JOURNAL OF THE KOREAN SOCIETY OF CIVIL ENGINEERS. April 2019. 335-344
https://doi.org/10.12652/Ksce.2019.39.2.0335

MAIN

• 1. Introduction

• 2. Waveform Micropile

• 3. Method of Analysis

•   3.1 Numerical Model for Pile Foundation

• 4. Results and Discussion

•   4.1 Single Pile Tests

•   4.2 Axial Stiffness of piles

•   4.3 Construction Procedure of Vertical Extension of an Existing Building

•   4.4 Load Settlement Response of Underpinned Foundation

• 5. Conclusions

1. Introduction

Because of rapid growth of population and limited land resources, building remodeling with vertical extension becomes an economical and effective way to enhance the utilization of existing buildings. In Korea, the government has published a statement that apartment buildings with more than 12 floors aged more than 15 years could be vertically extended up to 2-3 floors (MOLIT, 2013). In this case, foundation underpinning is essential to enhance the bearing capacity and reduce the settlement of an existing foundation in order to resist to applied loads from additional floors. Several technologies are available to underpin foundations (Thornburn and Littlejohn, 2014; Cole, 1993). Due to a limited spacing between existing piles and applicability for in-situ construction, micropile underpinning technology is widely used for existing foundation.

A micropile is a small-diameter, drilled, and grouted pile with a central steel bar. Generally, micropiles are between 100 and 300 mm in diameter, 20 m to 30 m in length, and 300 to 1000 kN in compressive or tensile service load (FHWA, 2005). The installation of micropiles causes minimal disturbance to adjacent structures, soils, and the environment. They can also be installed in restrictive conditions at any angle. The technology of micropililing was introduced by Lizzi in the early 1950s (Lizzi, 1982). Since then, it has been widely used to reinforce existing foundations in static and dynamic environments and support slopes since the 1980’s (Bruce et al., 1985; Han and Ye, 2006a; Han and Ye, 2006b; Isam et al., 2012; Sadek et al., 2004; Babu et al., 2004; Esmaeili et al., 2012).

In the design of underpinning for existing foundation subjected to additional loads, load sharing by existing and underpinning piles should be considered. Underpinning pile needs to share partial loads for existing piles in order to prevent exceeding existing pile’s allowable load. To optimize an efficient arrangement of underpinning piles, one effective way is to improve the underpinning pile’s load sharing capacity. Wang and Han(2017) has demonstrated that load sharing capacity of underpinning pile increased with its increasing stiffness by numerical analysis.

In Korea, a new type of micropile named waveform micropile was developed by Jang and Han (2014). Waveform micropile has wave-shaped grout by jet grouting method to enhance its skin resistance along the shaft of the pile. Bearing capacity and construction efficiency of waveform micropiles have been verified to be higher than those of conventional micropiles by full-scale field tests, centrifuge tests, and numerical analysis (Jang and Han, 2014; Jang and Han, 2015; Jang and Han, 2017; Jang and Han, 2018). However, the application of waveform micropile as an underpinning element has not been sufficiently investigated previously.

To enhance underpinning effect and construction efficiency of micropile during vertical extension, the main objective of this study was to evaluate underpinning effect of waveform micropile of in terms of reducing final settlement and load sharing ratio (LSR) by numerical analysis. Moreover, underpinning effects of three conventional micropiles of different lengths were also evaluated and compared to those of waveform micropiles.

2. Waveform Micropile

Waveform micropile is a new type of micropile with shear keys along the pile’s shaft. Shear keys, which can enhance the shaft resistance in compressive stratum, are constructed by jet grouting method. Due to constructability of jet grouting method, it is applied only in the soil layers. The construction process is shown in Fig. 1. It involves the following steps: (a) drilling, (b) injection of grout to develop waveform micropile, (c) installation of steel reinforcement, and (d) Completion. This construction method of waveform micropile has been demonstrated to be more economical than that of conventional micropile (Jang and Han, 2014). Fig. 2 shows schematics of conventional micropile and waveform micopile. For case of waveform micropile, diameter of shaft part is 300 mm, and diameter of shear key part is 500 mm, which is 1.7 times larger than the diameter of the pile’s shaft. Based on full-scale experimental results and centrifuge experimental results, it is demonstrated that bearing capacity of a waveform micropile is 50 % higher than that of conventional micropile at the same size (Jang and Han, 2017; Jang and Han, 2018).

Fig. 1.

Construction Method of Waveform Micropile (Jang and Han, 2017)

Fig. 2.

