Responses of a short column-supported highrise tower to adjacent deep excavations in water-rich sandy strata and dynamic optimization of protection plans

Abstract

A highrise tower atop short columns in Nantong, China was threatened by excavation of a subway station nearby. Although an elaborate protection plan composed of isolation piles, artificial recharge and underpinning was executed throughout the excavations, the tower underwent unacceptable settlements and notable inclinations. In combination of field measurements and numerical simulations, this paper investigates the tower’s responses to the adjacent excavations, examines the effects of adopted protection plans and explores potential effective protection plans. First, the responses of the tower and the effectiveness of the three implemented measures were examined, and the contributory factors triggering intolerable tower deformations were identified; then, the effects of primary protection parameters were quantified, including the length, stiffness and layout of isolation piles, the water level surrounding recharge wells after recharging and the depth and location of wells, and the length of underpinning piles. It reveals that the underpinning plan had the best protection effect, followed by isolation piles and recharging wells. Construction timing of protection measures and termination manners of recharging are two critical factors in restraining tower deformations. Moreover, underpinning the tower with 36-m long steel pipe piles solely before implementation of adjacent excavations could be another optimal protection scheme.

Graphical abstract

Keywords

highrise tower / excavation / piled-raft footing / protection / field performance / numerical simulation

1 Introduction

Along with rapid urbanization, underground space has been developed massively in many cities of China to solve urban problems [1–11]. Therefore, a large number of deep excavations have been constructed to fulfill increasing demands for infrastructures and other facilities, such as transportation lines, basements, parking garages, and underground oil storages [12–15]. For those excavations implemented in densely built urban environments, pre-existing structures and facilities in their proximity are prone to be damaged by excavation-induced ground movements [4,16–26]. Consequently, it is the first priority to take effective measures to guarantee the safety of these sensitive structures during excavation.

In practice, isolation piles or walls, artificial recharge, underpinning, compaction (compensation) grouting, and superstructure reinforcement have been extensively employed to safeguard pre-existing structures close to deep excavations and tunnels [27–36]. The protected objects of these previous studies mainly concentrated on low-rise brick-concrete superstructures founded on shallow foundations, rarely involving pile-supported superstructures. As disclosed in Tan et al. [31], if pile toes were within the excavation-caused ground displacement zone, buildings on piled foundations would be vulnerable to ground movements as well and hence were prone to be damaged. Furthermore, the preceding researches primarily focused on soft clayey soils and thus their protective experience might be inapplicable for safeguarding buildings sensitive to adjacent excavations in other complicated formation conditions, e.g., water-rich sandy strata. For excavations in permeable sandy strata with high groundwater levels, sand boiling, piping and through-wall leakage were encountered frequently owing to the inherent instability and erodible properties of sandy soils, often triggering substantial ground volume losses outside the excavations [3,37–40]; therefore, pre-existing structures and facilities in the vicinity of the failures were impaired [41]. To date, very few researches regarding protection of pile-supported superstructures close to deep excavations in water-rich sandy strata have been known in literature, in particular those atop short piled-raft footings. Therefore, it is worthwhile to study this issue, which could help professionals worldwide select appropriate protection plans for similar projects in the future.

One subway station in the city of Nantong, China was excavated close to a 193.5-m high TV tower founded on short concrete columns, providing a rare opportunity to investigate the aforementioned issue. Because of its great height and short piles, the TV tower would have a high risk of structural damage and sudden overturning failure if it underwent intolerable differential settlements and tilts. To ensure the tower’s safety, a series of protection measures consisting of isolation piles, artificial recharge and underpinning were carried out throughout excavation of the adjacent subway station. Although the TV tower was stabilized finally within the following four months after completion of the subway station, it suffered pronounced settlements up to 70.4 mm and excessive tilts of 1.343‰. This indicated the inadequate competence of the adopted protection plans. In light of this, a comprehensive parametric study employing the three-dimensional (3D) finite-element method (FEM) was conducted in this study to evaluate the capability of the three measures (isolation piles, artificial recharge and underpinning) for limiting tower settlements and tilts, to ascertain the critical factors helpful for restraining tower deformations, and to quantify the effects of primary parameters of those protection measures. Subsequently, an optimization protection alternative for the TV tower was suggested, which could save project costs without sacrificing protective performance.

2 Project description

2.1 Project overview

The investigated project was excavation of an interchange subway station, i.e., He-Ping-Qiao Station (HPQ) of Nantong Metro Line 1. It had a final excavation depth H_{\mathrm{e}} of 10.9–26.5 m. HPQ was composed of four individual excavation zones, i.e., pits 1–4 (Fig.1). The subsurface soils were comprised of a thin layer of fill in the upper 1.4–5.8 m below ground surface (BGS), underneath by thick sandy soils (aquifers) interbedded with thin impermeable silty clays (aquitards), as shown in Fig.2. The long-term water table in the perched aquifer (Aq 0) variated at 1.2–2.3 m BGS. As for the two detected confined aquifer layers (Aq I and Aq II) in the uppermost 70 m, their elevated pressure heads were approximately at 4 and 4.3 m BGS, respectively. The typical soil parameters are summarized in Tab.1. As indicated in Fig.2, the excavations of HPQ were executed within the permeable, erodible sandy soils. The Nantong TV tower located at the south-east side of HPQ was a 193.5-m high concrete tube-shape superstructure with one basement atop short concrete columns, see Fig.1 and Fig.3. The minimum distance, D_{\mathrm{m}}, between the TV tower and HPQ was 29.3–94.4 m (Fig.1); apparently, the TV tower was located within the potential influence zone of ground settlements induced by excavation of pits 2–4 but beyond that of pit 1 [42]. If the TV tower underwent intolerable damages stemming from excavation-induced ground movements and/or ground volume losses incurred by seepage accidents, it would crack, tilt and even overturn. In light of these risks, taking some auxiliary measures to guarantee the safety of the TV tower was imperative during construction of HPQ.

For this project, the subway excavations were constructed using a covered bottom-up (BU) method and supported by perimeter diaphragm walls (DWs) braced by multiple levels of struts, proceeding from pits 1 to 4 in sequence. The braced struts were composed of steel-reinforced concrete struts (CSs) and steel pipe struts (SPSs), as shown in Fig.2. To alleviate possible detrimental effects on the surrounding environments arising from drainage inside the pits and seepage failures, additional 1.5–8 m high plain concrete walls were supplemented below the toes of DWs and a rigid end-plate method was employed to construct the DW joints [3,40].

2.2 Review of the protection plans of the TV tower during excavation of He-Ping-Qiao Station

During construction of HPQ, the original protection plans (OPP) comprised of isolation piles, artificial recharge and underpinning were carried out in three phases to safeguard the TV tower. The layouts and parameters of these three measures can refer to Fig.1. As the detailed protection plans of the TV tower have been introduced by Liu et al. [13], they are only reviewed briefly herein. Fig.4 plots the measured maximum tower settlement, {\delta }_{\mathrm{b}\mathrm{m}}, and tilt, {\omega }_{\mathrm{b}}, over time throughout construction of HPQ. After completion of DWs and prior to soil removal, six recharge wells as well as a row of bored piles (the initial isolation piles) and high-pressure JG piles had been installed between HPQ and the TV tower, in which the JG piles were implemented as waterproof curtains to avoid potential seepage erosion under waterflow [13]. Despite this, the {\delta }_{\mathrm{b}\mathrm{m}} was very close to the control criteria ({\delta }_{\mathrm{b}\mathrm{m}}\le 24 mm and {\omega }_{\mathrm{b}}\le 1.4‰) when excavation of pit 3 reached the depth of 13–15 m BGS. Considering the notable tower settlements, another row of 20-seven MJS piles (the supplementary isolation piles) and two rows of bi-slurry piles were conducted alternately along the perimeter of the TV tower within the following month. Nevertheless, both the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} remained accelerated growth rates after the completion of pouring substructures (the base, middle, and roof slabs) at pit 3; of them, the {\delta }_{\mathrm{b}\mathrm{m}} exceeded its allowable magnitude and the {\omega }_{\mathrm{b}} was near its maximum allowable magnitude. Given this fact, a conservative protection plan of underpinning the TV tower with 48 28-m long end-bearing steel pipe piles was executed before excavation of pit 4. With the aid of the underpinning piles, the {\omega }_{\mathrm{b}} decreased significantly from 1.344‰ to 1.003‰ even if the {\delta }_{\mathrm{b}\mathrm{m}} had a conspicuous increase of about 10.5 mm. To save project costs, the contractors terminated the artificial recharge immediately after completing the substructures at pit 4. Because of abrupt drawdowns of groundwater levels along with rapid increases of effective soil stresses (compression of strata), the TV tower developed notable settlement and tilt increments and stabilized finally at {\delta }_{\mathrm{b}\mathrm{m}} = 70.4 mm and {\omega }_{\mathrm{b}} = 1.343‰ four months later.

