(Shandong Lubei Geological Engineering Survey Institute, Dezhou 2530 15)
Author's brief introduction: Chen Song (1961-), male, senior engineer, mainly engaged in geotechnical engineering survey and water conservancy environmental geological survey management.
On the basis of analyzing the engineering geological conditions of the site, according to the checking results of slope stability, the design of slope shotcrete and anchor support is put forward, which is proved to be feasible by engineering practice.
Keywords: engineering geological conditions; Slope stability; Shotcrete bolt support; Texas
1 project overview
Xinhua Mansion in Dezhou consists of a main building and a skirt building, with 12 floors above ground, three floors above ground and one floor underground, covering an area of about 3200m2. The absolute elevation of the natural terrace of the site is about18.8m, and the foundation pit excavation is 6.5m.
The communication building in the south of the proposed site is five stories, with a buried depth of 2.5m and a distance of about 8m from the slope. The west and north sides are three-story brick-concrete residential buildings built in 1950s, about 7m away from the foundation pit. Wenhua Road is on the north side, and there is a pipeline trench about 5m away from the foundation pit slope, which contains water supply pipelines, communication and power cables. There is no safe slope starting point for the foundation pit slope, and shotcrete and anchor support are adopted in the design to ensure the safety of the foundation pit slope. See figure 1 for the general plan.
Figure 1 construction site plan
The buried depth of groundwater in the site is1.8m. If the groundwater is not pre-treated, piping and quicksand will inevitably occur. According to the site geological conditions and site construction conditions, the well point dewatering method is designed to treat groundwater.
2 engineering geological conditions
The proposed site belongs to the alluvial-diluvial plain area in the lower reaches of the Yellow River in geomorphic unit. After artificial transformation, the ground is relatively flat, and the height difference is within1m. Relevant engineering geological parameters are as follows: ① Miscellaneous fill, the buried depth of bottom plate is 0.9~2.8m, the average thickness is 1.5m, r = 18kn/m3, c = 0, φ = 20; (2) Silty soil and silty clay interbedded. The buried depth of the floor is 7.9~8.7m, with an average thickness of 6.7m, mainly silty clay, with a single layer of silty clay of about 0.4m, with an average r = 18. 1kN/m3, c = 29kpa, and φ = 8. (3) Silty soil, the buried depth of the bottom plate is 9.2 ~ 10.6 m, the average thickness is 1.4m, r = 19 kn/m3, and k =1.2m/d; ④ Silty sand, the buried depth of bottom plate13.8 ~14.9m, with an average of 3.5m, r = 18.5kn/m3 and k = 3.5m/day. ..
3 Natural stability of foundation pit slope
3. 1 distribution of active earth pressure on slope
Due to the small space around the slope, the slope stability is calculated at 90, and the top of the slope is overloaded with q = 10 kPa. According to Rankine earth pressure theory, the calculation formula of active earth pressure is as follows:
Shandong environmental geology collection
See Figure 2 for the distribution of active earth pressure on the slope calculated by formula (1). The slope with active earth pressure greater than zero will be overturned and unstable, and the active fracture surface is the dotted line in Figure 2.
Fig. 2 Distribution of Active Earth Pressure on Slope
3.2 Slope anti-sliding stability
According to Taylor's theory of slope stability (Chongqing Construction Committee, 2003), cohesive soil slope is unstable and will slide with arc surface. According to Taylor's empirical curve, the most dangerous sliding surface of the slope can be found (Figure 3). According to the slice method, the anti-sliding safety factor of arc surface is calculated as follows:
Fig. 3 Schematic diagram of dangerous sliding surface
Shandong environmental geology collection
Where: wi is the gravity of the soil strip; Bi is the density of soil strips; Ai is the elevation angle of the tangent at the midpoint of the bottom edge of the soil strip; Ti is the length of the bottom edge of the soil strip; Ci and li are the shear radian parameters of the soil layer where the bottom edge of the soil strip is located.
According to formula (2), the anti-sliding safety factor of L1arc is:
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It can be seen that the natural anti-sliding stability is not enough.
