I. Overview of the Project
Building B, the reconstruction project undertaken by our unit, is a composite structure of φ300 bored pile and cap beam. The foundation is placed at the top of silty mudstone slope, and the height of the slope is about 12m. The silty mudstone has developed fractures, and the rock mass is in the landslide zone of 50 ~ 60. When the foundation is leveled, due to the construction in rainy season, the exposed rock mass is affected by rainwater infiltration, which leads to the local collapse of silty mudstone. Endangering the stability of building foundation and the safe operation of construction personnel. In this case, we immediately stopped the construction and reported to the general office of the company. After investigation and discussion with the company, in order to ensure the stability of the rock slope, we decided to anchor the rock slope with rivet anchor first, and then carry out the construction of bored pile in the second residential building. At that time, the treatment of dangerous rocks was generally to make a retaining wall outside after the anchor rod penetrated into the stable layer. In view of the fact that the rock layer here is not too broken, the sliding layer is riveted with rivet-type expanding head to prevent the dangerous rock from sliding down. This treatment method is very novel.
Rivet anchor, design diameter D= 125mm, each hole is welded with 3 secondary steel bars, and anchor mortar is M30 cement mortar. Five rows of rivet anchors are arranged, the vertical height is 12m, the horizontal distance of anchors is 1.65m, the mooring anchor section is not less than 5m, the free anchor section increases linearly with the height of rock slope, and the rivet concrete is C28.
Second, the anchorage construction preparation
Anchor rod is a new type of tension rod, one end of which is connected with the structure or retaining wall pile, and the other end is anchored in the bedrock, so that the rock and anchor rod bear all kinds of overturning forces together.
The construction difficulty of rivet anchor rod in this project lies in that the anchor rod can't collide with the vertical bored pile, and the drilling angle must be controlled accurately and can't deviate from left to right.
In view of the possible problems in construction, the following technical measures are formulated:
1. Set a vertical plane parallel to the foundation line of Building B to control the drilling angle and position.
2. Use a level gauge to control the level of the base, and use a self-made protractor to control the inclination angle and left and right deviation angle of the anchor rod. Make sure that the anchor rod forms an angle of15 with the horizontal plane.
3. Use φ 15mm galvanized water pipe threaded connection to detect drilling depth.
4. In order to ensure the low shrinkage and high durability of mortar, the water-cement ratio of mortar is controlled at 0.40, and 5‰ NNO is added.
Three, anchor construction procedures according to the actual situation, formulate process flow and strictly implement.
Fourth, the formation of pores.
After the steel pipe is erected, the position of the fixed hole is measured according to the design requirements, and the drilling technology of the anchor hole will directly affect the anchoring ability of the anchor rod. This project uses the rotary drilling rig commonly used in China to drill holes. Considering the development of cracks in the rock mass, and the rock mass is in the landslide zone of 50 ~ 55, in order to avoid the siltstone from further collapse, the drilling method is dry drilling. The machine is a 4m3 gas compressor, using air medium to clean holes, and using XY-2PB drilling rig produced by Chongqing Prospecting Machinery Factory. The weight of the whole machine is less than1.0t. The drilling rig can fix the rotating shafts with different angles to drill holes, and the angle between the anchor rod and the horizontal plane is effectively guaranteed to be 15 by using the self-made protractor. The bottom of the drilling rig is equipped with a walking frame, which is easy to move. The positioning of the drilling rig is accurate and convenient, which ensures the drilling quality.
The designed anchor hole diameter of the anchorage section and non-anchorage section is φ 125mm, and the dip angle is 15. Immediately after the hole is formed, check the hole, clean the anchor hole and grout the anchor.
Five, anchor grouting
Mortar is still used to block the non-anchored section, which reduces the special anti-corrosion treatment of the anchor rod. Grouting is the key stage of anchor construction, which directly affects the quality of anchor. When grouting, we should not only record relevant data, but also take effective measures to control grouting quality. In order to reduce the shrinkage of mortar during solidification, reduce the water-cement ratio as much as possible and increase the sand content, the mixing ratio is cement: sand: water =1:1.1:0.4, and add 5% NNO. The sand is ultra-fine sand with fineness modulus not less than 0.7 and silt content not more than 3%. The cement is 525 ordinary portland cement.
When grouting, insert the grouting pipe into the bottom of the hole and press the cement mortar into the hole with a mortar pump. Grouting pump's working pressure should be above 0.4MPa, and pressure grouting should be carried out until the overflow concentration at the orifice is consistent with the poured mortar. The demolition speed of grouting pipe should be controlled according to the mudstone area and hole depth, especially in the severely weathered mudstone area with developed cracks. According to the drilling records and the grouting situation of adjacent holes, the grouting pipe should be slowly demolished by vibration to ensure that it is always buried in mortar. When the grouting pipe is removed, the grouting mortar will vibrate slightly to reduce the generation of bubbles and prevent the grouting from loosening. After placing the grouting pipe, immediately insert the anchor skeleton and vibrate slightly. See section 3 for the installation position of steel skeleton. Then check whether the steel skeleton is in place. After the steel skeleton is in place, check the grouting situation, conduct secondary grouting and close the orifice. After the completion of the whole anchor construction, the strength can not be reached within three days, and the rivet construction of rivet anchor should be carried out immediately. See the detailed description of geometric dimension 5A for details. During construction, two pieces of steel mesh are added in the pile cap, and the spacing between steel bars is φ12 @100×100. After the pouring of the bearing platform is completed, the construction of the whole rivet anchor is announced.
Six, rivet anchor construction calculation
According to the rock stability analysis, determine the most unstable surface, and analyze the stress of the sliding surface according to Figure 6.
1, anchor resistance calculation:
H= 12m
l = h÷sin 55 = 12/0.8 19 = 14.65m
b = l×cos 55 = 14.65×0.574 = 8.4 1m
γ=2.5(t/m3) (rock bulk density)
A= 1.65 (m) (horizontal spacing of anchors)
F=0. 1 (coefficient of friction in sliding surface)
Rock gravity W=H? b？ Answer? γ/2
= 12×8.4 1× 1.65×2.5/2
=208 (ton)
F=F？ l？ A = 0./kloc-0 /×14.65×1.65 = 2.42 tons.
Application of engineering anchor tension N=K? P
K= 1.3 (safety factor against sliding)
P= anti-sliding tension of anchor rod
F= anti-sliding friction of rock mass
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