1, the existing problems
In recent years, with the advancement of urban modernization, the building density of cities has been increasing. With the continuous construction of high-rise buildings, the problems of deep foundation pit excavation and support are increasingly prominent. Therefore, engineers pay more and more attention to the excavation and support of deep foundation pit and its influence on adjacent buildings, roads and facilities, and have formulated many good measures. However, the excavation depth of deep foundation pit is getting deeper and deeper, and the excavation environment is becoming more and more complicated. Designers and constructors often encounter new problems and challenges, thus reducing the success rate of foundation pit engineering. Especially in big cities such as Shanghai and Shenzhen, the accident rate is higher. There are nearly 40 foundation pit accidents in Shanghai a year. A foundation pit accident on Guangdong Road in Shanghai caused 1.8m settlement, which caused serious damage to various underground pipelines, gas leakage and explosion, and smoked more than 20 people on the spot, resulting in direct economic losses of more than 50 million yuan and extremely bad social impact. 1998, a serious collapse accident occurred in a foundation pit in Shenzhen, several construction workers were buried, and several buildings around the foundation pit were seriously damaged, which caused a sensation throughout the country. This paper analyzes the present situation of deep foundation pit excavation and support, and puts forward some opinions and suggestions for reference in design and construction.
2. Characteristics and present situation of deep foundation pit engineering
(1) The foundation pit is dug deeper and deeper. In order to facilitate the use of expensive land or meet the requirements of urban management and civil air defense, construction investors have to go underground. Basements with 1 ~ 2 floors were not common even in big cities, let alone in medium-sized cities. Nowadays, in big cities and coastal areas, especially in special zones, 3 ~ 4 floors underground are very common, and 5 ~ 6 floors are also available. Therefore, the depth of foundation pit is mostly between 10 ~ 16m, and there are many around 20m.
(2) The engineering geological conditions are getting worse and worse. This is more prominent in some coastal economic development zones.
(3) The surrounding environment of the foundation pit is complex. Important high-rise and super-high-rise buildings are concentrated in densely populated and densely built places, and are close to important municipal roads. The original building structure here is out of date, and there are many underground pipelines on the ground. Therefore, foundation pit excavation should not only ensure the stability of the foundation pit itself, but also ensure that the surrounding buildings and structures are not destroyed.
(4) There are various supporting methods for foundation pit. Such as manual digging piles, precast piles, deep mixing piles, steel sheet piles, underground continuous walls, internal supports, various piles, plates, walls, pipes, combined supports with anchor support, and anchor nail walls.
(5) The success rate of foundation pit engineering is low. Once the foundation pit support fails, it will often lead to cracks in adjacent houses, underground pipelines and roads, causing engineering disputes and even serious damage, resulting in significant economic losses and casualties.
3. Accident analysis of deep foundation pit engineering
Because of the above characteristics of deep foundation pit engineering, deep foundation pit support has become the most difficult engineering problem. Through the investigation and analysis of engineering accidents, the following views are put forward on the reasons:
3. 1 Wrong design scheme
(1) Scheme selection error. There are many accidents in this kind of engineering. For example, a building in Jinan is located in a bustling urban area, with 23 floors above ground and 3 floors underground, and its foundation pit is 12m deep. The site is narrow, and the east, south and north sides are close to the building. The construction unit proposes to adopt the pile-anchor supporting system, which adopts large-diameter cast-in-place piles, with soil anchors and concrete ring beams at the top of the piles. Φ 800 cantilever cast-in-place pile is used partly, φ 150 steel pipe cantilever pile is used partly, and the cost of partial sloping scheme is 400,000. Results According to the plan of the construction unit, 57 Φ 800C30 cantilever cast-in-place piles with a length of 65,438 were poured in the east, south and northwest directions by using 1∶0.3 slope on the west side. The bottom of into the pit is 6m. There are 7 φ 150 steel pipe cantilever piles in the north, @ 1000, pile length 15m, cantilever 12m and into the pit bottom 3m. As a result, the pile broke several times and the collapse was fierce, all of which happened instantly. * * 3000m3 of earthwork was accumulated in the pit, and 23 piles were broken. We must pay attention to the foundation pit support, and we must never make a plan casually in order to save costs. After analysis, the original scheme proposed by the construction unit is still feasible. The construction unit makes plans at will, which is unscientific, resulting in waste of investment and delay of construction period. Haste makes waste.
(2) The implementation plan is inconsistent with the design plan.
