Influence of Underpinning Pile Drilling Construction on the Bearing Behavior of Existing Loaded Foundation Piles: Case Study

,e mechanism of drilling borehole and its influence on the bearing behavior of existing foundation piles are unclear, hindering the application of excavation techniques. ,is paper proposes a new excavation scheme for high-rise buildings in downtown and develops theoretical formulas to calculate the skin friction and end resistance of existing foundation piles affected by adjacent borehole drilling for installing underpinning piles. ,ese underpinning piles are installed and connected with the existing pile by the head cap before excavation. In a case study, the parameter analysis was then performed to understand the effect of underpinning construction on the bearing behavior of existing foundation piles in terms of skin friction and end resistance. ,e results show that borehole diameter, depth of drilling borehole, distance between the existing foundation pile and drilling borehole, and number of boreholes have a negligible influence on the end resistance of existing foundation piles. ,e effect of drilling parameters on the skin friction of existing foundation pile, as well as the influence extent, varies for different parameters. ,e depth of drilling borehole and the number of boreholes have significant influence and thus should be considered in real engineering design while the borehole diameter and the pile-pile distance have a negligible effect. ,e proposed new excavation technique could be potentially adopted for real engineering design of underneath excavation projects for high-rise buildings in downtown.


Introduction
As economies develop and urban populations rise, developers must meet the rapidly increasing demand for housing and, especially, the parking market. One way is to replace older buildings that have limited parking facilities with new structures with sufficient parking capacity, which is economically inefficient [1].
A more economical means to add parking spaces to existing buildings is retrofitting, for example, excavation beneath existing buildings. Qiu et al. [2] reported the structural design for renovation and extension of Beijing Music Hall. e corresponding techniques are challenging and still immature as limited applications of these techniques in fields have been reported. Wen et al. [3] proposed the deformation control techniques for excavation beneath existing buildings. Simpson and Vardanega [4] and Shan et al. [5] monitored the whole construction process of British Library excavation and the No. 3 subsection of Ganshuixiang Construction Project, respectively. e excavation beneath existing buildings, particularly high-rise buildings, inevitably causes the ground stress field, which leads to the stress change at the pile-soil surface and the pile end, thereby reducing the pile bearing capacity and integrity of the piled foundation. Given that the bearing capacity of piles after excavation fails to be practically measured in field, it is of great importance to conduct theoretical and numerical analysis to estimate its bearing capacity after excavation and the detrimental effects of excavation-induced soil movement on adjacent existing pile foundation, particularly during the engineering design stage. e influence of excavation on an adjacent single pile in different types of formation (e.g., soft clay and sand) has been investigated by several researchers [6][7][8][9][10]. ese studies generally show that bending moment and deflection occur, and pile head boundary condition and embedded depth of the wall, as well as formation level of the excavation, play an important role in influencing the pile response due to an adjacent excavation. As piles are often used in groups, researchers are also interested in exploring the influence of excavation on a pile within a pile group [11][12][13]. Generally, their results show that the interaction effect between piles depends on the arrangement of piles (e.g., either piles are arranged in a row parallel to the retaining wall or in a line perpendicular to the wall), pile spacing, pile number, and whether piles are capped-head or free-head ones. e excavation-induced bending decreases with the number of piles. e interior piles of the pile group experience lower bending moments than those of peripheral piles.
In practice, the applications of excavation techniques are under development, particularly for high-rise buildings in downtown, where limited construction space is allowed in most countries. In this study, a new excavation technique was proposed and its application was simulated in a real project. For this excavation scenario, the underpinning piles were installed and connected with the existing pile by the head cap before excavation. To understand the influence of drilling construction of underpinning piles on the bearing behavior of existing piles, this paper studies the influence of underpinning pile drilling construction on bearing behavior of foundation piles in a case study. Our results indicate that it is promising for designers to utilize the proposed excavation technique in future engineering projects. Figure 1 illustrates the construction process for adding extra space beneath the existing building. e whole process includes five steps, and they are described as follows:

Excavation Scheme.
(1) e retaining structure (wall) is installed to support the excavation, as shown in Figure 1(a). (2) As the bearing capacity of existing piles decreases with the excavation depth, the settlement of existing buildings can occur; underpinning piles in groups are installed and connected with existing piles via the cap before the excavation, as shown in Figure 1(b). (3) e soil is excavated layer by layer until the designed depth, and then bottom slabs of the new basement are cast, as shown in Figure 1(c). (4) Pillars are cast at the specific location of the new basement. e pillar is prepared to support the superstructure, as shown in Figure 1(d). (5) To maximize the space, the pile segments above the bottom slab of the new basement are cut and removed, as shown in Figure 1(e). After completing the above five steps, the main structure of the new basement is obtained. e corresponding stress analysis is then presented in the next section. Figure 2 shows the calculation model for the above pile structure during the drilling construction process. e vertical effective stress before underpinning pile drilling construction σ vb could be estimated using the following empirical equation:

Calculation Model for Stress.
where c ′ is the effective unit weight of soil and q is the soil load caused by the load of superstructure. e vertical effective stress after underpinning pile drilling construction σ va is where σ z is the vertical effective stress at the calculation point. e effective stress σ z at the calculation point could be estimated by the Mindlin solution [14][15][16], as shown in Figure 3.
where μ is Poisson's ratio; a is the width of the strip load; h is the drilling depth; z is the depth of the calculation point; z 1 , z 2 , r 1 , and r 2 are the calculation parameters, z 1 � z − h; z 2 � z + h; r 2 1 � a 2 + z 2 1 ; r 2 2 � a 2 + z 2 2 ; and p is the vertical effective stress at the borehole bottom of the underpinning pile. p � c′h. e effective stress σ zd due to the underpinning pile drilling construction could be calculated by the following formula using the superposition principle, as shown in Figure 4: where σ z1 and σ z2 are the effective stresses caused by a 1 and a 2 , respectively. It can be concluded from formula (3) that as long as the drilling depth, borehole diameter, and the distance between the borehole and the existing pile are determined, the effective stress of the soil at the calculation point is only related to its depth. Meanwhile, the excavation for installing underpinning piles should be conducted with carefulness to minimize the soil disturbance. e bearing capacity of a pile can be assessed by its skin friction and end resistance, the formulas of which are then derived in the next two sections.

Skin Friction.
In general, the distance between the underpinning pile and the existing foundation pile is small [17,18]. us, the process of underpinning pile borehole may affect skin friction along the existing foundation pile.
According to the suggestions of Burland [19] and Loukidis and Salgado [20], the unit ultimate skin friction along the pile f s can be written as where K is the coefficient of lateral earth pressure; σ v is the vertical effective stress; and δ is the angle of friction at existing foundation pile-soil interface. e product K tan δ is sometimes denoted by β, and the use of βσ v to calculate f s is known as the "β method" [21].
As for the coefficient of lateral earth pressure K, Zhang et al. [22] proposed the following expression: where φ is the internal friction angle. Zhang et al. [23] suggested that the value of K for different pile-soil interactions is (0.7∼4.0) K 0 based on the statistical data. Note that K 0 is the coefficient of earth pressure at rest. Generally, the pile-soil system has a long time for consolidation before the basement-supplementing retrofit engineering. erefore, this paper assumes the soil around the pile is in the normal consolidation state [24]. e coefficient of lateral earth pressure K can be expressed as  where φ ′ is the effective internal friction angle. e drilling construction of underpinning piles could be regarded as excavation, as shown in Figure 2. is would break the existing equilibrium state in the pile-soil system, which could not reach the new equilibrium timely. Consequently, the soil is in the state of overconsolidation. e coefficient of lateral earth pressure (K OCR ) is related to the overconsolidation ratio (OCR) [25]: It should be noted that the formula for K OCR applies up to a limit of e overconsolidation ratio is defined as the ratio of vertical effective stress before underpinning pile drilling construction to that after the pile drilling construction.

