A Field Study of Soil Plugging of Jacked Pile and Its Effect on the Pile Resistance

Abstract

Both a static pile load test and cone penetration test were conducted in a field to investigate the effect of soil plugging on the ultimate bearing capacity of a pile. Both the force equilibrium method and Terzaghi’s method were adopted for the theoretical analysis of the soil plugging effect during the pile setting process. Consequently, a new equation was proposed for the estimation of the height of soil plugging. It is observed that the process of pile settlement can be divided into three stages: (i) the soil plugging is initially formed; (ii) the soil plugging reaches its maximum height; and (iii) force equilibrium is reached with a constant soil plugging height. The proposed method in this paper provides an alternative method for the assessment of the height of soil plugging and its effect on the ultimate bearing capacity of the jacked pile.

1. Introduction

In offshore areas, jacked piles have been widely used due to their rapid construction speed, low engineering noise, and environmental pollution. From the point of view of sustainability, an accurate judgment of pile-sinking resistance before construction is a problem that scholars should be concerned about. Soil plugging is commonly created during the construction stage, and the effect of soil plugging on the long-term capacity of a pile requires extensive study.

Jacked piles are usually pressed into the soil at a constant speed of 1 to 2 m/min. This process is similar to the Cone Penetration Test (CPT), and the curves of the two are relatively similar. Based on the actual measured pressure and cone penetration data of jacked piles in soft soil areas in Shanghai, Li et al. [1] adopted the experience of using static cone penetration ratio average penetration resistance to estimate pile-sinking resistance. Kou et al. [2] used a full-scale model to simulate a jacked-pile and obtain the comprehensive correction coefficients α and β, considering the pile tip resistance and pile side friction resistance in order to workout the jacked-pile-sinking resistance with the specific coefficient pile size. Li et al. [3] used a penetrating centrifugal model to study the relationship between statically driven piles and Piezocone Penetration Test (CPTU) and analyzed the force mechanism of its sinking process. Liu at al. [4,5] combined the consequence with the result from a model-socketed pile test on reef lime stone and obtained the variation law of the bearing capacity with the pile tip displacement. Su et al. [6] conducted a numerical analysis on the effect of soil plugging and compared it with field measurements, and they found that the ultimate bearing capacity of the soil plug had a linear relationship with the soil parameters Standard Penetration Test(SPT) and CPT. In fact, prefabricated hollow pipe piles have become the most used due to their higher cost effectiveness. In the process of the settlement of statically driven hollow piles, the biggest difference compared with a static cone probe is that it will produce a soil plugging effect. However, the above-mentioned scholars rarely consider this. Therefore, more in-depth research is needed on the impact of the soil plugging effect on the estimation results of pile-sinking resistance.

This paper adopts the latest real-time monitoring equipment to collect pile-sinking resistance data on location. The obtained data is compared with a CPT curve, and the effect of the plugging effect is analyzed. Using the static balance method and the Terzaghi ultimate foundation bearing capacity theory, the static composition of a soil plug in a pile is analyzed, and the calculation formula for the height of the soil plug under the limit height is established. Based on the existing formula for the ultimate bearing capacity of a pile foundation, an estimation formula for pile-sinking resistance is proposed, considering the effect of soil plugging. The effectiveness of the method is proved by comparing it with the data measured on site.

2. Materials and Methods

At a construction site on the east coast of China, the whole process of jacked pile construction was monitored. The pile’s body concrete is C80, the pile’s outer diameter is 500 mm, the inner diameter is 280 mm, and the length is 18 m. The position of CPT is two meters away from the jacked pile. The site was divided into 6 geological engineering layers based on this. The soil parameters of the soil in each layer are taken from the Soil Investigation Report (No. B21031-1) and shown in

Table 1. Soil layer parameters.

Soil Layer		Average Thickness	Moisture Content	Natural Density	Void Ratio	Compression Modulus	Shear Strength
Sequence	Name						c	φ
		m	%	kN/m3		MPa	kPa	°
①	Silty Clay with Clay Silt	1.65	34.2	18.15	0.971	6.93	15.3	23.9
②	Silt with Sandy Silt	4.63	29.1	18.66	0.824	14.01	7.7	33.5
③	Sandy Silt with Silt	2.21	31	18.44	0.882	10.78	12.3	28.2
④	Silt	4.06	27.9	18.84	0.782	15.51	7.9	32.4
⑤	Sandy Silt with Silt	1.56	31	18.45	0.88	10.97	13.2	27.4
⑥	Silt-Fine Sand with Sandy Silt	7.15	27.4	18.9	0.765	17.15	7.6	34.4

.

