Experimental study on pile-end post-grouting piles for super-large bridge pile foundations

Abstract

The application of pile-end post-grouting piles for super-large bridge pile foundations in some important projects was introduced in this paper. There are totally 21 test piles. The maximum pile diameter varies from 2.5 m to 3 m, and the maximum length is 125 m; the bearing capacity of the post-grouting piles is over ten thousands tons. Based on the test results, the bearing capacity, displacement, and bearing characteristic before and after grouting were analyzed. The results show that the bearing capacity of the piles are increased in different degrees after grouting although the technical parameters, including the patterns of grouting pipes, pressure, dosages of cement, duration of grouting lasting time, are different. However, the obtained values are very discrete. In addition, the calculation formula for the post-grouting piles under specified grouting condition was deduced based on the statistics analysis results of 57 test piles. The research results have been applied in the design of bridge foundation.

Keywords

self-balanced testing technique / pile-end post-grouting / bearing capacity / grouting technology

Introduction

Pile-base post-grouting technique aims to increase capacity as well as decrease settlement. Firstly locate grouting pipes and devices to pile base, then press cement slurry through grouping pipes by high pressure injection pump after concrete reaches certain strength, which solidifies hole-base slime and mud cake around piles. By this method, certain inherent flaws of traditional construction technology for grouping piles are eliminated [1].

During grouting, grouting slurry diffuses into sediments and soil around pile base to reinforce pile base by permeating (coarse grained soil) and splitting (fine grained soil). Besides, grouting slurry diffuses upwards to a height of 10-15 m along mud cakes and soft turbulent layers to strengthen side friction. This suggests that pile-base grouting can not only enhance tip resistance but also increase side friction above the base within certain bounds.

In this paper, some engineering examples of pile-base post-grouting technique in super-long pile foundations of super-large bridges were presented, and an easy method for determining grouting quantity as well as calculation formulas of capacity after grouting was given [2-4].

Analysis of engineering example

Sutong Yangtze River Bridge

The main bridge of Sutong Yangtze River Bridge is a double-pylon cable-stayed bridge with a span of 1088 m, and the specific holes are 140 m+268 m+140 m continuous rigid frames. In addition, the approach bridge is composed of constant height prestressed concrete continuous beams with spans of 75 m, 50 m, and 30 m respectively. Foundations are bored piles. Ф2.5 m pile group foundation are used for the auxiliary piers of the main bridge and near pylons, Ф2.5 m bored pile for the far pylons and transition piers, Ф3.0 m bored pile for the main piers of the specific channel bridge, Ф1.8 m bored pile for the transition piers of the specific channel bridge and 75 m-span box girders, Ф1.5 m bored pile for the 50 m-span box girders, Ф1.2 m bored pile for the 30 m-span box girder and abutment. The lengths of piles are 57.5-118 m, and the number of piles is approximately 2580.

In order to obtain reliable construction parameters and design parameters of the Sutong Yangtze River Bridge, 4 tests on piles were taken, in which 9 contrastive piles were used. The parameters of the piles and ultimate capacity before and after grouting are shown in Table 1. The bearing capacity before and after grouting for SZ4 is shown in Fig. 1. According to the test results of the Sutong Yangtze River Bridge, the ultimate capacities are increased by 0.48-1.0 times after grouting.

Eastsea Bridge

Eastsea Bridge starts from the Luchao Harbour of Nanhui District in Pudong, Shanghai, China, spans over the northen part of the Hangzhou Bay, and reaches Xiaoyangshan Island, with a total length of 31 km. This project has complex natural surroundings such as wind, wave, and current, thereby it is difficult to construct bored piles offshore. Besides, for the limit of design and constructing experience for large diameter bored piles offshore, it is necessary to carry out tests on Φ2.5 m large bored piles of navigation opening bridge, guiding the design and construction of bored piles (see Table 2).

The self-balanced test method was used in the pile testing for the high capacity of large diameter bored piles in this project. In order to test the capacities of pile PM336 under main navigable spans before and after grouting, two load cells (upper load cell and lower load cell) were buried and tested separately. Both load cells were tested before grouting, and the lower was tested after grouting. The secondary navigable spans were tested with single load cell before and after grouting. The comparison of capacities for two test piles before and after grouting is available in Table 3. The test results of the piles before and after grouting are shown in Fig. 2.

It is obvious that the capacities are greatly enhanced after pile-base grouting. In Fig. 2, the Q-s curves before grouting decrease sharply under small loads and have great deviations from existed geological values, which attributes to long time interval between drilling and grouting, thick mud cake around piles and hole bases, and existing long-term stress relaxation for empty side. This leads to the decrease of side friction and tip resistance. After grouting, tip resistance is greatly increased. From testing and conversion curves, the capacity stability (bearing capacity and settlement) of the grouted piles far outweighs common ones.

