Research and Application of Anchorage Vertical Loading System in Building Reinforcement and Rectification

Abstract

In this paper, the settlement and tilt of a building in north China are taken as an example. The differential settlement of the building is not stable, and the tilt continues to increase. After the rectification measures of forced landing by digging out the soil were taken, the expected effect was not achieved. Then the auxiliary rectification was carried out by means of centralized and cyclic loading through the anchorage vertical loading system, and the automatic dipmeter and manual monitoring are adopted in the process of deviation rectification to achieve information construction. At present, the construction of this project has been completed and the effect of reinforcement and rectification is good, which can provide reference for similar projects and has certain promotion and guidance significance.

1 Introduction

For the reinforcement and rectification of existing buildings, the commonly used measures mainly include jacking method and crash landing method. Compared with the jacking method, which has a long construction period, high cost and difficulty, the crash landing method is relatively simple. The crash landing method mainly includes loading, precipitation, stress relief and excavation method, etc. Relatively speaking, excavation method is the most commonly used because of its simplicity, short construction period and strong controllability. However, a single excavation method often has certain limitations. If the site soil layer is dense, or gravel, gravel and other inclusives are more, the excavation method is not easy to achieve the desired results. Excessive excavation may lead to the risk of sudden increase of building settlement. At the same time, in the process of grouting hole sealing after rectification, the amount of grouting is not easy to grasp due to the looseness of the soil, which is easy to lead to the tipping of the building in the process of grouting [1, 2].

This paper takes a building in the north as an example, the vertical loading system of anchor cable was adopted to assist the rectification of tilting on the basis that the forced landing failed to reach the expected effect. The system made use of the original raft of the building to cut holes in the raft for anchor cable construction, and the anchor end of the anchor cable entered a good soil layer, so that the anchor cable and the original raft formed a good vertical loading system. Through a number of jacks to carry out concentrated circulation loading to crash landing and prevent tipping. At the same time, automatic dip meter combined with manual monitoring is used in the process of rectification, so as to achieve information construction. The project has achieved good results and can provide some reference for similar projects.

2 Engineering project

2.1 Project Overview

The project is located in a city in the north of our country, the use functions above ground are houses, underground use functions for warehouse, equipment houses and parking, etc. There are 11 floors above ground and 1 floor underground. The width of the foundation is 15.95 m and the length of the foundation is 48.30 m. The structure type is shear wall structure, the foundation type is graded sand and stone replacement foundation, the foundation type is flat raft foundation, the thickness of the raft is 600 mm, and the design grade of the foundation is Class B.

The original foundation treatment scheme is as follows: remove all the soil above the layer of ③ silt (or ③1 gravel), then use graded sand and stone with a thickness of no more than 300 mm to backfill to the design elevation, the compaction coefficient is 0.97.

According to relevant construction and monitoring data, since the structure capping up to now, the overall relative settlement of the southwest corner is 5.18 cm, and the overall relative settlement of the southeast corner is 6.18 cm. The inclination rate of the relative settlement of the south and north sides exceeds the requirement of the design code by 0.3%. Field monitoring data show that the settlement of the tower is not stable and still presents a trend of continuing to increase.

2.2 Settlement Cause Analysis

The building site is generally high in the northwest and low in the southeast, showing a gentle slope shape. According to the survey report, the lithologic soil of the site consists of loess-like silt and silty soil on the upper part, with round gravel layer distributed among them, and pebble layer on the lower part, which can be divided into 5 layers from top to bottom, namely plain filled soil, loess-like silt, silt, gravel and pebble in turn, among which loess-like silt layer has collapseability. The typical geological profiles are shown in Figs. 1 and 2.

Fig. 1

North basement lithology map

Fig. 2

South basement lithology map

No groundwater was revealed within the scope of the survey. Considering the buried depth of the foundation of the proposed building, the adverse effects of the site’s water on the construction of the project are not considered.

According to the survey report, combined with the calculation model and the actual situation of the site, the main reasons for the building tilt are analysed as follows.

According to the geological survey report, there are ③ silty soil and ③1 gravel layers under the replacement padding layer, and the compression modulus of the silty soil layer is 8 MPa, and the compression modulus of gravel layer is 30 MPa. The north side of the building has many interlayers of ③1 gravel, and the thickness is thicker. The south side of the building basically has no ③ gravel layer.

Through modeling analysis, the overall center of gravity is south, and the eccentricity ratio is 2.88.

