundation pit and correspondingly erecting a supporting structure. The method effectively solves the problem of potential surge of confined water during the excavation of the ultra-deep foundation pit, has small influence on the surrounding environment during construction, cannot cause the settlement of surrounding buildings, and can ensure the construction quality and the construction efficiency.

Description

Construction method for preventing inrush of ultra-deep foundation pit
Technical Field
The invention relates to the field of building construction, in particular to a construction method for preventing inrush of an ultra-deep foundation pit.
Background
Along with the rapid development of cities, more and more urban underground space resources are developed and utilized to meet the living needs of human beings, particularly, underground rail transit is being built in various big cities as if so, however, when ultra-deep foundation pits need to be excavated and buildings with extremely high settlement control requirements are arranged around the ultra-deep foundation pits, the process operations which can cause serious deformation of the surrounding environment of the foundation pits, such as confined water precipitation, high-pressure jet grouting reinforcement and the like, cannot be carried out, however, the maximum excavation depth of the foundation pits reaches below the confined water layer, the safety coefficient of anti-surging of the bottom of the foundation pit is very low on the premise of not adopting confined water control, the excavation cannot be carried out, and great potential safety hazards exist, so that the construction is influenced.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, provides a construction method for preventing the inrush of the ultra-deep foundation pit, solves the problem of the hidden danger of confined water inrush during the excavation of the ultra-deep foundation pit, has small influence on the surrounding environment during construction, cannot cause the settlement of surrounding buildings, and can ensure the construction quality and the construction efficiency.
The technical scheme for realizing the purpose is as follows:
the invention provides a construction method for preventing a surge of an ultra-deep foundation pit, wherein the bottom of the foundation pit to be excavated is close to a pressure-bearing water layer, and the construction method comprises the following steps:
s11, arranging an underground continuous wall around an area of a foundation pit to be excavated;
s12, pouring a waterproof reinforcing layer at the bottom of the underground continuous wall by using the MJS, wherein the periphery of the waterproof reinforcing layer is attached to the underground continuous wall;
s13, pouring a depressurization reinforcing layer at the bottom of the foundation pit to be excavated above the waterproof reinforcing layer by using MJS, wherein the periphery of the depressurization reinforcing layer is attached to the underground continuous wall;
s14, excavating a foundation pit and correspondingly erecting a supporting structure.
The invention adopts the construction method for preventing the inrush of the ultra-deep foundation pit to construct the ultra-deep foundation pit, and the underground continuous wall is arranged in the area to be excavated, and a pressure reduction reinforcing layer and a water-proof reinforcing layer are respectively arranged between the underground continuous walls and at the position close to the bottom of the foundation pit and the position at the bottom of the underground continuous walls, so that the pressure reduction reinforcing layer can offset the bearing force of the confined water to increase gravity to prevent the confined water from gushing suddenly, the waterproof reinforcing layer can form a waterproof curtain at the bottom of the underground continuous wall to form second protection, the water quantity between the waterproof reinforcing layer and the pressure reduction reinforcing layer can be controlled, further excavating the foundation pit to avoid confined water inrush in the excavation process, effectively solving the problem of confined water inrush hidden danger in the ultra-deep foundation pit excavation, and the influence on the surrounding environment is small during construction, the settlement of surrounding buildings cannot be caused, and the construction quality and the construction efficiency can be ensured.
The construction method for preventing the inrush current of the ultra-deep foundation pit is further improved in that when the waterproof reinforcing layer is poured, the construction method further comprises the following steps:
and pouring the bottom of the waterproof reinforcing layer to a position below the bottom of the underground continuous wall, wherein the part of the waterproof reinforcing layer lower than the underground continuous wall supports the bottom of the underground continuous wall.
The invention further improves the construction method for preventing the inrush of the ultra-deep foundation pit, and before forming the waterproof reinforcing layer by using MJS, the construction method also comprises the following steps:
calculating the bottom elevation of the waterproof reinforcing layer according to the set depth of the foundation pit to be excavated and the elevation position of the pressure-bearing water layer, wherein the calculation formula is as follows:
Σ h.gamma.s ≧ Fs.gamma.w.H, in which
h is the thickness (m) of each layer of soil between the bottom of the foundation pit and the top surface of the confined water layer,
gamma s from the bottom of the foundation pit to the top of the confined aquiferAverage weight of soil in each layer (kN/m)3)，
H is the height (m) of the confined water head above the top surface of the confined aquifer,
gamma.w is the water gravity (kN/m)3) Taking 10kN/m3，
And Fs is the anti-surge safety coefficient of the foundation pit, and is 1.00.
