Selection of support (enclosure) scheme
1. An adjacent project refers to another unit project developed on the same plot at the same time. The basement is about 10000 square meters, and the average depth of foundation pit excavation is 4.5 meters. It has the same basic type and started earlier than this project. The excavation of foundation pit adopts 1: 1 two-stage sloping, and basically no support (enclosure) is done. According to the deformation and displacement of slope during excavation and construction, it is found that landslides generally occur to varying degrees, and some areas are more serious.
2. Slope stability calculation Using the soil parameters, excavation depth and planned slope gradient in the foundation pit in the geological survey report, we checked and analyzed the slope stability of this project by using the slope stability analysis and calculation software SLOPEv 1.0 in the seven-star series software of Tongji University. Because we were fully prepared and the measures were in place, there was no accident during the construction.
3. Determination of the support (enclosure) scheme After comprehensive analysis and comparison, it is finally determined that the excavation scheme is two-stage sloping, with slopes of 1: 1 (uphill section) and 1: 1.2 (downhill section) respectively, and C 15 plain concrete is adopted. And do a good job of dewatering and drainage in the foundation pit, waterproof outside the foundation pit, and reduce the erosion and erosion of slope and foundation by groundwater and surface water.
Excavation test The foundation pit of this project meets the excavation test conditions: firstly, the foundation pit covers an area of 50,000 square meters with a circumference of about1.500m, which is a super-large foundation pit. Even if the test model is 10%, it is 5000 square meters, and the test data is reliable; Secondly, even if there is a landslide on the slope of the foundation pit during the test, the impact on the surrounding environment is not great; Third, the project is determined to be fully excavated after the Spring Festival. This commencement date creates conditions with time effect and climate effect for excavation test. Before and after the Spring Festival, there is the most concentrated season of rain, snow and frost in Jiaxing. After consulting the design, supervision and construction parties, a trial excavation of about 4,000 ~ 5,000 square meters of foundation pit was carried out with the designed excavation scheme by using the gap before the Spring Festival, and excavation, enclosure and dewatering were carried out according to the normal construction progress. The exposed and closed cushion was carried out for the foundation pit, and special personnel were assigned to regularly measure the displacement and deformation of the foundation pit, and observation was carried out as usual during the Spring Festival. After two months of freezing rain and snow baptism, the foundation pit is basically stable, which shows that the foundation pit excavation and enclosure scheme we designed is suitable for this project. However, cracks appear in concrete slope protection in some areas with poor geology, and the slope shows signs of sliding, which shows that similar areas should be encrypted with wooden stakes, and steel sheet piles or channel steel slope protection should be used if necessary. In addition, the water yield is basically consistent with the calculation.
Foundation pit excavation
1. Excavation sequence The foundation pit area of this project is relatively large. In order to avoid confusion and arrange flow construction reasonably, the reasonable excavation advancing direction is determined in advance, and it advances from D-A axis in D area to C area, and then it is divided into two parts, which advance to A area and B area respectively. The earthwork excavated in the later period of Area A and Area B is directly used to backfill the earlier excavation part of Area D. The foundation pit of this project was fully excavated on March 1 2005, and all earthwork excavation was completed on May 20th, which lasted 8 1 day, slightly ahead of schedule. The average daily unearthed amount is 3,700 cubic meters, and the maximum daily unearthed amount is 6,000 cubic meters.
2. Excavation technology Earthwork excavation is mainly mechanical, supplemented by labor. Backhoe excavators are used for digging, and dump trucks are used for earth transportation. The large-scale backhoe excavator is responsible for the earthwork excavation above the ground floor and the earthwork excavation transferred to the ground by the small excavator at the bottom of the foundation pit; The foundation pit adopts a medium-sized excavator, which is responsible for the earthwork excavation of the bearing platform under the bottom plate. Because of its small working radius, the excavated soil must be transferred to the working radius of a large excavator. The small excavator is responsible for the earthwork excavation of the foundation beam under the floor. Because of its small working radius, they must transfer the excavated soil to the working radius of large and medium-sized excavators. After the slope is leveled, the excavation opportunity will cooperate with the pile at the foot of the slope; Insert bamboo chips on the slope in time, pour slope protection concrete, and cover it with plastic film for heat preservation and maintenance.
The biggest feature of foundation pit treatment under special circumstances is its huge area, and the uneven distribution of geological conditions caused by it increases the difficulty of foundation pit construction, which needs to be treated according to specific conditions.
1. Fire elevator shaft and other partial deep foundation pit construction There are 5 fire elevators in this project, of which 4 elevator shafts have an elevation of -7.250, plus 1.6-meter-high cap and 200mm-thick cushion, which requires the depth of foundation pit to reach -9.050. According to the excavation situation, there is a lot of quicksand at the bottom of the pit, which must be closed as soon as possible, otherwise it will be extremely unfavorable to the project. We replaced 100 mm thick gravel cushion with 200 mm thick quick-hardening concrete to stop water, poured cushion on it, bound steel bar, poured platform and elevator shaft first, and reserved ground beam steel bar, which ensured the smooth construction of local deep excavation parts such as fire elevator shaft.
2. The local reinforcement is located in the northeast of the project site, and another two-story office steel plate trailer of the unit project is built. If the site is still excavated according to the normal slope, dig directly to the foot of the trailer. In order to keep the mobile home, the slope of this part of the foundation pit is increased, and a number of channel steel piles and wooden stakes are added to ensure its safety and avoid large deformation affecting its normal use.
conclusion
At present, the government is gradually shifting the focus of urbanization to suburbs or development zones, and the building density is far less than that of the city center. Large-scale projects as infrastructure are often accompanied by large or super-large foundation pit construction, and the most economical and simple slope excavation is bound to be the first choice. However, the geological distribution of super-large foundation pits is often uneven, which leads to the inconsistency of foundation pit construction schemes and the coexistence of various supporting (enclosure) methods. Optimizing allocation in time and space can save costs and improve efficiency. It is still very meaningful to do "excavation test" for conditional projects.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd