Experimental Study and Estimation of Groundwater Fluctuation and Ground Settlement Due to Dewatering in a Coastal Shallow Conﬁned Aquifer

Journal of Marine Science and Engineering

Article Experimental Study and Estimation of Groundwater Fluctuation and Ground Settlement due to Dewatering in a Coastal Shallow Conﬁned Aquifer

Jiong Li 1, Ming-Guang Li 1, Lu-Lu Zhang 1, Hui Chen 2, Xiao-He Xia 1 and Jin-Jian Chen 1,* 1 State Key Laboratory of Ocean Engineering, Department of Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] (J.L.); [email protected] (M.-G.L.); [email protected] (L.-L.Z.); [email protected] (X.-H.X.) 2 Shanghai Changkai Geotechnical Engineering Co., Ltd., Shanghai 200240, China; [email protected] * Correspondence: [email protected]; Tel.: +86-021-34207003

Received: 18 February 2019; Accepted: 25 February 2019; Published: 1 March 2019

Abstract: The coastal micro-confined aquifer (MCA) in Shanghai is characterized by shallow burial depth, high artesian head, and discontinuous distribution. It has a significant influence on underground space development, especially where the MCA is directly connected with deep confined aquifers. In this paper, a series of pumping well tests were conducted in the MCA located in such area to investigate the dewatering-induced groundwater fluctuations and stratum deformation. In addition, a numerical method is proposed for the estimation of hydraulic parameter, and an empirical prediction method is developed for dewatering-induced ground settlement. Test results show that groundwater drawdowns and soil settlement can be observed not only in MCA but also in the aquifers underneath it. This indicates that there is a close hydraulic connection among each aquifer. Moreover, the distributions and development of soil settlement at various depths are parallel to those of groundwater drawdowns in most areas of the test site except the vicinity of pumping wells, where collapse-induced subsidence due to high-speed flow may occur. Furthermore, the largest deformation usually occurs at the top of the pumping aquifer instead of the ground surface, because the top layer is expanded due to the stress arch formed in it. Finally, the proposed methods are validated to be feasible according to the pumping well test results and can be employed to investigate the responses of groundwater fluctuations and stratum deformations due to dewatering in MCA.

Keywords: pumping well test; groundwater ﬂuctuation; stratum deformation; micro-conﬁned aquifer

1. Introduction Shanghai is located at the riverfront and coastal plain of the Yangtze River deltaic deposit, where soft Quaternary deposits with a thickness of about 300 m are widely distributed [1,2]. In this region, an alternated multi-aquifer-aquitard system (MAAS) is formed due to complicated palaeoclimatic and palaeogeographic conditions as well as frequent transgressions and regressions in history [3–5]. The groundwater system in Shanghai is a part of the deltaic groundwater system of the Yangtze River [4] and mainly includes a phreatic aquifer group and five confined aquifers (labeled as Aq I to Aq V). Specifically, the phreatic aquifer group is composed of a phreatic aquifer (labeled as Aq0) and a micro-confined aquifer (labeled as MCA). All the aquifers are separated by seven aquitards (labeled as Ad0 to Ad VI). According to the distribution of MCA and its hydraulic connection with adjacent aquifers, the MAAS in Shanghai central city can be divided into five types (labeled as Type I to Type V), as depicted in Figure1d. A geological survey shows that the MAAS keeps abundant and high artesian groundwater in aquifer layers [6], easily causing adverse effects on the construction of the underground facilities as well as deep excavations in these soft deposits [7–10], especially the

J. Mar. Sci. Eng. 2019, 7, 58; doi:10.3390/jmse7030058 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 2 of 21 J. Mar. Sci. Eng. 2019, 7, 58 2 of 20 artesian groundwater in aquifer layers [6], easily causing adverse effects on the construction of the underground facilities as well as deep excavations in these soft deposits [7–10], especially the groundwatergroundwater stored stored in in MCR, MCR, Aq Aq I I andand Aq Aq II. II. In In past past few few decades, decades, the influence the inﬂuence of Aq I of and Aq Aq I II and has Aq II has beenbeen deeply deeply concerningconcerning and and widely widely studied studied by by both both researchers researchers and and engineers engineers [7,9–13], [7,9– whereas13], whereas literaturesliteratures concentrated concentrated on on the the inﬂuence influence of of MCA MCA areare rare.

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t ic tr P is udon D Mi nha Shanghai fu g Distr Di g New ng an ng st i ji t i ric Q g ic ct on tr t S is D Fengxian Jinshan District District

(a) (b) (c)

Aq0 Aq0 Aq0 MCA Ad0 MCA Ad0 Aq0 Ad0 Legend AdⅠ AdⅠ AdⅠ MCA AdⅠ AqⅠ AqⅠ +AqⅠ Urban area AdⅡ AqⅠ+Ⅱ AdⅡ AdⅡ AqⅡ AqⅡ AqⅡ Type Ⅰ AdⅢ AdⅢ AdⅢ AdⅢ Type Ⅱ AqⅢ AqⅢ AqⅢ AqⅢ AdⅣ AdⅣ AdⅣ AdⅣ Type Ⅲ AqⅣ AqⅣ AqⅣ AqⅣ Type Ⅳ AdⅤ AdⅤ AdⅤ AdⅤ AqⅤ AqⅤ AqⅤ AqⅤ Type Ⅴ AdⅥ AdⅥ AdⅥ AdⅥ River Base Rock Base Rock Base Rock Base Rock Test site (d1) (d2) (d3) (d4)

FigureFigure 1. Typical 1. Typical distribution distribution and and hydro-geological hydro‐geological proﬁle profile of of multi-aquifer-aquitardmulti‐aquifer‐aquitard system system (MAAS) (MAAS) in Shanghaiin Shanghai central city:central (a) locationcity: (a) oflocation Shanghai; of Shanghai; (b) plan view(b) plan for the view location for the of Shanghailocation of Administration Shanghai Region;Administration (c) type distribution Region; (c of) type MAAS distribution in Shanghai of MAAS central in city; Shanghai (d1) schematic central city; diagram (d1) schematic for MAAS of Typediagram I and Type for II;MAAS (d2) schematicof Type I diagramand Type for II; MAAS (d2) schematic of Type III;diagram (d3) schematic for MAAS diagram of Type forIII; MAAS(d3) of Typeschematic IV; (d4) schematic diagram for diagram MAAS of for Type MAAS IV; ( ofd4) Type schematic V. diagram for MAAS of Type V.