Although construction of HPQ had been completed safely, the intolerable tower settlements and prominent tilts caused by deep excavation implied that there exist some questions for protection of the TV tower, which are worth being explored further. 1) Could the tower deformations be minimized considerably if the MJS and bi-slurry piles had been completed before construction of HPQ instead of during excavation of pit 3? 2) What role did the termination manners of artificial recharge play in triggering the increases in tower deformations? 3) What were the optimum parameters of the three adopted protection measures? 4) Could the protection plan for the TV tower be optimzied further? To answer these questions, an in-depth and comprehensive parametric study via the 3D FEM was conducted, and the calculated results are to be discussed in the subsequent sections.

3 Numerical studies

3.1 Three-dimensional flow-solid coupled model

FEM has demonstrated its effectiveness in simulation of geotechnical problems [43–51] and hence was employed in this study to investigate the protection issues of the TV tower adjacent to the subway excavations. Because excavation of HPQ was a typical 3D problem, the following parametric analyses were performed using a commercial FEM software––Midas GTS NX 2017, specializing in 3D fluid-solid coupled simulations.

Considering that the deformations of the TV tower were primarily triggered by the construction of pits 2–4 in its close proximity, the 3D numerical model only duplicated the in situ construction scenarios of pits 2–4. Fig.5 presents the mesh and geometry of the whole model and inner structures and Fig.6 shows the details of this delicate model. This model consisted of two parts, i.e., the internal and external models. To diminish boundary effects, the horizontal distance between the vertical boundaries of the internal model and the DWs was 4H_{\mathrm{e}}; the ratio of horizontal distance between the vertical boundaries of the external model and the DWs to H_{\mathrm{e}} was set to be about 13.2–18.4 based on the results of field pumping tests (i.e., the radius of influence induced by pumping), comparable to that of Ong et al. [52]. The four vertical and bottom surfaces of the finite-element (FE) model were restrained as rolled and fixed boundaries, respectively. According to the site hydrological conditions, the initial total heads of Aq 0, Aq I, and Aq II in the numerical model were assumed to be 2.0, 4.0, and 4.3 m BGS, respectively (Fig.2); in addition, the groundwater level inside the pits was supposed to be 1.0 m below the excavation surface before each soil removal.

As the purpose of implementing artificial recharge was to maintain the groundwater level around the TV tower during dewatering inside the pits, the simulation of recharging water was achieved by setting a constant head boundary at the nodes of recharge wells in the FE model. Two observation wells (W1–W2 in Fig.1) close to the recharge wells showed that the measured groundwater level variated from 1.5 to 2.5 m BGS throughout recharging, as shown in Fig.7; hence, the initial total head at the nodes of recharge wells, H_{\mathrm{t}}, was assumed to be 2 m BGS when these wells were put into operation. As introduced in Liu et al. [13], the field implementation scenarios of the underpinning plan were very complicated, e.g., pressing segmented steel pipe piles into the dense strata below the tower basement and sealing the heads of press-in piles with three different methods (i.e., prestressed, conventional grouting and compressible plate methods). Therefore, it is extremely difficult to reproduce these construction processes by numerical modeling precisely. In light of this, the simulation of underpinning the TV tower was simplified, i.e., the scenario of sealing pile heads to actively rectify the tower tilts was not considered in the FE model. Because of this simplification, the maximum tower settlement and inclination calculated by the numerical simulations would be underestimated and overestimated to some extent, respectively.

To capture the nonlinear soil behaviors, the hardening-soil (HS) model [53], which considered shear and compression hardening and the stiffness deviations under loading and unloading conditions, was used in this FE model. Tab.1 and Tab.2 summarize the soil input parameters for numerical simulations. The responses of structural elements were simulated employing the conventional linear-elastic model. To expedite computing and facilitate model converging, the bored, MJS and bi-slurry pile walls were simplified as continuous walls with a certain thickness based on the equivalent stiffness method [54], and their wall thicknesses were 0.6, 2.2, and 0.7 m, respectively. The equivalent walls, DWs, and the tower were simulated by 3D solid elements, the base, middle, and roof slabs were simulated by two-dimensional plate elements, and the concrete columns, steel pipe piles, CSs, and SPSs were simulated by one-dimensional beam elements. The input parameters of these structural elements in the FE model are summarized in Tab.3. Interface elements complying with the Mohr–Coulomb (MC) model were utilized to capture soil-structure interaction, and the relevant strength reduction factors, R, are presented in Tab.2.

3.2 Calculation procedures

On the basis of field construction scenarios of HPQ, the calculation procedures of the FE model were divided into eight stages S0–S7: 1) S0, initialization of the geo-stress and seepage fields; 2) S1, construction of the DWs and the interior steel columns and piles; 3) S2, installation of the recharge wells and bored piles; 4) S3, excavation and casting of substructures at pit 2; 5) S4, excavation and casting of substructures at pit 3 and implementation of the MJS and bi-slurry piles when pit 3 was excavated to 15 m BGS; 6) S5, underpinning the TV tower with steel pipe piles; 7) S6, excavation and casting of substructures at pit 4; and 8) S7, instant termination of artificial recharge. The detailed construction sequences of each sub-pit can refer to Liu and Tan [3] and were not repeated herein.

3.3 Validation of the adopted numerical model with field data

Fig.8(a) depicts the computed and measured lateral wall deflection, {\delta }_{\mathrm{h}}, at completion of each excavation zone; Fig.8(b) presents the simulated and monitored increments of {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} at each construction stage except S5. As shown in the figure, the simulated {\delta }_{\mathrm{h}} matched reasonably with the field data both in shape and magnitude; good agreements between the calculated and measured tower settlement and tilt increments were also captured. Therefore, the subsequent parametric studies were conducted employing this validated FE model.

4 Parametric studies

4.1 Effects of isolation piles

To examine the effects of isolation piles on limiting the development of tower deformations, three sets of 3D FE analyses were carried out: 1) Plan 1, the OPP introduced previously (Fig.1 and Fig.4); 2) Plan 2, the OPP without isolation piles; and 3) Plan 3, all isolation and bi-slurry piles of the OPP were executed before excavation of HPQ. The calculated results showed that: 1) the {\delta }_{\mathrm{b}\mathrm{m}} of Plans 1 to 3 were 56.64, 62.04, and 52.07 mm, respectively; and 2) the {\omega }_{\mathrm{b}} of Plans 1 to 3 were 1.675‰, 1.870‰, and 1.578‰, respectively. Compared with Plan 2, the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} of Plan 1 were reduced by 8.7% and 10.4%, respectively, validating the effectiveness of isolation piles in protecting the TV tower. Fig.9 plots the contours of simulated soil deformations at completion of HPQ. Obviously, the propagation paths of excavation-induced ground movements behind the DWs were sectioned by the isolation piles between the pits and the TV tower; thus, both the ground horizontal movements and settlements were restrained effectively (Fig.10(a)–Fig.10(b)), accompanied by significant reductions of the tower settlements. However, the isolation piles rarely affected the lateral wall displacements and earth pressures (Fig.10(c)–Fig.10(d)). To investigate the importance of construction timing of isolation piles, this study further compared the modeling results of Plans 1 and 3. Compared with Plan 1, Plan 3 yielded reductions of 8.1% in {\delta }_{\mathrm{b}\mathrm{m}} and 5.8% in {\omega }_{\mathrm{b}}. These comparisons indicated that construction timing of protection measures is one of the critical factors in reducing the tower settlement and tilt.

4.2 Effects of artificial recharge

Considering its merits in diminishing the groundwater drawdowns and ground subsidence incurred by dewatering inside the excavations [11,32,55–58], artificial recharge was utilized to alleviate the drainage-associated adverse impacts on the TV tower in this project. To examine the availability of artificial recharge, two simulated conditions were conducted, i.e., recharging and without recharging. It was hypothesized that no other protection measure was executed for these two conditions. Fig.11(a) and Fig.11(b) show the distributions of groundwater drawdowns, \mathrm{\Delta }H, in the aquifers. Recharging water reduced the drops of groundwater levels in each aquifer; hence, the \mathrm{\Delta }H below the tower basement decreased and the related pore pressures increased (Fig.12(a) and Fig.12(b)), followed by decreases in the effective soil stresses and consolidation compression of the strata (Figs. Fig.12(c) and Fig.12(d)). The calculated results showed that recharging yielded notably smaller {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} compared with the case without recharging, as illustrated in Fig.11(c). Apparently, artificial recharge is one effective way to protect the pre-existing structure close to the deep excavations in permeable sandy strata.

Additionally, the field monitoring data indicated that the TV tower suffered significant increments of {\delta }_{\mathrm{b}\mathrm{m}} up to 12.03 mm and of {\omega }_{\mathrm{b}} up to 0.152‰ following the sudden termination of recharge wells, as shown in Fig.4. Zheng et al. [59] suggested that the rapid development of ground subsidence arising from the termination of recharging could be mitigated with the aid of stopping recharge progressively. Therefore, the effects of termination manners of artificial recharge on tower deformations were investigated herein. The number, N, of stages to terminate artificial recharge was varied from 0 to 8 in each set of FE analyses. Fig.13 presents variations of the increments of {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} at S7 with N. It is observed that both of them decreased with increasing N to a threshold value of 6, beyond which the tower deformations reduced slightly with continuous increases of N. Evidently, the termination manner of recharging water is another critical factor in restricting tower settlement and inclination.

4.3 Effects of underpinning the TV tower

Underpinning is an effective, expensive solution to protect pre-existing structures threatened by adjacent deep excavations [28,31]. Six different scenarios were simulated to evaluate the effects of underpinning in safeguarding the TV tower and their effects were compared with the tower performance in the case of the other protection measures implemented in situ. Tab.4 displays the details of these scenarios and their corresponding calculated {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}}. Increases of 46.5% and 20.0% in the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} were obtained when the protection plans were altered from Scenarios 2 to 1; it indicated that the tower settlements and tilts could be diminished substantially by underpinning the TV tower. Fig.14(a)–Fig.14(c) show the distributions of ground compression underneath the tower without underpinning piles (Scenario 1). It is noticed that the existing short concrete columns underneath the tower basement close to the excavations were entirely within the excavation-induced ground settlement trough with large magnitudes (> 21 mm) throughout construction of HPQ. However, if the underpinning piles were implemented (the white dash lines in Fig.14), they would penetrate into the underlying strata which were disturbed slightly by excavation; hence, the bearing capacity of the tower footing could be improved effectively by the underpinning piles and the tower deformations could be minimized. Moreover, the axial forces and bending moments of the existing short columns could be lessened after underpinning (Fig.15(a) and Fig.15(b)).

Furthermore, the comparisons between Scenarios 2 and 3 showed that Scenario 3 with implementation of underpinning prior to subway excavation solely had much smaller {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} magnitudes. Once again, this finding validated the significance of timing of implementing protection measures. Then, the calculated results among Scenarios 3–6 were discussed. Compared with Scenario 6, Scenarios 3–5 yielded reductions in the {\delta }_{\mathrm{b}\mathrm{m}} of 61.3%, 19.0%, and 11.8% and in the {\omega }_{\mathrm{b}} of 51.2%, 22.3%, and 6.1%, respectively. Obviously, underpinning the TV tower had the best protection performance, succeeded by isolation piles and artificial recharge.

4.4 Effects of the parameters of isolation piles

4.4.1 Length of isolation piles

The first investigated parameter of isolation piles is the length, L_{\mathrm{i}}, ranging from 16 to 48 m. For each set of 3D FE analyses, the OPP having different L_{\mathrm{i}} was employed to safeguard the TV tower while the L_{\mathrm{i}} of bored and MJS piles was kept consistent. Fig.16(a) plots the simulated results at various L_{\mathrm{i}}. As shown in this figure, both the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} decreased nonlinearly as L_{\mathrm{i}} increased; both of them reduced sharply at 16 m 32 m while gradually became gentle at 32 m \le L_{\mathrm{i}}\le 48 m.

4.4.2 Stiffness of isolation piles

The second investigated parameter is the stiffness of isolation piles, S_{\mathrm{i}}, varying from 0.5E_{0\mathrm{i}} to 4E_{0\mathrm{i}} (E_{0\mathrm{i}} = the initial stiffness of isolation piles). Similarly, the TV tower was assumed to be protected following the OPP under five distinct S_{\mathrm{i}}. Fig.16(b) presents the relationship between the tower deformations and the normalized stiffness of isolation piles, S_{\mathrm{i}}/E_{0\mathrm{i}}. The simulation results showed that no noticeable reduction in {\delta }_{\mathrm{b}\mathrm{m}} or {\omega }_{\mathrm{b}} took place as S_{\mathrm{i}}/E_{0\mathrm{i}} increased. This simulation result implies that it would not be effective or economical to enhance the protective effects of isolation piles by increasing their S_{\mathrm{i}}.

4.4.3 Layout of isolation piles

To explore the effects of the layout of isolation piles, three arrangement plans (Layouts 1–3) were simulated in this section (Fig.16(c) and Tab.5). With respect to these three layouts, they featured L_{\mathrm{i}} = 32 m; in addition, the protection plans were solely composed of isolation piles and artificial recharge, implemented before excavation of HPQ. Fig.16(c) plots the calculated tower deformations under different configurations of isolation piles. Tab.5 summarizes the distance, d, between the TV tower and the isolation piles at various layouts. The simulation results showed that even if most of the bored piles with a large d between the tower and pit 2 in Layout 1 were replaced by the MJS piles with a much smaller d in Layout 2, no apparent decrease of {\delta }_{\mathrm{b}\mathrm{m}} or {\omega }_{\mathrm{b}} would happen. In contrast, the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} of Layout 3 could be significantly reduced if the remaining bored piles between the tower and pits 3–4 in Layout 2 were supplanted by the MJS piles in Layout 3. It can be inferred that both d and layout of isolation piles are crucial for protecting the TV tower. Because the development of {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} resulted mainly from construction of pits 3–4 (Fig.4), the tower settlements and tilts could be reduced considerably by reducing the d of those isolation piles between the TV tower and pits 3–4. Based on this analysis result, isolation piles with a small d are suggested to be installed along the parts of pre-existing buildings which are potential to suffer the worst damage caused by adjacent excavations.

4.5 Effects of the parameters of recharge wells

4.5.1 Water level around wells after recharge

The utilization of artificial recharge in this project aimed to reduce drops in groundwater levels around the TV tower during the adjacent excavations, which were closely related to both the injection flow quantity and pressure of recharge wells [36,57,60,61]. The influences of recharge flow quantity and pressure on tower deformations could be reflected by exploring the effects of water levels around wells after recharging. The total head at the nodes of recharge wells, H_{\mathrm{t}}, was varied from 0 to 4 m BGS in the numerical simulations to explore the effectiveness of the OPP under various H_{\mathrm{t}} for protecting the TV tower. As shown in Fig.17(a), both {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} increased nonlinearly with increasing of H_{\mathrm{t}}; the tower settlements and tilts increased slowly when H_{\mathrm{t}}\le 3 m BGS but increased rapidly when H_{\mathrm{t}} > 3 m BGS.

4.5.2 Depth of recharge wells

The depth of recharge wells, L_{\mathrm{r}}, was altered from 20 to 60 m, keeping the OPP as the protection plan for the TV tower. Fig.17(b) summarizes the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} at various L_{\mathrm{r}}. It is observed that both {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} decreased significantly as L_{\mathrm{r}} increased when L_{\mathrm{r}}\le 40 m, whereas increasing L_{\mathrm{r}} further only incurred negligible reduction in both {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} when L_{\mathrm{r}}> 40 m.

4.5.3 Location of recharge wells

When isolation piles and artificial recharge were implemented concurrently, isolation piles as the barrier would block the groundwater flowing from the TV tower toward the excavations; hence, the seepage field around the tower would be altered along with the relative positions of these two protection measures, accompanied by variations of the tower settlements and tilts. To compare the efficiencies of recharge wells installed at different locations, two typical scenarios were simulated, i.e., plan A: all recharge wells were installed between the excavations and the isolation piles, and plan B: all recharge wells were installed between the TV tower and the isolation piles. In these two sets of numerical simulations, it was assumed that only isolation piles and recharge were carried out before excavation of HPQ. As shown in Fig.18(a) and Fig.18(b), compared with plan A, plan B yielded a smaller groundwater drawdown and more considerable pore pressure of aquifers. These simulation results implied that effective soil stresses and consolidation settlement due to the decline of groundwater level could be lessened under plan B (Fig.18(c)). However, the simulation results showed that both {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} along with soil settlements in the case of plan B were more significant than those of plan A (Fig.17(c) and 18(d)). In general, the ground and tower settlements derived from the following two parts: 1) consolidation settlements caused by dewatering and 2) vertical and lateral displacements induced by the adjacent excavations. Therefore, it can be reasonably postulated that the TV tower underwent more pronounced settlements arising from excavation-induced ground movements in the case of plan B.

Inherently, the tower settlement and tilt were associated with ground movement underneath its footing. In this project, the ground movements outside the pits resulted primarily from the lateral displacements of DWs and/or isolation piles; thereby, the simulated {\delta }_{\mathrm{h}} and horizontal displacements of isolation piles, {\delta }_{\mathrm{i}\mathrm{h}}, were analyzed subsequently. Fig.19 illustrates the calculated lateral wall deflections {\delta }_{\mathrm{h}} at several typical surveyed points. Compared with plan B, plan A yielded a greater {\delta }_{\mathrm{h}}; thus, it can be postulated that {\delta }_{\mathrm{h}} was not the contributory cause leading to the larger {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} of plan B. Thereafter, the {\delta }_{\mathrm{i}\mathrm{h}} data at plans A and B were examined. The simulated results showed that the maximum {\delta }_{\mathrm{i}\mathrm{h}} of plan B was 8.82 mm larger than that of plan A (Fig.20). This was consistent with the previous comparisons of tower deformations and ground settlements for plans A and B. As a result, it can be concluded that the increases in the horizontal displacements of isolation piles under plan B resulted in greater lateral and vertical displacements of the subsoils below the tower footing, then incurring larger tower settlements and tilts.

To further explore the triggering mechanism of the {\delta }_{\mathrm{i}\mathrm{h}} increase at plan B, the computed pore pressure P_{\mathrm{w}} and total lateral earth pressure {\sigma }_{\mathrm{h}} on both sides of the isolation piles were examined. Fig.21 plots distributions of the \mathrm{\Delta }{P}_{\mathrm{w}} and \mathrm{\Delta }{\sigma }_{\mathrm{h}} under plans A and B, in which \mathrm{\Delta }{P}_{\mathrm{w}} and \mathrm{\Delta }{\sigma }_{\mathrm{h}} denote the deviations of P_{\mathrm{w}} and {\sigma }_{\mathrm{h}}, respectively. Apparently, the \mathrm{\Delta }{P}_{\mathrm{w}} and \mathrm{\Delta }{\sigma }_{\mathrm{h}} at plan B were larger than that of plan A. It means that under plan B, the isolation piles undertook larger lateral water and earth pressures because of the higher groundwater levels on the tower side; hence, the horizontal displacements of isolation piles toward excavation increased; ultimately, the TV tower experienced a larger {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}}. These findings suggest that installation of recharge wells between isolation piles and the excavation rather than between isolation piles and pre-existing structures would be more effective in protecting the adjacent structures.

