There Are Some Tips for Troubleshooting Foundation Pit Hazards. Please Accept This Construction Secret

Due to the development of high-rise buildings and underground spaces, foundation pit projects, especially deep foundation pit projects, have large scale and great depth, which have brought many technical difficulties to construction. It is also the area where accidents occur most frequently in engineering construction. The key points of foundation pit construction are summarized below. Let’s take a look.

1. Identification and control of hazardous sources in foundation pit engineering

Major hazard sources

(1) Excavation construction will inevitably have a safety impact on adjacent buildings (structures) and facilities or require special protection.

(2) The design service life has been reached and it is planned to continue to be used.

(3) The current design plan is changed to deepen, expand, or change the conditions of use.

(4) Adjacent engineering construction, including piling and foundation pit excavation and dewatering construction, affects the safety of foundation pit support.

(5) The foundation pit is adjacent to the water.

General sources of danger

(1) There are inferior materials, quality defects, component damage, or other unfavorable conditions that affect the safety and applicability of the foundation pit project.

(2) The vibration and shearing caused by the construction of the supporting structure and engineering piles may cause soil flow, soil liquefaction, and seepage damage.

(3) The water-stop curtain may leak seriously.

(4) The main traffic road is located within the influence range of the foundation pit excavation, or the pipelines of the buildings and municipal pipelines around the foundation pit may leak or water may accumulate in the trench.

(5) During the rainy season, the soil nail wall and the prestressed anchor rods installed in the shallow layer may fail or their bearing capacity may be seriously reduced.

(6) The side wall is a miscellaneous fill or special rock and soil.

(7) The foundation pit excavation may produce excessive bulge.

(8) There is a vibration load on the side wall of the foundation pit.

(9) The internal support fails or is continuously damaged due to various reasons.

(10) Lateral impact loads may be generated on the supporting structure.

(11) Typhoons, heavy rains, or heavy rainfall may cause power outages for construction and the foundation pit drainage system fail.

(12) Creep of soil nails and anchor rods causes excessive deformation and ground cracks.

Key points for hazard source control

(1) Preliminary control: When preparing the construction organization design or special construction plan before the project starts, formulate prevention and control measures for various sources of danger in the project.

(2) Construction process control: During the construction process, strictly supervise and inspect following regulations and conscientiously implement rectification.

Strengthening comprehensive management of production safety

(1) Seriously implement the safety production responsibility system at all levels, establish and improve various management systems, and prevent all man-made accidents.

(2) Strengthen safety education for employees, and improve the quality of operators and their awareness of self-protection in safety production.

(3) Enhance the safety responsibility awareness of managers at all levels and strengthen safety professional knowledge training.

(4) Strictly strengthen various hazardous sources and management work, and implement corresponding preventive control measures for confirmed hazardous sources in combination with project characteristics.

2, Selection of Foundation Pit Support Methods

Shallow foundation pit engineering

Shallow foundation pit projects are generally projects with a foundation pit depth of less than 5m. Common support methods include anchor support, inclined column support, short pile diaphragm support, temporary retaining wall support, steel pile diaphragm support, etc.

(1) Anchor support and inclined column support are often used for excavating large and shallow foundation pits or using mechanical excavation.

(2) Short pile diaphragm support and temporary retaining wall support are often used for excavating wide foundation pits or when the lower slope of some sections is not enough.

(3) Steel pile diaphragm support is often used in general clay or sand layers with low groundwater levels and shallow depths.

Deep foundation pit engineering

Deep foundation pit projects generally have an excavation depth of more than 5m (including 5m) or more than 3 basement floors (including 3 floors), or projects with a depth of less than 5m but with particularly complex geological conditions, the surrounding environment, and underground pipelines. Common support methods include pile support, underground continuous wall, cement soil pile wall, reverse arch wall, etc.

(1) Pile support and underground continuous wall are often used for deep foundation pits with a safety level of one to three for the foundation pit side wall.

(2) Cement soil pile wall is often used for deep foundation pits with a safety level of two or three for the foundation pit side wall.

