Keywords

1 Introduction

PHC pile has reliable quality and is widely used in soft foundation treatment. And the compaction effect of PHC pile during installation is significant. The pile penetration makes the soil around the pile compact which will induce lateral extrusion and vertical uplift of the soil and in turn causes tilting or floating of the adjacent pile, even crack and overturning of the adjacent buildings [1,2,3] (a. Zhang et al.2006 b. Liu et al. and c. Wang et al.). In addition, compaction effect leads to the increase of excess pore pressure, and the dissipation of the induced excess pore pressure, i.e., consolidation, results in considerable soil settlement and negative surface friction around the pile. Field test is a commonly used research method by many scholars because of its authenticity and reflecting the engineering practice. Li et al. (2011) [4] and Liu et al. (2012) [5] studied the influence of pile installation process on foundation deformation, excess pore pressure and plugging effect through field tests. Xing et al. (2009) [6] found that the lateral displacement of the soil mass at the distance of 16 d (d is the pile diameter) from the edge of the pile group edge was the largest, equal to15 mm. Wei et al. (2020) [7] investigated the effect of the pre-drilled hole and isolation effect in PHC pile installation; Wan et al. (2020) [8] studied the effect of soil properties and pile driving speed on soil compaction during pile installation. In summary, the PHC piles has been widely researched and lots of useful results were obtained.

However, the existing research on PHC pile driving is basically within 10 m long, lacking of research on the extra-long PHC pile which usually exceeds 40 m and requires pile extensions more than three times. For extra-long PHC pile, the pile installation process has a more s13ignificant impact on the lateral deformation than the normal pile [9,10,11,12]. In the southeast regions of China, the thickness of the soft foundation often reaches tens of meters, even nearly 100 m which acquires longer pile length. The existing research showed that the excess pore pressure is positively related to the penetration depth. Thus, it is necessary to study the compaction effect of extra-long PHC piles. In addition, previous studies mostly focused on the excess pore pressure and lateral deformation of soil caused by driving piles, and there has no research about the compaction effect on the lateral deformation of adjacent piles.

In this paper, field test was conducted to study the compaction effect of extra-long PHC piles. Thorough this test, excess pore pressure changes, lateral displacements of the soil and pile shaft during the pile installation were monitored. And the compaction effect caused by the penetration of extra-long PHC piles was analyzed assisted with the measured data.

2 Field Test

2.1 Test Site

The field test was carried out in an expansion project of express way in Guangdong province in China. The test site is relatively flat but with very deep soft clay deposits. At the top, there is a layer of sand filling with a thickness of 2.5 m, and the followed by 34 m – thick clay layer, including silty clay, a shell layer and a layer of silty, the bottom of a 8 m silty sand located on the moderately weathered rock stratum. The main physical parameters are shown in Table 1.

Table 1 Physical–mechanical indexes of soil
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2.2 Tested Program

The moderately weathered rock stratum below the fine sand was designed as the bearing stratum of the piles. The design length of the pile is ranging from 45 to 150 m, the pile has an outer diameter of 400 mm and an inner diameter of 310 mm, thus the thickness of the annular of the PHC pile is 90 mm. The material of the pile was C80 concrete. A total of 79 piles were installed. The piles were installed in a rectangular shape. The pile space is 3 × 2 m along the longitudinal and transversal direction. The pile location layout and installation route procedure are illustrated in Fig. 1. The PHC pile installation in the test area started on A-1 (5 May) and ended on V-16 (4 June), and lasted for 31 days. The layout of the test instruments is shown in Fig. 1. Two inclinometer tubes were installed, designed as CX1 and CX2 with a depth of 42 m and 44 m, respectively. CX1 was used for lateral deformation measurement of the soil within piles. And CX2 was used for the pile shaft and placed inside the hollow of the pile I-5. In addition two of the piezometers were set at K1 points with a buried depth of 12 m and 18 m respectively. As shown in Fig. 1, K1 has an equal radial distance of 3.5 m from the surrounding 4 piles.

Fig. 1
A layout diagram. The starting point starts at 1, 6 on May 5 and ends at 16, 5 on June 4. It features pipe, inclinometer, and piezometer.