A real-time monitoring system is used to obtain the pile pressure resistance. The system connects a pressure transmitter to a static pressure machine and measures accurate pressure data through the Internet of wireless technology (Figure 1). In order to obtain the accidental soil layer distribution in the test, more than 800 jacked piles were monitored during the whole process.

However, compared with the precast pile, the cone penetration tester shows a large size difference, and the data obtained is not suitable for a direct comparison. Therefore, the normalization method is used to process the data to ensure its comparability. The comparison chart is shown in Figure 2. It is noted that the CPT resistance denotes the sum of the end resistance and side friction of the probe.

It was found that although the trend of the two curves is similar, there is a significant difference in value. The value of each point in the static cone penetration curve is almost always smaller than the actual value of the pile-sinking resistance at the same depth. Therefore, there is bound to be an error when directly linking the static cone penetration data with the pile-sinking resistance. We need to conduct further analysis on this.

3. Results

3.1. Analysis of Soil Plug in Pile

3.1.1. Formation and Force of Soil Plug

Jacked piles are soil-squeezing piles. By applying a pile pressure on the top of the pile, the surrounding soil is broken through the resistance of the soil layer, and finally pressed to the target bearing layer. The biggest feature of hollow piles is that during the process of squeezing the soil, soil will be continuously squeezed into the center of the pipe piles to form a soil plug, which is quite different from general closed piles. In fact, the pressing time of a single pile (an average of 15 m into the soil) is generally about 12 min, and the water in the soil plug is hard to drain. It can be assumed that the soil plug has been in an undrained state during the pile-sinking process. According to this, the force of the soil plug during the pressing process is shown in Figure 3. The pile-sinking process can be divided into the following three stages according to the change process of the soil plug. In the first stage, the early stage of soil plug formation, under the action of the foundation bearing capacity P_u at the end of the pile, a large amount of soil is poured into the pile to form a soil plug. As the soil plug increases, both sides of the soil plug come into contact with the inner wall of the pile, generating internal friction f_s′ to prevent the soil plug from increasing. When the height of the soil plug reaches a certain level, under the combined action of gravity G, internal friction and the bearing capacity of the bottom foundation, the soil plug reaches a temporary equilibrium. As the pile sinks, when the soil layer changes, the bearing capacity of the pile tip foundation also changes, which will increase the soil plug again until it reaches equilibrium. At this time, because the jacked pile is in a state of uniform decline, one can consider that the pile-sinking resistance is equal to the pile pressure P. In this stage, the pile pressing force always maintains a dynamic balance with the pile side friction Q_s, the reaction force f_s generated by the soil plug internal friction and the pile end resistance Q_A.

In the second stage, according to the effective soil plug height theory proposed by Randolph [7], after the first stage of action, the height of the soil plug reaches a limit value, namely the effective height of the soil plug, and the soil plug above this height will be loose. In this state, although downward gravity G′ is provided, it cannot generate frictional resistance with the inner wall of the pile. At this time, the effective area of the internal friction resistance will no longer change, but the value will continue to increase with the change of the earth plug gravity at the effective height. During the process of sinking, the combined force of the soil’s own weight, overload and side friction finally reach the value of the ultimate bearing capacity of the bottom, and at this time the soil plug no longer changes and enters a static equilibrium.

In the third stage, as the soil plug continues to increase, the combined force of the soil’s own weight, overload and side friction reaches the value of the ultimate bearing capacity of the bottom foundation P_u. At this time, the soil plug will enter a static equilibrium and no longer increase. The internal friction resistance will not change with the sinking, and the value of the soil plug will have a constant resistance value [8].

3.1.2. Effective Height of Soil Plug

In order to distinguish between the first stage and the second stage, the effective height of the plug needs to be introduced. Many scholars have conducted research on the effective height of soil plugs [9,10,11,12,13,14,15,16,17,18,19]. According to existing research results, the effective height can be six times the size of the pile’s outer diameter.

3.1.3. Determination of the Limit Height of the Soil Plug

As it constitutes the critical point at which the soil plug no longer increases, the question of how to determine the limit height of the soil plug has a high research value. In order to determine the value, the soil plug is considered separately. The force, when it is at the limit height’s equilibrium, is shown in Figure 4.

The static balance expression of the entire soil plug should be:

$$G+f_s{}^{\prime }+G^{\prime }=p_u\pi r^2$$

(1)

The dead weight within the effective height of the soil plug is:

$$G=\gamma \pi r^{2}h,$$

(2)

where γ is the weight of the soil inside the plug, which can be calculated according to the average weight of the height of each soil layer; r is the radius of the pile; and h is the effective height of the plug.