(3) Improper strength of water-stop curtain. For example, Nanjing Bank of Communications Building has 28 floors above ground, basement 1 floor and foundation pit depth of 6.7m The design scheme is: support? 800 cantilever cast-in-place pile, @ 1000, pile length 14m, with 800×500mm ring beam at the top of the pile and 8.8m； embedded in the pit bottom; Waterproof and dewatering: set high-pressure jet grouting concrete after pile arrangement to form a water-stop curtain. The east side of the foundation pit is 42m long, about 15m away from the house. 1:1is adopted for slope excavation. There are three tube wells in the pit with a depth of 20m, which are used as dewatering wells. The implementation scheme is as follows: the foundation pit is deepened by 0.7m to 7.4m, the pile length is changed to 13m, the pile is buried at the bottom of the pit by 5.6m due to site limitation, and the slope is changed to 1: 0.3 ~ 0.5. In order to speed up the progress, the ring beam at the top of the pile began to be dug before the construction, and the design elevation was dug at one time. After the excavation of the foundation pit, a large number of mud gushing and quicksand appeared between the piles in the southeast corner, and the supporting structure shifted to the inner side of the foundation pit by more than 20cm. After piling, a ground fissure of 5 ~ 10 cm was formed, the slope area was unstable and slipped, and the dewatering well failed, which caused serious cracking and damage to the Heping Cinema in the southeast and forced to stop the demolition. The pavement of Hunan Road on the north side cracked and was forced to be reinforced with soil anchors, resulting in direct economic losses of 1 10,000 yuan. It can be seen that the failure to construct according to the original design scheme and to form a waterproof curtain between cast-in-place pile and sprayed concrete is the main cause of foundation pit accidents.
3.2 Design calculation error
(1) Bolt calculation error. For example, a high-rise building in Shijiazhuang has a construction area of 65,438+10,000 square meters, 28 floors above ground and 4 floors underground, with a foundation pit depth of 20.5.5m, east-west length 120m and north-south width 100m. The foundation pit adopts φ600 cast-in-place pile, @ 1000, with a pile length of 20m and a concrete strength of 5m. 130 anchor: the length of the first anchor is 15.5m, @ 2000; The second anchor rod is 20m long, @1500; The length of the third anchor rod is 18m, @ 1000. Channel steel is combined with retaining piles. 1September, 993 12, after the construction of the cushion at the bottom of the west pit was completed, the construction managers found that there was broken soil falling between the retaining piles in the west of the foundation pit and there was water seepage. The brick wall at the top tilted outward, and cracks appeared on the ground at the top. September 65438. Part of the anchor nut is loose. Construction personnel weld channel steel and tighten nuts. Part of the pit top is excavated and unloaded. At about 5 pm on September 16, the retaining wall structure of about 50m on the west and south sides of the foundation pit collapsed rapidly, and 48 reinforced concrete piles were broken. The collapse edge is about 13m away from the pit edge. The retaining wall pile is divided into three sections, with the breaking points at the second and third layer anchors respectively, and the first layer anchor is taken out of the soil. Through analysis and calculation, the anchorage length of the first anchor is 25.6~30m, and that of the second anchor is 22 ~ 25m. It can be seen that the main reasons for the collapse are the complete pull-out in the design, the pull-out of the anchor heads of the second and third floor anchors, and the torsion of the waist beam. Through analysis and calculation, the anchorage length of the first anchor rod needs to be 25.6~30m, and the anchorage length of the second anchor rod needs to be 22 ~ 25m. It can be seen that the main cause of collapse is design and calculation errors.
(2) The buried depth of supporting piles is not enough. Deep cement mixing pile is used for foundation pit support of a project in Shanghai. The excavation depth of the foundation pit is 5 ~ 7m, the pile length is 12m, and the buried depth is 5m. There was no accident when the excavation was 5m, but piping, sand gushing and water gushing occurred when the excavation was 7m. Due to the appearance of a large amount of sand, the supporting structure finally collapsed. The investment in reinforcement alone increased by 300,000 yuan (the original support cost).
(3) The safety factor is small. In many foundation pit designs, in order to pursue the cost simply, many factors are ignored, which makes the safety factor of the project very small. In case of rain or a small amount of unexpected loading near the pit, the foundation pit will be unstable.
3.3 Stability checking calculation is not conducted.
From many engineering accidents, it can be seen that it is not enough to design the foundation pit support or choose the scheme, but also to check the stability to ensure the overall and local stability of the foundation pit, especially in soft soil areas.
3.4 Problems in Construction Management
(1) seriously overbreak, failing to follow the principle of layered and segmented excavation; (2) Too much accumulation near the pit; (3) Management confusion.
4. Suggestions and countermeasures
4. 1 Adhere to the principle of layered and segmented excavation and support.
