Keywords

1 Introduction

Adopting new pile foundation reinforcement scheme for existing buildings foundation reinforcement and correction is more common reinforcement scheme, and each part of the existing buildings foundation quality accident is often accompanied by foundation uneven settlement, and the problem of excess tilt, therefore, in the process of strengthening full tilt, with uneven sheet pile can reduce the deformation, optimization of basal counterforce distribution difference.

2 Project Overview

The main buildings of this project are shear wall structure, raft foundation, foundation buried depth of about 2.5–3.5 m. Among them, Building B has 15 floors above ground and 1 floor underground, and the basement is connected to the underground garage. The surface layer of the site is plain fill soil, and the underlying layer is Quaternary and Tertiary Marine sedimentary soil. From top to bottom, it is divided into 5 geotechnical engineering layers. The measured buried depth of stable water level is about 3 m. The typical geological profile is shown in Fig. 1, and the physical and mechanical parameters of the soil layer are shown in Table 1.

Fig. 1
An illustration of 3 vertical beams Z K 17, Z K 18, and Z K 19 with 5 layers in between that are labeled Q m, Q m 4, Q m d, and N 2.

Typical geological profile

Full size image
Table 1 Physical and mechanical parameters of soil layer
Full size table

The basement soil of Building B is layer ② which is Medium sand with a bearing capacity characteristic value of 120 kPa. The superstructure design requires that the bearing capacity after foundation treatment is 300 kPa and 250 kPa, respectively. The bearing capacity and deformation of natural foundation can not meet the requirements of superstructure. The original foundation treatment scheme uses cement soil mixing pile composite foundation.

The design parameters of composite foundation of raw soil–cement mixing pile in Building B are as follows: The characteristic value of bearing capacity of single pile is 300kN, the characteristic value of bearing capacity of composite foundation is 250 kPa, the pile length is 15−18 m, the pile end falls in the fourth layer gravel sand layer, the pile spacing is 1 m and 1.2 m, the pile diameter is 500 mm, the strength grade of cement body of the pile is not less than 5.1 MPa, the thickness of mattress is 250 mm.

Since the construction of the main structure in July 2012, up to October 2013 before the foundation reinforcement, the settlement of the two buildings has been too large, with the settlement of building A reaching 30 cm and the settlement of Building B approaching 20 cm, and the inclination reaching 4.76‰. The settlement and differential settlement have no convergence trend. Therefore, it can be judged that there are defects in the construction quality of soil–cement mixing pile of this project, which has not reached the expected purpose of design.

3 Calculation Method and Results

When the existing building is reinforced and the new piles under the base pass through the soft soil layer and enter the distributed friction piles of relatively good soil layer, the settlement of the middle point of the reduced-sink composite pile foundation can be calculated according to the following formula [1, 2].

$$ S = \psi \left( {S_{s} + S_{sp} } \right) $$
$${s}_{s}=4{p}_{0}\sum_{i=1}^{m}\frac{{z}_{i}{\overline{\alpha }}_{i}-{z}_{(i-1)}{\overline{\alpha }}_{(i-1)}}{{E}_{sic}}$$
$${s}_{sp}=280\frac{{\overline{q} }_{su}}{{\overline{E} }_{s}}\cdot \frac{d}{({s}_{a}/d{)}^{2}}$$
$$ p_{o} = \eta_{p} \frac{{F - nR_{a} }}{{A_{c} }} $$

s- settlement amount at the center of pile foundation;

ss– midpoint settlement caused by additional pressure of foundation soil at the bottom of cap (Fig. 2.);

Fig. 2
An illustration of the layers of the foundation. The natural ground elevation is followed by the elevation of the base, 3 vertical beams, and layers i plus 1 and i with Z i minus 1, Z i, and Z n from top to bottom. The average additional stress coefficient curve is along the layers.

Stratification diagram of settlement calculation of reduced sink composite pile foundation

Full size image

ssp– settlement caused by pile-soil interaction;

po– the hypothetical average additional pressure of natural foundation calculated by the combination of quasi-permanent values of load effects (kPa);

Esic– The compression modulus of the soil under the bottom of the cap shall be the modulus of the soil loaded after unloading;

m– The number of soil layers in the depth range of foundation settlement calculation; The calculated depth of settlement can be determined by stress ratio or deformation ratio.

qsu、Es– the average thickness-weighted ultimate friction and average compression modulus of the pile side within the range of the pile;

d– Pile diameter, when it is a square pile, d = 1.27b(b is the length of the section side of the square pile);

sa/d– equivalent distance to diameter ratio;

zi,zi-1– the distance between the bottom of the cap and the bottom surface of the i and i-1 layers;

αi,αi-1– the average additional stress coefficient of corner points from the bottom of cap to the bottom of the i and i-1 soil layers; Rectangle aspect ratio a/b and depth aspect ratio zi/b = 2zi/Bc were calculated according to equivalent cap area, which were determined by Appendix D of Technical Code for Building Pile Foundation (JGJ 94–2008). The equivalent width of the cap: Bc = B \(\sqrt{{A}_{C}}\)/L,B、L is the width and length of the outer edge plane of the building foundation;

F– Total additional load (kN) acting on the bottom of the bearing platform under the combination of quasi-permanent value of load effect;

ηp– Influence coefficient of foundation pile penetration deformation; According to the soil quality of the bearing layer of pile end, the sand is 1.0, the silt is 1.15, and the viscous soil is 1.30.