Instrument layout and construction sequence diagram

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3 Test Results and Analysis from Single Pile Driving

3.1 Pore Water Pressure

In the section, the compaction effect of three piles with different radial distance on the target pile I-5 was investigated. In this paper, pile II-4, pile III-4 and pile VI-4 are selected and these piles are representative of the pile installation at close distance, middle distance and far distance. The excess pore pressure variation with pile installation time was shown in Fig. 2(a)–(c) for the above three piles, respectively. Note the excess pore pressure plotted in Fig. 2. are the incremental pressure induced by the penetration from an initial time. Figure 2(a) shows the relationship between the excess pore water pressure in K1 and the driving time of pile II-4. It can be seen that when the pile tip depth is less than 13 m, the pore water pressure of K1–12 increases rapidly. When the pile depth exceeded 13 m, the excess pore water pressure of K1-12 basically remains unchanged, keeps at 6 kPa. For K1-18, the excess pore pressure is zero when the pile depth is within 13 m. When the pile depth ranges within 13 –37 m, the excess pore pressure rises rapidly, with the maximum value of 15.8 kPa. It is interesting to find that the excess pore water pressure starts to slowly decrease as the pile tip depth is beyond 37 m, and the excess pore water pressure of K1-18 is 13.3 kPa after pile installation.

Fig. 2
3 graphs for excess pore pressure per k P a versus pile tip depth in meters versus pile driving time in minutes plot 3 fluctuating curves for K 1-12, K 1-18, and pile tip depth.

Curve of excess pore water pressure generated by single pile installation

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Figure 2. (b) shows the relationship between the excess pore pressure and the driving time for pile III-4. The total penetration depth of the PHC pile is 45 m. Basically, the trend of pore pressure with time is consistent with pile II-4. For both piles, i.e., II-4 and III-4, the excess pore pressure rises rapidly when the pile tip reaches near the measuring point. And the induced excess pore pressure measured at the deep point (18 m) is larger than that at the shallow point (12 m). For K1-12, the peak value of the induced excess pore pressure is 2.0 kPa, and the residual value after installation is 0.6 kPa. For K1-18, the peak value is 8.5 kPa, and the residual value is 8.5 kPa. Figure 2. (c) shows relationship between the excess pore water pressure and the driving time for pile VI-4. The total penetration depth is 38 m with a peak excess pore pressure 1.4 kPa and a residual value of 0.9 kPa at K1-12, and a peak value of 1.6 kPa and a residual value of 1.54 kPa at K1-18. Clearly, as the distance of increasing between the PHC pile and the measuring point, the excess pore water pressure caused by the single pile installation decreases gradually. For example, the excess pore water pressure during the penetration of pile VI-4 is very small generally indicating no significant compaction effect due to pile driving at farther distance. Therefore, the horizontal distance between pile VI-4 and the measuring point, e.g., K1, can be taken as the impacting radius, which is 10.7 m in this study.

3.2 Lateral Displacement of Soil

The pile II-6 and II-8 were selected to study the impact of penetration on the lateral deformation of soil between piles. Figure 3. shows the lateral displacement with depth in X and Y directions after each pile’s installation. The lateral displacement were obtained from the inclinometer at CX1. In general, the pile installation pushing the soil mass away results in positive displacement in Y direction and negative displacement in X direction. In Y direction, the maximum lateral displacement of pile II-6 is 3.8 mm at the depth of about 24 m and the value induced by pile II-8 is basically close to 0. And the maximum lateral displacement caused by pile II-6 and II-8 in X direction is 4.8 mm and 2.5 mm, respectively. Definitely, the lateral displacement of soil induced by the PHC pile installation decreases with the distance of the driven pile. It also can be seen from Fig. 3. that the lateral displacement of soil in the Y direction is significantly smaller than that in the X direction. Due to the existence of the in-service expressway, there is a constraint effect on soil deformation induced by pile installation in the Y direction. In addition, the lateral displacement at the upper and lower parts in Y direction are small, while the displacement in middle height is relatively larger. Contrast to Y direction, the soil displacement in X direction shows a continuous increase from bottom to the top. As mentioned above, the existing embankment has significant resistant effect on the soil deformation in Y direction, but it is not true in X direction.

Fig. 3
A graph of depth in meters versus lateral displacement in m m plots 4 curves, 2 each of Y-2-6 along with squares and y-2-8 along with circles.

