【Excerpt】Comprehensive summary of foundation pit engineering construction

        In the process of foundation pit construction, a reasonable support scheme should be selected according to the geological conditions of the site, the hydrological environment and the characteristics of the foundation pit, and relevant measures should be applied to control the deformation of the foundation pit.

Key points of foundation pit construction

        (1) Precipitation in the foundation pit. The construction drainage should be carried out continuously. When the main structure project does not have the anti-floating conditions, the drainage shall not be stopped. It should be noted that the precipitation cables shall not be rubbed with the well walls or other sharp objects.

        (2) insert steel plate pile to prevent tilt. Before inserting the steel plate pile, the iron wedge or hardwood wedge should be inserted into the lower end of the uninserted lock to prevent the mud and sand from blocking the lock when sinking. When the steel plate pile is inserted in the solid land belt, the pile tip can be cut into a certain Angle, and the reaction force is used to make the inclined steel plate pile gradually return to normal. Steel plate pile lock deformation and leakage due to improper insertion. The remedial measure is to cast fine cinder on the outside of the cofferdam at the leakage lock, and the leakage can be blocked when the cinder sinks to the height of the leakage.

        (3) Safety control of earthwork excavation. When the excavator digs the soil, it should be dug after the fuselage stops. When the bucket does not leave the working surface, no action such as turning or walking shall be allowed. Use the rotary brake and do not reverse the steering clutch. After the operation, the excavator shall not be parked near the high slope and the filling area, but should be parked in a solid, flat and safe area, the bucket shall be put flat on the ground, all the joystick shall be set in the middle, and the control room shall be closed.

        (4) The construction of deep foundation pit must solve the groundwater level. Generally, light well points are used to pump water, so that the groundwater level drops to 1m below the bottom of the foundation pit. The spacing between multiple excavators shall be greater than 10m, and the excavation shall be carried out from top to bottom layer by layer. When stacking materials and moving construction machinery on the side of the deep foundation pit, a certain distance should be kept from the edge of the excavation. When the soil is good, the distance shall be 0.8m away, and the height shall not exceed 1.5m.

        (5) The deep foundation pit backfill should be symmetrically backfilled around, and cannot be filled and extended, and layered compaction.

        (6) In the rainy season, drainage measures must be set for the ground water around the pit to prevent rainwater and ground water from flowing into the deep foundation pit. The earthwork excavated in the rainy season should leave 15~30cm soil above the elevation of the foundation pit, and the excavation shall be made after the sky is clear.

        (7) The layout of the monitoring points shall meet the monitoring requirements, and the objects to be protected within the excavation depth range of 1~2 times outside the edge of the foundation pit shall be taken as the monitoring objects. The number of displacement observation reference points is not less than two points, and should be set outside the range of influence. Deep foundation pit works shall be subject for horizontal and vertical displacement monitoring. Meanwhile, horizontal displacement monitoring shall include displacement amount, displacement rate and direction. The monitoring project shall measure the initial value before the foundation pit excavation, and shall not be less than 2 times. The time interval of each monitoring can be determined according to the construction process. When the deformation exceeds the relevant standard or the change rate of the monitoring results is large, the observation times should be encrypted.

Foundation pit support selection

shallow foundation pit project

        Shallow foundation pit engineering is generally the engineering with the foundation pit depth of less than 5m. Common supporting methods include anchor pull support, inclined column support, short pile transverse partition plate support, temporary retaining wall support, type steel pile transverse baffle support, etc.

        (1) Anchor pull support and inclined column support are often used in the excavation of large and deep foundation pits or the use of mechanical excavation.

        (2) Short pile transverse partition support and temporary retaining wall support are often used in the excavation of the foundation pit with large width or the lower part of the section is not enough.

        (3) The steel pile transverse baffle support is often used in the general clay layer or sand layer with low underground water level and small depth.

Deep foundation pit project

        Deep foundation pit engineering excavation depth is more than 5m (including 5m) or more than 3 layers of basement (including 3 layers), or the depth is not more than 5m, but the geological conditions and surrounding environment and underground pipelines are particularly complex. The common support methods are row pile support, underground continuous wall, water-soil pile wall, reverse arch wall, etc.

        (1) Pile pile support and underground continuous wall are often used in deep foundation pits with the safety level of the side wall of the foundation pit.

        (2) Water-soil pile walls are often used in deep foundation pits with a safety grade of two and three levels on the side walls of the foundation pit.

        (3) The reverse arch wall is often used in the deep foundation pit with three safety grade of the side wall of the foundation pit, but it is not suitable for silt soil.

Foundation pit accident and prevention measures

01 Foundation pit system displacement is too large and collapsed

        The foundation pit system is displaced by too much. According to the monitoring data provided by the monitoring unit, such as foundation pit displacement is too large, close to or reach the warning value, stop on the side of the earthwork excavation, adjust the earthwork excavation position, to the excavation of the opposite, such as displacement continues to expand, stop the earthwork excavation, the part of backfill, backfill material can use sandbags, local soil method.

        Foundation pit collapse. If the foundation pit collapse occurs, make temporary support for the collapsed slope immediately. No.14 channel steel @200 row pile is used, and the 80mm thick C20 concrete is sprayed on the outside of the row pile to ensure water leakage into the foundation pit.

02 The bottom of the pit uplift, the surrounding soil gushing

        (1) If the bottom of the pit rises, stop the construction immediately, and the prepared sandbags and foundation pit soil are used to reinforce the pit foundation soil according to the actual situation, and then dig to the elevation. Add up the settlement monitoring points outside of the foundation pit.

        (2) The surrounding soil gushing occurs, and the plate pile is inserted along the pit to prevent the external soil from squeezing into the pit, increase the soil depth of the envelope structure, treat the soil at the bottom of the pit, reduce the dynamic water pressure, and prevent the occurrence of soil gushing.

03 Defects of the envelope structure

        (1) If the leakage point is limited to above the excavation surface, and the leakage is not large, it is appropriate to use the method of double fast cement pumping pressure polyurethane plugging.

        (2) If the leakage point is limited to above the excavation surface, and the amount of leakage is large, it is appropriate to penetrate the drain pipe at the leakage point, plug the drain pipe with steel pipe and double fast cement, and close the drain pipe valve after the plugging material reaches the surrounding strength.

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

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

        (5) If the leakage point is unknown and the water flows up from the excavation surface, the excavation should be stopped immediately, partially backfill until the leakage stops, and then take the above measures of double slurry injection outside the foundation pit.

        (6) If the leakage water flow is cloudy and the leakage time is long, attention should be paid to the serious soil loss near the leakage point and the cavity. At this time, it is strictly prohibited for heavy machinery to approach, and the vibration ator grouting method should be immediately used to fill the hole.

04 surrounding houses cracking and uneven settlement

        (1) Immediately stop the construction, please invite experts to analyze the cracking and settlement of the house, find out the reasons, formulate the protection plan of the building and organize the implementation, and continue the construction after dealing with the hidden dangers.

        (2) Immediately stop the excavation of the foundation pit and strengthen the foundation pit support. The ground strengthening measure is to use 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.

——Source: Foundation Work

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