###### Abstract  
This paper presents a field study on the axial behavior of four large-diameter drilled shafts embedded in coarse sand. The grouting and loading test procedures were reported. The bearing capacity of shafts (TS1 and TS2) and grouted drilled shafts (TS3 and TS4) were herein determined by the bi-directional static test and top-down load test, respectively. The enhancement mechanism of bearing characteristics of the grouted shafts was discussed in detail. The test results indicate that the bearing characteristics and load transfer mechanisms of the test shafts were significantly affected by the quantity of pressurized cement slurry and the mechanical properties of the soil surrounding the shafts. Furthermore, the tip resistance of shaft can be mobilized more rapidly and fully after grouting, the side and tip resistance are mobilized in a more synchronized and coordinated manner due to the pre-mobilization of the grouted cement. Additionally, the standard penetration test (SPT) prediction model was introduced to calculate and predict the SPT blow counts of soil after grouting. The results show that the post grouting has a more obvious improvement on the strength of cohesionless soil. drafted shaft; bi-directional static test; top-down load test; standard penetration test (SPT) show counts; enhancement mechanism; load transfer characteristic +
Footnote †: journal: Applied sciences  
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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. [PERSON] et al. [12] discussed the effects of construction methods and drilling slurries on observed side shear and end bearing load transfer. [PERSON] and [PERSON] [13] investigated the load transfer mechanisms of drilled shafts installed in sandy soils after grouting. [PERSON] et al. [14] analyzed the axial loading performance on various sized grouted piles by experiment and FEM method. [PERSON] 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. [PERSON] et al. [16] described a method for quantifying improvements due to pre-mobilization and ground improvement performed on grouted drilled shafts. Subsequently, [PERSON] 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 [18; 19; 5; 15; 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.  
Figure 1: Drilled shaft load transfer after grouting [23]. (**a**) Factor 1. (**b**) Factor 2. (**c**) Factor 3. (**d**) Factor 4. 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.