The critical sliding surface (L2 in Figure 3) of k≈ 1 can be calculated by slice method, and its anti-sliding safety factor is:
Shandong environmental geology collection
All the soil above L2 may slip and lose stability.
Stability of pit bottom against uplift
When the soft soil slope exceeds a certain high pressure, it will uplift and lose stability because the weight of the side soil exceeds the ultimate bearing capacity of the bottom soil. The safety production coefficient of pit bottom uplift resistance is calculated as follows:
Shandong environmental geology collection
Where, Nc is the bearing capacity coefficient of foundation, calculated according to Formula (3):
Shandong environmental geology collection
Therefore, anti-floor heave instability will not occur.
5 Slope shotcrete and anchor support structure design
According to the analysis of the natural stability of the slope, the design of the bolt-shotcrete support structure of the slope is shown in Figure 4. * * * Set 5 rows of anchor rods, 1 row in miscellaneous fill, which one shall be adopted? 48×3.5 steel pipe grouting anchor, 2 ~ 5 rows of anchor are bored grouting anchor, the aperture is 120mm, and the lacing is? 25 rebar is made of 32.5 ordinary silicon cement, 0.5 water cement ratio mixed with early strength agent, and grouted with cement slurry. The designed anchoring force is 12kn/m ... A row of steel pipe piles is pressed from 4m to play the role of advanced support.
Fig. 4 Schematic diagram of slope shotcrete and anchor support arrangement
32.5 ordinary silicon cement is used for shotcrete, and the aggregate is crushed stone and medium sand with the particle size of 5 ~ 15 mm, and the mixing ratio is cement: sand: stone: water = 1: 2: 2: 0.5, and the accelerator is added and erected. Anchor head 6.5 @ 200× 200 steel mesh? 16 transverse edge joint of reinforcement.
Stability analysis of slope supported by bolting and shotcreting
6. 1 Analysis of overturning stability
According to Figures 2 and 4, the overturning moment of the slope to the toe of the slope is the sum of the moments of the main dynamic earth pressure to the toe of the slope, and the overturning moment of the shotcrete-anchor support structure to the toe of the slope is the sum of the moments of the effective anchoring force outside the active fracture surface to the toe of the slope. The overturning safety factor can be obtained by the graphic method as follows:
Shandong environmental geology collection
Therefore, the slope supported by shotcrete and bolt will not overturn and lose stability.
6.2 Anti-sliding stability
When the slope slides along any cambered surface, the anchor rod outside the sliding surface is an effective anchoring section, and the sum of the effective anchoring forces on the sliding surface is the anti-sliding force of the anchor rod. The anti-sliding safety factor of bolting and shotcreting support structure can be calculated according to the following formula:
Shandong environmental geology collection
Where: Fj is the effective anchoring force of J-row bolt; βj is the included angle between J row anchor and sliding surface.
According to Figure 2 and Figure 4, the anti-sliding safety factor of L 1 and L2 sliding surfaces can be calculated according to Formula (5):
Shandong environmental geology collection
Shandong environmental geology collection
It can be seen that the bolting and shotcreting support structure meets the requirements of anti-sliding stability.
7 the stability of the construction process
Bolt-shotcrete supporting structure not only requires to ensure the stability of the slope after construction, but also must ensure the safety at any stage in the construction process. Shotcrete anchor support is excavation first and then support. When the slope is excavated to the bottom and the bottom row anchor is not constructed, the slope stability safety factor is the lowest. After checking by the above method, the safety factor of the slope at this time is:
Shandong environmental geology collection
Shandong environmental geology collection
It can be seen that the shotcrete-anchor supporting structure is safe in the construction process.
8 Application and conclusion
8. Application of1
The monitoring results of the day before foundation pit backfilling are as follows:
Shandong environmental geology collection
8.2 Conclusion
The project started at 5438+0 on April 5, 2006, and the backfill was completed on September 10 of the same year, which lasted for 5 months. Although it is rained in rainy season, the slope is basically stable, which shows that the supporting scheme of this project is feasible and the design is successful.
refer to
Chongqing Municipal Construction Committee. 2003. Technical Code for Building Slope Engineering. China Building Industry Press.