The micro-confinedThe micro‐confined aquifer aquifer in in Shanghai Shanghai is is locatedlocated at at the the top top of of MAAS MAAS and and is the is lower the lower sublayer, sublayer, underneathunderneath the phreaticthe phreatic aquifer, aquifer, of of Holocene Holocene phreatic phreatic aquifer aquifer group. group. The The aquifer aquifer is discontinuously is discontinuously distributeddistributed in the in the horizon horizon direction direction and and is is usually usually buried at at a adepth depth of ofonly only 15 15to 22 to m 22 [4]. m For [4]. this For this reason,reason, the piezometricthe piezometric head head of of the the groundwater groundwater in MCA MCA is is easily easily affected affected by bymeteorological meteorological and and hydrological conditions [3] and periodically varies from 3 to 11 m below the ground surface [14,15]. hydrological conditions [3] and periodically varies from 3 to 11 m below the ground surface [14,15]. The thickness of the aquifer generally varies from 5 to 20 m in most regions, whereas in the region of The thickness of the aquifer generally varies from 5 to 20 m in most regions, whereas in the region of Type V, MCA is directly connected with Aq I due to the hiatus of Ad I and its thickness can reach 40 Typeto V, 50 MCA m [4,16,17]. is directly Additionally, connected the with MCA Aq in I dueShanghai to the is hiatusmainly of composed Ad I and of its silty thickness sand and can silty reach clay 40 to 50 mand [4,16 its,17 hydraulic]. Additionally, conductivity the MCA is relatively in Shanghai lower is than mainly those composed of the deep of silty confined sand andaquifers. silty The clay and its hydraulichydraulic conductivity conductivity of is relativelyMCA is variable lower between than those (3~6) of × the 10−5 deep~10−3 cm/sec confined while aquifers. those of The Aq I hydraulic and conductivityAq II usually of MCA vary between is variable (3~6) between × 10−4~10 (3~6)−3 cm/sec× 10 and−5 ~10(2~6)− ×3 10cm/sec−3~10−2 cm/sec, while those respectively of Aq [14]. I and In Aq II usuallyaddition, vary betweenthe maximum (3~6) specific× 10− discharge4~10−3 cm/sec capacity and of MCA (2~6) is about× 10− 43.23~10 m−3/day2 cm/sec,‐meter, respectivelywhich is also [14]. In addition,smaller thethan maximum those of deep specific confined discharge aquifers capacity with the of maximum MCA is aboutbeing greater 43.2 m than3/day-meter, 720 m3/day which‐meter, is also smallere.g., than Aq II those and Aq of deepIV [4]. confined aquifers with the maximum being greater than 720 m3/day-meter, As aforementioned, the artesian head in the MCA is high, and the burial depth of MCA is small, e.g., Aq II and Aq IV [4]. causing deep excavations to be more easily affected by the MCA, especially the MCA in Type V. As aforementioned, the artesian head in the MCA is high, and the burial depth of MCA is Moreover, with rapid development of ocean economy and coastal industry in recent decades, an small,increasing causing deepnumber excavations of municipal to beand more commercial easily affectedinfrastructures, by the e.g., MCA, metro especially tunnels theand MCA stations, in Type V. Moreover,were or are with being rapid constructed development in the ofcoastal ocean soft economy deposits of and Shanghai coastal [4,18–21], industry resulting in recent in decades,the an increasingincrease of number excavation of municipal scale and depth and commercial [5,22–24]. In infrastructures,some projects, the e.g., depth metro of excavation tunnels and reaches stations, were or are being constructed in the coastal soft deposits of Shanghai [4,18–21], resulting in the increase of excavation scale and depth [5,22–24]. In some projects, the depth of excavation reaches the top of MCA [15]. As excavation depth is increased, the remaining bottom soil is insufficient to counteract the artesian head underneath the excavation, leading to seepage and inrushing damage to the excavation [9,25,26]. This is particularly true for the excavations above the coastal MCA, for which J. Mar. Sci. Eng. 2019, 7, 58 3 of 20 the critical excavation depth is usually less than 10 m according to the Shanghai design specification for the requirement of surge resistance [14]. Consequently, dewatering measurements should be adopted in excavations to reduce or even eliminate the adverse effects due to artesian groundwater [8,10,26–28]. However, regardless of many advantages of dewatering for protecting excavations, it has been widely accepted that groundwater extraction can induce stratum deformation in the vicinity of the excavations [5,7,8,18,19,29–33] as well as secondary hazards to surrounding structures [34–37]. Many scholars and engineers have been devoted to investigating the dewatering-induced environment influence in Shanghai. As a matter of fact, these studies were primarily concentrated on the influence of groundwater extraction in deep confined aquifers [10–12], such as Aq I and Aq II, whereas few literatures were dealt with that in the shallow-buried MCA. As above mentioned, the hydrological and geological conditions (e.g., burial depth, compressibility, hydraulic conductivity, and specific discharge capacity) of the MCA are distinguished from those of deep confined aquifers. Additionally, the MCA is more easily affected by meteorological and hydrological conditions than deep confined aquifers due to its shallow burial depth [3], resulting in complex hydrological conditions in the MCA. Moreover, hydrological conditions can be more complex if there is aquitard hiatus, e.g., in the MAAS of Type V, where the MCA is directly connected with Aq I due to the hiatus of Ad I. Considering these factors, the environmental risk from excavation dewatering in a coastal MCA is still uncertain. The objective of this study is to investigate the response of groundwater fluctuations and ground settlement induced by dewatering in the coastal MCA of Shanghai. To achieve this aim, a series of field pumping well tests were conducted at a construction site located at Pudong New Area District, where the MCA is connected directly with Aq I. To help analyze the responses, the following are addressed:

1. How are MCA and Aq I hydraulically connected and how does the hydraulic connection affect the responses of groundwater fluctuations and strata deformation? 2. What is the correlation between stratum deformation (ground settlement, stratum compression) and groundwater fluctuations? 3. How to estimate the hydrogeological parameters of the MCA based on pumping well tests if the MCA is directly connected with the confined aquifer. 4. How to predict the ground settlement induced by dewatering in the MCA when the MCA is directly connected with the confined aquifer.

2. Study Area To investigate the responses of groundwater ﬂuctuations and stratum deformation as well as the inﬂuence on excavation, three groups of single-well pumping test and a group of multi-well pumping test were conducted.

2.1. Engineering Geology The test site is located in the northwest of Pudong New Area District (as shown in Figure1c), and the elevation at the test site varies from 4.57 m to 5.66 m. The soil distributed in the influence depth of the foundation is characterized as a depositional soil layer of the coastal plain from Quaternary Holocene to Pleistocene, mainly including clay, silty soil, and silty sand. The columns in the left part Figure2 plot the soil profile of the construction site. The first layer is an artificial layer in the upper about 3.76 m below the land surface, underlain by silty clay, mucky silty clay, mucky clay, and clay to the depth of about 19.85 m. The following layer is sandy silt to the depth of about 25.65 m overlying the layer of silty clayey silt to the depth of about 30.17 m. The next layer is a silt layer extending to a depth of about 41.42 m, followed by sandy silt mixed up with silty clay to a depth of about 64.90 m. Underneath the above layers is silty sand to a depth of about 74.98 m. Vertically below all these layers is the interbedded strata of silty clay and silty sand until the termination depth of about 82.14 m. Figure3 presents the geotechnical parameters for each layer at the test site. The grain size distribution indicates that the content of silty sand and sand in deep confined aquifers is higher than J. Mar. Sci. Eng. 2019, 7, 58 4 of 20

that in the MCR. The initial void ratio, e0, was determined based on the physical properties of the soils at different depths tested from the laboratory tests. The vertical and horizontal hydraulic conductivity, kv and kh, of soils were obtained based on laboratory tests and injection tests. The compression J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 4 of 21 index, a0.1−0.2, was determined by laboratory oedometer tests. The water content of each stratum was usuallyall close these to layers its liquid is the limit,interbedded whereas strata the of plasticsilty clay limit and variedsilty sand little until along the termination with the depth of and was approximatelyabout 82.14 21%. m.

Soil φ650 φ650 φ650 φ650 φ650 Hydrogeology profile(m) φ273 φ273 φ273 φ273 φ273 profile 3.76 ① 1.71 ② PA 5.41 ③ 14000 14000 1400 22000 LayerA 5.26 ④ 5000 5000 5000 ADⅠ 3.71 ⑤1 3000 3000 39000 6000 6.80 ⑤2 12000 LayerB 15000 59000 13000 4.52 ⑤3-1 12000 (Upper MCA)

1000 11.45 ⑤3-2 1000 1000 G5-2-1 LayerC 5000 15000 MCA C5-2-2 C5-2-4 G5-2-2 2000 (Middle MCA) G5-2-3 C5-2-1 9000 C5-2-3 ⑤ 23.48 3-3 1000 LayerD 18000 (Lower MCA) C5-3-1 5000 G5-3-1 2000 Legend: 6000 Settlement ⑦ Gravel pack High-quality clay 1000 CA 10.08 monitoring LayerE Magnet ring Sedimentary tube C7-1 hole X1-X4 G7-1 7.16 ⑧ Screen pipe Clay G7-2 ADⅡ

Note: Layer①=Miscellaneous fill, Layer②=Silty clay, Layer③=Mucky silty clay, Layer④=Mucky clay, Layer⑤ 1=Clay, Layer⑤2=Sandy silt, Layer⑤31=Silty clay mixed with sandy silt, Layer⑤32=Silty sand, Layer⑤33=Sandy silt mixed with silty clay, Layer⑦=Silty sand, Layer⑧=Interbedded strata of silty clay and silty sand, PA=Phreatic aquifer, AD=Aquitard, MCA=Micro-confined aquifer, CA=Confined aquifer J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 5 of 21 FigureFigure 2. Proﬁle 2. Profile of geological of geological and and hydrogeological hydrogeological section section and and well well structure. structure.