Hangzhou Bay Sea-Crossing Bridge

The Hangzhou Bay Sea-Crossing Bridge lies between Haiyan, Jiaxing, Cixi, and Ningbo in Zhejiang Province. Spanning over Hangzhou Bay with six bi-directional lanes, it has a width of 33 m and a total length of 36 km. There are two channels, including northern channel and southern channel. The northern channel bridge with a main span of 448 m is a diamond-shape cable-stayed bridge with double-pylon and double-cable planes, and the southern channel bridge with a main span of 318 m is an A-shape cable-stayed bridge with single pylon and double-cable planes. Besides, the prestressed concrete continuous box girders with spans from 30 m to 80 m were used for the approach bridges.

In order to verify the validity of geological drilling data, the self-balanced test method was used to test the bearing capacity of single pile, the side friction of different layers, and the top resistance of pile foundation before and after grouting, which provides necessary basis for the optimization of pile designs. A total of 7 grouted piles were tested, the parameters are shown in Table 4.

Shanghai Yangtze River Bridge Project of Chongming Crossing-river Channel

The Shanghai Yangtze River Bridge Project of Chongming Crossing-river Channel connects Chongming Island and Changxing Island. It has super long-large diameter bored piles. To ensure successful construction and reliable structures and provide scientific evidence for foundation design and construction, according to the parameters given by Chinese Code, 4 piles were tested with the self-balanced test method, 2 load cells were buried to test the ultimate capacity before and after grouting.

The properties of soil layer in the pile areas are given as follows: ④layer, grey silt clay; ⑤1-1 layer, grey clay; ⑤1-2 layer, grey sandy clay with silt; ⑤2 layer, grey clayey silt; ⑦1 layer, grey sandy silt; ⑦t layer, grey sandy clay with silt; ⑦2 layer, grey silty sand; ⑨2 layer, grayish yellow-gray fine sand containing gravel; ⑨2t layer, grey-celadon silt clay; ⑩layer, grey-brown blue-grey silty clay; ⑪layer, grey silty sand with gravel; ⑪t layer, grey-brown silt clay. The parameters of grouting for the piles and the capacity are given in Table 5.

From Table 5, the bearing capacities of pile 61＃and 62＃are enhanced by 98.9% and 73.0% respectively after grouting. It is shown that grouting eliminates the effects of thick base sediment on pile 61# and greatly increases the tip resistance for both 61# and 62# piles; the upward penetration of cement slurry eliminates the impact of mud cake around pile within a certain range, improves the properties of pile-soil contacting interface, and enhances the total side friction. According to equivalent curves for contrast before and after grouting, the curves after grouting are more gentle than before grouting, and the capacities of pile are improved.

Calculation method of bearing capacity after grouting

The cases mentioned above suggest that although the grouting pipe style, grouting pressure, cement quantity, and duration of grouting are various during the process, and the soil layers at pile base are also different, the ultimate capacities of piles are increased to different levels after grouting. However, the increase of ultimate capacity are various, thus post-grouting process should be designed before construction for major engineering projects or multiple-base projects.

So far, the theoretical study of pile-base post-grouting technique has lagged behind its practical application, and its mechanism, effect, process optimization and durability have not been studied deeply. Therefore it is meaningful to establish an accurate and reliable estimating formula of large diameter base grouting bored piles for design and construction of real projects. The reasons are as follows:

1) Pile-base post-grouting technique has complicated mechanism but definite research object, i.e. the increase of bearing capacity for piles caused by the improvement of the strength of soft soil surrounding piles and the properties of pile-soil contacting interface;

2) Pile-base post-grouting technique has been broadly applied in bored piles, especially in large diameter bored piles, and its effects have been verified in many real projects.

3) Pile-base post-grouting process is relatively perfect. There are more than ten sorts of pile-base grouting devices today, and the principle of controlling construction is scientific and rational. The success ratio of construction is high.

4) Pile-base post-grouting technique has been widely used in foundation constructions of civil architectures and large bridges, so the results of bearing capacity for piles after grouting are important for constructors, designers and pile testers.

The contribution of pile-base post-grouting technique to pile capacity depends on soil features, such as bearing stratum, pile length, pile diameter, grouting quantity, and grouting pressure. Under normal conditions, the most important soil feature influencing grouting is bearing stratum. As the soil of bearing stratum turns from fine to coarse, the effects of grouting could be enhanced.

The experimental data of 57 piles using pile-base post-grouting technique were gathered (all of the test piles have rather large diameter and top displacement). Based on statistical analysis and referring to the study of vertical bearing capacity nowadays in China [5-8], for proper large diameter bored piles, the allowable bearing capacity of single pile after grouting can be calculated by the following equation:

