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1. Foundation pit wall form Before foundation pit construction, firstly, according to the requirements of the code, the grade of the foundation pit wall should be determined according to the seriousness of the consequences after the foundation pit wall damage, and then according to the safety level of the pit wall and the surrounding of the foundation pit. The form of the pit wall is selected by factors such as environment, excavation depth, engineering geology and hydrogeology, construction equipment and conditions during the construction season.
When there is no important building (structure) at the top of the pit foundation, and the site has grading conditions and the pit depth is ζ10m, the slope rate method can be used preferentially. The slope value of the temporary excavation should meet the requirements of Table 1G420114-2. When the construction site cannot meet the requirements of the design slope value, support measures should be taken for the pit wall. The safety level of the pit wall is suitable for the retaining wall of the digging hole. The safety level of the pit wall is suitable for the wall of the nail wall.
2. Construction height The excavation height of the foundation pit shall be determined according to factors such as soil quality, soil conditions and construction methods. The order and method of earthwork excavation must be consistent with the design conditions, and follow the principle of "grooving support, first support and then excavation, foundation layer excavation, super excavation prohibited". The foundation pit must be stratified and balanced for excavation, and the height of the foundation should not exceed 1m. Generally, the anchor-pull support should also adopt the method of layered excavation and layered support. The regulations stipulate that the soil nail support should be constructed according to the layered excavation depth and operation sequence specified in the design, and the soil nail on the upper work surface is completed. Excavation at the next depth shall not be carried out until the concrete is sprayed.
The excavation depth shall also ensure the ability of the bare slope to remain self-standing during the period of completion of the support (spinning and concrete spraying). Therefore, the foundation pit should also be considered for excavation in the horizontal direction.
(2) Guarantee the quality of survey and correct design 1. Ensure the quality of survey.
2. The geotechnical test and geotechnical parameters are determined correctly.
3. Choose the design calculation method correctly.
(III) Construction and protection of foundation pit support structure Soil wall support is the pressure directly facing the foundation soil, so the determination of its structural form and construction operations must be taken seriously. Selection and design should be carefully based on excavation depth and width, soil and groundwater conditions, excavation methods, adjacent buildings, etc.
According to the requirements of the regulations, during the excavation of the foundation pit, measures shall be taken to prevent the collision support structure, the engineering pile or the disturbing base undisturbed soil. Most of the foundation pit collapse accidents are caused by insufficient support or insufficient support. Because the deformation of the pile top is very sensitive to the surrounding environment, special attention must be paid to the support structure (including the column) to maintain sufficient strength and rigidity, and to leave sufficient wealth factor. (usually up to 1. 2~ 1. 4). Control the horizontal displacement of the pile top. It is not allowed to make holes and holes in the closed structure with water blocking function because of the convenient construction; because the construction period needs to bear the load when the tear concrete of the pile and other structures is not strong enough; or the collision and disturbance support structure Damage and damage to the structure and reduce the carrying capacity.
(4) Strengthening the control of surface water, eliminating the impact of water habitat and the safety of foundation pits.
Before the foundation pit construction, the pipe network around the foundation pit should be found out to avoid damage to the pipe network during the construction process, and explosion or leakage may occur. At the same time, in order to reduce the infiltration of surface water into the soil of the pit wall, concrete sealing is applied around the top of the foundation pit. A surface drainage system should be provided in the construction site to organize the discharge of rainwater, construction water, groundwater extracted from the precipitation well, etc. The puddle and precipitation grit chamber should be waterproofed to prevent leakage. When grading and excavation, measures should be taken to reduce drainage at the top of the slope, on the slope and at the foot of the slope.
A drain hole shall be provided for the pit wall adopting the supporting structure to ensure that the water pressure in the soil inside the retaining wall can be eliminated in time, the soil moisture content is reduced, and the surface water in the soil around the foundation pit is also conveniently observed, and measures are taken in time. The slope of the drain hole should not be less than 5%, the spacing should be 2~3m, and it should be arranged in the shape of plum.
(5) On-site monitoring of the supporting structure The monitoring of the supporting structure is an important means to prevent the supporting structure from being collapsed. The monitoring requirements shall be proposed during the design of the supporting structure, and the monitoring plan shall be prepared by a qualified monitoring unit and implemented after design, supervision and approval. The monitoring plan shall include the purpose of monitoring, monitoring items, test methods, arrangement of measuring points, monitoring period, monitoring item alarm value, information feedback system and on-site raw status data records.
The contents of the monitoring project include: horizontal displacement and vertical displacement at the top of the foundation pit, and deformation of the top of the foundation pit (structure).
The selection of monitoring items should take into account the safety level of the foundation pit, the deformation control requirements of the supporting structure, and the characteristics of the geological and supporting structures. The monitoring plan can be determined based on factors such as design requirements, wall stability, surrounding environment and construction progress. The monitoring unit shall notify the construction unit and the supervision unit of the monitoring situation on a regular basis. When the monitoring value exceeds the alarm value, the design, construction and supervision units shall be notified immediately, the reasons shall be analyzed, and measures shall be taken to prevent the occurrence of the accident.
(1) Choosing a suitable foundation pit wall form and excavation height