γ e k k a w FigureSP 3HP presentsGZ the geotechnical 0 parametersv for eachh layer0.1-0.2 at the testn site. PL The grain LL size 3 -1 distribution indicates (%) that the(kN/m content) of silty sand(cm/s) and sand(cm/s) in deep(MPa confined) (%) aquifers(%) is higher (%) than 0 50 10015 17 190.6 1.2 1E-8 1E-41E-8 1E-40120 35 50 15 21 27 30 40 50 that 0in the MCR. The initial void ratio, e0 , was determined based on the physical properties of the ① soils at ②differentPA depths tested from the laboratory tests. The vertical and horizontal hydraulic 10 ③ conductivity, kv and kh , of soils were obtained based on laboratory tests and injection tests. The ④ ADⅠ compression index, a , was determined by laboratory oedometer tests. The water content of each 20 ⑤1 0.1 0.2 stratum⑤ was2 usually close to its liquid limit, whereas the plastic limit varied little along with the depth and30 was⑤3-1 approximately 21%. ⑤3-2 40 MCA Depth (m) Depth 50 ⑤3-3 60

70 ⑦ CA

80 ⑧ ADⅡ >0.075mm Average ⑨ 0.005～0.075mm Maximum 90 <0.005mm Minimum Note：SP=soil profile, HP= hydrogeology profile, GZ=grain size, PL=plastic limit, LL=liquid limit,γ=unit weight,

e0=void ratio, kv=vertical hydraulic conductivity, kh=horzontional hydraulic conductivity, wn=water content, a =compression coefficient, PA=phreatic aquifer, AD=aqutard, MCA=micro-confined aquifer, CA=confined aquifer 0.1-0.2

FigureFigure 3. Soil 3. Soil proﬁle profile and and propertiesproperties at at the the construction construction site. site.

2.2 Hydrogeology There are mainly three types of groundwater stored in the influence depth of the proposed project, and they are phreatic water referred to as phreatic aquifer, feeble confined water known as MCA, and confined water dubbed as confined aquifer (Aq I), respectively. As can be seen in Figure 2, the phreatic aquifer and the MCA are separated by an aquitard (Ad0) while MCA is adjacent to the confined aquifer directly and there is a certain hydraulic connection between the two aquifers. The phreatic aquifer is mainly composed of the silty clay with a thickness of about 9 m, and the water head of it is variable from 0.75 m to 1.70 m below the ground surface. The MCA is primarily stored in the sandy silt (Layer ⑤2), the silty sand (Layer ⑤3‐2), and the silty clay (Layer ⑤3‐3) with an aggregate thickness of about 44.2 m. The artesian head in these layers varies from −5.1 m to −6.8 m compared to the ground surface. In addition, the confined aquifer is mainly composed of silty sand (Layer ⑦) with a thickness of about 9.8 m and the water level varies from −8.9 m to −9.7 m.

3. Pumping Well Tests

3.1. Well Installation In the tests, twelve test wells, including seven pumping wells and five observation wells, were employed. The layout of the wells is plotted in Figure 4, and the distance between each well is also labeled in the figure. Each test well was composed of a steel tube, screen pipe, and sedimentary pipe. The steel pipe was installed at the upper part of a well and outside the steel pipe were sealed with clay, high‐quality clay, and gravel pack from top‐down to prevent the groundwater from the upper aquifers flowing into test wells. The screen was installed underneath the steel pipe and outside the screen was backfilled with gravel to ensure the groundwater flowed into the well smoothly. The sedimentary pipe with a length of 1 m was installed at the bottom of the well to prevent the screen pipe being clogging by the sediment in the groundwater. The structures of the test well are depicted in Figure 2, and the associated parameters are also presented in the figure.

J. Mar. Sci. Eng. 2019, 7, 58 5 of 20

2.2. Hydrogeology There are mainly three types of groundwater stored in the influence depth of the proposed project, and they are phreatic water referred to as phreatic aquifer, feeble confined water known as MCA, and confined water dubbed as confined aquifer (Aq I), respectively. As can be seen in Figure2, the phreatic aquifer and the MCA are separated by an aquitard (Ad0) while MCA is adjacent to the confined aquifer directly and there is a certain hydraulic connection between the two aquifers. The phreatic aquifer is mainly composed of the silty clay with a thickness of about 9 m, and the water head of it is variable from 0.75 m to 1.70 m below the ground surface. The MCA is primarily stored in the sandy silt (Layer 5 2), the silty sand (Layer 5 3-2), and the silty clay (Layer 5 3-3) with an aggregate thickness of about 44.2 m. The artesian head in these layers varies from −5.1 m to −6.8 m compared to the ground surface. In addition, the confined aquifer is mainly composed of silty sand (Layer 7 ) with a thickness of about 9.8 m and the water level varies from −8.9 m to −9.7 m.

3. Pumping Well Tests

3.1. Well Installation In the tests, twelve test wells, including seven pumping wells and five observation wells, were employed. The layout of the wells is plotted in Figure4, and the distance between each well is also labeled in the figure. Each test well was composed of a steel tube, screen pipe, and sedimentary pipe. The steel pipe was installed at the upper part of a well and outside the steel pipe were sealed with clay, high-quality clay, and gravel pack from top-down to prevent the groundwater from the upper aquifers flowing into test wells. The screen was installed underneath the steel pipe and outside the screen was backfilled with gravel to ensure the groundwater flowed into the well smoothly. The sedimentary pipe with a length of 1 m was installed at the bottom of the well to prevent the screen pipe being clogging by the sediment in the groundwater. The structures of the test well are depicted in FigureJ. Mar.2, and Sci. Eng. the 2019 associated, 7, x FOR PEER parameters REVIEW are also presented in the figure. 6 of 21

C5-2-4 C5-2-4 Deformed Steel Bars

Observation Plastic Bushing D2 D3 Point D1C5-3-1 G5-3-2 G5-2-3 D5 G5-2-1 G5-2-2 Ground Surface

20000 20000 20000 300 C7-1 G7-1 G7-2

X3 X4 D X1 X2 16 D15 1000 C5-2-2 C5-2-3 D14

D 13 D12

(a) D (c) 11 D10

Pumping center

D30 D29 D28 D27 D26 D25 D24 D1 D2 D3 D4 D5 D6 D7 D8 D9

17 D 18 D Pumping well 19 D Observation well D20 Plain concrete D21 Artificial layer 22 D D1-D30 Ground settlement monitoring point 23 X1-X4 Deep soil settlement monitoring hole (b) D

FigureFigure 4. Layout 4. Layout of test of test wells wells and and ground ground settlement settlement monitoring points: points: (a) (Layouta) Layout of test of wells; test wells; (b) (b) LayoutLayout of ground of ground settlement settlement monitoring monitoring points; points; ( (cc)) proﬁleprofile of of ground ground settlement settlement monitoring monitoring points. points.

In the tests, there were four pumping wells (labelled C5‐2‐1~C5‐2‐4, at a depth of 35 m, 38 m, 35 m, 38 m, respectively) and three observation wells (labelled G5‐2‐1~G5‐2‐3, at a depth of 35 m) installed in Layers ⑤2~⑤3‐2, a pumping well (labelled C5‐3‐1, at a depth of 56 m) and an observation well (labelled G5‐3‐1, at a depth of 56 m) installed in Layer ⑤3‐3, a pumping well (labelled C7‐1, at a depth of 73 m) and two observation wells (labelled G7‐1 and G7‐2, at a depth of 73 m) installed in Layer ⑦. The detailed structural parameters for each well are listed in Table 1. The external radius for pumping well was identical to that for observation well and was 650 mm, whereas the internal radius for pumping well was 273 mm while that for observation well was 168 mm. The length of the screen for C5‐2‐1~C5‐2‐4 was 14 m, 17 m, 14 m, and 12 m, respectively, for G5‐2‐1~G5‐2‐3 it was 12 m, for C5‐3‐1 and G5‐3‐1 it was 9 m, for C7‐1 and G7‐1～G7‐2 it was 6 m.

Table 1. Detailed characteristics of test wells.

Buried Buried Depth of Internal External Pumping/ Well Depth of Well Screen (m) Well type Diameter Diameter Monitoring Number Well Bottom (mm) (mm) Upper Bottom Stratum (m) C5‐2‐1 35 273 650 20 34 ⑤2~⑤3‐2 C5‐2‐2 38 273 650 20 37 ⑤2~⑤3‐2 Pumping C5‐2‐3 35 273 650 20 34 ⑤2~⑤3‐2 well C5‐2‐4 38 273 650 20 37 ⑤2~⑤3‐2 C5‐3‐1 56 273 650 46 55 ⑤3‐3 C7‐1 73 273 650 66 72 ⑦ G5‐2‐1 35 168 650 22 34 ⑤2~⑤3‐2 Monitoring G5‐2‐2 35 168 650 22 34 ⑤2~⑤3‐2 well G5‐2‐3 35 168 650 22 34 ⑤2~⑤3‐2

J. Mar. Sci. Eng. 2019, 7, 58 6 of 20

In the tests, there were four pumping wells (labelled C5-2-1~C5-2-4, at a depth of 35 m, 38 m, 35 m, 38 m, respectively) and three observation wells (labelled G5-2-1~G5-2-3, at a depth of 35 m) installed in Layers 5 2~ 5 3-2, a pumping well (labelled C5-3-1, at a depth of 56 m) and an observation well (labelled G5-3-1, at a depth of 56 m) installed in Layer 5 3-3, a pumping well (labelled C7-1, at a depth of 73 m) and two observation wells (labelled G7-1 and G7-2, at a depth of 73 m) installed in Layer 7 . The detailed structural parameters for each well are listed in Table1. The external radius for pumping well was identical to that for observation well and was 650 mm, whereas the internal radius for pumping well was 273 mm while that for observation well was 168 mm. The length of the screen for C5-2-1~C5-2-4 was 14 m, 17 m, 14 m, and 12 m, respectively, for G5-2-1~G5-2-3 it was 12 m, for C5-3-1 and G5-3-1 it was 9 m, for C7-1 and G7-1~G7-2 it was 6 m.

Table 1. Detailed characteristics of test wells.

Buried Depth of Well Well Buried Depth Internal External Pumping/ Well Type Screen (m) Number of Well Bottom Diameter Diameter Monitoring (m) (mm) (mm) Upper Bottom Stratum

C5-2-1 35 273 650 20 34 5 2~ 5 3-2 C5-2-2 38 273 650 20 37 5 2~ 5 3-2 Pumping C5-2-3 35 273 650 20 34 5 2~ 5 3-2 well C5-2-4 38 273 650 20 37 5 2~ 5 3-2 C5-3-1 56 273 650 46 55 5 3-3 C7-1 73 273 650 66 72 7

G5-2-1 35 168 650 22 34 5 2~ 5 3-2 G5-2-2 35 168 650 22 34 5 2~ 5 3-2 Monitoring G5-2-3 35 168 650 22 34 5 2~ 5 3-2 well G5-3-1 56 168 650 46 55 5 3-3 G7-1 73 168 650 66 72 7 G7-2 73 168 650 66 72 7

3.2. Test Scheme Three single-well pumping tests and a multi-well pumping test were performed successively from July 26, 2014, to August 26, 2014. Table2 shows the detailed process of the tests. The single-well pumping tests were performed in Layers 5 2~ 5 3-2 using well C5-2-1 at a rate of 9.96 m3/h from 9:00, July 26 to 18:37, July 27 lasting for 2017 min, in Layer 5 3-3 using well C5-3-1 at a rate of 9.77 m3/h from 12:00, August 5 to 13:00, August 7 lasting for 2940 min and in Layer 7 using well C7-1 at a rate of 26.7 m3/h from 8:00 August 9 to 10:00 August 10 lasting for 980 min. The multi-well pumping test was conducted in Layers 5 2~ 5 3-2 from 12:00, August 12 to 15:00, August 20 and consumed 10930 min using well C5-2-1~C5-2-4 at the discharge rate of 7.28 m3/h, 15.64 m3/h, 11.68 m3/h, and 10.87 m3/h, respectively. It should be noted that enough time should be left for groundwater recovery between each test and in this test they were 2920 min, 2828 min,1020 min, and 8120 min, respectively.

Table 2. Process of the pumping well tests.

Pumping Well s t t Q Test Type w t t p r Aquifer Number (m) 0 e (min) (min) (m3/h)

5 2~ 5 3-2 C5-2-1 8.94 09:00 26 Jul. 18:37 27 Jul. 2017 2920 −9.96 Single- 5 3-3 C5-3-1 31.21 12:00 5 Aug. 13:00 7 Aug. 2940 2828 −9.77 well 7 C7-1 10.21 08:00 9 Aug. 10:00 10 Aug. 980 1020 −26.7 C5-2-1 10.87 −7.28 Multi- C5-2-2 7.82 −15.64 5 2~ 5 3-2 12:00 12 Aug. 15:00 20 Aug. 10930 8120 well C5-2-3 7.63 −11.68 C5-2-4 9.69 −10.87

Note: sw = drawdown in pumping well; t0 = start time; te = end time; tp = pumping time; tr = recovery time; Q = discharge rate.

3.3. Stratum Deformation To obtain the responses of the ground settlement and deep stratum deformation induced by multi-well dewatering, 30 ground settlement monitoring points (labelled D1 to D30) and four deep J. Mar. Sci. Eng. 2019, 7, 58 7 of 20 soil settlement monitoring holes (labelled X1 to X4) were installed at the test site. The layout of all the monitoring points is shown in Figure4. The ground settlement monitoring points were arranged in a radial shape with the center of pumping area (D1) as its endpoint, and the distance between two adjacent points was 10 m. The burial depth of each monitoring point was 1.3 m to protect the point from external disturbance. The deep soil settlement monitoring holes were laid close to the monitoring point D1, D2, D3, and D5. The proﬁle of the four monitoring holes is plotted in Figure2. Each of the monitoring holes was 68 m deep and had four deep soil settlement monitoring points installed from Layer 5 2 to Layer 7 at a depth of 22 m, 35 m, 50 m, and 68 m, respectively.

4. Results and Analysis

4.1. Responses of Groundwater Level As aforementioned, the MCA concerned in this study is directly connected with Aq I. For this reason, dewatering in the MCA can also induce the groundwater drawdowns in its adjacent aquifers. To investigate the hydraulic connection between MCA and Aq I as well as its influence on groundwater fluctuations, the responses of groundwater level in different aquifers during the field tests are analyzed in this section.

