What caused the foundation pit collapse?

What is the cause of foundation pit collapse? Please read the article edited by Zhong Da Consulting Company.

The serious overload at the top of foundation pit slope is one of the main reasons for the collapse of foundation pit. The recent vivid case is that the top of the foundation pit is seriously overloaded, which leads to the overall collapse of the foundation pit.

Design drawings of foundation pit support

Second, the overall instability.

Integral instability refers to the formation of sliding surface in soil, and the retaining structure is unstable with the soil outside the foundation pit and at the bottom of the pit. The general instability form is that the upper part of the retaining structure escapes from the pit, the bottom of the retaining structure moves into the pit, the soil at the bottom of the pit rises and the ground outside the pit sinks.

Example: Longtan Hanging Garden Foundation Pit Accident

On August 3, 2005, the top of the slope cracked and settled about 30 meters wide in the morning, and the cement-soil mixing pile at the foot of the slope broke. It began to rain heavily at 7 o'clock in the morning, and the road collapsed and slipped half an hour later. The main reason is that there is overbreak in the landslide area at the east end of the north side of the foundation pit, and the time left after excavation is long; A large amount of accumulated water in the pit was not pumped out in time; The soil layer at the foot of the slope is soaked by water, which reduces the strength of the soil layer and inevitably leads to the creep deformation of the slope; When the sewer pipe near the pit leaks for a long time and the creep deformation of the slope accumulates to a certain extent, the sewer pipe under the road surface at the top of the slope cracks and a large amount of water is immersed in the soil of the slope, which leads to the instability of the slope.

On September 3rd, 2005 12, Jinbang Mingyuan on Pengliuyang Road in Wuchang District has been excavated to the design depth of 5.2m, and the slope about 40m long is in danger of collapse. Click here to download the construction technical data for free.

Third, the pit bottom uplift.

(1) The uplift at the pit bottom is upward displacement, which is caused by the unloading rebound of deep soil and the plastic flow of soil caused by the pressure difference caused by excavation.

(2) Because the soil is a continuum, the uplift of the pit bottom and the horizontal displacement of the retaining structure will inevitably lead to the settlement and horizontal displacement of the soil outside the pit, thus driving the adjacent buildings or municipal facilities to tilt or flex. These additional deformations may lead to cracks in structural members or pipes, which will affect the use and endanger safety.

(3) The general solution is to reinforce the passive area, improve the soil resistance, reduce the deformation, and solve the problems of overall stability and pit bottom uplift.

Example: Sanjin New Town International Block C accident.

Four. Overturning instability of retaining structure

Overturning instability of retaining structure mainly occurs in gravity structure or cantilever retaining structure. Under the action of active earth pressure outside the pit, the retaining structure of gravity structure rotates around a certain point at its lower part, and the top of retaining structure collapses into into the pit. The moment to resist overturning instability is mainly formed by the gravity of the retaining structure itself, and the passive resistance at the bottom of the pit is also a factor to resist the moment.

Example: Wuhan Torch Building Accident

The excavation depth of Wuhan Torch Building is 10m, and the upper part is old drilled soil and the lower part is bedrock. The foundation pit is supported by ￠900mm manually dug rock-socketed piles. After excavation to the design elevation, the strength of the old clay is partially soaked, and the earth pressure increases sharply. Because the piles are embedded in the rock, the deformation is not easy to reconcile, which leads to the fracture of more than ten supporting piles, endangering the adjacent six-story comprehensive building and suspending the stairwell of the building. After emergency backfilling and adding anchor rods. Settle down

Verb (abbreviation of verb) Instability of bearing capacity of foundation at the bottom of retaining structure

The instability of foundation bearing capacity at the bottom of retaining structure refers to the instability caused by excessive pressure at the bottom of gravity retaining structure and insufficient foundation bearing capacity. Because the earth pressure still acts on the outside of the retaining structure, the resultant force is inclined. Under the action of inclined load, the foundation soil will be squeezed into the into the pit, and the retaining structure will have uneven settlement, which may lead to cracking and damage of some retaining structures.

Example: Tianheng Building accident

The excavation depth of Tianheng Mansion is about 5m, and the thickness of silt and mucky soil is nearly 20 m. The engineering piles are drilled with 1000m rock-socketed piles, and the excavation and support scheme adopts lattice cement-soil gravity retaining wall, and the passive area at the bottom of the pit adopts lattice cement-soil concealed support.

At that time, the construction period was tight. More than a dozen cement-soil retaining walls and hidden supports have been built day and night by powder jet grouting piles, and the bite of piles and the quality of piles are not good. In the case of insufficient age, hasty excavation, combined with unfavorable factors such as improper stacking at the pit side, local excavation and pile connection, and rainstorm attack, led to large-scale slope instability and pit bottom uplift. Most engineering piles in the pit are skewed, the tower crane foundation is empty, and the piles under the foundation are cracked.

After comprehensive inspection, the integrity of inclined piles was confirmed, the independent bearing platform was changed to raft, and 56 bored piles were added. At the same time, the mud at the bottom of the pit was reinforced by grouting. After nearly half a year's efforts, the hidden dangers of accidents have been successfully dealt with.

Six, retaining structure sliding instability

The sliding instability of retaining structures also mainly occurs in gravity structures. Under the action of active earth pressure outside the pit, the retaining structure translates into the pit. The anti-sliding force is mainly composed of the friction force at the bottom of the retaining structure and the internal passive earth pressure. When the soil at the bottom of the pit is weak or the soil at the bottom of the retaining structure is softened, the wall will slip and lose its stability.