Under normal circumstances, the slope failure has a process of starting locally and gradually expanding. First, local failure is the breakthrough. When the soil stress in a certain part reaches or exceeds its strength, the breakthrough point begins to fail, which causes the change of the mechanical properties of the surrounding soil and the appreciation of the stress in the adjacent parts, thus expanding the failure surface. With the development of urban high-rise buildings, the depth of foundation pit is increasing day by day. The slope is steeper and steeper (generally 80 ~ 90). At present, various theoretical calculation models of slope stability are established around 60, which is quite different from the initial stress state of steep slope. After slope excavation, the original three-dimensional stress state of natural soil is destroyed, resulting in a high energy zone near the excavation surface. Part of the energy is transferred to the surrounding soil, and part of it becomes the power to deform the soil. For slopes, it is almost vertical. The accumulated energy is very large, which may become the breakthrough point of failure and lead to collapse. Therefore, it is necessary to control the length and depth of the excavation face and carry out rapid support during construction, so that the support can play its role as soon as possible and achieve the purpose of controlling and eliminating the breakthrough point of damage. Layered and segmented excavation support is beneficial to the release of slope energy. Part of the energy in the pre-excavation interval is transferred to the deeper soil layer through the anchor body. Due to the influence of the building panel, part of it remains in the shallow part of the slope. After the next layer is excavated, it is absorbed and released by the excavation section behind. Therefore, the construction method of layered and segmented excavation support is also a process of energy release, which ultimately leaves the total excavation energy on the slope, which is beneficial to the stability of the whole fault plane.
The excavation size of slope layer should be regarded as an important part of design. Through the analysis of the mechanical properties of soil, the distribution of additional load of groundwater and slope, the possible breakthrough point position is predicted, which is an important basis for stratification. On this basis, the excavation map of each slope layer is drawn as the construction basis, and it is adjusted according to the specific situation in the construction.
4.2 Information feedback is an important part of foundation pit construction.
The so-called information feedback in the construction process basically refers to two aspects: first, it refers to the information feedback of geological structures exposed during slope excavation, changes in groundwater distribution and unknown underground buildings; Second, it refers to the information feedback of slope displacement and stress monitoring during construction. Among them, the causes of lateral displacement during construction are as follows: (1) The fuzziness of soil mechanics: the layered structure of soil is changeable, with many influencing factors and large dispersion of physical and mechanical properties. Its structural calculation principle and various parameters are fuzzy, so it is impossible to calculate them at one time. (2) Deformation under external force. (3) Instability in the construction stage.
4.3 Innovation of supporting structure
(1) Change the structural form from the structural stress. The closed arch ring retaining soil and the multi-arch foundation pit support both change the plane structure into the spatial support structure. Using the arch frame, on the one hand, the lateral pressure of the soil on the pile is reduced, on the other hand, the structure is bent into an arch ring to be compressed, and the compression characteristics of concrete are fully exerted to reduce the project cost.
(2) Change the construction method. The top-down method of pile-wall integrated basement is to combine the foundation pit supporting piles with the basement wall, use the beam and slab of the basement as the support, start construction from the top of the basement, and construct the basement exterior wall at the same time. Its advantage is saving investment, and waterproof curtain is still needed in areas where groundwater is rich and it is not easy to lower the water level.
(3) Develop new support methods. In recent years, shotcrete anchor net support and anchor nail wall support have been applied in engineering, and have shown remarkable economic benefits. They do not need piles, plates, pipes and braces, and completely abandon the traditional methods and their passive support concepts, so as to maintain, significantly improve and make maximum use of the inherent mechanical strength of the soil on the side wall of the foundation pit and turn the soil load into a part of the supporting structure system. They actively support the soil. Generally, the construction period is shortened by 30 ~ 60 days compared with the traditional method, and the project cost is reduced by 10% ~ 30%. The maximum vertical pit depth of support is 18m, and the depth of construction silt foundation pit is 10m.
4.4 further study the theory of foundation pit support
It can be seen that with the rapid development of national economy and the process of urban modernization, the reliability of foundation pit engineering has become an urgent problem for high-rise buildings. Therefore, it is urgent for engineering practice to further explore the methods and calculation theories of foundation pit support, especially the calculation theory of new support methods. Such as spray anchor net support method, anchor nail wall method and so on.
4.5 Discuss the emergency rescue technology of retaining wall of foundation pit.
As mentioned above, the failure rate of foundation pit engineering is high. Therefore, it is very necessary to discuss the emergency rescue technology of foundation pit wall protection combined with the monitoring and information feedback technology of construction process. At present, it is found that the retaining wall of foundation pit is ineffective, and the method adopted is to stop excavation or backfill earthwork, which has little effect. Therefore, the emergency measures of foundation pit wall protection must be considered when designing support or determining construction scheme. For example, the chemical grouting rescue technology for water leakage of foundation pit curtain is simple and economical. Quick and effective, it is the best emergency measure for water leakage and sand gushing in foundation pit at present.
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