4.1.1. Test Results Figure5 presents the discharge rates of the pumping wells and the groundwater level obtained in the observation wells. In the C5-2-1 pumping test, the groundwater head in Layers 5 2~ 5 3-2 had a rapid decline at the beginning and then decreased slowly until it reached a steady value, whereas the drawdowns observed in Layer 5 3-3 and Layer 7 were relatively small. Speciﬁcally, the maximum drawdowns obtained using G5-2-1~G5-2-3 in Layers 5 2~ 5 3-2 were 1.75 m, 1.18 m and 0.74 m, respectively, while those monitored by G5-3-1 in Layer 5 3-3 and G7-1 and G7-2 in Layer 7 were almost negligible, with the maximum drawdown of 0.14 m, −0.22 m, and −0.1 m, respectively, where minus meant head increment. A similar phenomenon was also observed from the C5-3-1 and the C7-1 pumping well test. The maximum drawdowns monitored by G5-2-1~G7-2 in the C5-3-1 pumping well test were 0.44 m, 0.23 m, 0.12 m, 2.04 m, 0.56 m, and 0 m, respectively, while those in the C7-1 pumping well test were 0.01 m, 0.02 m, −0.01 m, 0.03m, 1.23 m and 0.94m, respectively. In addition, once the dewatering was interrupted, the groundwater head recovered immediately. When the multi-well pumping test was conducted in Layers 5 2~ 5 3-2 using well C5-2-1~C5-2-4, besides obvious drawdowns observed in the pumping aquifer, maximum drawdowns of 8.18 m monitored in well G5-2-1, 7.25 m in well G5-2-2, and 6.26 m in well G5-2-3, an inconspicuous but not negligible drawdown of 1.59 m monitored in well G5-3-1 was also observed in the underneath Layer 5 3-3 as well as a 0.05-m-deep drawdown monitored in G7-1 and a 0.13-m-deep drawdown monitored in G7-2 traced in Layer 7 . The groundwater drawdown rate in Layers 5 2~ 5 3-2 was large at the beginning and then declined gradually until it reached a steady level. The groundwater head in Layer 5 3-3 and Layer 7 also followed the same development law. After the multi-well pumping test was shut down, the groundwater head in each aquifer recovered immediately. In addition, the groundwater head in Layers 5 2~ 5 3-2 increased unexpectedly after the test continued for about 65 h due to power failure. J. Mar. Sci. Eng. 2019, 7, 58 8 of 20 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 9 of 21

C5-2-1 C5-3-1 C7-1 C5-2-1～C5-2-4 Pumping Pumping Pumping Pumping

） 28 (a)

/h C5-2-1 C5-3-1 C7-1 Recovery 3 24 m 20 Recovery Recovery C5-2-2 C5-2-3 C5-2-4 （ 16 12 Recovery 8 4 Discharge -4 (b) -6 G5-2-1 G5-2-2 G5-2-3 -8 -10 Power failure -12 Mixed Layer⑤2～⑤3-2 -14

） -16

m -4 (c) （ -6 -8 -10 -12 Layer ⑤3-3 -14 G5-3-1 -16 Groundwater level Groundwater -4 (d) -6 -8 -10 -12 Layer ⑦(AqⅠ) -14 G7-1 G7-2 -16 14/7/26 7/27 7/29 8/5 8/7 8/9 8/10 8/12 8/20 2014/8/26 Time (YY/MM/DD) Figure 5. The time-history curves for discharge rate and groundwater level: (a) curves for discharge Figure 5. The time‐history curves for discharge rate and groundwater level: (a) curves for discharge rate of pumping well; (b) curves for groundwater level in Layer 5 2~ 5 3-2; (c) curves for groundwater rate of pumping well; (b) curves for groundwater level in Layer ⑤ 2 ～⑤3‐2; (c) curves for level in Layer 5 3-3; (d) curves for groundwater level in Layer 7 . groundwater level in Layer ⑤3‐3; (d) curves for groundwater level in Layer ⑦. 4.1.2. Analyses 4.1.2. Analyses As aforementioned, in the single-well pumping test, remarkable groundwater drawdowns could As aforementioned, in the single‐well pumping test, remarkable groundwater drawdowns could be observed in the pumping aquifers, while small but not negligible drawdowns could also be detected be observed in the pumping aquifers, while small but not negligible drawdowns could also be in other aquifers especially in the adjacent aquifers, indicating that there were hydraulic connections detected in other aquifers especially in the adjacent aquifers, indicating that there were hydraulic and a leakage effect among each aquifer. The hydraulic connection and leakage effect were more easily connections and a leakage effect among each aquifer. The hydraulic connection and leakage effect observed in the multi-well pumping test because the accumulated discharge rate and operation time were more easily observed in the multi‐well pumping test because the accumulated discharge rate were much larger than those in the single-well pumping tests. In real field work, the dewatering is and operation time were much larger than those in the single‐well pumping tests. In real field work, usually performed using multi-well for several months or even more than a year [7], consequently the dewatering is usually performed using multi‐well for several months or even more than a year causing apparent drawdowns in the pumping aquifer as well as its adjacent aquifers due to the [7], consequently causing apparent drawdowns in the pumping aquifer as well as its adjacent aquifers hydraulic connection and leakage effect. Hence, during the excavation dewatering, although the due to the hydraulic connection and leakage effect. Hence, during the excavation dewatering, 5 5 pumpingalthough wellsthe pumping were only wells installed were only in Layers installed2~ in Layers3-2, the ⑤ groundwater2~⑤3‐2, the groundwater drawdowns drawdowns and stratum 5 7 deformationand stratum in deformation Layer 3-3 andin Layer Layer ⑤3‐should3 and alsoLayer be ⑦ considered should also because be considered of the hydraulic because connection of the andhydraulic leakage connection effect. Otherwise, and leakage an underestimate effect. Otherwise, of water an underestimate inflow and land of subsidencewater inflow may and emerge, land resultingsubsidence in anmay adverse emerge, effect resulting on the safetyin an ofadverse the excavation effect on and the thesafety surroundings. of the excavation and the surroundings. 4.2. Responses of Ground Settlement 4.2. Responses of of Ground ground Settlement settlement induced by dewatering in deep confined aquifers [8–12] has been widelyResponses investigated, of ground whereas settlement few literatures induced focus by ondewatering those due in to deep groundwater confined extractionaquifers [8–12] in shallow has confinedbeen widely aquifers. investigated, In this section, whereas the few ground literatures settlement focus dueon those to dewatering due to groundwater in the MCA extraction as well as in the shallow confined aquifers. In this section, the ground settlement due to dewatering in the MCA as

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well as the correlation between ground settlement and groundwater drawdowns is analyzed to correlation between ground settlement and groundwater drawdowns is analyzed to investigate the investigate the responses of ground settlement induced by dewatering in shallow confined aquifers. responses of ground settlement induced by dewatering in shallow conﬁned aquifers. 4.2.1. Results 4.2.1. Results In Figure 6, the distributions of ground settlements obtained by the 30 monitoring points (D1 to In Figure6, the distributions of ground settlements obtained by the 30 monitoring points (D1 D30) during the multi‐well pumping test are depicted. As can be seen in Figure 6, the ground tosettlement D30) during increased the multi-well gradually pumping with the proceeding test are depicted. of the pumping As can be test seen and inreached Figure its6, maximum the ground settlementvalue at the increased end of the gradually test (Aug. with 20th). the After proceeding that, a remarkable of the pumping rebound test of and ground reached settlement its maximum could valuebe observed. at the end Besides, of the testit is (Aug.notable 20th). that the After distributions that, a remarkable of ground rebound settlement of at ground different settlement stages of couldthe betest observed. were similarly Besides, shown it is notable in bell that‐shaped the distributionsdistribution. The of ground settlement settlement was larger at different when the stages distance of the testof were monitoring similarly point shown to the in bell-shaped pumping center distribution. was smaller The settlement and reached was its larger maximum when thevalue distance at the of monitoringmonitoring point point to D24. the pumping Additionally, center the was settlement smaller andat the reached left side its of maximum D1 was apparently value at the larger monitoring than pointthat D24. at the Additionally, right side, which the settlementwas inconsistent at the with left the side groundwater of D1 was apparently drawdown. larger The possible than that reason at the rightresponsible side, which for this was may inconsistent be the soil with erosion the groundwaterand ground collapse drawdown. induced The by possible high‐speed reason groundwater responsible forflow. this may be the soil erosion and ground collapse induced by high-speed groundwater ﬂow.

(a) Ground Settlement Monitoring Points (b) Ground Settlement Monitoring Points D30 D29 D28 D27 D26 D25 D24 D1 D2 D3 D4 D5 D6 D7 D8 D9 D16 D15 D14 D13 D12 D11 D10 D1 D17 D18 D19 D20 D21 D22 D23 0

） 0 ） mm -5 mm （（ -5 -10 13 August -10 13 August -15 14 August 14 August 15 August 15 August -15 -20 18 August 18 August Ground Settlement 20 August Settlement Ground 20 August -25 21 August -20 21 August 25 August 25 August -30 Figure 6. The distributions of ground settlement: (a) curves for ground settlement of D1–D9 and Figure 6. The distributions of ground settlement: (a) curves for ground settlement of D1–D9 and D24– D24–D30; (b) curves for ground settlement of D1, D10–D16, and D17–D23. D30; (b) curves for ground settlement of D1, D10–D16, and D17–D23. Furthermore, the time-history curves for the settlement of D1, D3, and D5 as well as the Furthermore, the time‐history curves for the settlement of D1, D3, and D5 as well as the drawdowns in G5-2-1, G5-2-2, and G5-2-3 (next to D1, D3, and D5, respectively) are observed in drawdowns in G5‐2‐1, G5‐2‐2, and G5‐2‐3 (next to D1, D3, and D5, respectively) are observed in FigureFigure7. As7. As can can be be seen, seen, the the development development of of the the settlementsettlement was similar to to that that of of the the drawdown drawdown observedobserved in thein adjacentthe adjacent well. well. The rateThe of rate ground of ground settlement settlement varied in varied proportion in proportion to that of groundwaterto that of drawdowns,groundwater and drawdowns, a larger ultimate and a valuelarger ofultimate groundwater value of drawdown groundwater would drawdown cause a would greater cause ultimate a valuegreater of ground ultimate settlement. value of ground Once the settlement. multi-well Once pumping the multi was‐well terminated, pumping thewas groundwater terminated, the level recoveredgroundwater immediately, level recovered and the immediately, surface subsidence and the surface rebounded subsidence subsequently. rebounded subsequently. As can be seen As in Figurecan be7, slightseen in hysteresis Figure 7, slight can be hysteresis observed can between be observed the development between the development of groundwater of groundwater drawdowns anddrawdowns ground settlement. and ground The settlement. possible The reason possible responsible reason responsible for this phenomenon for this phenomenon could becould that be the aquitardthat the and aquitard phreatic and aquifer phreatic overlying aquifer the overlying pumping the aquifer pumping limit theaquifer delivery limit and the accumulationdelivery and of theaccumulation stratum deformation. of the stratum deformation.

J. Mar. Sci. Eng. 2019, 7, 58x FOR PEER REVIEW 1110 of of 2021 ） m -1.0 Pumping stage Recovery stage （ -3.0 G5-2-1 G5-2-2 G5-2-3 -5.0 -7.0 Drawdown -9.0

） 0.0 D1 D3 D5

cm -0.4 （ -0.8 -1.2 -1.6

Settlement -2.0

12 17 18 23 24 25 8/158/168/ 8/ 8/218/228/ 8/ 08/ 08/1308/140 0 0 0 08/1908/200 0 0 0 08/ 08/26 Date (MM/DD) Figure 7. The time-history curve of ground settlement and groundwater drawdowns. Figure 7. The time‐history curve of ground settlement and groundwater drawdowns. 4.2.2. Analyses 4.2.2. Analyses Groundwater extraction would reduce the artesian head and pore pressure in the pumping aquiferGroundwater as well as its extraction adjacent aquifers,would reduce causing the the artesian increase head of effective and pore stress pressure and finally in the resulting pumping in theaquifer compression as well as of its pumping adjacent aquifers, causing which is the the increase primary of reason effective for stress ground and settlement finally resulting [3]. Thus, in needlessthe compression to say, the of distributionspumping aquifers, of the which ground is settlement the primary should reason be for parallel ground to thosesettlement of drawdowns. [3]. Thus, Accordingneedless to to say, test the results, distributions the conclusion of the isground tenable settlement in the most should areas ofbe the parallel test site to those except of the drawdowns. immediate vicinityAccording of pumping to test results, wells, wherethe conclusion the ground is settlementtenable in was the amost little areas larger, of e.g., the the test settlement site except of D24. the Asimmediate aforementioned, vicinity theof pumping possible reason wells, maywhere be soilthe erosionground andsettlement ground was collapse. a little In fact,larger, the e.g., average the dischargesettlement rate of D24. of pumping As aforementioned, well C5-2-2 the was possible 15.64 m reason3/h with may the be maximum soil erosion value and ofground over 25collapse. m3/h, makingIn fact, the the average groundwater discharge flow torate C5-2-2 of pumping at a higher well speed. C5‐2‐2 This was high-speed 15.64 m3/h flow, with carrying the maximum along plenty value of soilover particles, 25 m3/h, flowedmaking out the of groundwater underground flow through to C5 pumping‐2‐2 at a higher wells, consequentlyspeed. This high causing‐speed ground flow, collapsecarrying andalong unexpected plenty of settlement. soil particles, Moreover, flowed the out collapse-induced of underground settlement through was pumping unrecoverable. wells, Thus,consequently the settlement causing couldground not collapse rebound and sufficiently unexpected as settlement. the drawdown Moreover, did in the the collapse recovery‐induced stage (seesettlement Figure 7was). unrecoverable. Thus, the settlement could not rebound sufficiently as the drawdown did in the recovery stage (see Figure 7). 4.3. Responses of Deep Soil Deformation 4.3. Responses of Deep Soil Deformation Both engineering practices and theoretical researches have revealed that the dewatering-induced settlementBoth engineering of deep soil canpractices be larger and than theoretical that of the researches ground surface have [8revealed]. In this that section, the to dewatering investigate‐ theinduced responses settlement of deep of soil deep deformation, soil can be thelarger soil than settlement that of at the different ground depths surface as [8]. well In as this its correlationsection, to toinvestigate the groundwater the responses fluctuations of deep is soil analyzed. deformation, the soil settlement at different depths as well as its correlation to the groundwater fluctuations is analyzed. 4.3.1. Results 4.3.1.On Results the left part of Figure8 are the history curves for deep soil settlement monitored by X1~X3. The dataOn the of left X4 werepart of absent Figure due 8 are to the technical history failures. curves for As deep can besoil seen, settlement during monitored the pumping by X1~X3. stage, theThe soil data at of various X4 were depths absent and due positions to technical firstly failures. subsided As gradually can be seen, until during the pumping the pumping was shut stage, down the andsoil at shortly various afterwards depths and rebounded positions progressively firstly subsided with gradually groundwater until the recovery. pumping At thewas same shut depth,down and the subsidenceshortly afterwards of the monitoring rebounded point progressively was larger with as its groundwater distance to therecovery. pumping At centerthe same became depth, closer. the Insubsidence the same of monitoring the monitoring hole, the point soil was at a depthlarger ofas 22 its m distance suffered to the the largest pumping subsidence center became for most closer. of the time,In the followed same monitoring by that at hole, a depth the ofsoil 35 at m a (in depth X1 and of 22 X2) m orsuffered the surface the largest soil (in subsidence X3), whereas for thatmost at of a depththe time, of 68followed m held by the that smallest at a depth deformation, of 35 m smaller(in X1 and than X2) that or at the a depthsurface of soil 50 m. (in X3), whereas that at a depthSubsequently, of 68 m held the stratum the smallest is divided deformation, into five layerssmaller by than the deepthat at soil a depth settlement of 50 monitoring m. points and theySubsequently, are Layer Athe (0 stratum m to −22 is m), divided Layer Binto (− 22five m layers to −35 by m), the Layer deep C (soil−35 settlement m to −50 m),monitoring Layer D points and they are Layer A (0 m to −22 m), Layer B (−22 m to −35 m), Layer C (−35 m to −50 m), Layer