4.6 Effects of the length of underpinning piles

Many preceding studies [27,31] emphasized that the length of underpinning piles, L_{\mathrm{u}}, is the foremost parameter in design. The bearing capacity of building foundations could be improved substantially by increasing L_{\mathrm{u}}, but it means a notable increase in construction costs. Therefore, five sets of 3D FE analyses were executed on underpinning piles by altering the bearing stratum of pile toes from layers 3-2 to 5-3, i.e., L_{\mathrm{u}} was varied from 15 to 45 m to obtain the optimal L_{\mathrm{u}} for underpinning (Fig.22). For these numerical analyses, the protection plans of the TV tower were assumed to be identical to the OPP. Fig.22 shows that both {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} reduced significantly with increasing of L_{\mathrm{u}} to a threshold value at 36 m, i.e., the piles were toed in Layer 5-2; beyond this critical value, the decreases of {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} by increasing L_{\mathrm{u}} could be ignored.

5 Optimization of protection schemes for the TV tower

The preceding comprehensive parametric analyses validated the effectiveness of isolation piles, artificial recharge and underpinning in protecting the TV tower and their primary optimal parameters were obtained. It should be clarified herein that these optimized results might not apply to all projects in water-rich sandy strata due to stratigraphic variability. To optimize the protection plans of the TV tower, another four sets of 3D FE analyses were carried out. The details of these four protection scenarios are summarized in Tab.6. Fig.23 presents the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} at different protection plans. As shown in this figure, the magnitudes of {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} for Schemes 1 and 2 were out of their specified control criteria. It implied that regardless of the locations of isolation piles, the protection scheme merely consisting of isolation piles and artificial recharge was incapable of ensuring the safety of the TV tower. On the contrary, the {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} of Schemes 3 and 4 were within the scope of the specified control criteria. Therefore, underpinning the TV tower with long end-bearing steel pipe piles was indispensable for safeguarding the tower in this case. Moreover, the comparisons between Schemes 3 and 4 showed that if underpinning had been implemented, the additional isolation piles and artificial recharge could only cause slight reduction in {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}}. Obviously, Scheme 3, i.e., only underpinning the TV tower with steel pipe piles (L_u = 36 m) before excavation of HPQ, is the most cost-effective scheme for protecting the TV tower.

6 Conclusions

Via 3D FE analyses, this paper performed a comprehensive parametric study on the protection plans for the 193.5-m high TV tower adjacent to subway excavations in water-rich sandy strata. The effectiveness of isolation piles, artificial recharge and underpinning in protecting the tower was evaluated, and the critical factors that help restrain tower deformations were identified. Then, the primary parameters of these three measures for protection were optimized, and an optimal protection scheme for the TV tower was proposed finally. Based on these analyses, the following major conclusions and suggestions can be drawn.

1) In terms of diminishing the excavation-induced tower deformations, the underpinning technique performed best, followed by implementation of isolation piles and artificial recharge.

2) The construction timing of protection measures and termination manners of recharge are two critical factors in limiting tower deformations. Protection measures should be completed prior to excavation instead of during excavation. Besides, both {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} decrease nonlinearly as the number of stages to terminate recharge increases.

3) Both {\delta }_{\mathrm{b}\mathrm{m}} and {\omega }_{\mathrm{b}} are negatively correlated with the length of isolation piles (L_{\mathrm{i}}), the depth of recharge wells (L_{\mathrm{r}}), and the length of underpinning piles (L_{\mathrm{u}}) while positively correlated with the water level around recharge wells after recharging (H_{\mathrm{t}}), in which the suggested optimal L_{\mathrm{i}}, L_{\mathrm{r}}, L_{\mathrm{u}}, and H_{\mathrm{t}} for this project are 32 m, 40 m, 36 m, and 3 m BGS, respectively. In contrast, tower deformations are insensitive to the stiffness of isolation piles.

4) Isolation piles are suggested to be installed close to the parts of pre-existing structures susceptible to adjacent deep excavations as close as possible. Additionally, if isolation piles and artificial recharge are executed concurrently as protection measures, recharge wells should be installed between isolation piles and excavation instead of between isolation piles and protected structures.

5) For this project, if only isolation piles and/or artificial recharge were conducted as the protection measures for the TV tower, its safety could not be ensured; however, both the tower settlements and inclinations could be limited within their performance specifications if the underpinning plan was completed before excavation of HPQ. Moreover, once underpinning had been implemented prior to excavation, additional isolation piles and artificial recharge could only result in negligible improvement in protecting adjacent buildings.

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