The construction method for preventing the inrush of the ultra-deep foundation pit is further improved in that when the bottom elevation of the waterproof reinforcing layer is calculated, the construction method further comprises the following steps:
the thickness of the decompression reinforcing layer is 3m, and the thickness of the waterproof reinforcing layer is 10 m.
The construction method for preventing the inrush of the ultra-deep foundation pit is further improved in that when the bottom elevation of the waterproof reinforcing layer is calculated, the construction method further comprises the following steps:
calculating the water jacking force of the pressure-bearing water layer,
Pw＝(-H3+H4)*γw*Fs＝(-H3+H4)*10.0*1.0；
calculating the soil layer pressure between the bottom of the foundation pit and the top surface of the confined water layer,
Ps＝∑hi×γsi＝(-H2+H1)*γs1+(H2-H3)*γs2；
wherein H1 is the elevation of the bottom of the foundation pit,
h2 is the bottom elevation of the reduced pressure reinforcement layer,
h3 is the bottom elevation of the water-blocking reinforcing layer,
h4 is the initial water level elevation of the pressure water layer,
gammas 1 is the weight of the reduced-pressure reinforcement layer,
γ s2 is the gravity of the soil layer.
The construction method for preventing the inrush of the ultra-deep foundation pit is further improved in that gamma s in a pressure calculation formula from the bottom of the foundation pit to the top surface of the pressure-bearing water layer1Take 20kN/m3，γs2Take 19.4kN/m3；
Expanding to obtain Ps 3 × 20+ (H2-H3) × 19.4;
and 3 × 20+ (H2-H3) × 19.4 ≧ (-H3+ H4) × 10.0 × 1.0 is used for obtaining the value range of H3 so as to determine the bottom elevation of the water-resisting reinforcing layer.
The construction method for preventing the inrush of the ultra-deep foundation pit is further improved in that before the foundation pit is excavated, the construction method further comprises the following steps:
and (3) drilling a dewatering well leading to the space between the depressurization reinforcing layer and the water-resisting reinforcing layer, extracting the confined water between the depressurization reinforcing layer and the water-resisting reinforcing layer by using the dewatering well, and observing the change condition of the water level of the confined water between the depressurization reinforcing layer and the water-resisting reinforcing layer, thereby inspecting the water-resisting effect of the depressurization reinforcing layer and the water-resisting reinforcing layer.
The construction method for preventing the inrush of the ultra-deep foundation pit is further improved in that when the foundation pit is provided with the supporting structure, the method further comprises the following steps:
and sequentially erecting inner supports inside the foundation pit along with the excavation depth of the foundation pit so as to reinforce two side walls of the foundation pit.
The construction method for preventing the inrush of the ultra-deep foundation pit is further improved in that the MJS is an omnibearing high-pressure jet grouting method.
Drawings
FIG. 1 is a flow chart of the construction method for preventing the inrush current in the ultra-deep foundation pit according to the invention.
FIG. 2 is a schematic view of foundation pit reinforcement in the construction method for preventing the inrush current in the ultra-deep foundation pit according to the present invention.
FIG. 3 is a schematic diagram of the foundation pit position corresponding to the confined water layer in the construction method for preventing the inrush of the ultra-deep foundation pit according to the present invention.
Detailed Description
The invention is further described with reference to the following figures and specific examples.
Referring to fig. 1, the present invention provides a construction method for preventing a gush of an ultra-deep foundation pit, by installing an underground diaphragm wall in an area to be excavated, and a pressure reduction reinforcing layer and a water-proof reinforcing layer are respectively arranged between the underground continuous walls and at the position close to the bottom of the foundation pit and the position at the bottom of the underground continuous walls, so that the pressure reduction reinforcing layer can offset the bearing force of the confined water to increase gravity to prevent the confined water from gushing suddenly, the waterproof reinforcing layer can form a waterproof curtain at the bottom of the underground continuous wall to form second protection, the water quantity between the waterproof reinforcing layer and the pressure reduction reinforcing layer can be controlled, further excavating the foundation pit to avoid confined water inrush in the excavation process, effectively solving the problem of confined water inrush hidden danger in the ultra-deep foundation pit excavation, and the influence on the surrounding environment is small during construction, the settlement of surrounding buildings cannot be caused, and the construction quality and the construction efficiency can be ensured. The construction method for preventing the inrush current of the ultra-deep foundation pit according to the present invention will be described with reference to the accompanying drawings.