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(D− (50−50 m m to to− −6868 m) m) and and Layer Layer E E ( ≤−(≤−6868 m), m), respectively, respectively, as can be seen in Figure 2 2.. Here,Here, LayerLayer A A refers refers to thethe soilsoil layerslayers overlyingoverlying the the MCA, MCA, Layers Layers B, B, C, C, and and D D refer refer to to the the upper, upper, middle, middle, and and lower lower part part of theof the MCA, MCA, respectively, respectively, while Layerwhile ELayer refers E to therefers soil to layers the underlyingsoil layers theunderlying MCA. The the deformation MCA. The of eachdeformation layer can of be each obtained layer can by subtracting be obtained the by displacement subtracting the at itsdisplacement bottom by that at its of bottom its top, by and that positive of its valuestop, and mean positive soil expansion values mean while soil negative expansion values while mean negative soil compression. values mean The soil results compression. are depicted The inresults the right are depicted part of Figure in the8. right As can part be seen,of Figure there 8. were As can usually be seen, four layers,there were including usually Layer four B, layers, C, D, andincluding E, compressed Layer B, C, to D, varying and E, degrees compressed and one to varying layer, Layer degrees A, expanded and one layer, and the Layer expansion A, expanded decreased and asthe the expansion distance decreased to the pumping as the center distance increased. to the pumping Among thecenter compressed increased. layers, Among Layers the compressed B, C, and D locatedlayers, inLayers the pumping B, C, and aquifer D located (MCA) in usually the pumping had relatively aquifer larger (MCA) deformation usually whilehad relatively layer E in larger Layer deformation7 suffered the while smallest layer compression. E in Layer ⑦ suffered the smallest compression.

0 X1 1 -1 0

-2

-3 X1 -1 ） Pumping stage Recovery stage cm

-4 -2 （） 0 X2 cm 1

（ -1 0

-2 X2 -1 -3

Settlement -4 -2 0 X3 Pumping stage Recovery stage 1 -1 deformation Stratum 0

-2 X3 -3 d=68 m d=50 m d=35 m Layer A Layer B Layer C -1 d=22 m d=0 m Layer D Layer E -4 -2 3 7 1 5 3 7 1 5 1 1 2 2 /1 /1 /2 /2 / / / / 8 8 8 08 08/15 08 08/19 08 08/23 08 08 08/15 0 08/19 0 08/23 0 Date (MM/DD)

Figure 8. The time time-history‐history curves of deep soil settlement and stratum deformation.

Additionally, the time-historytime‐history curves for the soil compression of Layers B, D, and E in X3 and the corresponding drawdowns monitored by G5 G5-2-2,‐2‐2, G5 G5-3-1,‐3‐1, and G7 G7-1‐1 (next to X3) are depicted in Figure9 9.. AsAs demonstrated,demonstrated, thethe developmentdevelopment ofof soilsoil compressioncompression inin thethe pumpingpumping stagestage was irregular,irregular, whereas thatthat in in the the recovery recovery stage stage showed showed good correlation good correlation with the developmentwith the development of the groundwater of the ﬂuctuationsgroundwater in fluctuations the same layer. in the The same larger layer. the The drawdown larger the was, drawdown the more was, severely the more the soilseverely compressed, the soil alsocompressed, indicating also that indicating dewatering-induced that dewatering drawdown‐induced was andrawdown important was reason an forimportant ground settlement.reason for ground settlement.

J.J. Mar. Sci. Eng. 2019,, 7,, 58x FOR PEER REVIEW 1312 of of 2021

0.0 ）

m -2.0 （ -4.0 Pumping stage Recovery stage -6.0 -8.0 Drawdown G5-2-2 G5-3-1 G7-1 -10.0 0.0 -0.2 -0.4 -0.6

-0.8 Layer B Layer D Layer E Compression (cm) -1.0 1 4 /14 /17 /20 2 /23 2 08/1208/1318 08/1508/1608 08/1808/1908 08/ 08/2208 08/ 08/25 Date (MM/DD) Figure 9. The time-history curves of deep soil compression and groundwater drawdowns. Figure 9. The time‐history curves of deep soil compression and groundwater drawdowns. 4.3.2. Analyses 4.3.2. Analyses As aforementioned, there was expansion in Layer A, and that was why the maximum subsidence usuallyAs aforementioned, occurred at the top there of was the expansion pumping aquiferin Layer instead A, and that of the was ground why the surface. maximum The subsidence reason for soilusually expansion occurred could at the be top attributed of the pumping to dewatering-induced aquifer instead differential of the ground drawdowns surface. The and reason compression for soil inexpansion the pumping could aquifer. be attributed During to thedewatering test, although‐induced there differential were no obviousdrawdowns drawdowns and compression in Layer A,in whereasthe pumping to satisfy aquifer. deformation During coordination, the test, although uneven there downward were no displacement obvious drawdowns took place at in the Layer bottom A, ofwhereas Layer A,to whichsatisfy induced deformation the rotation coordination, of principal uneven stress downward and the formation displacement of stress took arch place in at Layer the Abottom due to of surrounding Layer A, which constraint induced and the caused rotation the of expansionprincipal stress of Layer and A.the In formation addition, of as stress observed arch in FiguresLayer A8 dueand 9to, the surrounding distributions constraint and development and caused of the the expansion stratum deformation of Layer A. were In addition, relatively as irregular. observed Thein Figures possible 8 reasons,and 9, the including distributions compressibility, and development body force, of the and stratum stress history deformation of soil, were piping relatively erosion, andirregular. ground The collapse possible due reasons, to high-speed including ﬂow compressibility, as well as the inﬂuence body force, of partially and stress penetrating history well, of soil, etc., arepiping various erosion, and complex.and ground Therefore, collapse further due to study high on‐speed this issueflow isas still well imminently as the influence required. of partially penetrating well, etc., are various and complex. Therefore, further study on this issue is still 5.imminently Back Analysis required. of Groundwater Fluctuations and Ground Settlement

5.1.5. Back Hydrogeological Analysis of Parameter Groundwater Estimation Fluctuations Based on and Pumping Ground Well Settlement Test 5.1.1. Limitations of the Analytical Methods 5.1. Hydrogeological Parameter Estimation Based on Pumping Well Test For the prediction model of groundwater drawdown and stratum deformation, in addition to accurate5.1.1. Limitations mechanisms of the of Analytical groundwater Methods seepage, the precise parameters of pumping aquifers play significantFor the roles prediction for the finalmodel results. of groundwater Several mathematical drawdown and models stratum and correspondingdeformation, in analytical addition orto semi-analyticalaccurate mechanisms solutions of havegroundwater been proposed seepage, for aquiferthe precise parameter parameters estimation of pumping [38–46]. Theseaquifers models play cansignificant consider roles the for influence the final of results. leaky aquifers Several [ 40mathematical,41] or variable models discharge and corresponding [42,45] to a certain analytical degree. or However,semi‐analytical the preceding solutions models have been assume proposed homogeneous, for aquifer isotropic, parameter and estimation laterally-unbounded [38–46]. These aquifers. models Nevertheless,can consider the in reality,influence the of aquifer leaky isaquifers much more[40,41] complicated or variable thandischarge the previously-assumed [42,45] to a certain degree. aquifer aforementioned.However, the preceding Real aquifers models are assume generally homogeneous, characterized isotropic, by anisotropy and laterally and leakage‐unbounded effect, resultingaquifers. inNevertheless, the above models in reality, being the not aquifer accurate is much enough more for the complicated description than of groundwater the previously flow‐assumed and estimation aquifer ofaforementioned. aquifer parameters. Real aquifers are generally characterized by anisotropy and leakage effect, resulting in the above models being not accurate enough for the description of groundwater flow and 5.1.2. Parameter Estimation using Numerical Method estimation of aquifer parameters. This research mainly focuses on an aquifer system consisting of several micro-confined and confined5.1.2. Parameter aquifers. Estimation Considering using the Numerical complexity Method of the hydrogeology and the limitations of the analytical

J. Mar. Sci. Eng. 2019, 7, 58 13 of 20 methods, the numerical method is more reliable and recognized for parameter estimation [47–50]. In this study, a three-dimensional numerical model is developed in Visual Modflow [51] for groundwater drawdown calculation. In the model, the leaky aquifers are considered by simulating the real site conditions, whereas the variable injection rate is considered by setting a pumping schedule in the software. Moreover, the parameter estimation program PEST of Visual Modflow has been adopted for hydrogeological parameter estimation [51]. The following steps are undertaken for the parameter estimation using numerical method: Step 1: Develop a numerical model based on the site condition and calculate the groundwater drawdowns in observation wells due to single-well pumping with the parameters obtained using the Hantush–Jacob solution. Step 2: Call PEST for the parameter optimization of Layers 5 2~ 5 3-2 and update the parameters. Step 3: Call PEST for the parameter optimization of Layer 5 3-3 and update the parameters. Step 4: Recalculate the groundwater drawdowns in Layers 5 2~ 5 3-2 and compare the discrepancy between the results of Step 4 and Step 2. Repeat step2 and step 3 until the discrepancy is sufficiently small. Step 5: Call PEST for parameter optimization for Layer 7 and check the influence of parameter optimization on groundwater level in other aquifers. Repeat Step 2 and Step 3 until the influence can be negligible.