Lateral displacement curve of soil caused by single pile installation

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3.3 Lateral Deformation of PHC Pile

The influence of pile installation on the lateral deformation of adjacent piles mostly researched by numerical simulation or model test [13,14,15,16] (a. Yao et al. b. Zhan et al. and c. Wei et al. d Luo et al.) at present. Rare reports about field tests on this topic were found up to date. In this paper, by setting inclinometer in the pile the lateral deformation of the pile was directly measured. Thus the impact of the compaction effect of subsequent pile installation on the existing pile can be directly estimated. As shown in Fig. 1, pile I-5 is selected to be the target pile with a 44 m-long inclinometer. First, four consecutive piles (II-5, III-5, IV-5 and V-5) are selected as the research object along the Y direction. Figure 4. shows the curve of lateral displacement of pile I-5 with depth as the four piles (II-5, III-5, IV-5 and V-5) were penetrated. Among them, the penetration of the nearest PHC pile II-5 has the greatest impact on the lateral displacement of the pile shaft, and the maximum lateral displacement is 4.3 mm at the depth of 22 m. In addition, the pile’s displacement mainly occurs at 1/3–2/3 of the pile length. Considering the connection position of the prefabricated pile shaft, the lateral displacement turning points (1/3 and 2/3 depth) exists consistent with the connection points. It is possible that the joint of the pile shaft is a relatively weak point resulting in the bending and deformation of the whole pile due to installation of near pile. Therefore, special attention should be paid to the joint of shaft of a PHC pile during driving. The results also show that the influence of driving for pile V-5 on the deformation of pile I-5 can be ignored. The influence range of pile installation on the deformation of adjacent piles is about 4 times the pile spacing, generally equal to 10.7 m.

Fig. 4
A graph of depth in meters versus lateral displacement in m m plots 4 curves for the y-direction as follows. 2-5, 3-5, 4-5, and 5-5. To the left, a vertical pile is given with connections and an inclinometer inside it.

Lateral displacement of pile shaft caused by single pile installation

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3.4 Influencing Radius

Figure 5. shows excess pore pressure with horizontal distance from pile-center to the K1. Note that the excess pore water pressure in Fig. 5. (a) is the maximum value after penetration of each pile. It can be seen from Fig. 5. (a) that the maximum excess pore water pressure caused by single pile installation at a depth of 12 m is 26 kPa, and the value at 18 m is 80 kPa at horizontal distance of 1.8 m. As mentioned above, deeper soil yields larger pore water pressure. As the horizontal distance increases, the pile driving induced excess pore water pressure caused by single pile installation gradually decreases. When the horizontal distance exceeds 10.7 m, the PHC pile installation will not produce any excess pore water pressure. In this case, the influence range of excess pore pressure generated by single pile installation on soil mass is about 10.7 m, that is, 27 times the pile diameter. In Fig. 5. (b), pile driving induced pore water pressure was normalized by the effective stress. The effective stress at 12 and 18 m was estimated respectively as 120 and 180 kPa by assuming a fully saturated condition and a unit weight of 20 kN/m3. It was found the normalized results for pore pressure at both 12 and 18 m can be identically expressed as Eq. (1):

$$ \frac{u}{\sigma_v } = 0.8R^{^{ - 1.6} } $$
(1)
Fig. 5
2 graphs. A, excess water pore pressure versus distance from pile plots 2 concave-up curves for K 1-12 and K 1-18. B, standardized pore pressure versus distance from pile plots scattered circles and squares for K 1-18 and K 1-12, respectively.

Curve of induced excess pore water pressure changing with horizontal distance between pile and measuring point

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\(u\): excess pore water pressure (kPa)

\(\sigma_v\): effective stress (kPa)

\(R\): distance between measuring point and pile axis (m)

By using Eq. (1), the pile driving induced pore water pressure can be estimated with known radial distance and depth.

4 Conclusions

In this paper, based on a high-speed reconstruction and expansion project, a field test study on the compaction effect of PHC piles in super deep soft foundation is carried out. According to the test results, the following conclusions are drawn:

  1. 1.

    The value of excess pore water pressure caused by single pile installation is mainly related to the linear distance between the pile tip and the measuring point. The shorter the distance is, the greater the excess pore water pressure induced by the pile installation process is. When the pile tip crosses the measuring point, the excess pore pressure will not increase. In this test, the horizontal influence radius of PHC pile soil compacting on excess pore pressure is about 10.7 m.

  2. 2.

    The induced pore water pressure increases with the depth and is obviously affected by the stratum property. In this study, the peak value of the induced pore water pressure is 165 kPa. The pore water pressure cannot be accumulated in the vicinity of soil with large permeability coefficient, such as thin sand layer or silty fine sand layer.

  3. 3.

    The existing subgrade has obvious restraint effect on the soil between piles and the lateral deformation of PHC piles. The deformation of PHC piles is small at the top and bottom, and large in the middle. The inflection point of deformation occurs at the pile connection position.

  4. 4.

    The relationship between excess pore water pressure and horizontal distance can be obtained by normalization. Pile driving induced pore water pressure can be estimated with known radial distance and depth.