End
Resistance. e Hoek-Brown criterion is shown in Figure 5, and its formula is as follows [26,27]: where σ 1 and σ 3 are the major and minor failure stresses, respectively; σ c is the compressive strength of the intact rock; and m and s are, respectively, the parameters of the Hoek-Brown criterion. e strength modulus β and tensile strength coefficient of the rock mass ξ are where β and ξ are directly on the parameters m and s. Lambe's variables (dimensionless) p and q could be expressed by the instantaneous angle of friction ρ.
e average overburden load in ground h m (dimensionless) is [28] where c s and c R are effective weights of the soil and the rock, respectively; H s is the thickness of the soil; and H R is the depth of the pile embedded into the rock. e underpinning pile drilling construction would influence the ultimate bearing capacity at the tip pile. Before the underpinning pile drilling construction, the average overburden load in ground h m is [29] After the underpinning pile drilling construction, the average overburden load in ground h m is where σ hd is the effective stress at the tip of the pile due to the underpinning pile drilling construction. It could be calculated by formula (4). e vertical stress σ v exerted upon boundary 1 is where α is the angle of inclination for the assumed virtual failure surface. e stress component σ v in the plane direction that represents boundary 1 (t 1 ) and the one that runs perpendicular to it (s 1 ) are as follows: e following formula holds for Mohr's circle at boundary 1 (see Figure 6): where p 1 and q 1 are variables of Lambe that verify the Hoek-Brown criterion expressed by formulas (13)-(14), depending on the instantaneous angle of friction (ρ).
Taken into formulas (19)- (20), the angle α for virtual boundary 1 is expressed as follows: e angle of inclination for the main major stress for boundary 1 regarding vertical axis ψ 1 is where ε is the angle that forms the main major stress with the inclination of the ground. It can also be deduced from Figure 6 that angle ε is expressed by Given this, angle ε and angle ψ 1 can be expressed as a function of angle α and thus as a function of angle ρ 1 (ρ 1 is the instantaneous angle of internal friction for boundary 1) according to formula (22). e angle θ 1 that forms the family of characteristic line 1 with the ground is Riemann's invariant I(ρ) is a function of ρ, as defined by the following formula: roughout the length of any characteristic line that surrounds the singular point O, the following relation could be verified [30]: is relationship enables one to transfer the stress state from boundary 1 to boundary 2 because where ρ 2 and ψ 2 are the instantaneous angle of internal friction for boundary 2 and angle of inclination of the main major stress for boundary 2 (considering the vertical axis). e pile tip load is always vertical; therefore, ψ 2 � 0. Advances in Civil Engineering e embedment ratio n can be expressed by where B is the diameter of the pile. e following formula could be expressed by the OBC triangle, OAB Prandtl's plastified radial zone, and OMA triangle: e above formula could be solved by iteration calculation.
e ultimate bearing capacity under two-dimensionality hypothesis σ * h is Once the ultimate bearing capacity has been obtained under the plane strain hypothesis, it is necessary to take into account the three-dimensional geometry of the pile. e shape factor S β is where ρ m is an average angle of friction. It is established by the following formula: e ultimate bearing capacity at the tip of the pile σ hp is e ultimate bearing capacity at the tip of the pile during the pile drilling construction could be calculated by replacing formulas (16)-(17) with formula (15).

Case Study.
A typical old building in downtown was selected as a prototype structure for excavation analysis. is old building, i.e., Zhejiang Hotel, was built in 1997 and open to the public in 1999. It is located in the Central Business District, Hangzhou downtown, China, as shown in Figure 7.
e main building and the attached building of Zhejiang Hotel are 12 floors and four floors, respectively. ere was one layer of the basement. e pile-raft foundation with a diameter from 0.6 m to 0.9 m was used. e pile was cast with the strength level of C30 (concrete strength grade in China) and its rock-socketed depth is more than 1.0 m. e building site is a typical soft soil area, and the mechanical parameters of soils are shown in Table 1. e groundwater table at this site was about 0.5 m below the ground surface. To calculate the end resistance of piles, for highly weathered andesite (Table 1) in our case study, m and s are 0.09 and 10 −5 , respectively [31,32]. Note m and s are, respectively, the parameters of the Hoek-Brown criterion (see formula (10)).
As Zhejiang Hotel is located in the most flourishing merchandise street, there is no available place for parking. e existing one layer of basement failed to meet the increasing parking requirements. us, the owner of Zhejiang Hotel decided to excavate another one layer of basement beneath the existing one. en, the design basement would add an area of 2,800 m 2 . e design of this excavation project is completed, but the engineering work has not started. e new excavation technique proposed in this paper is applied in this case project, followed by a parameter analysis regarding the influence of underpinning pile diameter, depth of drilling borehole, distance between the existing foundation pile and drilling borehole, and number of boreholes on the skin friction and end resistance of the existing pile during the underpinning pile construction.

Parameter Analysis and Discussion.
According to the excavation design of the case project, the initial diameter, length, and rock-socketed depth of the existing foundation pile are 0.9 m, 34 m, and 1.1 m, respectively. e load applied at the head of the existing foundation pile is 4,330 kN. e buried depth of existing basement is 5.1 m. e formation layer along its depth is mucky silty clay, mucky clay, clay, silty clay, and clay, as shown in Figure 8.