Comparison of Conventional Micropile (CMP) and Waveform Micropile (WMP)

3. Method of Analysis

3.1 Numerical Model for Pile Foundation

A 3D numerical model was developed to simulate a series of cases of foundation underpinning with finite element code PLAXIS 3D (Plaxis, 2005). Fig. 3 exhibits a schematic sketch of the numerical model developed in this investigation. The model consisted of a 4×4×1 m raft with four existing piles (EP) and one underpinning pile (UP). General prestressed concrete piles (PCP) widely used in 1990s as foundation components were modeled as existing piles. Three conventional micropiles (CMP) with different lengths and one waveform micropile (WMP) were used as underpinning piles for comparison with the micropile’s underpinning effect. Details of the size of these piles are presented in Table 1.The mesh was extended in both horizontal direction to a width of 10 m and vertical direction to a height of 20 m. Soils are divided into layers in this analysis: 0-8 m of sand layer and 8-20 m of rock layer. Soil behavior was determined as Mohr- Coulomb model. Not considering material failure in this analysis, prestressed concrete piles and micropiles were modeled as linear-elastic model. As conventional micropiles consist of grout materials and a central steel bar, to simplify the simulation model, composite young’s modulus Etot combined with material of grout and steel bar was used. It is defined as follows:

Fig. 3.

Geometry of Piled Foundation

Table 1. Size of Test Pile

 Test pile Length (m) Diameter (mm) PCP 8 350 WMP 8 D1:300/ D2: 500 CMP1 8 300 CMP2 10 300 CMP3 12 300

 $$E_{tot}=\frac{E_{grout}A_{grout}+E_{steel}A_{steel}}{A_{tot}}$$ (1)

Where Etot is composite young’s modulus of waveform micropile; Atot is the area of waveform micropile; Egrout and Esteel are young’s modulus of grout and steel used for waveform micropile, respectively; Agrout and Asteel is the area of grout part and steel part of waveform micropile, respectively.

For waveform micropile, due to complex configuration of the pile, models of grout component and steel bar were built separately. Grout was modeled as linear-elastic solid model while the steel bar was modeled as beam elements. Fig. 4 shows the geometry of an example of a conventional micropile and a waveform micropile in numerical analyses. Material properties of soils and piles used in these analyses are shown in Tables 2 and 3. They were taken from Wang et al. (Wang et al., 2017; Wang et al., 2018a; Wang et al., 2018b). The soil-pile interface was described by Rinter, the interface strength reduction factor (Plaxis, 2005). Rinter was defined as:

Fig. 4.

Modeling of Micropile and Foundation in Plaxis

 $$c_i=R_{inter}c_{soil}$$ (2)
 $$tan\phi_i=R_{inter}tan\phi_{soil}$$ (3)

Where ci and 𝜙i are cohesion and frictional angles of the interface; csoil and 𝜙soil are cohesion and frictional angles of the soil; Rinter = 0.67, a representative value in Plaxis 3D.

4. Results and Discussion

4.1 Single Pile Tests

Fig. 5 presents the load-settlement behavior of five single piles under compression. The curve clearly shows that the WMP has higher load capacity than conventional micropiles. Because of no significant failure point shown in the curve, the ultimate bearing capacity of each pile was estimated corresponding to pile head settlement of 25.4 mm (Terzaghi and Peck, 1967; Touma and Reese, 1974). Failure mechanism is considered to be developed only in the soil due to the assumption that no failure occurred in the pile material. A factor safety of 3 was applied to calculate the allowable bearing capacity of single piles. Table 4 summarizes the proposed allowable bearing capacities of PCP, CMP1, CMP2, CMP3, and WMP (567 kN, 328 kN, 400 kN, 483kN, 877 kN), respectively. Shear keys along the pile’s shaft enhanced shaft resistance in both compressive stratum and bearing stratum compared to conventional micropiles for which the shaft resistance was mainly mobilized in the bearing stratum. Bearing capacity of waveform micropile was 1.5 times more compared to prestressed concrete pile and 2 ~ 4 times more compared to conventional micropiles depending on pile’s length. Trends of numerical results in the present study are in agreement with those reported by Jang and Han (2018).

Fig. 5.

Load Settlement Response of Single Piles

4.2 Axial Stiffness of piles

Axial stiffness kv of a pile is defined as the slope of load- settlement curve. It can be obtained from single pile loading test under compression based on properties of the pile and soils (Randolph, 1994) or empirical equation based on numerous field data (KHS, 2008; Koichi et al., 1996). The empirical equation proposed by Korea Highway Bridge Design Standard (2008) is shown below:

 $$k_V=\alpha\frac{A_PE_P}L$$ (4)

Where kv is axial stiffness of a pile (kN/m); Ap, Ep, and L are pile’s area (m2), Young’s modulus (KPa), and length (m), respectively.

𝛼 is stiffness factor depending on the type and construction method of piles as follows:

 $$Driven\;pile:\alpha=0.014\left(\frac LD\right)+0.72$$ (5)
 $$Vibration\;pile:\alpha=0.017\left(\frac LD\right)-0.014$$ (6)
 $$Cast\;in\;situ\;pile:\alpha=0.031\left(\frac LD\right)-0.15$$ (7)
 $$Bored\;pile:\alpha=0.010\left(\frac LD\right)+0.36$$ (8)

In this study, prestressed concrete pile (PCP) used as existing pile is a kind of precast driven pile. Eq. (4) was used to calculate PCP’s axial stiffness. Micropile used as underpinning pile is a kind of cast in situ pile. The value of Young’s modulus and area of each pile for calculation is in accordance to the data used in numerical analysis shown in Table 2. Eq. (7) was applied to calculate MP’s axial stiffness. Piles’ axial stiffness estimated by Eq. (5), kvs, and estimated by slope of load-settlement curve based on Fig. 5, kve, are calculated in Table 5. It is seen that kvs obtained in the loading test is good agreement with that proposed by KHS for WMP and PCP. However, for comparison of conventional micropiles, the value of kvs showed that stiffness of conventional micropile is significantly affected by its socket length, but it is not considered in Eq.(4).