2.3 The Original Rectification Plan for Reinforcement

• (1) Reinforcement Scheme

The original reinforcement scheme adopts the form of external raft and micro pile to control the continued settlement of the building.

The design parameters of micro-pile are as follows, effective pile length is 19 m, the raft anchored in the upper part is no less than 0.5 m, ordinary welded pipe with DN80 wall thickness of 3 mm is selected for steel pipe, the total length of welded pipe is no less than 19.5 m, the diameter of micro-pile is 200 mm, the distance between piles is 1000 mm, PSA 32.5 cement slurry is injected into the hole and inside the welded pipe, and the water-cement ratio should be 0.5 ~ 0.55.

The width of the south external raft is 1.5 m, the concrete strength grade is C35. The rib plate is set on the new foundation to connect with the outer wall of the original basement, and the rib plate is set at the place of the original shear wall.

• (2) Inclination correction plan

The original rectification plan adopted the form of forced landing.

Two small drill hammers with equal spacing from east to west are used to cut soil simultaneously. For the first time, dig a deep hole every 2 m with a depth of 8 m and a diameter of no more than 8 cm (the diameter of drill pipe should be selected according to the site test drilling). The second dig is the middle position of the first dig hole, and the dig depth is 5 m. The third excavation is the middle position of the excavation holes after the two sides are encrypted, that is, the spacing of the excavation holes is 0.5 m, and the depth of the holes is 8 m. The fourth excavation is in the middle of the above excavation holes, with a depth of 10 m; Adjust whether to encrypt and increase the depth of excavation hole according to the effect of excavation and building tipping. When encountering the location of elevator shaft and catchment hole, dig under the catchment hole for excavation construction, with the depth of excavation not less than 12 m.

2.4 Effect Evaluation of the Original Reinforcement and Rectification Scheme

According to the reinforcement and rectification plan, the construction of micro pile and external raft was carried out first. The monitoring data showed that the settlement of the south side of the building increased more than that of the north side at the initial stage, indicating that the uneven settlement and tilt were still intensifying and did not show a convergence trend. After the construction of micro pile and external raft, the differential settlement trend gradually slows down.

After the construction of the external raft is completed, the north side of the work begins to dig. The excavation work is carried out symmetrically and synchronously with the micro-drill hammer. The depth, speed and position of excavation are determined according to the requirements of the program and the actual situation on site. Monitoring data show that with the progress of excavation work, the north side of the settlement is obvious, showing a relatively obvious inclination trend on the whole. With the progress of excavation work, the inclination trend gradually becomes stable, and the maximum inclination rate is inclined from 5.2‰ to about 4.0‰, which is still a certain gap from the standard requirements.

3 Anchorage Vertical Loading System Scheme

In view of the dense soil layer in the site of this project, there are gravel and boulders in the original stratum, and the original foundation treatment plan adopts graded sand and gravel backfill, resulting in more gravel and boulders in the excavation are. The excavation method is not easy to achieve the desired results, and excessive excavation may lead to the risk of sudden decline of building settlement. Therefore, in order to further accelerate the settlement rate of the north side of the building and avoid the tilting rebound when the grouting hole is closed after the building is rectified, the anchorage vertical loading system is adopted to assist the tilting correction in the northeast and north side of the raft overhanging area.

In this project, 6 anchor cables are set at the northeast side of the anchor cable loading system, with a spacing of 1 m and a locking value of 500kN. On the north side, set 24 anchor cables with a spacing of 1.5 m and a locking value of 500kN. The diameter of the anchor cable is 150 mm, three bundles of 15.2 steel strands are used, and the depth of the anchor cable is 20 m. The cement is P.O. 42.5 cement, and the water-cement ratio is 0.5 ~ 0.55. When the construction age of the anchor cable is satisfied, the 6 cables shall be taken as a group for simultaneous grading tensioning. The 6 cables on the east side shall be taken as a group, and the 6 cables on the north side (separated by 3 knots) shall be taken as a group. After the completion of the first group of tensioning, the next group of cables shall be taken until the tensioning of all the anchor cables is completed. In the tensioning process, automatic monitoring equipment is used for real-time monitoring, so as to achieve dynamic design and information construction. At the same time, the tensioning is carried out timely according to the slack situation of the anchor cable. After the correction is complete, the anchorage loading system will be locked as a permanent anchor cable.