The internal friction of the soil plug is:

$$f_s{}^{\prime }=U\beta {\displaystyle {\int }_0^h\left\{\gamma \pi r^2\left[(l-h)+z\right]\right\}dz},$$

(3)

where U is the perimeter of the plug; z is the distance from any point within the effective height of the plug to the effective height of the plug; and β is the conversion coefficient, which is the difference between the vertical effective stress and the horizontal effective stress σ_n. The ratio needs to be calculated using the Mohr–Coulomb theory and introducing a stress circle (Figure 5):

$$\beta =\frac{\sin \phi \sin (\partial -\delta )}{1+\sin \phi \cos (\partial -\delta )},$$

(4)

where ψ is the friction angle in the soil; δ is the friction angle between the plug and the inner wall of the pile; and ∂ is the angle between the friction angle and the stress circle. The conversion relationship of each angle is:

$$\sin \partial =\frac{\sin \delta }{\sin \phi },$$

(5)

For the general surface roughness of pile soil, $\frac{\tan \delta }{\tan \phi }$ can be taken as 0.8, and β can be calculated as 0.19 by taking (5) into (4).

The dead weight outside the effective height of the soil plug G′ is:

$$G^{\prime }=\gamma \pi r^2(l-h),$$

(6)

where l is the limit height of the soil plug.

According to the nature of the soil layer, local shear failure of the foundation soil should occur. For the circular foundation, the formula for the bearing capacity of the Terzaghi foundation is:

$$p_u=0.8c{N}_c^{\prime }+q{N}_q^{\prime }+0.6\gamma b{N}_{\gamma }^{\prime },$$

(7)

where N_c′, N_q′, N_γ′ are the foundation bearing capacity coefficients, which can be calculated from the internal friction angle ψ of the soil; c is the cohesion of the soil; q is the load on both sides of the foundation; and b is the foundation width.

Substituting Formulas (2), (3), (6), (7) into Formula (1), we can obtain:

$$\gamma \pi r^{2}h+U\beta {\displaystyle {\int }_0^h\left\{\gamma \pi r^2\left[(l-h)+z\right]\right\}dz}+\gamma \pi r^2(l-h)=p_u\pi r^2$$

(8)

$$\gamma \pi r^{2}l+2\pi r\beta \gamma \pi r^2(l-h)h+\pi r\beta \gamma \pi r^2{h}^2=p_u\pi r^2$$

(9)

When solving Formula (9), one can obtain:

$$l=\frac{p_u+\pi r\beta \gamma h^2}{\gamma +2\pi r\beta \gamma h},$$

(10)

From this, the expression formula for the limit height of the soil plug can be obtained. Because of all the parameters in the formula, the effective soil plug height, effective soil weight, pipe pile inner diameter, pipe diameter perimeter u, soil cohesion c, and conversion coefficient β can be calculated by methods such as in-situ experiments. The limit height of the soil plug can be calculated based on information on the soil layer and the inner diameter of the pile, and it has nothing to do with other factors. Therefore, the limit height of the earth plug can be calculated by an in situ test before the pile-sinking in the field and used to estimate the pile-sinking resistance.

4. Discussions

4.1. Estimation Formula of Pile-Sinking Resistance

According to the discussion in the previous section, the resistance in the process of pile driving should be dynamically balanced with the pressure of the pile. Thus, finding the resistance of pile driving is essential to determining the value of the pressure of the pile. The pile pressing force applies pressure on top of the pile, and this form of action is similar to the upper load after the pile is arranged. The existing pile foundation bearing capacity formula is:

$$Q_{uk}=Q_{sk}+Q_{pk}=u{\displaystyle \sum q_{sik}{l}_i}+p_{sk}{A}_p,$$

(11)

where Q_uk is the ultimate bearing capacity of a single pile; Q_sk is the side friction resistance of the pile; Q_pk is the pile tip resistance; u is the perimeter of the pile body; q_sik is the standard value of the ultimate side friction resistance of the i-th layer of soil around the pile, measured by a cone penetration experiment; l_i is the thickness of the i-th layer of soil; p_sk is the average value of the specific penetration resistance near the pile end; and A_p is the area of the pile end.

From the derivation of the force of the soil plug in the previous section, the friction f_s acting on the pile body and the friction resistance acting on the soil plug are opposite to each other, and the acting positions are the same. Soil plugging effect expression:

$$Q_{fk}=\{\begin{array}{c}\beta \gamma \pi r^{2}L,L\le h\\ \beta \gamma \pi r^{2}h+\frac{\beta \gamma \pi r^2(L-h)h}{L},h<L\le l\\ \beta \gamma \pi r^{2}h+\frac{\beta \gamma \pi r^2(l-h)h}{l},l<L\end{array}$$

(12)

where Q_jk is the friction resistance of the soil plugging effect, and L is the actual depth of penetration; the meaning of the other parameters is the same as above.