Referring to fig. 1, fig. 1 is a flowchart of a construction method for preventing a sudden inrush of an ultra-deep foundation pit according to the present invention. The construction method for preventing the inrush current in the ultra-deep foundation pit according to the present invention will be described with reference to fig. 1.
As shown in fig. 1, 2 and 3, the construction method for preventing the inrush of the ultra-deep foundation pit of the present invention, the bottom of the foundation pit 11 to be excavated is close to the confined water layer 21, and the construction method includes the following steps:
s11, arranging an underground continuous wall 112 around the area of the foundation pit 11 to be excavated; then, step S12 is executed
Step S12, pouring a waterproof reinforcing layer 13 at the bottom position of the underground continuous wall 112 by using MJS, and attaching the periphery of the waterproof reinforcing layer 13 to the underground continuous wall 112; then, step S13 is executed
Step S13, utilizing MJS to pour a pressure reduction reinforcing layer 12 at the bottom of the foundation pit 11 to be excavated above the waterproof reinforcing layer 13, and the periphery of the pressure reduction reinforcing layer 12 is attached to the underground continuous wall 112; then, step S14 is executed
And S14, excavating the foundation pit 11 and correspondingly erecting a supporting structure.
The waterproof reinforcing layer 13 can block the bottom of the underground continuous wall 112 to prevent pressure-bearing water from entering a space between the waterproof reinforcing layer 13 and the underground continuous wall 112, the content and pressure of the pressure-bearing water in the space can be better controlled to prevent the pressure-bearing water from excessively flowing into the space between the underground continuous walls 112, the depressurization reinforcing layer 12 is poured at the bottom of the foundation pit 11 to be excavated, the pressure above the pressure-bearing water is increased by the depressurization reinforcing layer 12 to compensate the pressure of the excavated soil of the foundation pit 11 after excavation, and the pressure-bearing water can be pressed to prevent the pressure-bearing water from suddenly flowing to break the bottom of the foundation pit 11; the soil layer is located between the waterproof reinforcing layer 13 and the depressurization reinforcing layer 12, the surrounding of the underground continuous wall 112, the waterproof reinforcing layer 13 and the depressurization reinforcing layer 12 prevents the communication between the soil layer and the pressure-bearing water layer between the waterproof reinforcing layer 13 and the depressurization reinforcing layer 12, and avoids the situation that the groundwater in the pressure-bearing water layer counts into the soil layer and the pressure-bearing water layer between the waterproof reinforcing layer 13 and the depressurization reinforcing layer 12, so that the water content in the soil layer between the waterproof reinforcing layer 13 and the depressurization reinforcing layer 12 is ensured to be constant, the pressure of the depressurization reinforcing layer 12 is lower, the linkage effect of the waterproof reinforcing layer 13 and the depressurization reinforcing layer 12 is realized, the pressure of the pressure-bearing water is controlled more accurately, the bottom of the foundation pit 11 is ensured not to gush suddenly, and the stability of the main structure can be ensured.
Specifically, when pouring water proof back up coat 13, still include: the bottom of the waterproof reinforcing layer 13 is poured to a position below the underground continuous wall 112, and the part of the waterproof reinforcing layer 13 lower than the underground continuous wall 112 supports the bottom of the underground continuous wall 112, so that the bottom of the underground continuous wall 112 can be blocked, and the confined water is prevented from flowing between the depressurization reinforcing layer 12 and the waterproof reinforcing layer 13 to a large extent.
Preferably, the MJS is an all-around high-pressure jet grouting method, and the method has small influence on surrounding main body structures and can effectively prevent ground settlement.
As a preferred embodiment of the present invention, before forming the water-blocking reinforcing layer 13 using MJS, the method further includes:
calculating the bottom elevation of the waterproof reinforcing layer 13 according to the set depth of the foundation pit 11 to be excavated and the elevation position of the pressure-bearing water layer 21, wherein the calculation formula is as follows:
Σ h.gamma.s ≧ Fs.gamma.w.H, in which
h is the thickness (m) of each layer of soil between the bottom of the foundation pit and the top surface of the confined water layer,
gamma s is the average gravity (kN/m) of each layer of soil between the bottom of the foundation pit and the top surface of the confined aquifer3)，
H is the height (m) of the confined water head above the top surface of the confined aquifer,
gamma.w is the water gravity (kN/m)3) Taking 10kN/m3，
And Fs is the anti-surge safety coefficient of the foundation pit, and is 1.00.
Preferably, when calculating the bottom elevation of the water-resisting reinforcing layer 13, the method further includes:
the thickness of the decompression reinforcing layer is 3m, and the thickness of the waterproof reinforcing layer is 10 m.
Further, when calculating the bottom elevation of the water-resisting reinforcing layer 13, the method further includes:
calculating the water jacking force of the pressure-bearing water layer 21,
Pw＝(-H3+H4)*γw*Fs＝(-H3+H4)*10.0*1.0；
calculating the soil layer pressure between the bottom of the foundation pit 11 and the top surface of the confined water layer 21,
Ps＝∑hi×γsi＝(-H2+H1)*γs1+(H2-H3)*γs2；
wherein H1 is the elevation of the bottom of the foundation pit,
h2 is the bottom elevation of the reduced pressure reinforcement layer,
h3 is the bottom elevation of the water-blocking reinforcing layer,
h4 is the initial water level elevation of the pressure water layer,
gammas 1 is the weight of the reduced-pressure reinforcement layer,
γ s2 is the gravity of the soil layer.
Specifically, gamma s in the pressure calculation formula from the bottom of the foundation pit to the top surface of the confined water layer1Take 20kN/m3，γs2Take 19.4kN/m3And (-H2+ H1) the thickness of the decompression reinforcing layer is 3 m;
expanding to obtain Ps 3 × 20+ (H2-H3) × 19.4;
3X 20+ (H2-H3) 19.4 ≧ -H3+ H4) 10.0X 1.0,
the H2 is known data, H1+3 is known data, H4 is also known data, a value range of H3 can be obtained through an inequality, and a proper numerical value is selected to determine the bottom elevation of the waterproof reinforcing layer.
Further, before excavating foundation ditch 11, still include:
and (2) constructing a dewatering well leading to the space between the depressurization reinforcing layer 12 and the water-resisting reinforcing layer 13, extracting the confined water between the depressurization reinforcing layer 12 and the water-resisting reinforcing layer 13 by using the dewatering well, and observing the change condition of the water level of the confined water between the depressurization reinforcing layer 12 and the water-resisting reinforcing layer 13, so that the water-resisting effect of the depressurization reinforcing layer 12 and the water-resisting reinforcing layer 13 is tested, the construction safety is further ensured, and the condition that the settlement threat to the peripheral buildings cannot be caused can be ensured.
Further, when erecting bearing structure to foundation ditch 11, still include:
multiple horizontal inner supports 111 are sequentially erected inside the foundation pit 11 along with the excavation depth of the foundation pit 11 to reinforce two side walls of the foundation pit 11, so that influence on surrounding buildings due to collapse inside the foundation pit 11 is prevented.
The specific embodiment of the invention is as follows:
taking the barrier removing wells on two sides of a rail transit station as an example, a foundation pit to be constructed is a barrier removing well, the peripheral building of the foundation pit is the station, so that the requirement on ground settlement is extremely high, the settlement range is not more than 2mm and not more than 5mm, the elevation of the top surface of a pressure-bearing water layer of a construction area is-23.92 to-26.31 m, the deepest excavation depth of the foundation pit reaches 23.6m and is very close to the pressure-bearing water layer, and according to field geological exploration data and water exploration feedback, the pressure-bearing water lowering depth needs to reach 13.6m to meet the safety requirement of foundation pit excavation, and four foundation pits need to be excavated in the project;
calculating the bottom elevation of the waterproof reinforcing layer according to the condition that the sigma H & gamma s is more than or equal to Fs & gamma w & H, wherein the thickness of the waterproof reinforcing layer is 10m, the thickness of the pressure reduction reinforcing layer is 3m, and the influence of the confined water in the first confined water layer 211, the second confined water layer 212 and the partition layer 213 in the confined water layer 21 is considered simultaneously during calculation;
for the second confined water layer 212, the water pressure is Pw (-H3+ H4) × 10.0 × 1.0, where H4 has an actual value of-1.50 m, so Pw (-H3-1.50) × 10.0 × 1.0 and the soil pressure is Ps ∑ Hi×γsiBased on actual H1, H2-H1 +3 and the site ground level of 3.79m, H3 and the actual installed depth are calculated as 3 × 20+ (H2-H3) × 19.4, in meters as shown in the following table:
Figure GDA0003208379930000061
Figure GDA0003208379930000071
aiming at the first pressure-bearing water layer 211, the elevation of the first pressure-bearing water layer 211 is-24.58 m, and the water pressure is
Pw is (24.58-1.50). times.10.0. times.1.0 is 230.80kPa, and the soil pressure is shown in the above table
1#：Ps＝∑hi×γsi＝3*20+(24.58-22.047)*19.4＝109.14kPa，
2#：Ps＝∑hi×γsi＝3*20+(24.58-22.363)*19.4＝103.01kPa，
3#：Ps＝∑hi×γsi＝3*20+(24.58-22.271)*19.4＝104.79kPa，
4#：Ps＝∑hi×γsi＝3*20+(24.58-22.096)*19.4＝108.19kPa，
The depth of the water level of the confined water layer needing to be reduced is obtained through calculation, and is shown in the following table:
Figure GDA0003208379930000072
according to the data, the absolute elevation of the top surface of the confined water in the confined water layer is determined to be-36 m to-37 m and then reach a balance point, and the elevation of the ground on site is +3.79m, so that the depth of the bottom of the waterproof reinforcing layer needs to reach 40.384m to 41.016m, namely about 2m to 3m below the underground continuous wall;
the bottom of the underground continuous wall 112 is positioned above the bottom of the second confined water layer 212, so that the confined water layer 21 cannot be completely cut off, the water-proof reinforcing layer 13 covers the bottom of the underground continuous wall 112 to seal the bottom of the underground continuous wall 112, so that confined water cannot permeate from the lower part of the underground continuous wall 112, according to the calculated data, the top of the water-proof reinforcing layer 13 is arranged at a position 4m above the bottom of the underground continuous wall 112, the bottom of the water-proof reinforcing layer 13 is arranged at a position 1-4m below the bottom of the underground continuous wall 112, the depressurization reinforcing layer 12 is poured to a position 3m below the bottom of the foundation pit 11 from the bottom of the foundation pit 11 to be excavated, and the depressurization reinforcing layer 12 and the water-proof reinforcing layer 13 are sprayed by adopting a phi 2000mmMJS construction method;
constructing a precipitation well in the range of an interlayer formed by the depressurization reinforcing layer 12 and the water-resisting reinforcing layer 13 to verify the confined water-resisting effect, respectively arranging a precipitation well at two angular points of each foundation pit 11, and verifying the water-resisting effect of the water-resisting reinforcing layer through water pumping quantity;
the water suction pump is utilized to pump water and observe the height of the water surface of the confined water, and the water level observation time interval is as follows: 1 ', 5', 10 ', 15', 20 ', 25', 30 ', 40', 50 ', 60', 90 'and 120', observing once every 30min later, observing once every 60min after 480 ', observing once every 2h after 1200', stopping pumping water after the water level is stable, observing and recovering the water level after stopping, carrying out the same pumping test at time intervals, detecting the high precision and the high density of the water level by adopting an automatic water level collector, and adjusting in time when the flow change observed twice before and after and exceeds +/-5%;
after the surface of the confined water reaches the design requirement, a foundation pit 11 is excavated, an inner support 111 is arranged inside the foundation pit 11, and the inner collapse of the foundation pit 11 is prevented from affecting the stability of surrounding buildings.
While the present invention has been described in detail and with reference to the embodiments thereof as illustrated in the accompanying drawings, it will be apparent to one skilled in the art that various changes and modifications can be made therein. Therefore, certain details of the embodiments are not to be interpreted as limiting, and the scope of the invention is to be determined by the appended claims.

Claims (7)

1. A construction method for preventing sudden surge of an ultra-deep foundation pit is characterized in that the bottom of the foundation pit to be excavated is close to a pressure-bearing water layer, and the construction method comprises the following steps:
s11, arranging an underground continuous wall around the area of the foundation pit to be excavated;
s12, pouring a waterproof reinforcing layer at the bottom of the underground continuous wall by using MJS, pouring the bottom of the waterproof reinforcing layer to a position below the bottom of the underground continuous wall, wherein the part of the waterproof reinforcing layer lower than the underground continuous wall supports the bottom of the underground continuous wall, so that confined water is prevented from entering a space between the waterproof reinforcing layer and the underground continuous wall, and the peripheral edge of the waterproof reinforcing layer is attached to the underground continuous wall;
s13, pouring a depressurization reinforcing layer at the bottom of the foundation pit to be excavated above the waterproof reinforcing layer by using MJS, attaching the periphery of the depressurization reinforcing layer to the underground continuous wall, drilling a dewatering well leading to the depressurization reinforcing layer and the waterproof reinforcing layer, extracting confined water between the depressurization reinforcing layer and the waterproof reinforcing layer by using the dewatering well, and observing the change condition of the water level of the confined water between the depressurization reinforcing layer and the waterproof reinforcing layer, thereby inspecting the waterproof effect of the depressurization reinforcing layer and the waterproof reinforcing layer;
s14, excavating the foundation pit and correspondingly erecting a supporting structure.
2. The construction method for preventing the inrush current in the ultra-deep foundation pit according to claim 1, wherein before the forming of the water-blocking reinforcing layer using the MJS, the construction method further comprises:
calculating the bottom elevation of the waterproof reinforcing layer according to the set depth of the foundation pit to be excavated and the elevation position of the pressure-bearing water layer, wherein the calculation formula is as follows:
Σ h.gamma.s ≧ Fs.gamma.w.H, in which
h is the thickness (m) of each layer of soil between the bottom of the foundation pit and the top surface of the confined water layer;
gamma s is the average gravity (kN/m) of each layer of soil between the bottom of the foundation pit and the top surface of the confined aquifer3)；
H is the height (m) of the confined water head above the top surface of the confined aquifer;
gamma.w is the water gravity (kN/m)3) Taking 10kN/m3；
And Fs is the anti-surge safety coefficient of the foundation pit, and is 1.00.
3. The construction method for preventing the inrush current in the ultra-deep foundation pit according to claim 2, wherein when calculating the bottom elevation of the water-blocking reinforcing layer, the method further comprises:
the thickness of setting step-down reinforcing layer is 3m, sets up the thickness of water proof reinforcing layer is 10 m.
4. The construction method for preventing the inrush current in the ultra-deep foundation pit according to claim 3, wherein when calculating the bottom elevation of the water-blocking reinforcing layer, the method further comprises:
calculating the water jacking force of the pressure-bearing water layer,
Pw＝(-H3+H4)*γw*Fs＝(-H3+H4)*10.0*1.0；
calculating the soil layer pressure between the bottom of the foundation pit and the top surface of the confined water layer,
Ps＝∑hi×γsi＝(-H2+H1)*γs1+(H2-H3)*γs2；
wherein H1 is foundation pit bottom elevation, H2 is depressurization reinforcing layer's bottom elevation, H3 is water proof reinforcing layer's bottom elevation, H4 is confined water layer initial water level elevation, gammas1For reducing the severity of the reinforcing layer,. gamma.s2Is the heavy soil layer.
5. The method as claimed in claim 4, wherein γ s is calculated by pressure between the bottom of the pit and the top of the pressurized water layer1Take 20kN/m3，γs2Take 19.4kN/m3；
Expanding to obtain Ps 3 × 20+ (H2-H3) × 19.4;
and 3 × 20+ (H2-H3) × 19.4 ≧ (-H3+ H4) × 10.0 × 1.0 is used for obtaining the value range of H3 so as to determine the bottom elevation of the water-resisting reinforcing layer.
6. The construction method for preventing the inrush current in the ultra-deep foundation pit according to claim 1, further comprising, when the supporting structure is erected for the foundation pit:
and sequentially erecting inner supports inside the foundation pit along with the excavation depth of the foundation pit so as to reinforce two side walls of the foundation pit.
7. The construction method for preventing the inrush of the ultra-deep foundation pit as set forth in claim 1, wherein the MJS is an all-around high-pressure jet grouting method.
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