(3) Reverse arch wall is often used for deep foundation pits with a safety level of three for the foundation pit side wall, but is not suitable for silty soil.

3. Key Points for Foundation Pit Excavation

Excavation of foundation pit without support

Generally speaking, direct layered slope excavation has a large construction space and a short construction period. It is often used in open environments and excavation of foundation pits with small excavation depths. The foundation pit unsupported method is often used for excavation.

(1) Before excavation, the stability of the foundation pit slope should be determined based on the soil quality, foundation pit depth, excavation method, retention time, slope load, drainage conditions, and site size to ensure the safety of foundation pit excavation construction.

(2) The layer thickness of the soft soil foundation should be controlled within 2m, and the excavation depth should not be too large and should be controlled within 6~7m.

(3) The layer thickness of hard soil should be controlled within 5m, but its excavation depth is not limited.

(1) Basin excavation method. The basin excavation method is often used for large foundation pits where support or anchoring operations are difficult and slopes cannot be laid. During construction, the soil in the middle of the foundation pit is usually excavated first, and soil slopes are reserved on the four sides, and the soil slopes are excavated last. At the same time, since the basin excavation method generally has the characteristics of a small support amount and low cost, the surrounding soil slopes can play a supporting role on the retaining wall, which is conducive to reducing the deformation of the retaining wall and ensuring the stability of the foundation pit.

(2) Center island excavation method. The Center island excavation method is often used for large foundation pit excavation with a large space. During construction, the soil around the foundation pit should be excavated first, and the soil around the foundation pit should be temporarily reserved. The soil in the middle is used as a fulcrum to build a trestle. At the same time, although the center island excavation method has the characteristics of fast excavation and soil transportation speed, it may increase the deformation of the support structure during construction, which has an adverse effect on the stress of the support structure. Therefore, relevant requirements should be strictly followed during construction.

Construction points

(1) When excavating the foundation pit, the principle of "layered excavation of earthwork and pouring of cushion layer as it is excavated" should be followed. When excavating the foundation pit with support, the principle of "grooving support, support first and then excavation, layered excavation, and over-excavation is strictly prohibited" should be followed.

(2) When excavating the foundation pit, slope repair must be combined. After excavating a certain depth, the corresponding earthwork excavation slope should be repaired before the next layer of excavation.

(3) When machinery cannot be used, manual excavation should be used, and the excavation speed of the excavator should be appropriately slowed down so that manual excavation can keep up with the speed of the excavator as much as possible. At the same time, the two should be staggered in time and place to ensure the safety of excavation. The excavated earthwork should be filled and transported out in time.

(4) When the foundation pit is excavated to 200~300mm above the pit bottom elevation, the excavation should be stopped and manual soil repair should be carried out. At the same time, the excavation procedure should be strictly followed during construction, and the order and method of earthwork excavation must be consistent with the design conditions. Over-excavation is strictly prohibited to ensure construction safety.

(5) Before excavating a deep foundation pit, it is necessary to monitor the foundation pit project, that is, to observe and monitor the supporting structure and the surrounding environment to accurately understand the actual situation of the soil layer and take effective measures to protect the environment around the foundation pit promptly, thereby ensuring the safety of the foundation pit project.

4. Foundation Pit Accidents and Prevention Measures

Excessive displacement of the foundation pit system and landslides

Excessive displacement of the foundation pit system

According to the monitoring data provided by the monitoring unit, if the foundation pit displacement is too large and approaches or reaches the warning value, stop the earthwork excavation on the excessive side, adjust the earthwork excavation position, and excavate on the opposite side. If the displacement continues to expand, stop the earthwork excavation and backfill the area. The backfill material can be sandbags, local soil, etc.

Foundation pit collapse

If the foundation pit collapses, immediately make temporary support for the collapsed slope, use 14# channel steel @200 to drive piles, and spray 80 thick C20 concrete on the outside of the piles to ensure that water leaks into the foundation pit.

The pit bottom is bulging and the surrounding soil is gushing out

(1) If the bottom of the pit is bulging, stop construction immediately, use the prepared sandbags and foundation pit soil for backfilling, reinforce the bottom of the pit according to the actual situation, and then dig to the elevation. Set up settlement monitoring points outside the foundation pit.

