1. Introduction
Drilled shafts have a long history of use where soil strata immediately below the shallow ground are not strong enough to prevent bearing failures or excessive settlements. Drilled shafts develop loading resistance from a combination of tip and side resistances that are mobilized as the shafts are loaded. Unfortunately, a significant portion of a drilled shaft’s tip resistance is unusable due to the vertical displacement required for mobilization [
1,
2,
3]. This leads to larger diameter and deeper shafts or more numbers of shafts to be utilized than would be needed if the shaft tip capacity were also incorporated into the design. However, due to factors related to construction difficulties and higher costs, continuous attempts are being made to improve the performance of drilled shafts without increasing the diameter of the shafts [
4,
5].
Post grouting techniques have gradually become a trend to be used as a method to improve shaft performance. In this procedure, the compaction grout is pumped into the soil around the shaft; then, the mud around the shaft and the sediments at the tip of the shaft are penetrated, compacted and separated by the cement slurry. As a result, the mechanical properties of the soil around the shaft are improved and the bearing performance of the shaft is further enhanced [
6,
7,
8,
9,
10].
Full scale field tests have been carried out to study the reinforcement mechanism of post-grouting for drilled shaft by many researchers. Sinnreich and Simpson [
11] presented fourteen full-scale load tests (including nine grouted and five ungrouted shafts) conducted in various parts of the United States. They found that the improvement of tip grouting on the stiffness and/or overall capacity could not be shown in all circumstances. This could be due to differences in factors such as geological conditions, grouting equipment, grout properties and grouting pressure. Thus, further research into the enhancement mechanism of bearing characteristics of the grouted shafts is needed. Frizzi et al. [
12] discussed the effects of construction methods and drilling slurries on observed side shear and end bearing load transfer. Ruiz and Pando [
13] investigated the load transfer mechanisms of drilled shafts installed in sandy soils after grouting. Thiyyakkandi et al. [
14] analyzed the axial loading performance on various sized grouted piles by experiment and FEM method. Dai et al. [
15] conducted in situ static load test to illustrate the grouting effect of a large diameter grouted drilled shaft. The test results of drilling and coring showed that the return height of grout reached 15 m, and the number of standard penetration test (SPT) blows with grouting was generally increased. Day et al. [
16] described a method for quantifying improvements due to pre-mobilization and ground improvement performed on grouted drilled shafts. Subsequently, Wen et al. [
17] clarified that the bearing capacity of steel pipe micropiles embedded in marine soft clay soils can be significantly influenced by post-grouting craftwork.
In conclusion, the mechanism of post-grouting technique to improve the bearing performance of drilled shafts has been summarized into four factors [
5,
15,
18,
19,
20,
21,
22]: (1) compression of the soil under the shaft tip, (2) increase of the shaft tip area due to the formation of the grout bulb, (3) redistribution of residual stresses along the shaft due to the prestressing of the tip, and (4) grout is migrated and infiltrated along the axis by pressurization so that the soil mechanical properties on the shaft side are strengthened. These effects are summarized on
Figure 1.
Unfortunately, the aforementioned research mainly focuses on the drilled shaft in which only the shaft tip was grouted. However, there are few reports on the field test of large diameter tip and side combined grouting shaft. Therefore, full-scale load tests were reported herein to compare and discuss the bearing performance of four drilled shafts (three shafts were post-grouted and one shaft as control). Subsequently, the load-displacement response, load transfer mechanisms and mobilization of shaft resistance in large diameter drilled shafts after combined tip and side grouting are further explored through detailed measurement data of the shafts.
2. Site Description and Test Shafts Details
The field tests were conducted in Tai’an, China. All load tests were carried out at the same location. The geology of this area mainly consists of silty clay and coarse sand layers. Sufficient boreholes were sunk to obtain details of the soil layers distributions. Detailed soil profiles and corresponding physical characteristics along the axis of each test shaft can be obtained from
Figure 2 and
Table 1, respectively. As shown in
Table 1,
ω is the moisture content,
γ is the unit weight of soil,
c and
φ are cohesion and internal friction angle of soil, respectively. Sample preparation, test equipment, and test methods were all in accordance with Chinese standard GB/T 50123-1999 [
24].
Test shafts TS2, TS3 and TS4 were combined tip-and-side post grouted shafts, the ungrouted drilled shaft (TS1) was load-tested in the same site to quantify the improvement of shaft capacity after grouting. The bi-directional static test and top-down load test methods were adopted to determine the bearing capacity of shafts TS1, TS2 and TS3, TS4, respectively. The details of the four test shafts can be obtained by
Table 2. The elastic modulus of the concrete for the four test shafts
Ec is 31 GPa. Furthermore, the elevations of the strain gauges for test shafts TS1 and TS2, the ring-shaped grouting pipes along shafts TS2, TS3 and TS4 are all presented in
Figure 2. It should be noted that the grouting devices arranged for test shafts TS2, TS3 and TS4 are different. As shown in
Figure 2, two ring-shaped grouting pipes were fixed at the side of test shaft TS2, while three for test shafts TS3 and TS4, the spacing between the ring-shaped grouting pipes of grouted shafts are all 10 m. Moreover, three straight grouting pipes were fixed to the bottom of test shaft TS2, while four straight grouting pipes were fixed to the bottom of test shafts TS3 and TS4, respectively, and the bearing layers of the four test shafts were all sand layers with similar properties.