Keywords

1 Introduction

Waterproof curtain is the key process for the success of deep foundation pit support scheme. Triaxial mixing pile is a common waterproof curtain method because of its ease to construct and good integration. However, due to the complex and changeable geological and hydrological conditions, the waterproof curtain often fails, resulting in leakage of the foundation pit. Leakage is one of the biggest construction risks of deep foundation pit. It seriously affects the safety of the foundation pit and construction quality [1].

Triaxial mixing piles were used for the waterproof curtain of a deep foundation pit. When the construction reached 1880 elevation, there were many water and sand gushing on the side wall of the foundation pit, causing local failure of the triaxial waterproof curtain of said foundation pit, resulting in large settlement, deformation and cracking of buildings and roads around it. It created imminent danger to personal and property safety. This research status summarizes the disease treatment scheme adopted by the foundation pit, which is intended to provide experience reference for similar projects.

The shortcomings of the research status is that the causes of foundation pit water leakage are not discussed. The reason is that the causes of foundation pit water leakage are complex, and no agreement has been reached in many rounds of expert argumentation.

2 Project Overview

The total land area of the project is about 7644.08 m2 (about 11.5 mu). Two 35-storey high-rise buildings with a height of 99.7 m and a shear wall structure were proposed. The total building area is about 73827 m2, and the underground area is about 17827 m2. The buildings had rotary pile foundations with three-story basements. The foundation pit has an excavation depth of 16.8–17.4 m and is rectangular in shape, approximately 340 m in perimeter, 99 m in length and 72 m in depth. The east side of the foundation pit is adjacent to Yongle Road, a municipal road with many pedestrian and vehicular traffic. There are municipal water supply, drainage pipelines, communication cables and power cables under the road. The south side of the foundation pit is adjacent to a six-story concrete residential building, and the excavation boundary of the foundation pit is 10.51 m away from the outer wall of the building. The west side of the foundation pit is adjacent to a 16-storey concrete commercial and residential building with a basement. The excavation sideline of the foundation pit is 8.97 m away from the outer wall of the commercial and residential building. The north side of the foundation pit is adjacent to Yunnan Baiyao Pharmaceutical Factory, which is home to many old brick-concrete buildings. The excavation boundary of the foundation pit is only 5.23 m away from the nearest external wall of those old buildings. The current situation around the foundation pit is complex, so the settlement and deformation of the foundation pit need to be strictly monitored and controlled.

3 Geological and Hydrological Profile

3.1 Geological Conditions

According to the survey report, the stratum of the proposed site is mainly composed of artificial fill layer and Quaternary alluvial-proluvial and alluvial-lacustrine stratum, and the stratum involved in foundation pit support from top to bottom is described as follows:

  • Miscellaneous fill: grayish white, gray, loose, slightly wet, mainly composed of gravel, broken bricks, concrete blocks, etc., with loose structure, low strength and uneven soil quality, distributed in the whole site;

  • Clay: brown yellow, locally mixed with brown gray and gray, hard plastic state, slightly wet, medium compressibility, general soil uniformity, distributed in the site;

  • Silty clay: gray, brown gray, locally brown yellow, gray with brown yellow, plastic, slightly wet, medium compressibility, medium dry strength and toughness, distributed in some sections of the site;

  • Silt: gray, light blue gray, locally gradually changed into silty sand, wet, slightly dense, medium compressibility, slight shaking response, distributed in the site;

  • Gravelly sand: light gray, light blue gray, local gray, purple gray, saturated, slightly dense–medium dense. It contains a small amount of round gravel, the content is generally 20%–40%, the particle size is 0.5–2.0 cm, the main components are moderately weathered sandstone and basalt, and most of them are round and sub-round; Calcareous cementation layer and thin layer of cohesive soil and silt are occasionally intercalated between layers; It is distributed in the site;

  • Silty clay: locally clay, light gray, purple gray, light blue gray, slightly wet, plastic state, medium toughness and dry strength, medium compressibility, general soil uniformity, distributed in some sections of the site;

