1.3 the integrity of the retaining structure of the foundation pit shall be designed to make the retaining structure closed as much as possible, especially when there is a berm, attention shall be paid to the sealing of the retaining structure of the foundation pit, otherwise it is easy to be damaged at the place where the foundation pit is not closed, thus endangering the safety of the whole foundation pit.
[Key words] soil nailing wall support; Design; Construction 0 Introduction soil nailing wall support, as a new method of foundation pit support, has developed rapidly because of its fast and simple construction, and its cost is far lower than other support methods.
Based on several years’ experience in design and construction, the author talks about some experiences in the design and construction of soil nailing wall.
This requires that earthwork construction must be carried out in accordance with the requirements of support construction.
After the first layer of shotcrete anchor support construction is completed, it is forbidden to excavate downward immediately.
In order to ensure full grouting, the grout stop plug and exhaust pipe shall be set at the orifice, and the grouting pressure shall be maintained at 0.4 ~ 0.6MPa.
It shall be excavated in other sections, and it can only be excavated downward after curing for 1 ~ 2 days.
When the surface load is less than 20kn/m2, the value shall be taken as 20kn/m2.
In this case, the designer can modify the design scheme in time according to the site construction and stratum conditions.
The support construction shall be accelerated when the excavated slope surface is exposed.
As soil nailing has certain requirements for soil layer, it is not suitable to use soil nailing alone in the case of loose sand, soft plastic, fluid plastic cohesive soil and rich groundwater resources, and it is best to combine it with other support methods.
Before grouting, the residual and loose waste soil in the hole shall be cleaned, and the grouting pipe shall be inserted to 0.5m ~ 1.0m from the hole bottom.
The excavation depth of each layer of earthwork shall be consistent with the depth of soil nailing arrangement (generally 0.5m below the depth of soil nailing arrangement).
The determination of parameters should consider the change of groundwater level, the influence of winter and rainy seasons on the strength of slope soil mass, and the influence of leakage of underground pipe network.
(4) Placement and grouting of soil nails: when the length of soil nails is less than 12M, the full-length reinforcement shall not be connected in principle.
(3) As the soil nailing wall support construction is to support one layer after excavation, the construction of the lower layer can only be carried out after the upper layer support is stable.
When there are important underground pipelines around the foundation pit, special measures can be taken to control the displacement of the slope, To ensure the safety of underground pipelines.
(1) Change the space shape of the foundation pit, shorten the side length of the foundation pit, increase the corners of the foundation pit and reduce the deformation.
According to the comprehensive analysis of the geological data, when the soil layers have little difference, the thickness weighted average value of each soil layer index can be used as the shear strength index.
1.2 the geological data of the site shall be fully understood.
1.5 considering the space effect, the space effect shall be considered in the design of foundation pit support, so as to save the supporting structure materials and reduce the cost on the basis of increasing the safety of foundation pit.
One soil nail locator every 2.0m to ensure the thickness of protective layer of soil nail reinforcement.
(2) If the site conditions permit, the first grade grading shall be changed to the second grade bench sloping excavation to reduce the earth pressure, support workload and project cost.
If there is any infringement, please contact and delete it! [Abstract] soil nailing wall support is a new type of support method, which has been widely used at home and abroad.
1.6 add monitoring scheme to ensure the safety of foundation pit, the monitoring scheme of foundation pit shall be determined during the design of foundation pit support, and the layout of monitoring points, monitoring items and monitoring alarm values shall be specified.
If it is necessary to extend, the binding strip shall be used to weld the length, and each weld shall not be less than 5D.
At the connection between the end of the soil nail and the surface layer, the end of the soil nail protruding from the hole can be bent and welded with the reinforcing bar attached to the reinforcing bar mesh..
A certain gap (preferably 30mm) should be left between the reinforcing bar mesh and the slope.
However, because the design and construction of foundation pit involve many problems such as geological conditions, site environment, engineering requirements, climate change, construction methods and so on, there are many engineering accidents caused by design and construction errors, and the lessons are also very profound.
Author: Shi Meicheng source: AVIC survey, design and Research Institute website.
The inversion analysis can also be carried out according to the existing engineering data, and the parameters can be determined in combination with the engineering.
(5) Connection between reinforcing bar, reinforcing mesh and soil nail: the reinforcing bar mesh is fixed with longitudinal and horizontal reinforcing bars and short reinforcing bars inserted into the soil.
1.4 in addition to the dead weight of the soil, the additional load to be considered in the design shall also include the surface load such as the load caused by the stacking and lifting transportation of vehicles and materials, as well as the load imposed by the foundation of nearby ground buildings and underground structures, and the actual action value of the load shall be taken as the calculation value.
(2) The material of soil nailing wall support: fine aggregate should be medium and coarse sand, otherwise it will affect the good bonding between cement and aggregate surface; The coarse aggregate should be pebble or crushed stone with particle size of 5 ~ 15mm (preferably pebble), and the crushed stone should be continuously graded, otherwise the concrete mixture is easy to separate and the rebound will increase.
1 design aspect 1.1 before the design of soil nailing wall, the site must be surveyed to understand the surrounding environment of the site and the burying of underground pipelines, so that the interaction between the foundation pit and surrounding buildings during the excavation of foundation pit can be considered in the design, and the underground pipelines should be avoided when arranging soil nails.
This paper briefly introduces some problems that should be paid attention to in the design and construction of soil nailing wall support.
Construction (1) the design and construction of soil nailing support should be undertaken by one unit, and the designer should also be the construction technology director.
(3) When the site cannot meet the soil grading conditions, the pit wall can be excavated with different slopes.
The excavation length of each layer depends on the slope area that can ensure the stability of the slope during cross construction (generally 10 ~ 20m is appropriate).
Generally, the support construction shall be completed within 16 hours after the slope surface is exposed.
For example, the upper loose miscellaneous fill and soft ~ flow plastic silty soil shall be excavated with gentle slope, and the lower ~ hard plastic cohesive soil shall be excavated with steep slope.
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