4 Construction Process Control

The anchorage vertical loading system uses the original raft to make holes in the original raft, the construction equipment adopts the anchor rig, the construction technology adopts the double casing followed by hydraulic drilling into the hole, the pile end bearing layer into the pebble layer, so that the anchor cable and the original raft form a good vertical loading system, the raft is cyclically loaded by several jacks, thus increasing the overall settlement of the north side. At the same time, the anchor cable is tensioned twice a day, and the tensioning process is also carried out synchronously for multiple groups of anchor cables, so as to solve the situation of automatic unloading of the lock on the anchor cable after the raft settlement. After the rectification work is completed, in order to prevent the building from tilting back when grouting holes are sealed, each anchor cable would be locked.

5 Monitoring Data Analysis

In order to control the deformation of the building in the whole process of tilting correction, the monitoring means of automatic inclinometer combined with manual monitoring and dual control are adopted in the process of reinforcement and tilting correction. The two monitoring methods verify each other, complement each other with advantages, increase the credibility of monitoring data, and enable the monitoring to better serve the tilting correction construction and truly achieve dynamic design and information construction.

Manual tilt monitoring adopts the drop method for tilt measurement, and the measuring instrument is the total station. Before construction, the used instrument shall be inspected and calibrated according to the standard requirements. Each observation shall be carried out according to the same observation Angle and height interval, the same instrument shall be used, and the observation personnel shall be fixed.

Automatic incline monitoring is performed using a Geokon BGK6150-1 automatic incline meter, which uses a uniaxial sensor to accurately measure incline using a incline sensor attached to the monitored structure. A dip meter was set in the southwest corner and the northeast corner of the building respectively during the rectification process. The data collector adopted the BGK Logger data acquisition system, which was set to automatically collect data every 6 h for analysis. During the construction of the anchorage vertical loading system, real-time monitoring of the dip meter data with each level of load tension can guide the construction and control of vertical loading of the anchor cable, so that the tilting correction of the building can be controlled. The manual and automatic monitoring curves are shown in Fig. 3, 4, 5.

Fig. 3

Artificial tilt monitoring curve

Fig. 4

Automatic tilt west side monitoring curve

Fig. 5

Automatic tilt east side monitoring curve

At present, the rectification work has been completed. The whole rectification process has realized safety and control, and the overall tilt of the floor meets the standard requirements. The above data are the curves of manual monitoring and automatic monitoring.

Figure 3 shows the tilting change curve of the main body of the building monitored manually. It can be seen from the figure that the tilting trend is obvious in stage 1–1, that is, from the excavation on July 12 to around August 10, and the tilting trend gradually becomes gentle in stage 2–2 due to stratum conditions and other reasons. In stage 3–3, the tilting effect is obvious after the vertical anchor loading system combined with the crash landing. And the four corners all returned to within 0.3%, meeting the requirements of the specification. In Stage 4–4, the overall tilt trend tends to be stable after the grouting sealing in mid-September.

Figure 4 and Fig. 5 are the trend charts of automatic monitoring curves, among which Fig. 4 is the trend chart of the west side and Fig. 5 is the trend chart of the east side. The two curves are the trend graphs of the tilt curve after the vertical loading system of the anchor cable is adopted. According to the figures, the overall tipping effect is obvious after the vertical loading system of the anchor cable is adopted, and the trend tends to be consistent with the manual measurement.

6 Conclusion

Combined with an example of a building in northern China, when the original reinforcement and rectification measures failed to achieve the desired effect, the vertical loading system of anchor cable was adopted to rectify the tilting measures, and the effect was good. The main conclusions are as follows.

• (1) Appropriate tilting rectification measures should be selected according to the soil condition. For the dense soil layer, or the stratum with more gravel and boulders, it is not easy to achieve the expected effect with a single excavation method, and the anchorage vertical loading system can be used to assist tilting rectification.

• (2) The anchorage vertical loading system can be used to rectify the tilting of buildings. It uses the original raft to form a good loading system, and adopts several jacks to load the original raft in circulation, thus increasing the overall settlement of the building block. In complex geological conditions, excessive excavation may lead to a sudden increase in the risk of building settlement, is a very good measure to rectify the tipping.

• (3) The vertical loading system locks each anchor cable as a permanent anchor cable after the end of the tilting correction, which can prevent the inclined rebound in the process of building grouting hole sealing and the subsequent settlement stabilization process of the building.

• (4) Information construction should be carried out in the process of rectification and reinforcement, manual monitoring and automatic monitoring should be combined, and the two monitoring methods should verify each other, complement each other’s advantages, increase the credibility of monitoring data, so that the monitoring can better serve the rectification and rectification construction, and truly achieve dynamic design and information construction.