  • Silt; Light gray, purple gray, light blue gray, wet, medium dense, low dry strength, slight shaking response, medium compressibility, distributed in some sections of the site;

  • Round gravel; Gray, light gray, brown gray, locally blue gray, saturated, moderately dense, containing a small amount of pebbles, the content of gravel and pebbles is uneven, generally 50%–60%, locally up to 70%, the particle size is 0.2–2.0 cm, locally 2.0–5.0 cm, the parent rock is composed of moderately weathered sandstone and basalt, filled with silt, locally filled with silty clay, mostly round and sub-round. Calcareous cementation layer is occasionally intercalated between layers and distributed in some sections of the site;

  • Clay: locally silty clay, gray and dark gray, locally purple gray and light gray, slightly wet, plastic, locally hard plastic, medium toughness and dry strength, medium compressibility, distributed in some sections of the site.

3.2 Hydrological Overview

During the survey, the depth of the mixed stable groundwater level in the site measured by drilling is 0.6–1.8 m, the elevation is 1887.07–1888.32 m, and the water level variation difference is 1.25 m. The groundwater type of the proposed site is mainly pore phreatic water and the miscellaneous fill distributed on the surface contains a small amount of perched water. Pore water mainly exists in , silt; gravel sand and round gravel. It is replenished by the infiltration of atmospheric precipitation and surface water.

There is pressured underground water on the site, and the top of the water table is about 6 m below ground surface. According to the results of the foundation pit pumping test, the inrush volume of the foundation pit is 1737.3 m3/d, and the underground water has a great influence on the construction of this foundation pit, so it is necessary to protect the foundation pit from underground water during the design and construction phase.

4 Foundation Pit Support Design

According to the engineering conditions, excavation depth and surrounding conditions of the foundation pit, it was determined that the safety level of the retaining structure of the foundation ditch is Level I, and the structural importance coefficient is 1.1.

Foundation pit support scheme: “triaxial support curtain + prestressed combined steel support + local concrete support”; Drainage and drainage scheme: Φ850 × 850 × 850 triaxial mixing piles form a curtain, and local triaxial mixing piles can not be applied to the ground.See Fig. 1 Foundation Pit Support Plan, Fig. 2 Support Layout Plan and Fig. 3 Drain off water and Fig. 4 Foundation Pit Support Profile for details of the scheme of “drainage of blind ditch at the bottom of the pit + drainage of intercepting ditch at the top of the pit + drainage of collecting well + monitoring and recharge of recharge well outside the pit”.

During the excavation of the foundation pit, a temporary collection well shall be created to allow water to be pumped and discharged. The bottom of the foundation pit shall be provided with a 300 × 300 mm blind drain along the excavation line of the foundation pit bottom; 800 × 800 × 800 mm dirt collection wells shall be set every 25 m, and 300 × 300 dirt intercepting ditches shall be set at the top of the foundation pit to guide the dirt on the ground to the periphery of the foundation pit, so as to ensure that no dirt accumulates inside the foundation pit during construction. The surrounding surface tritium can be discharged in time to minimize the amount of tritium in the seepage stratum. In the process of dewatering and drainage of the foundation pit, the level outside the pit shall be controlled at 2.0 m below the ground surface, and the level inside the pit shall be controlled at 0.55 m below the bottom of the foundation pit. 17 recharge wells with a depth of 26 m shall be arranged outside the pit, and one well shall be arranged at 20 m along the periphery of the excavation.

Fig. 1.
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Layout plan of foundation pit support

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Fig. 2.
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Support layout plan

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Fig. 3.
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Drain off water

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Fig. 4.
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Section of foundation pit support

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5 Leakage of Foundation Pit

During the excavation at 1880.00 m, a great amount of water and sand mixture gushed in through the side wall of the foundation pit, causing the local failure of the original triaxial waterproof curtain of the foundation pit, and resulting in large amount of settlement, deformation and cracking of buildings and roads around the foundation pit. Data collected through the monitoring program data reflected the process of foundation pit deformation, as shown in Figs. 5, 6, 7, and 8. At this stage, the cumulative value and rate of settlement and deformation of the foundation pit exceeded the warning value. Especially on April 14–15 and June 22–23, two major dangerous situations occurred. The horizontal displacement of some crown beams increased suddenly, the groundwater level dropped rapidly, and the surrounding buildings subsided obviously.