Example: Huarui Building accident

Huarui Building is located at the intersection of Zhuodaoquan South Road and Xiongchu Street. It is a 26-story high-rise building, with the buried depth of foundation about-10.8m. The foundation pit support is about 6m below the ground, and the slope ratio is 1: 03, and the manual digging pile anchor support is below 6m. On June 26, 2005, the west side of the foundation pit collapsed, the supporting piles were seriously tilted inward, and some slope protection piles were broken. The top of the west slope sinks and the slope rises; The ground on the south and east slopes (including cracks on sidewalks) is in serious danger. The main cause of the accident is that the strength of red clay layer decreases rapidly after encountering water, which leads to shallow landslide.

At noon on June 4, 2004, the slope protection of Xinhua Haoting foundation pit of Xinhua Xia Road in Hankou suddenly collapsed, and the maintenance workshop of Zhongxin Automobile Maintenance Company separated by a wall collapsed.

Seven, "kicking" instability

Kicking instability may occur in a single-supported foundation pit, such as pivot rotation, the upper part of the supporting structure falling out of the pit, and the lower part of the supporting structure turning up, which is vividly called kicking instability. Generally speaking, apron instability will not occur in multi-support enclosure structure unless all other supports fail and only one support works.

Example: Haizhu City Plaza Area B accident.

On July 2, 2005, around noon 12, a foundation pit collapsed at the site of Zone B of Zhongzhaizhu Square, Jiangnan Avenue, Haizhu District, Guangzhou. The supporting structure on the south side of the foundation pit collapsed and the diagonal braces on the southeast corner fell off. The collapse range of foundation pit support is about 104.55 linear meters, with an area of about 2007 square meters. The foundation pile of the south seafarer hotel broke and slipped, and the structure partially collapsed. At the same time, 3 people were killed and 8 people were injured.

Reason:

(1) The construction is inconsistent with the design, the foundation pit construction time is too long, and the foundation pit support is damaged and invalid, which constitutes a major accident hidden danger.

(2) The rock stratum on the south side inclines to the foundation pit, and there are water seepage and debris flow phenomena in the weak and strongly weathered interlayer. The design and construction scheme were not adjusted in time during the construction, and the opportunity to eliminate the danger was missed.

(3) The top of the foundation pit is seriously overloaded, which causes the balance of the south support of the foundation pit to be destroyed and the top of the slope to crack.

(4) The deformation of the foundation pit is obviously increased, and when the cracks are enlarged, the foundation pit is not reinforced in time.

Eight, the structure of the retaining structure damage

The failure of envelope refers to the failure mode that the envelope itself cracks, breaks, shears or buckles, resulting in the loss of bearing capacity of the structure. Improper support system or unclosed enclosure structure; It may also be that the load is underestimated or the strength of structural materials is overestimated during design and calculation, and the section of support or purlin is insufficient, which leads to failure; In addition, improper handling of structural nodes will also lead to overall failure due to local instability, especially in steel support systems, where there are many nodes and the quality of processing and installation is difficult to control. Joint treatment includes the joint between the bearing and the wall, such as no purlin or insufficient connection strength.

For example, the subway construction accident in Hu Xiang section of Xiaoshan, Hangzhou.

On June 5th, 2008165438+1October 15: 20, the subway construction site in Hu Xiang section of Xiaoshan, Hangzhou collapsed. Fengqing Avenue collapsed to form a deep pit with a length of 75 meters, a width of 2 1 meter and a depth of 15.5 meters. The nearby river burst its banks and the river flowed backwards, with a water depth of more than 6 meters. 1 1 The vehicle that was driving on the road fell into a deep pit, and dozens of subway construction workers were buried, and the number of workers killed reached 2 1. At the same time, Fengqing Avenue was interrupted. The West Primary School in Xiaoshan District, which is separated from the scene of the accident by a wall, has all collapsed. A series of chain damage effects, such as tilting damage of nearby houses and underground pipeline damage.

The failure form of foundation pit is preliminarily determined. The whole foundation pit is unstable, and the pit bottom is uplifted, which makes the retaining wall inclined. However, the connection stiffness between steel support and retaining wall is very weak, which can basically be considered as hinge. When the axial forces on both sides of the support are not in a straight line, the steel support is prone to instability and failure, resulting in domino effect, which leads to the final instability and failure of the foundation pit, the collapse of soil around the pit and the failure of the support.

IX. Instability and damage of anchorage system

The instability and failure of anchorage system includes two different failure modes. The failure of anchor is mainly manifested in the pull-out, fracture or prestress relaxation of anchor, and the failure of soil anchor is mostly local; The instability of the support is likely to be holistic, and its shape varies from system to system. Most support systems are statically indeterminate, and local failure will cause overall instability, especially for steel support systems, where the failure probability of local nodes is relatively high.

Ten, water curtain function failure and seepage deformation damage at the bottom of the pit

The failure of water-stop curtain function and seepage deformation at the bottom of pit refer to the loss of water-stop curtain function, which leads to leakage, water gushing, soil flow or sand flow. Due to soil erosion, the ground outside the foundation pit sinks and collapses, resulting in cracking and damage of adjacent buildings.

The main reason for the failure of water-stop curtain function of envelope structure is construction factor, followed by design factor and material factor. Due to poor construction quality, holes or cracks appear in the water-stop curtain, which is the most common phenomenon. The design of water-stop curtain is too short, and the permeable layer is not completely separated, which is also a possible reason for water leakage. Excessive hydraulic gradient caused by the failure of water-stop curtain causes seepage deformation and damage at the bottom of the pit. If it is not stopped in time, the displacement and settlement of soil outside the pit caused by seepage deformation will be very serious.

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