(2) If soil gushing around occurs, insert sheet piles along the perimeter of the pit to prevent the external soil from squeezing into the pit, increase the depth of the retaining structure into the soil, dewater the soil at the bottom of the pit, reduce the dynamic water pressure, and prevent soil gushing.

Defects in the building envelope

(1) If the leakage point is limited to above the excavation surface and the leakage volume is not large, it is advisable to use the method of double-fast cement troughing and polyurethane injection to seal it.

(2) If the leakage point is limited to above the excavation surface and the leakage volume is large, it is advisable to drive a drain pipe at the leakage point, use a steel pipe and double-fast cement to seal the drainage pipe around it and close the drain pipe valve after the surrounding sealing material reaches strength.

(3) If the leakage point extends from above the excavation surface to below the excavation surface, a double-liquid slurry should be injected near the leakage point outside the foundation pit. The grouting is pressure-controlled and the maximum pressure shall not exceed 0.3MPa.

(4) If the leakage point extends from above the excavation surface to below the excavation surface and the flow rate is large, the foundation pit should be partially backfilled until the flow rate decreases, and polyurethane should be injected near the leakage point outside the foundation pit.

(5) If the leakage point is unknown and water flows upward from below the excavation surface, the excavation should be stopped immediately, and the partial backfilling should be carried out until the leakage stops, and then the above-mentioned double-liquid slurry injection measures should be taken outside the foundation pit.

(6) If the leakage water is turbid and the leakage time is long, attention should be paid to the possibility of serious soil loss and voids near the leakage point. At this time, heavy machinery is strictly prohibited from approaching, and the vibration pipe grouting method should be used immediately to fill the void.

Cracking and uneven settlement of surrounding houses

(1) Stop construction immediately, invite experts to analyze the cracks and settlement of the house, find out the reasons, formulate and implement a protection plan for the building, and continue construction after dealing with the hidden dangers.

(2) Stop excavation of the foundation pit immediately, strengthen the foundation pit support, and strengthen the ground by grouting within 5.0m around the foundation pit. The ground grouting material is pure cement slurry, the grouting pressure is 0.5~1.0MPa, and the soil reinforcement depth is 8.0m.

5. Foundation Pit Construction Monitoring

Layout of foundation pit monitoring points

(1) The horizontal and vertical displacement monitoring points at the top of the foundation pit slope should be arranged along the perimeter of the foundation pit, and monitoring points should be arranged in the middle of the perimeter and at the sun corners.

(2) The horizontal and vertical spacing between monitoring points should not be greater than 20m, and the number of monitoring points on each side should not be less than 3.

(3) The horizontal and vertical displacement monitoring points should be shared points, and the monitoring points should be set on the top of the retaining wall or the top of the foundation pit slope.

Key points for monitoring foundation pit engineering

(1) Monitoring of horizontal displacement of deep soil. Generally, inclinometers are pre-buried in the wall or soil, and the horizontal displacement of the retaining wall or the soil around the pit at various depths is monitored by an inclinometer. In the project, after the inclinometer probe approaches the temperature inside the pipe, two measurements should be made in each monitoring direction, forward and reverse, to determine the change in the pipe mouth coordinates and make corrections in time. The accuracy of the inclinometer system should be no less than 0.25mm/m, and the resolution should be no less than 0.02mm/500mm.

(2) Monitoring of groundwater level. Generally, a water level pipe is set in the hole, and the distance from the water surface in the water level pipe to the pipe mouth is measured by a water level meter and other instruments. The absolute elevation of the water level pipe mouth is then measured, and the absolute elevation of the water surface in the water level pipe is calculated. Water level monitoring should be carried out once every 1d, and the diameter of the pressure water level pipe should be 50~70mm, and the filter pipe section should be no less than 1m. At the same time, its monitoring accuracy should be no less than 10mm, and attention should be paid to distinguishing between shallow diving monitoring and deep-pressure water level monitoring.

===Thanks===Kelly Bar Factory