Fig. 5.
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Horizontal displacement of crown beam

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Fig. 6.
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Vertical displacement of crown beam

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Fig. 7.
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Change of groundwater in adjacent surroundings

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Fig. 8.
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Settlement of adjacent peripheral buildings

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6 Cause Analysis of Leakage Disease

After the occurrence of water and sand gushing in through the side wall of the foundation pit, well-known experts in the province were invited to analyze, survey and demonstrate many times. At first, there were different opinions on the causes of foundation pit leakage, and no consensus could be reached. Through half a year's observation and several rounds of thematic meetings for analysis, investigation and demonstration, we have gradually reached a consensus that the leakage of foundation pit was caused by the influence of groundwater runoff and the loss of some cement slurry during construction, which resulted in the local failure of the original triaxial waterproof curtain of foundation pit and leads to leakage.

7 Construction Measures for Treatment of Leakage Disease

Due to the serious leakage of water from the side wall of the foundation pit, the roads and buildings around the foundation pit obviously settled and cracked. Geophysical prospecting showed that the local geological structure around the foundation pit had changed, and a large cavity had appeared, which endangered the safety of the surrounding people and property. Therefore, it was urgent to control the damages. See Fig. 9. Cracking of roads and buildings.

Fig. 9.
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Cracking of roads and buildings

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After the demonstration of the special meeting, because the project support had been completed and the foundation pit backfilling conditions were not available, the treatment scheme of “external sealing and internal blocking, combination of dredging and blocking, local strengthening” has been adopted. After the implementation, the leakage was stopped in time, the deformation of the foundation pit was reduced, and the safety of the foundation pit was ensured. Specific measures are as follows:

7.1 Treatment of the Position Outside the Foundation Pit Corresponding to the Water Leakage Point

High-pressure rotary jet grouting is carried out on the outer side of the top beam of the foundation pit at the water leakage point between the corresponding supporting piles. A proper amount of water glass is added. The depth of high-pressure rotary jet grouting is 5 to 10 m deeper than the water leakage position. If the high-pressure jet grouting drilling encounters underground obstacles and cannot be lowered, the geological drilling rig shall be used to guide the hole before the high-pressure jet grouting construction, and the depth of the hole shall be determined according to the actual situation of the site.

7.2 Treatment of the Inner Side of the Foundation Pit Corresponding to the Leakage Point

A drainage tube is first made at a water leakage hole, two galvanized pipes are buried in advance, the water leakage hole is plugged by watertight, polyurethane is injected by a gear pump after the watertight strength of the plugging is reached, and finally the drainage tube is plugged to play a role of quickly stopping water and polyurethane foaming can also fill empty land, and then a double-liquid grout machine is adopted to spray cement slurry and wat glass aiming at that water leakage area, so that the cement slurry is quickly solidified, and then the wat leakage area is quickly subjected to mesh spraying construction [2].

If water gushing is accompanied by sand gushing, in order to avoid holes and ground subsidence, it would be necessary to block the water leakage port with cotton quilt to filter the sand and reduce the loss of sand, and use sandbags to backpressure the water leakage area in layers, with the backpressure height 2–5 m higher than the leakage point to the upper purlin.

If the water leakage of the foundation pit is accompanied by sand and holes occur, the outside of the foundation pit shall be filled with graded gravel while grouting, so as to reduce the settlement around the foundation pit caused by water leakage.

7.3 Treatment of Underground Cavity Location

Holes have appeared under the ground surface on the east and south sides of the foundation pit, and the buildings on the ground have been inclined. The first thing is to evacuate the people in the ground building immediately, close the area with a cordon, and prompt the passing people and vehicles to detour. Secondly, the soil shall be reinforced by densely distributed high-pressure jet grouting piles with a diameter of φ 500 @ 1000 mm and a depth of 26 m [3].

8 Construction Technology

8.1 Watertight and Polyurethane

Main construction parameters: the watertight adopts the quick-setting type; the polyurethane adopts a hydrophilic single-liquid PU polyurethane foaming agent.

Construction procedures: clean up the leakage point → bury the aqueduct → fill the water-tight plugging agent → inject the polyurethane foaming agent → plug the aqueduct → moisturize and cure → check and repair the leakage → hang the net and spray the surface.

  1. (1)

    Clean up the leakage point

    1. 1)

      When cleaning the leakage point, the soft soil, silt and loose sand around the leakage point must be completely cleaned up. The treated base surface shall be fully wet to saturation, but without visible water, and shall be firm, clean and flat. The uneven part can be leveled with water without leakage.

    2. 2)

      The whole process of cleaning the leakage point should be timely and rapid;

    3. 3)

      During cleaning, pay attention to the change of soil around the leakage point at all times, and take emergency measures.

  2. (2)

    Filling watertight

    1. 1)

      Watertight materials are easy to get damp and should be sealed and stored in a dry place before use;

    2. 2)

      When mixing watertight materials, special rubber gloves shall be worn to prevent skin burns;

    3. 3)

      The temperature of the water used for watertight mixing is generally controlled at 15–30 ℃. When the ambient temperature is lower than 5 ℃, nitrous acid accounting for 1–3% of the weight of the powder shall be added, and warm water (about 50 ℃) shall be used for mixing below 10 ℃;

    4. 4)

      The sequence of watertight filling should be from top to bottom, and it is not suitable to fill from the periphery to the center, otherwise the water plugging effect is not good, and the watertight material is wasted. The thickness of watertight filling should not be less than 10 cm;

    5. 5)

      During the use of watertight plugging, it is not allowed to add water twice;

    6. 6)

      When the construction environment is under the scorching sun, humidification and curing are required.

  3. (3)

    Embedding of aqueduct

    1. 1)

      The length of the aqueduct should not be too long, so as not to affect the construction of the next process such as earthwork excavation, and the length should be 0.6 m;

    2. (2)

      The number and diameter of buried aqueducts can be adjusted according to the amount of water leakage, generally 1–5 aqueducts are suitable, and the diameter is 0.15 m–0.3 m;

    3. 3)

      a ditch is dug around the water leakage point, and the water flowing out of the aqueduct is introduced into the ditch;

    4. 4)

      The accumulated water in the foundation pit shall be pumped away in time to reduce the impact of the accumulated water on the stability of the foundation pit.

  4. (4)

    Filling polyurethane foaming agent under high pressure

    1. 1)

      Before drilling with an electric drill, ensure that the watertight has been completely solidified (10 min after laying the watertight), drill holes on the watertight after final setting, and lock the water stop needle;

    2. 2)

      before that mix liquid is injected into the wall body through the wat stop needle, the power supply is started to extrude the residual diluent in the material placing barrel and the pipe until the mix liquid is sprayed out;

    3. 3)

      During the matching and spraying of polyurethane and polyurethane paint, operators must wear special gloves to avoid contact with human body;

    4. 4)

      Polyurethane plugging agent is a water-swellable material. Wear protective equipment such as gloves and sunglasses when working. If it splashes into the eyes, wash it with clean water immediately and then send it to the hospital.

  5. (5)

    Key points for operation of high pressure plugging and filling machine

    1. 1)

      Operate the high-pressure leak-stopping filling machine in strict accordance with the instructions;

    2. 2)

      Continuous injection of polyurethane should not exceed 0.5 h to prevent parts from being worn due to overheating of the fuselage;

    3. 3)

      When the injection of polyurethane is stopped for more than 30 min or the construction is completed, the machine shall be cleaned in time with diluent. The cleaning method is to pump all the slurry in the charging barrel and high-pressure pipe back to the container, and then pour 300 ml of diluent into the charging cup to push out the polyurethane in the high-pressure pipe. After the polyurethane is completely ejected, 300 ml of diluent is poured in, and the filling material is put into a material placing cylinder for circular cleaning; After cleaning for 2 to 3 min, the cleaning agent is pumped into the container, and then a proper amount of engine oil is poured into the container for circulation for maintenance and lubrication, and the cleaning is completed;

    4. 4)

      The filling machine shall be checked regularly before use (the gearbox shall be filled with grease regularly), and any abnormality shall be repaired immediately to prevent failure during construction.