Ψ– The experience coefficient of settlement calculation is 1.0 if there is no local experience.

Building B has 14 floors above ground and 1 floor underground. The settlement amount at the maximum point (north side of the building) is 299.96 mm, at the minimum point (south side of the building) is 240.88 mm, and the maximum differential settlement amount from south to north is 59.08 mm. Therefore, uneven pile distribution is adopted to reduce subsidence and correct deviation in the north and south areas of the project. The steel pipe pile is φ299 mm/φ245 mm directly, the pile length is about 18 m, and the pile end falls on the gravel sand layer in layer 4. The bearing capacity of φ245 mm single pile is 250 kN, and that of φ299 mm single pile is 300 kN. The pile replacement ratio on the north side of the foundation is about 62%, and the pile replacement ratio on the south side is about 35% (Fig. 3.).

Fig. 3
A layout plan has multiple blocks of different sizes and beams. The top portion of the plan is highlighted with a rectangular box.

Layout Plan of pile reduction for Building B ( the area is the north side of the foundation)

Full size image

According to the results, the reaction force of the base on the south side of the foundation is about 50 kPa−200 kPa, and the reaction force of the base on the north side of the foundation is about 50 kPa−110 kPa, and the stress concentration state appears at the base side (Fig. 4, Fig. 5).

Fig. 4
A graphical representation with multiple irregular rectangles nested within one another. An irregular semicircle is at the center, within the rectangles.

Reverse diagram of basement soil of Building B

Full size image
Fig. 5
A line graph of base pressure plots a decreasing line. The values of base pressure along the line include 120, 50, 60, 50, 100, and 180 kilopascals. Values are estimated.

Pressure curve of the axial base of Building B (vertical)

Full size image

Through the adjustment of uneven pile distribution, the settlement gradually decreases from south to north. The new settlement in the south of the foundation is about 36 mm, and the north is about 21 mm, the settlement difference in the central axis is 15 mm. The foundation tilts to the south, and the foundation tilts back about 0.0013L. It shows that the variable stiffness leveling design of uneven pile can adjust the uneven settlement of foundation (Fig. 6, Fig. 7).

Fig. 6
A graphical representation on a grid plane plots concentric irregular semicircles.

New deformation of Building B after reinforcement

Full size image
Fig. 7
A line graph of z plots a decreasing line. The values of z along the line include 20, 25, 28, 30, 34, and 36 millimeters. Values are estimated.

New settlement curve of the central axis of Building B (vertical)

Full size image

4 Comparative Analysis of Engineering Measurement and Design [3]

After the settlement of the building stabilized, the average measured soil stress between piles on the north side of the foundation of Building B was 45 kPa, on the south side was 85 kPa, and the average stress on the top of the mixing pile was 149 kPa. According to the results, the base reaction on the north side of the foundation was 80 kPa, and on the south side was 105 kPa (Fig. 8, Fig. 9).

Fig. 8
A multiline graph plots prestress of soil between piles versus dates with 12 increasing lines. The line for 4155 is above all other lines.

In-situ stress-time curve of soil between piles in Building B

Full size image
Fig. 9
A double line graph of pile head stress versus time plots 2 increasing lines for 4156 and 4142. The lines start from (May 5, 2016, 40) and (May 5, 2016, 20) respectively, and overlap from (March 3 2017, 120). Values are estimated.

Stress-time curve of pile top of cement soil pile in Building B

Full size image

In November 2013, the north pile was connected with the foundation and sealed. By May 2017, the measured post-construction deformation was 32 mm−46 mm. Excluding the settlement generated according to the original settlement rate, the construction disturbance settlement was about 15 mm, and the new settlement was about 17 mm−31 mm.

5 Conclusion

After the uneven pile distribution method was adopted in this project, the average measured soil stress between piles on the north side of the foundation was 45 kPa, on the south side was 85 kPa, and the average stress on the top of the stirred pile was 149 kPa. The pile replacement ratio of the north side of the foundation (the side with large settlement) is 62%, the calculated base reaction value of the north side of the foundation is 80 kPa, the pile replacement ratio of the south side of the foundation (the side with small settlement) is 35%, the calculated base reaction value of the south side of the foundation is 105 kPa. The new settlement in the south of the foundation is about 36 mm, in the north is about 21 mm, and the foundation receding is about 0.0013L. The results show that the distribution of basal reaction can be adjusted successfully by the method of uneven pile distribution, and the effect of subsidence reduction and deviation correction can be achieved.