Keywords

1 Introduction

In recent years, with the continuous development of China’s construction industry, more and more construction project site selection involves deep and complex fill areas. Due to the uneven nature of deep and complex fill, loose structure, large thickness difference, low bearing capacity, high porosity, strong permeability and other factors, there are great risks in slope support. At present, the most widely used slope support method is pile-anchor support, which has good effect, simple construction, strong adaptability and strong economy [1,2,3,4].

This paper takes a project in northern China as an example. The project has a deep and complicated filling up to 20 m, which has great risks. In order to ensure the construction quality and safety of the project, the construction of ordinary anchor and double-pipe splitter were carried out respectively, and the multi-stage overtension test was carried out respectively, and the effect of improving the quality of anchor was comprehensively analyzed through the test data and quantitative indicators. Subsequently, numerical simulation combined with actual monitoring data was used for comparative analysis, the overall effect is good.

2 Engineering Project

2.1 Project Overview

The project is located in a city in northern China, the above-ground use function is a residence, the above-ground 4 floors, the underground 1–3 floors. The project is a permanent slope support project, including supporting piles, permanent anchor rods and a variety of retaining wall forms.

The proposed site is originally a mountain trench, the terrain is complex, the height difference is large, the site has been filled with soil treatment, the thickness of the site is about 20 m, the backfill time is about one year from the project construction, the site has been DDC construction treatment, after backfilling to form a permanent slope of 9−12 m.

In this project, the slope support system is adopted, and 3–5 anchors are set according to the slope height, the anchor aperture is 200 mm, the anchor length is 20–38 m, the steel strands are 4–5 1 * 7–17.8 (1860 MPa), the construction Angle is 35–40°, and the standard tension value is 290–570 kN. The locking value is 390–770 kN, the bolt adopts P.O 42.5 cement, the water-cement ratio is 0.5–0.55, and the construction technology adopts the casing follow-up bolt drill to make holes and the secondary split grouting.

2.2 Engineering Geological Condition

The special soil of this project is artificial fill and weathered bedrock.

The artificial fill is mainly mixed fill and gravel fill. The thickness of the layer is 0.5–20.0 m. According to the investigation, the artificial soil filling was formed when the site was leveled. The soil layer mainly includes mixed fill soil, gravel mixed fill soil, fully weathered sandstone, strongly weathered sandstone. The artificial filling time is short, about one year, the artificial filling composition is complex, the distribution is uneven, and the mechanical properties are poor. No groundwater was found within 20.0 m of the site.

2.3 Anchor Design Scheme

The design cement is P.O 42.5, and the water-cement ratio is 0.5–0.55. The first grouting pressure is 0.5 MPa, and the second high pressure grouting pressure is 2.0–2.5 MPa. When grouting, the grouting pipe should be inserted 250–500 mm away from the bottom of the hole, and the mixed cement slurry should be injected into the bottom of the hole and pumped outward from the bottom of the hole. Due to the shrinkage of the cement slurry, the grouting treatment should be done in time after grouting to make the hole section full of slurry.

In this test, the fifth anchor rod in the double-row pile area was selected. The anchor rod was composed of 4–17.8 (1860 MPa) steel strands. The anchoring section length was 15 m, the free section length was 5 m, the standard pulling force value was 290 kN, and the locking value was 390 kN.

According to the drawings, the maximum load of acceptance test is 1.5 Nak, and the maximum load of acceptance test of anchor rod on this road is 435 kN. In the process of tensioning lock acceptance, the multistage overtension verification test effect will be carried out according to the actual tensioning situation.

According to CECS 22 Technical Specification for Ground Anchors, the formula for calculating bolt displacement is shown as follows:

$$ \Delta {\text{s}} = \frac{{{\text{P}}*({\text{Lf}} + \frac{{{\text{La}}}}{2})}}{{{\text{E}}*{\text{A}}}} = \frac{435*(0.8*5)}{{1.95*4*193}}\sim \frac{435*(5 + 15/2)}{{1.95*4*193}} = 1.16{\sim}3.61\,{\text{cm}} $$
(1)

According to Article 9.4.6, “The total displacement measured by tension type anchor rod under the maximum test load shall exceed 80% of the theoretical elastic elongation value of the length of the free section of the rod body under the load, and be less than the theoretical elastic elongation value of the sum of the length of the free section of the rod body and the length of the 1/2 anchoring section”, In this project, the displacement of the bolt at the maximum load value of 435 kN is between 1.16 and 3.61 cm.

3 Double Tube Splitting Test

In view of the situation that the backfill soil in the field area is thick and the soil is loose and easy to collapse the hole, two ways of ordinary anchor rod and double pipe splitting are carried out respectively in this project. The double-pipe splitting technology is to add a second splitting pipe on the basis of the original one-time grouting and secondary splitting. Through the effect of double splitting, it can make up for the shortcomings that cement slurry is easy to lose in loose soil, so as to increase the solid holding force of anchor bolt and improve the overall anchoring strength.

In order to verify the test results, the maximum load value was increased to 1.95Nak during the tensioning process. The test records and curves are as follows Fig. 1 and Fig. 2.

Fig. 1.
figure 1

Q-s curve of ordinary anchor

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Fig. 2.
figure 2

Q-s curve of double pipe split bolt

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Through the comparative analysis of the above two sets of curves and corresponding load-displacement data, the results show that the displacement of the double-pipe split grouting test bolt is significantly higher than that of the ordinary anchor, which indicates that the construction quality of the anchor can be improved by the double-pipe split grouting method for the loose soil and other easily collapsing holes.

In this project, the construction technology of double-pipe split-grouting is adopted in the deep and complicated fill areas. At present, all the anchor rods in the site have been constructed, and the construction quality is good. The basic test and acceptance test meet the requirements.

4 Numerical Simulation

Taking a section of pile and anchor area as an example, the numerical simulation of pile and anchor support is carried out, and the numerical simulation results are compared with the measured results.

In the numerical simulation area, the pile diameter is 1.2 m, the pile length is 19 m, and the pile spacing is 2 m. The top of the pile has a bolt, the bolt bore is 200 mm, composed of 4 steel strands, the anchoring section length is 25 m, the free section length is 7 m, the standard pulling force value is 400 kN, the locking value is 540 kN. The soil layer in the pile and anchor supporting area consists of fill soil layer, gravel layer and strong weathered sand rock layer from top to bottom. The thickness from the top elevation of the pile to the bottom is 15,5 and 20 m, and there is no groundwater in the depth range.

After the establishment of the slope support model, the software including gravity self-stability, pile construction, anchor cable construction and slope excavation will carry out corresponding calculations to solve the internal force and deformation increment of the support system, and realize the controllability of simulated excavation. The simulation and analysis results are as follows Fig. 3, 4, 5, 6 and 7.

Fig. 3.
figure 3

Pile-anchor support model

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Fig. 4.
figure 4

Displacement cloud map of advance support of slope protection pile

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Fig. 5.
figure 5

Cloud map of advance support of anchor cable

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Fig. 6.
figure 6

Horizontal displacement contour map

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Fig. 7.
figure 7

Maximum horizontal displacement curve

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The maximum horizontal displacement generated by the set monitoring record is shown in the Fig. above. It can be seen from the simulation results that the maximum horizontal displacement of the pit wall occurs in the middle and lower part of the foundation pit slope, and the horizontal displacement of the soil behind the slope gradually decreases in the direction away from the supporting structure. The simulated horizontal displacement of the slope top is 8.7 mm and the maximum horizontal displacement is 10.1 mm.

5 Monitoring Data Analysis

Up to now, the maximum value of horizontal displacement monitoring of the actual supporting section slope top in the test area of this project has reached about 8 mm and gradually tends to be stable, which is similar to the calculated results of numerical simulation and more conservative than the results of numerical simulation, and generally meets the requirements of monitoring and alarm value. According to its changing trend, the horizontal displacement gradually increases with the construction of pile protection, anchor cable and slope excavation. With the completion of soil consolidation on the pile side, the curve gradually becomes stable.

6 Conclusion

In this paper, combined with a deep and complex soil filled slope project in north China, the double-pipe splitting scheme was first used to study the improvement of the construction quality of anchor rod, and then numerical simulation combined with actual monitoring was used for comparative analysis, with good results. The main conclusions are as follows.

  1. (1)

    Deep and complicated fill is harmful to building slope, so it should be paid attention to and measures should be taken.

  2. (2)

    For the pile and anchor support system of slope in deep and complex soil filling area, the displacement and deformation of double pipe split grouting are reduced compared with that of ordinary bolt. For loose soil and other strata prone to collapse holes, the construction quality of bolt can be improved by double pipe split grouting.

  3. (3)

    FLAC3D software was used to numerically simulate the support situation of pile and anchor slope. After the advance support of slope protection pile and anchor cable, soil was excavated, and the excavation and displacement conditions were studied. Through the comparison between the simulated situation and the actual monitoring situation, it was found that the numerical simulation was basically consistent with the actual situation.