(1)R_{\mathrm{a}}={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}\left[u{\displaystyle \sum _{i=\mathrm{1}}^n{\beta }_{\mathrm{s}i}{q}_{\mathrm{s}i}{l}_i}+{\beta }_{\mathrm{p}}{q}_{\mathrm{p}}{A}_{\mathrm{p}}\right],

where R_{\mathrm{a}} is the allowable value of single pile bearing capacity after grouting (kN); {\beta }_{\mathrm{s}i} is the reinforcement coefficient of side resistance of the ith layer of soil, and the value can be taken by referring to Table 6; modify pile side resistance in the range of 10 m to 15 m above pile base when grouting in saturated soil; modify pile side resistance in the range of 4 m to 5 m above pile base when grouting in unsaturated soil; {\beta }_{\mathrm{s}i} ＝ 1 for the range out of reinforcement area; {\beta }_{\mathrm{p}} is the reinforcement coefficient of pile base resistance, the value can be taken by referring to Table 6; q_{\mathrm{s}i} is the normal value of friction resistance between pile side and relevant soil layer (kPa); q_{\mathrm{p}} is the bearing capacity of soil at the pile base (kPa); l_i is the thickness of each layer (m); u is the perimeter of pile body (m); A_{\mathrm{p}} is the cross-sectional area of pile base (m²); n is the number of soil layers.

According to statistics, the technical standards of post-grouting technique are as follows:

1) Water cement ratio of grout shall be determined by saturation degree and permeability of soil. Water cement ratio is 0.5-0.7 for saturated soil, and 0.7-0.9 for unsaturated soil (0.5-0.6 for loosening gravel soil and gravel sand); water reducing agent shall be added to grout with low water cement ratio; accelerator shall be added to grout if the groundwater is flowing.

2) The end pressure of pile-base grouting shall be determined by the characteristic of soil layer and the depth of grouting point. Take 5-10 MPa for weathered rock, unsaturated clay soil, and silt, 1.5-6 MPa for saturated soil, the low value for soft soil and the higher value for compact soil. The time of loading should be no less than 5 min.

3) Flow rate of grouting should be no more than 75 L/min.

4) The design of grouting quantity shall take pile diameter, pile length, soil layer properties of pile base and pile side, and the increase of single pile bearing capacity into consideration. The grouting quantity can be figured out by the following formula:

G_{\mathrm{c}}＝{\alpha }_{\mathrm{p}}d.

The values of {\alpha }_{\mathrm{p}} are given in Table 7, d stands for the diameter of pile.

The comparison between the experimental value and calculated value is shown in Table 8. The maximum reinforcement coefficient of side resistance and top resistance should be adopted when calculating. According to Table 8, the errors between the experimental value and calculated value are small, proving that the formula is feasible.

Conclusions

The application of pile-base post-grouting technique in pile foundation of super-large bridges was discussed. After analyzing the results of the total bearing capacity, the side friction and top resistance before and after grouting from the static loading test, the conclusions are as follows:

1) Although grouting pipe style, grouting pressure, cement slurry quantity and duration of grouting are various in post grouting technique, and soil layers at pile top are various, grouting could enhance ultimate capacity in different degree. But the obtained values are discrete. Based on the statistical results, the calculation formula of bearing capacity under specified condition after grouting is given.

2) The values of ultimate capacity after grouting are increased by 8.4%-100% than before through load-bearing tests with 18 piles. Compared with un-grouted piles, the bearing capacity and load transfer behavior of piles are obviously improved by pile-base post-grouting. Based on the test results of post grouting, the technique shortens pile length, optimizes design, and achieves remarkable economic benefits.

3) Post grouting technique has evidently enhanced top resistance and side friction around pile-tops.

4) Pile-base post-grouting technique has a wide application prospect in bored piles of large heavy bridges, however the mechanism of post grouting in super-long piles and action effects of group piles needs further study.

References

[1] Bruce D A. Enhancing the performance of large diameter piles by grouting. Ground Engineering, 1986, 19(4): 9-15

[2] Fellenius B H, Kulesza R, Hayes J. O-Cell testing and FE analysis of 28-m-deep barrette in Manila, Philippines. Journal of Geotechnical and Geoenvironmental Engineering, 1999, 125(7): 566-575

[3] Gong Weiming, Dai Guoliang, Jiang Yongsheng, Xue Guoya. Theory and practice of self-balanced loading test for pile bearing capacity. Journal of Building Structure, 2002, 23(1): 82-88 (in Chinese)

[4] Dai Guoliang, Gong Weiming, Cheng Ye. Application of self-balanced testing and post grouting to large diameter and super-long piles. Chinese Journal of Geotechnical Engineering, 2005, 27(6): 690-694 (in Chinese)

[5] The Professional Standards Compilation Group of People’s Republic of China. Specifications for Design of Groundsill and Foundation of Highway Bridges and Culverts (JTJ024–85). Beijing: China Communications Press, 1986 (in Chinese)

[6] The Professional Standards Compilation Group of People’s Republic of China. Technical Code for Building Pile Foundations (JGJ94 – 94). Beijing: China Architecture and Building Press, 1995 (in Chinese)

[7] The National Standards Compilation Group of People’s Republic of China. Fundamental Code for Design on Railway Bridge and Culvert (TBJ10002.5-2005). Beijing: China Railway Press, 2005 (in Chinese)

[8] The Professional Standards Compilation Group of People's Republic of China. Code for Design of Building Foundation (GB 50007-2002). Beijing: China Architecture and Building Press, 2002 (in Chinese)

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg