Effect of Base Grouting on the Bearing Capacity of Bored Piles

Abstract

Post-grouting is a widely used technology for ground improvement and the strengthening of bored pile foundations. In situ experiments have shown that post-grouting is effective in improving the bearing capacity of bored piles. However, due to the cost and complexity of the in situ tests, the total number of test piles is limited. Therefore, it is not surprising that uncertainty is always associated with the collected data. This may significantly affect the conclusion of analyses, and most current studies have neglected this effect. In this paper, statistical analyses were carried out in order to investigate the effect of uncertainties in the data and the impact of post-grouting on the bearing behaviors of bored piles. The results show that post-grouting can significantly improve the bearing behavior of piles. This study is expected to provide technical guidance for post-grouting in foundation engineering.

1. Introduction

Bored pile foundations are commonly adopted in projects such as high-rise buildings, bridges, and viaducts due to the heavy load from the superstructure [1]. During construction, post-grouting is usually applied for the optimization of the pile design or the strengthening of the existing pile foundations. In China, bentonite mud is commonly used to prevent the collapse of drilled holes, and some of the mud may be retained between the pile and the soil. The mud skin out of the pile may weaken the skin friction of the pile [2,3]. The post-grouting technique, which injects premixed cement grout into the soil around the pile, can improve the mechanical properties of the soil and strengthen the pile-bearing capacity [4,5,6]. Grouting technology has been widely used across the world and adopted in many codes of practice, such as the British Standard “Execution of special geotechnical work—Bored piles” (BS EN 1536: 2000), the Chinese Standard “Technical Code for Building Pile Foundations” (JGJ 94-2008) and “Specifications for Design of Foundation of Highway Bridges and Culverts” (JTG 3363—2019). Zhu, et al. [7] proposed a new model to characterize the anisotropic behavior of nonlinear flow in fractured media, which could be used to describe the movement of slurry used for grouting. Kwaw, et al. [8] examined the effect of bond water on solute transport. They found that with increasing the bond water content to 16.4%, the transport pattern of the solute will turn from moderately non-Fickian to highly non-Fickian, which could shed light on solute transport in slurry. Liang, et al. [9] evaluated the influence of water on the mechanical properties of sandstone, which could provide insights into the impact of grouting on the base rock or soil. However, the majority of related research is based on in situ tests. Huang and Gong [10,11,12] conducted comparative analyses for large-diameter bored piles with and without grouting in the Sutong Yangtze River Bridge project. The comparison results showed that the bearing capacity of the pile could be significantly improved by adopting post-grouting technology. Dai, et al. [13] conducted bidirectional loading tests for six large-diameter bored piles in the second phase of the Sutong Yangtze River Bridge project and obtained similar observations. Employing bidirectional loading tests in the Hangzhou Bay Bridge project, Zhang, et al. [14] observed that base grouting (injecting cement grout at the pile toe) could improve the base-bearing capacity of the pile and reduce the settlement of the pile head. In the Suramadu Strait Bridge project, Huang, et al. [15] observed that post-grouting technology could increase the base-bearing resistance by 24.7–40.9%, the side friction by 9.2–32.1%, and the ultimate bearing capacities by 16.6–33.7%. Dai and Wan [16] conducted ten pile load tests in the Taizhou Bay Bridge project and found that base grouting could improve both the end-bearing capacity and the side skin friction. Xu, et al. [17] investigated the long-term bearing behaviors of bored piles with base grouting and found that the pile with base grouting had less settlement than the pile without base grouting within the same period of monitoring. However, due to the high cost and additional labor involved in the static pile load test, the total number of test piles is limited. The test data are scattered within a certain range. The analyses that were carried out on the small amount of data might lead to uncertainty in practical engineering, and this uncertainty has not been extensively studied.

In this paper, bidirectional tests were conducted on the pile in the Haimen–Tongzhou section of the Tongxi Expressway in Nantong City. The bearing capacity of the pile with and without the base grouting were determined and compared. Subsequently, statistical analyses were conducted to evaluate the performance of the base grouting on the improvement of the bearing capacity of the bored pile.

2. Background of the Project

The Haimen–Tongzhou section of the Tongxi Expressway is located in Nantong, Jiangsu Province, China, and plays a significant role in local transportation. It is a crucial part of the Nantong–Wuxi Expressway network. In general, this project is divided into three districts: the first one includes the main viaduct line in Haimen City, the G40 super overpass, Tongqi Canal Bridge, and the overpass across Wangchuan Line; the second one includes the Super Bridge over Tonglu Canal, the overbridge across the middle of Y211 and the overbridge across 228; and the third one consists of Shijiang Bridge, Dredging Channel Bridge, Jianghai River Bridge, and the overpass across the main line of G15. The typical geological strata information of these districts are shown in

Table 1. Information of strata.

District Number	Description of the Strata	Depth Distributions of the Stratum
1	① layer, loose plain fill	2.40 m~0.02 m
	② layer, soft plastic silty clay mixed with silt	0.02 m~−1.28 m
	③1 layer, slight dense silt blended with silty clay	−1.28 m~−11.28 m
	③1a layer, flow plastic muddy silty clay mixed with silt	−11.28 m~−21.28 m
	③2 layer, slight dense to medium dense silty sand incorporated with silt	−21.28 m~−28.78 m
	③2a layer, soft plastic silty clay interbedded with silty sand	−28.78 m~−41.88 m
	④1 layer, soft plastic to flow plastic muddy silty clay	−41.88 m~−45.38 m
	④2 layer, soft plastic silty clay blended with silty sand	−45.38 m~−47.60 m
	④2a layer, medium dense silty sand blended with silty clay	−47.60 m~−49.40 m
	④2b layer, medium or dense silty sand incorporated with silt	−49.40 m~−55.80 m
	⑤ layer, dense fine sand	−55.80 m ~−65.91 m
	⑤a layer, dense medium sand	−65.91 m~−85.00 m
2	①1 layer, slightly wet and plastic silty clay	3.90 m~2.27 m
	①2 layer, flow plastic muddy silty clay	2.27 m~−0.63 m
	①2a layer, slightly wet and plastic silty clay	−0.63 m~−3.33 m
	①2c layer, slight dense and very wet silt	−3.33 m~−12.33 m
	①3 layer, slight dense and saturated silty sand	−12.33 m~−22.33 m
	②3 layer, medium dense and saturated silty sand	−22.33 m~−32.60 m
	②3a layer, wet and plastic silty clay mixed with silty	−32.60 m~−34.10 m
	③2 layer, wet and plastic silty clay	−34.10 m~−41.13 m
	③2c layer, medium dense and saturated silty sand	−41.13 m~−54.60 m
	③3 layer, dense and saturated silty sand	−54.6 m~−65.00 m
3	①1 layer, plain fill	3.60 m~0.65 m
	①2 layer, silty clay	0.65 m~−2.85 m
	①2c layer, silt	−2.85 m~−4.55 m
	①3 layer, silt	−4.55 m~−11.35 m
	②3 layer, silt sand	−11.35 m~−18.55 m
	②3a layer, silty clay	−18.55 m~−36.40 m
	③2 layer, clay	−36.40 m~−46.90 m
	③2c layer, silty sand	−46.90 m~−60.00 m

.

In this project, 23 static loading tests were conducted. All the tested piles had a diameter of 1800 mm. In the first district, three piles without grouting and six piles with base grouting were selected for the test. In the second district, three piles without grouting and another three piles with base grouting were selected. In the third district, four piles without routing and four piles with base grouting were tested. The properties of the pile and grouting are illustrated in

Table 2. Properties of the test piles and quantities of grout for each pile.

District Number	Pile ID	Reference Bore Hole	Pile Length (m)	Bearing Stratum	Design Load (kN)	Grouting or Not	Quantities of Grout (t)
1	107# Pier/4# Pile	QSZK28	67	dense medium sand	5700	Non-grouting	—
	107# Pier/6# Pile	QSZK28	67	dense medium sand	5700	Grouting	4.1
	106# Pier/5# Pile	QSZK28	67	dense medium sand	5700	Grouting	4.1
	29# Pier/2# Pile	QSZK42	75	dense medium sand	7400	Grouting	4.9
	29# Pier/4# Pile	QSZK42	80	dense medium sand	8500	Non-grouting	—
	29# Pier/6# Pile	QSZK42	80	dense medium sand	8500	Grouting	4.9
	13# Pier/Left 1# Pile	QSZK40	65	dense fine sand	6000	Non-grouting	—
	13# Pier/Left 3# Pile	QSZK40	65	dense fine sand	6000	Grouting	4.1
	14# Pier/Left 2# Pile	QSZK40	63	dense fine sand	5500	Grouting	4.1
2	12# Pier/1# Pile	ZK3036	60	dense and silty sand	5504	Non-grouting	—
	12# Pier/6# Pile	ZK3036	60	dense and silty sand	5504	Grouting	4.1
	2# Pier/1# Pile	ZK3050	61	dense and silty sand	4218.3	Non-grouting	—
	2# Pier/3# Pile	ZK3050	61	dense and silty sand	4223	Grouting	3.3
	7# Pier/1# Pile	ZK3003	60	dense and silty sand	5426.7	Non-grouting	—
	8# Pier/6# Pile	ZK3003	60	dense and silty sand	5428.8	Grouting	4.1
3	17# Pier/1# Pile	ZK6127	59	silty sand	5700	Non-grouting	—
	22# Pier/3# Pile	ZK6127	57	silty sand	5670	Non-grouting	—
	16# Pier/1# Pile	ZK6160	57	silty sand	5670	Grouting	4.0
	22# Pier/6# Pile	ZK6160	57	silty sand	5670	Grouting	4.0
	26# Pier3# Pile	ZK3098	55	silty sand	5765	Non-grouting	—
	27# Pier/1# Pile	ZK3098	55	silty sand	5736	Grouting	4.0
	24# Pier/4# Pile	ZK6207	58	silty sand	5445	Non-grouting	—
	24# Pier/3# Pile	ZK6207	58	silty sand	5445	Grouting	4.0

. As shown in Table 2, the lengths of the tested piles vary from 55 m to 80 m, the designed loads vary from 4218.3 kN to 8500 kN, and the quantities of the grout for each pile range from 3.3 t to 4.9 t.

3. Program of Grouting and Bidirectional Load Test

3.1. Program of Grouting

The flowchart of the grouting for the bored piles is illustrated in Figure 1. The grouting hose was installed before casting, and then cast in the pile. When the concrete strength reached the design strength, the cement grout was injected through the grouting hose. The external high-pressure grouting system is shown in Figure 2.

In this project, the cloud control system was adopted for the termination of grouting. The cement (P.O 42.5) was temporarily stored in the cement tank and premixed with water and other admixtures in the mixer. Subsequently, the premixed cement grout was transferred to the slurry pump and injected to the side and base of the piles through the grouting hose.

3.2. Program of the Bidirectional Load Test

The bidirectional load test, as shown in Figure 3, was adopted in this paper. As shown in Figure 3, the main component of the bidirectional load test is the loading box cast in the pile at a preselected depth [18]. This loading box separates the pile into two portions: the upper portion and the lower portion. The load is applied to the pile by pumping the oil into the loading box, and the loading box transmits the load to the upper portion and lower portion simultaneously. The pile side friction force and the self-weight of the pile balance the load transmitted from the loading box to the upper portion, while both the side friction and the end-bearing resistance balance the load transmitted from the loading box to the lower portion [19]. The equivalent top-loaded Q-s curve is gained by combining the Q-s curves of both the upper and lower portions.

In this project, all the tests were performed following the procedure proposed by the “Static loading test of foundation pile—Self-balanced method” (JT/T 738-2009). The maximum loads were taken as 2 times the ultimate bearing load estimated by the method recommended by “Technical Code for Building Pile Foundations” (JGJ94-2008). The displacement data were collected every 5th min, 10th min, 15th min, 30th min, 45th min, and 60th min by displacement sensors for each step. The next step was started when the settlement in the previous step became stable.

4. Results and Discussion

The Effect of Base Grouting on the Ultimate Bearing Capacity of Piles

The original Q-s curves are illustrated in Figure 4, Figure 5 and Figure 6. The settlement is set as positive when the pile moves upward and negative when it moves downward. The upper curves characterize the displacements of the upper portions, while the lower curves represent the displacements of the lower portions. By using the method recommended in the “Static loading test of foundation pile—Self-balanced method” (JT/T 738-2009), the Q-s curves from the bidirectional load test were converted into Q-s curves (as shown in Figure 7, Figure 8 and Figure 9) for the conventional loading condition (i.e., applying the load on the pile head).

Based on the Q-s curve shown in Figure 7, Figure 8 and Figure 9, the ultimate bearing capacity (UBC) of the piles is computed and illustrated in

Table 3. The UBCs for the piles with and without the grouting based on the Q-s curve.

District	Pile ID	Designed Load (kN)	UBC Computed from Q-s Curve (kPa)	Percentage of Increase in UBC Due to Grouting
1	107# Pier/4# Pile	5700	13,743	0%
	107# Pier/6# Pile	5700	15,556	13.19%
	106# Pier/5# Pile	5700	15,665	13.99%
	29# Pier/4# Pile	8500	23,618	0%
	29# Pier/2# Pile	7400	29,334	24.20%
	29# Pier/6# Pile	8500	32,451	37.40%
	13# Pier/Left 1# Pile	6000	13,518	0%
	13# Pier/Left 3# Pile	6000	15,312	13.27%
	14# Pier/Left 2# Pile	5500	15,381	13.78%
2	12# Pier/1# Pile	5504	13,743	0%
	12# Pier/6# Pile	5504	15,808	15.03%
	2# Pier/1# Pile	4218.3	10,794	0%
	2# Pier/3# Pile	4223	11,937	10.59%
	7# Pier/1# Pile	5426.7	13,923	0%
	8# Pier/6# Pile	5428.8	15,987	14.82%
3	17# Pier/1# Pile	5700	13,496	0%
	16# Pier/6# Pile	5710	15,485	14.74%
	22# Pier/3# Pile	5670	13,658	0%
	22# Pier/6# Pile	5670	15,786	15.58%
	26# Pier/3# Pile	5765	13,970	0%
	27# Pier/1# Pile	5736	16,067	15.01%
	24# Pier/4# Pile	5445	13,952	0%
	24# Pier/3# Pile	5445	15,995	14.64%

. Table 3 indicates that the ultimate bearing capacity of the pile with grouting increased by 10.6% to 37.4%, compared with that of the piles without grouting.

The end-bearing resistance of the piles versus the applied load are shown in Figure 10, Figure 11 and Figure 12 for districts 1 to 3, respectively. As shown in the figures, the end-bearing resistances of the piles with grouting are higher than those without grouting.

The end-bearing capacity (EBC) for each pile was obtained using the method explained in Appendix A and illustrated in

Table 4. The side friction and end-bearing capacity of the piles with and without the grouting based on the Q-s curve.

District	Pile ID	EBC (kN)	Percentage of Increase in EBC Due to Grouting
1	107# Pier/4# Pile	2203	0%
	107# Pier/6# Pile	4905	122.65%
	106# Pier/5# Pile	4850	120.15%
	29# Pier/4# Pile	4642	0%
	29# Pier/2# Pile	10,962	136.15%
	29# Pier/6# Pile	11,100	139.12%
	13# Pier/Left 1# Pile	2366	0%
	13# Pier/Left 3# Pile	4946	109.04%
	14# Pier/Left 2# Pile	4827	104.02%
2	12# Pier/1# Pile	2354	0%
	12# Pier/6# Pile	4469	89.85%
	2# Pier/1# Pile	1701	0%
	2# Pier/3# Pile	3096	82.01%
	7# Pier/1# Pile	2189	0%
	8# Pier/6# Pile	4009	83.14%
3	17# Pier/1# Pile	2338	0%
	16# Pier/6# Pile	5002	113.94%
	22# Pier/3# Pile	2308	0%
	22# Pier/6# Pile	4713	104.20%
	26# Pier/3# Pile	2373	0%
	27# Pier/1# Pile	4786	101.69%
	24# Pier/4# Pile	2580	0%
	24# Pier/3# Pile	4982	93.10%

. Table 4 indicates that the EBC of the pile can be increased by 82.01% to 139.12% by adopting base grouting. These observations are consistent with the conclusions from Dai, et al. [20], Huang and Gong [21] and Zheng [22]. This implies that the UBC of the pile was mainly increased due to the increase in the end-bearing capacity of the piles.

5. Statistical Analysis

As shown in Table 3, the percentages of increase in UBC for the piles with the same bore hole are inconsistent. Therefore, the significance of the improvement in UBC was evaluated using a hypothesis test for the mean of two normal populations. Ott and Longnecker [23] introduced the following criteria for the hypothesis test: (1) the tested variables should be random variables; (2) the tested variables should have the same variance; and (3) the tested variables should follow the normal distribution [23]. To meet these requirements, a few assumptions were made: (i) the tested UBC was considered a random variable; (2) the expected value and standard deviation of the tested UBC were proportional to the design capacity; and (3) the geological conditions were consistent for the same district.

Based on the above assumptions, the random variable (UBC) for the pile with base grouting has an identical variance compared with the pile without grouting in the same district. The quotient defines the ratio between the determined UBC and the design load. In addition, the effect of the quantity of grout incorporated was determined by transferring the quotient based on the same amount of grout as the target pile through linear interpolation. For example, the equivalent quotient of the 29# Pier/2# Pile can be derived from 2.78 + 4.1 × (3.96 − 2.78)/4.9 = 3.77. In this study, 4.1 t was treated as a referenced amount of grout, and all the quotients were converted into equivalent quotients. As a result, the quotients and equivalent quotients are summarized in

Table 5. The quotient of piles.

District	Pile ID	Reference Bore Hole	Quotient	Equivalent Quotient
1	107# Pier/4# Pile (no grouting)	QSZK28	2.41	2.41
	107# Pier/6# Pile (grouting)	QSZK28	2.73	2.73
	106# Pier/5# Pile (grouting)	QSZK28	2.75	2.75
	29# Pier/4# Pile (no grouting)	QSZK42	2.78	2.78
	29# Pier/2# Pile (grouting)	QSZK42	3.96	3.77
	29# Pier/6# Pile (grouting)	QSZK42	3.82	3.65
	13# Pier/Left 1# Pile (no grouting)	QSZK40	2.25	2.25
	13# Pier/Left 3# Pile (grouting)	QSZK40	2.55	2.55
	14# Pier/Left 2# Pile (grouting)	QSZK40	2.80	2.80
2	12# Pier/1# Pile (no grouting)	ZK3036	2.50	2.50
	12# Pier/6# Pile (grouting)	ZK3036	2.87	2.87
	2# Pier/1# Pile (no grouting)	ZK3050	2.56	2.56
	2# Pier/3# Pile (grouting)	ZK3050	2.83	2.89
	7# Pier/1# Pile (no grouting)	ZK3003	2.57	2.57
	8# Pier/6# Pile (grouting)	ZK3003	2.94	2.94
3	17# Pier/1# Pile (no grouting)	ZK6127	2.37	2.37
	16# Pier/6# Pile (grouting)	ZK6127	2.71	2.71
	22# Pier/3# Pile (no grouting)	ZK6160	2.41	2.41
	22# Pier/6# Pile (grouting)	ZK6160	2.78	2.78
	26# Pier/3# Pile (no grouting)	ZK3098	2.42	2.42
	27# Pier/1# Pile (grouting)	ZK3098	2.80	2.80
	24# Pier/4# Pile (no grouting)	ZK6207	2.56	2.56
	24# Pier/3# Pile (grouting)	ZK6207	2.94	2.94

.

Before the hypothesis test, the conformity of the variables to the normal distribution was verified. Shaphiro and Wilk [24] explained that the Shapiro—Wilk test is suitable for a hypothesis with a limited sample size. Therefore, the Shapiro—Wilk test (the procedure of the computation is introduced in Appendix B) was chosen in this paper. The results of the Shapiro–Wilk test are shown in

Table 6. Results of Shapiro–Wilk test.

Variables	p Values
Vg1 *	0.051
Vn1	0.570
Vg2	0.537
Vn2	0.253
Vg3	0.621
Vn3	0.223

* V_xi: the first index indicates grouting or non-grouting, ’g’ stands for grouting, and ‘n’ stands for non-grouting. The second index, ‘i’, shows the district number.

.

As shown in the table, all p values were greater than 0.05. Therefore, at the significance level of 0.05, it can be concluded that all the variables follow a normal distribution.

The hypotheses were set as follows: H₀: μ₂ − μ₁ ≤ 0; H₁: μ₂ − μ₁ > 0 (μ₁ is the mean of quotients for the pile with base grouting; μ₂ is the mean of the quotient for the pile without the grouting). These hypotheses were tested in all three districts at the significance level of 0.05 by using the Statistical Package for the Social Sciences (SPSS). The confidence intervals (as illustrated in Appendix C) were adopted for the hypothesis test. Zhai, et al. [25] showed that the variability in the geotechnical properties of soil was very high. If the conventional confidence level of 95% was adopted for the test, then the confidence interval would be very wide and might not be suitable for the hypothesis test with a sample with a small size. Therefore, a confidence level of 90% rather than 95% was adopted in this study. If the computed confidence interval is greater than zero, then base grouting has a significant effect on the improvement in the UBC of the pile. The results of the hypothesis tests are shown in

Table 7. Results of hypothesis test.

Variables	Means	Sample Standard Deviation	Confidence Interval with the Significant Level of 0.1	Results
Vg1	3.042	0.526	[0.094, +∞]	Reject H0, and accept H1
Vn1	2.480	0.272
Vg2	2.900	0.036	[0.310, +∞]	Reject H0, and accept H1
Vn2	2.543	0.038
Vg3	2.808	0.096	[0.276, +∞]	Reject H0, and accept H1
Vn3	2.440	0.083

. Table 7 indicates that the UBC of the pile can be significantly improved by base grouting.

6. Conclusions

Both the UBC and EBC of the piles with and without base grouting were analyzed and compared. It is observed that base grouting can significantly improve the UBC and EBC of the bored piles. In addition, the Shapiro—Wilk test was adopted for the evaluation of the performance of base grouting on the UBC for the entire district. The results of the hypothesis test conducted in this paper show that the UBC of piles can be significantly improved by base grouting. The effect of base grouting on the side skin friction and the pile settlement need further study.