8.2 Hanging Net and Spraying Wall

Construction procedures: earth excavation → manual cleaning → drilling and planting reinforcement on supporting pile → hanging of finished steel mesh → spraying fine aggregate concrete → concrete curing → fabrication and installation of reinforcement mesh → welding of reinforcing bar → spraying fine aggregate concrete → concrete curing.

  1. (1)

    Preparation before net hanging

    Remove the loose rocks, dangerous rocks and floating soil between the supporting piles, and fill the larger cracks and pits to make the slope smooth and tidy.

  2. (2)

    Fixing of reinforcement mesh

    Firstly, according to the construction drawings, drill holes (depth of 250 mm) at 1500 mm/1600 mm horizontally (corresponding to the spacing of the supporting piles in this section) and 1500 mm vertically on the supporting piles for rebar planting, and anchor C14 L-shaped mesh rebar with a total length of 800 mm;

    A 600 mm long C14 reinforcement nail is driven into the center of the soil between the supporting piles (the same as the horizontal position of the L-shaped mesh reinforcement) to fix the reinforcement mesh between the piles;

    The reinforcement mesh (two-way φ 6 @ 150 mm × 150 mm) is then fixed by welding (10 d on one side) the C14 transverse tie bars (i.e., stiffeners, @ 1500 mm/1600 mm × 1500 mm) to the L-shaped mesh reinforcement embedded in the pile, pressing the entire mesh.

  3. (3)

    Raw materials and mix proportion

    Raw materials: PO42.5 ordinary Portland cement; Silty sand shall be used, and the water content of the sand shall be 2%–3%; Adopt melon seed stone as that stone; Clean water is used for water; That mix amount of the accelerator is 5% of the cement amount; Watertight.

    Selection of C20 fine aggregate concrete mix proportion (mass ratio) (dry material): cement: sand: gravel: water: accelerator: watertight = 1: 1.6: 2.8: 0.55: 0.05: 0.1.

8.3 Double-Liquid Grouting

  1. 1)

    Grouting pipe embedding

    When the drainage pipe is buried at the leakage point, the galvanized pipe shall be buried. The buried depth of the galvanized pipe shall be as deep as possible according to the leakage situation of the leakage point. One end shall be sealed with adhesive tape to prevent silt from blocking the grouting pipe.

  2. 2)

    Grout configuration

    Before slurry preparation, field test shall be carried out according to cement slurry concentration, water glass parameters and water temperature. The initial setting time of the double-liquid slurry is usually 20 s to 40 s, and the ratio of the slurry before each grouting is determined (1:1 to 1:2).

  3. 3)

    Double-liquid grouting

    Before grouting, double pipes shall be injected with water to ensure that the embedded grouting pipe is unblocked. After confirming that the grouting pipe is unblocked, water glass and cement slurry shall be injected at the same time. At the beginning of grouting, the drainage pipe will flow out of the mixture of cement slurry and water glass. Continue grouting until the drainage pipe is blocked. At the same time, continue grouting. The amount of grouting shall be judged according to the time and size of water leakage.

9 Conclusion

  1. (1)

    The geological structure is local and the geological conditions are unevenly distributed, so sometimes the geological survey report cannot fully reflect the geological conditions. Before construction, the geological and hydrological conditions around the plot should be fully understood and studied [4].

  2. (2)

    The complexity of geological structure shall be fully considered in the design of waterproof curtain, the design scheme shall be put forward pertinently, the possible problems and remedial schemes shall be clarified in key links, and the feasibility of the design scheme shall be fully verified by relevant experiments before implementation.

  3. (3)

    In case of abnormal conditions during construction, analyze the causes in time to avoid blind construction, and fully verify the reliability of the previous process before special process replacement. Emergency plans shall be prepared before implementation, and solutions shall be put forward and implemented in time when unexpected situations occur.