Drilling Borehole Diameter.
e influence of borehole diameter on the skin friction and end resistance of the existing foundation pile is analyzed in this section, and the results are shown in Figure 8 and Table 2. e depth of borehole is 17.5 m and space between existing foundation piles is 0.5 m. e influence of borehole diameter on the skin friction and end resistance of the existing pile was investigated by varying the borehole diameter from 0.26 m to 0.34 m while maintaining the other parameters constant. e above parameter values are input into formulas (5) to (9) to obtain the skin friction of the existing pile and input into formulas (10) to (34) to obtain the end resistance of the existing pile. Figure 8 shows the influence of borehole diameter on the skin friction of the existing pile. e skin friction varies with different soil layers around the pile. It decreases slowly with an increase in the borehole diameter within the depth of borehole (17.5 m). is decrease reaches the maximum at the interfacial zone between mucky silty clay and mucky clay formation when the borehole diameter increases from 0.26 m to 0.34 m. e drilling construction, similar to the excavation, could be regarded as the unloading of soil on the pile, which causes the reduction of vertical effective stress and thus the skin friction, according to formula (5). e higher the drilling borehole diameter, the larger the excavation produced. Consequently, the skin friction decreases with the increasing drilling borehole diameter, as shown in Figure 8.
e skin friction values are decreasing to negative values along with the depth of the mucky clay zone. Overall, the skin friction decreases with the increase of diameter within the depth of the neutral point, while it increases with the increase of diameter beyond the neutral depth. When the buried depth is 22.0 m, the negative friction reaches maximum of −13.91 kPa. en, the skin friction increases rapidly. e influence of borehole diameter on the skin friction of the existing pile becomes smaller and smaller within the clay, silty clay, and clay. Table 2 shows the influence of borehole diameter on the end resistance of the existing pile. is influence is indicated by the R d value, which is the ratio of end resistance with different borehole diameters to the end resistance of the existing pile when it has not been drilled. It shows that the R d value decreases very slowly with the increase of borehole diameters; thus, the borehole diameter is insensitive to the end resistance of the existing pile. Note that the average overburden load in ground h m changes with the excavation, causing the variation of vertical stress σ v , and thus the stress component σ v in the plane direction that represents boundary 1 (t 1 ) and the one that runs perpendicular to it (s 1 ) change. However, their consequent effect on the end resistance due to the variation of borehole diameter is negligible.

Depth of Drilling Borehole.
During an excavation, stress relief could occur; the bearing behavior of the existing pile could be impacted. Figure 9 and Table 3 show the influence of depth of borehole on the skin friction and end    Advances in Civil Engineering resistance of the existing pile, respectively. e diameter of borehole is 0.3 m, and the distance between existing piles is 0.5 m. e skin friction of the existing pile distributes differently within different soil layers, but the trend is similar in different soil layers, as shown in Figure 9. e skin friction increases generally with the increase of drilling depth. e influence of drilling depth on the skin friction within clay could be negligible. e results also show that the negative friction appears about 3 m above the borehole tip, except when the drilling depth is 10.85 m. e skin friction changes from negative friction to positive friction quickly near the borehole tip. Table 3 shows the influence of depth of drilling borehole on the end resistance of the existing pile. is influence is indicated by the R h value, which is the ratio of end resistance with different drilling depths to the end resistance of the existing pile when it has not been drilled. It shows that the R h value decreases rather slowly with the drilling depth of borehole increasing from 10.85 m to 27.6 m. us, the end resistance of the existing pile is insensitive to the variation of drilling depth.

Distance between the Existing Foundation Pile and Drilling Borehole.
e distance between the existing foundation pile and drilling borehole is a key parameter. As expected, the smaller the distance, the greater the influence of bearing behavior of the existing foundation pile. Figure 10 shows the influence of distance between the existing pile and borehole on the skin friction of the existing pile. e depth and diameter of drilling borehole are 17.5 m and 0.3 m, respectively. As shown in Figure 10, the influence could be negligible within the depth of borehole. For example, the distance of drilling borehole increases from 0.3 m to 0.9 m, and the corresponding skin frictions of the existing foundation pile at the burial depth are 10.50 kPa, 10.54 kPa, 10.54 kPa, 10.55 kPa, and 10.55 kPa, respectively. e negative skin friction would appear below 2.3-4.9 m of borehole tip. As the distance increases, the extreme point decreases to some extent.
is phenomenon also occurs in the clay. Generally, the influence curves of distance on skin friction of the existing foundation pile are coincident but negligible. Table 4 shows the influence of distance of drilling hole on the end resistance of the existing foundation pile. is influence is indicated by R s , which is the ratio of end resistance with different distances to the end resistance of the existing pile when it has not been drilled. It shows that R s increases slowly with the increase of distance, but the increment is rather small.

Number of Boreholes.
During the underpinning construction, each pile cap is connecting with several underpinning piles. us, the number of boreholes is also an important parameter. In this section, the number of boreholes is analyzed as shown in Figure 11 and Table 5. e depth and distance of borehole are 17.5 m and 0.5 m, respectively.
e results show that the skin friction of the existing foundation pile decreases rapidly with number of boreholes from one to four within the mucky silty clay. For example, the skin friction around mucky silty clay is positive when there are one or two boreholes; the negative skin friction occurs when there are three boreholes; the negative skin friction reaches the minimum value when there are four boreholes. e tendency of skin friction around the mucky clay is like the mucky silty clay. However, the skin friction      Table 5 shows the influence of number of boreholes on the end resistance of the existing foundation pile. is influence is indicted by R n , which is the ratio of end resistance with different number of boreholes to the end resistance of the existing pile when it has not been drilled. e ratio of end resistance is 0.9650 regardless of borehole number. us, there is no influence of number of boreholes on the ratio of end resistance.
To summarize, all the parameters including borehole diameter, depth of drilling borehole, distance between the existing foundation pile and drilling borehole, and number of boreholes have a negligible impact on the end resistance.
is is expected according to our derivation for theoretical formulas of the end resistance. e details can be found from formulas (10) to (34). However, different patterns were found regarding the effect of different parameters on the skin friction of the existing foundation pile, and the influence extent for different parameters is also different. Among the four parameters, the borehole diameter and the pile-pile distance have much lower impact on the skin friction of the existing foundation pile compared to the depth of drilling borehole and number of boreholes. is result indicates that the depth of drilling borehole and number of boreholes should be considered in the design of the proposed excavation scheme. For example, the sensitivity analysis of these two parameters should be performed to optimize the excavation design. We also found that the drilling parameters could have opposite impacts on the skin friction of existing piles. For example, in the upper soil layers, as the number and diameter of boreholes increase, the skin friction decreases, while in the lower soil layers, the skin friction increases with the number and diameter of boreholes.
Our study shows that the new excavation technique produces a reasonable result of bearing behavior of existing foundation piles regarding skin friction and end resistance in the case study. e parameter analysis could contribute to the understanding of key parameters (the depth of drilling borehole and the number of boreholes) and their value range during the excavation process. ese parameters could thus be used as design ones in the engineering design work. As the Zhejiang Hotel project is still under planning stage, our new excavation scheme has not been verified in real engineering projects. Future reports will be issued once the project is completed after using the proposed excavation technique.

Conclusions
Given the excavation technique is still under development for high-rise buildings in downtown, this paper proposed a new excavation scheme that satisfies the limited construction space. By implementing the excavation, the underpinning piles are installed and connected to the cap of existing foundation piles. A case study (the Zhejiang Hotel project) was performed to illustrate the effectiveness of this excavation scheme. Also, a parameter analysis was conducted to investigate the influence of underpinning construction on the bearing behavior of existing foundation piles regarding the skin friction and end resistance of existing foundation piles. e case study results conclude that the influence of borehole diameter, depth of drilling borehole, distance between the existing foundation pile and drilling borehole, and number of boreholes on the end resistance could be negligible. e skin friction of the existing pile decreases with the increase of the number and diameter of borehole in the upper soil layers. However, it increases with the number and diameter of borehole in the lower soil layers. e influence of borehole diameter and the distance between the existing pile and borehole on the skin friction could be ignored, while the depth of drilling borehole and the number of boreholes should be considered in the design of the   Figure 11: Effect of number of boreholes on the skin friction of the existing foundation pile.
Advances in Civil Engineering 9 proposed excavation scheme. e proposed new excavation technique could be potentially used for real engineering design of underneath excavation projects for high-rise buildings in downtown.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.