Substituting Formula (12) into Formula (11), the formula for estimating the pile-sinking resistance P_u can be obtained:

$$P_u=Q_{sk}+Q_{pk}+Q_{fk}=\{\begin{array}{c}u\xi {\displaystyle \sum q_{sik}{l}_i}+\alpha p_{sk}{A}_p+\beta \gamma \pi r^{2}L,L\le h\\ u\xi {\displaystyle \sum q_{sik}{l}_i}+\alpha p_{sk}{A}_p+\beta \gamma \pi r^{2}h+\frac{\beta \gamma \pi r^2(L-h)h}{L}),h<L\le l\\ u\xi {\displaystyle \sum q_{sik}{l}_i}+\alpha p_{sk}{A}_p+\beta \gamma \pi r^{2}h+\frac{\beta \gamma \pi r^2(l-h)h}{l},l<L\end{array},$$

(13)

From this, the formula for estimating the pile-sinking resistance P_u can be obtained. Compared with the previous formula, new parameters are added: the soil side friction resistance, pile tip resistance and soil layer thickness can also be calculated by in-situ experiments; the pile tip area and the body circumference can be obtained after selecting the pile type. Therefore, this formula can be used to determine the applicable pile type when the pile driver model has been determined before construction.

4.2. Case Calculation Analysis

4.2.1. Comparison of Project Examples

In order to further verify the reliability of using Equation (13) to estimate the pile-sinking resistance and in order to compare and analyze the relationship between the estimated resistance and the measured resistance, the geological parameters measured in Table 1 are used for calculation. Figure 6a shows the law of pile side resistance, pile tip resistance, and soil plug resistance during sinking process. Compared with the measured data of static pressure piles at the construction site, the comparison results are shown in Figure 6b.

4.2.2. Analysis of Comparative Results

From the comparison results in Figure 6, we can see that the estimated resistance curve and the actual measured pile-sinking resistance curve show the same changing trend along the depth direction, and the value deviation is small. This shows that the idea of using the soil plug effect to improve the ultimate bearing capacity formula to estimate the pile-sinking resistance is completely feasible. Before the pile end enters the bearing layer, the pile-sinking resistance slowly rises with the penetration depth. When the soil layer is poor, the side friction resistance and the end resistance are low, and the pile-sinking resistance may even decrease. When the pile tip enters the bearing layer, the pile-sinking resistance increases rapidly and reaches a maximum when it reaches the target depth.

Because the estimation formula is composed of three parts, some laws in the process of pile settlement can be obtained from the calculation process. When the penetration depth is shallow, the pile-sinking resistance is mainly composed of the pile end resistance. As the depth increases, the pile side friction resistance plays an increasing role. The effect of soil plugging will increase slowly when it is within the effective height and will increase rapidly after the effective height is exceeded, and it will remain unchanged after reaching the limit height. The size of the soil plug is similar to the resistance value of the pile tip, and both are related to the soil properties of the soil layer. In addition, the limit height of the soil plug, calculated using Equation (10), is 6.62 m. Detailed calculation are attached in the Appendix A. This is more consistent with the data obtained from the actual measurement of the piles, which verifies the accuracy of the soil plug limit height formula.

5. Conclusions

Based on the large amount of jacked-pile-sinking data obtained from field measurements, this paper analyzes the force during the pile-sinking process, and we used the static balance method and the Terzaghi ultimate foundation bearing capacity formula to analyze the soil plugging effect. From the analysis and derivation on the basis of the ultimate bearing capacity formula for the pile foundation, we mainly draw the following conclusions:

• Through the normalized comparison results of the actual measured pile-sinking resistance curve and the static cone penetration curve, it is found that there is a difference between the two curves. It is judged that the difference is caused by the soil plug effect;
• Using the static balance method as the main solution, a stress analysis of the soil plug formed by the prefabricated pipe piles during the static pressure process was carried out. A detailed description of its force composition is given, and the pile penetration process is divided into three stages according to its changing law;
• On the basis of the existing formula, an estimation formula for pile-sinking resistance based on the influence of the soil plug is proposed. Using this formula in comparison with the actual measured pile-sinking resistance curve, it is found that the fit is higher and can be used in actual projects, having a higher reference value.

Through the division and refinement of the calculation formula for pile-sinking resistance, the relationship of the values of the pile side friction resistance, pile tip resistance and soil plug resistance with the penetration depth is obtained.

The above research results are based on a proper simplification of the actual situation. Therefore, there are still some differences to the actual situation. The estimated result is slightly larger than that from the actual situation. Further research is needed.