Table 2. Properties of Piles

 Description EP UP Raft PCP CMP WMP Material Concrete Grout Steel Grout Steel Concrete Diameter (mm) 350 300 63.5 D1: 300 D2: 500 63.5 4 X 4 Unit Weight (kN/ m3) 23.5 23.5 78.5 23.5 78.5 23.5 Young's Modulus (GPa) 24 32.3 24 210 24 Material Model Linear Elastic Beam Linear Elastic

Table 3. Properties of Soils

 Description Sand Weathered Rock Depth (m) 0-8 8-20 Unit Weight (kN/ m3) 19 21 Material Model MC MC Interface strength factor, Rinter 0.67 0.67 Frictional Angle, 𝜙 (°) 34 39 Cohesion, c (kN/ m2) 10 30 Dilantacy Angle,𝜓(°) 4 9 Young's Modulus, E (KPa) 3.5E4 3.0E5 Poisson Ratio, v 0.3 0.28

Table 4. Summary of Bearing Capacity of Single Piles

 Type Qult (kN) Qall (kN) UP CMP1 985 328 CMP2 1200 400 CMP3 1450 483 WMP 2630 877 EP PCP 1700 567

Table 5. Summary of Stiffness of Single Piles

 Type kvs*(kN/m) kve**(kN/m) K*** UP WMP 194761 193116 1.28 CMP1 146126 193116 0.96 CMP2 154140 201678 1.01 CMP3 209800 207386 1.38 EP PCP 152000 169881 -
*kve : axial stiffness of a pile which is calculated by Eq. (4).
**kvs : axial stiffness of a pile which is estimated by initial slope of load-settlement curve.
*** K is the stiffness ratio of underpinning pile to existing pile based on kvs.

4.3 Construction Procedure of Vertical Extension of an Existing Building

4.4 Load Settlement Response of Underpinned Foundation

Fig. 6.

Load Settlement Behavior of Underpinned Foundation with Different Micropiles

Load sharing ratio (LSR) was used to describe the percentage of carried load of a pile divided by the applied load to the foundation. The definition of LSR for a pile is shown below:

 $$LSR_{Tot}=\frac{Carried\;load\;of\;each\;pile}{Total\;applied\;load}$$ (9)
 $$LSR_{Add}=\frac{Carried\;load\;of\;each\;pile}{Additional\;applied\;load}$$ (10)

Fig. 7.

Total Load Sharing Ratio of Existing and Underpinning Pile

Fig. 9.

Normalized Load Sharing vs. Normalized Stiffness of Underpinning Pile to Existing Pile

 $$\lambda=\frac{LSR_{addUP}}{LSR_{addEP}}$$ (11)
 $$K=\frac{k_{vsUP}}{k_{vsEP}}$$ (12)

5. Conclusions

In this study, FEM numerical analysis was carried out to investigate waveform micropile’s underpinning effect during building remodeling with vertical extension. Results were compared to those of foundation underpinning by conventional micropiles with different lengths.

(1) Bearing capacity and axial stiffness of a pile were firstly estimated by single pile loading test. Waveform micropile showed stiffer behavior and higher bearing capacity than conventional micropile with the same diameter and length. It had similar behavior to a conventional micropile with longer length with 1.5 times of waveform micropile length.

(2) Results of numerical analysis demonstrated that underpinning with micropile could reduce the total settlement of foundation and carry partial loads from existing piles. The underpinning performance of conventional micropile increased with increasing socket length of a pile. This is because socket length plays an important role in increasing axial stiffness of a pile.

(3) When waveform micropile and conventional micropile with the same length were compared, total settlement and load sharing in case of foundation underpinned with waveform micropile were 10 % less and 40% higher than those with conventional micropile under design loads of the vertical extension, which the additional load is 20 %. Waveform micropile had similar underpinning performance to conventional micropile with longer socket length with 1.5 times of waveform micropile length. These results imply that, in practical construction, waveform micropile would be a more economical and effective method for foundation underpinning than conventional micropile to save construction and material cost.

(4) The ratio of load sharing by underpinning pile to existing pile increased linearly with increasing stiffness ratio K of underpinning pile to existing pile if underpinning pile is stiffer than existing pile. This finding is useful for providing a proper underpinning method considering pile’s axial stiffness for vertical extension work.

In addition, as the limited numerical results in this study, field tests or centrifuge model tests will be carried out in order to obtain more accurate results.

Acknowledgements

This research was supported by a grant from “Development of Technologies for Structural safety on Vertical Extension for Existing Apartment Buildings” which is funded by the Korea Agency for Infrastructure Technology Advancement. An earlier version of this paper was presented at KSCE 2018 CONVENTION and was published in this Proceedings.

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