5.1.3. Results and Analyses In this section, the proposed numerical method is employed for parameter estimation as well as an analytical method based Theis solution. Table3 lists the results for parameter estimation. The estimation value using the analytical method is larger in hydraulic conductivity and smaller in storage than that using the numerical method in Layer 5 3-3 and Layer 7 , while it is the opposite in Layers 5 2~ 5 3-2. Moreover, the drawdowns are calculated in Visual Modflow using the two group of parameters, and the results are depicted in Figure 10 as well as the observed drawdowns. As can be seen, the calculated drawdowns using the proposed numerical method match the observed drawdowns well except at the early stage of pumping when the discharge rate is unstable, whereas those using the analytical method present significant discrepancy with the observed drawdowns, indicating that the proposed numerical method is more reliable for parameter estimation and drawdown calculation in complex aquifer systems. As mentioned earlier, there are significant discrepancies between the numerical results and the analytical ones. The primary reason for this is the hydraulic connection among each aquifer. When pumping in Layer 5 3-3 or Layer 7 , the head in the pumping aquifer decreased rapidly, and head difference was formed, causing the groundwater from the adjacent aquifers to flow into the pumping aquifer. When pumping in Layers 5 2~ 5 3-2, significant drawdowns could be widely observed in the pumping aquifer. However, the natural head in this layer was relatively larger than that in Layer 5 3-3 before dewatering, and this held true for most areas during dewatering, resulting in the groundwater flowed into Layer 5 3-3. In analytical methods, the aquifers were assumed to be entirely isolated. Thus, the groundwater flowing into or out of the pumping aquifer is oversimplified, causing the hydraulic conductivity in Layer 5 3-3 and Layer 7 was overrated and that in Layers 5 2~ 5 3-2 was underestimated. Whereas, in the numerical method this characteristic could be considered by developing a unified numerical model according to the site condition. Consequently, the numerical method is more accurate for parameter identification in complex aquifer systems. J. Mar. Sci. Eng. 2019, 7, 58 14 of 20

J. Mar. Sci.Table Eng. 3. 2019Comparison, 7, x FOR PEER between REVIEW the parameter obtained by the analytical and numerical method.15 of 21

Estimation Method Theis Method Numerical Solution k (cm/s) 3.73 × 10−3 3.58 × 10−3 v −3 −2 kh (cm/s) 3.73 × 10 1.02 × 10 S 4.07 × 10−4 −3 8.90×10−4 −3 Layers 5 2~ 5 3-2 kv (cm/s) 3.73 × 10 3.58 × 10 −3 −4 −4 −4 Skh (cm/s) 1.68 4.07× 10× 10 4.51 × 10 8.90×10 ⑤ −3 −3 −4 −4 Layer 3‐3 kh (cm/s)kv (cm/s) 1.68 1.68× 10× 10 1.50 × 10 4.51 × 10 −3 −4 Layer 5 3-3 kv (cm/s)S 2.44 1.68× 10×−4 10 9.12 × 10−4 1.50 × 10 S 2.44 × 10−4 9.12 × 10−4 −2 −3 kh (cm/s) 4.86 × 10 9.21 × 10 k (cm/s) 4.86 × 10−2 9.21 × 10−3 ⑦ h −2 −3 Layer kv (cm/s) 4.86 × 10 −2 3.84 × 10 −3 Layer 7 kv (cm/s) 4.86 × 10 3.84 × 10 S (10−S4 )(10−4) 9.80 9.80× 10×−6 10−6 1.44 × 10−3 1.44 × 10−3

0.1 1.0 2.0 Observation Observation -0.1 Theis solution 0.5 Theis solution 1.5 ）

m -0.3 Numerical solution 0.0 Numerical solution 1.0 （ -0.5 -0.5 0.5 -0.7 -1.0 0.0 Observation Theis solution Drawdown -0.9 -1.5 -0.5 Numerical solution -1.1 -2.0 -1.0 -1.3 -2.5 -1.5 0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.4 0.8 1.2 1.6 2.0 2.4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 (a) Time （d） (b) Time （d） (c) Time （d） Figure 10. Comparison between observed and calculated drawdowns: (a) drawdowns in observation Figure 10. Comparison between observed and calculated drawdowns: (a) drawdowns in observation well G5-2-2; (b) drawdowns in observation well G5-3-1; (c) drawdowns in observation well G7-1; well G5‐2‐2; (b) drawdowns in observation well G5‐3‐1; (c) drawdowns in observation well G7‐1; 5.2. Ground Settlement Prediction Induced by Dewatering 5.2. Ground Settlement Prediction Induced by Dewatering 5.2.1. Basic Assumptions 5.2.1. Basic Assumptions In this section, a simple prediction method for ground settlement induced by dewatering is proposedIn this and section, discussed. a simple In this prediction method, themethod conventional for ground Theis settlement Model is induced employed by todewatering calculate theis drawdownsproposed and due discussed. to dewatering, In this andmethod, the unidirectional the conventional compression Theis Model formula is employed is utilized to calculate to estimate the drawdowns due to dewatering, and the unidirectional compression formula is utilized to estimate the soil compression. Hence, the assumptions used in this method are identical to those in the Theis the soil compression. Hence, the assumptions used in this method are identical to those in the Theis Model except that the compression of the pumping aquifer is assumed to be completed instantly and Model except that the compression of the pumping aquifer is assumed to be completed instantly and equal to the ground settlement. This assumption is conservative considering the expansion of the equal to the ground settlement. This assumption is conservative considering the expansion of the overlying non-pumping layers. Besides, the assumptions above cannot consider many factors related overlying non‐pumping layers. Besides, the assumptions above cannot consider many factors related to the geological condition, such as the body force and stress history of soil. However, these factors to the geological condition, such as the body force and stress history of soil. However, these factors can be considered indirectly by calibrating the predicting result using the observation data. can be considered indirectly by calibrating the predicting result using the observation data. 5.2.2. Ground Settlement Prediction Based on Pumping Well Test 5.2.2. Ground Settlement Prediction Based on Pumping Well Test In application, the following steps may be taken to calculate the dewatering-induced ground In application, the following steps may be taken to calculate the dewatering‐induced ground settlement: settlement: Step 1: Calculate the groundwater drawdown induced by single-well pumping based on Theis Step 1: Calculate the groundwater drawdown induced by single‐well pumping based on Theis Formula. Formula. Step 2: Acquire the groundwater drawdown caused by multi-well pumping using the Step 2: Acquire the groundwater drawdown caused by multi‐well pumping using the superpositionsuperposition principle. principle. StepStep 3: 3: Obtain Obtain the the additional additional effectiveeffective stress due to groundwater drawdown drawdown by by the the theory theory of of effectiveeffective stress. stress. StepStep 4: 4: Develop Develop the the prediction prediction expressionexpression for settlement utilizing unidirectional unidirectional compression compression formula.formula. Step 5: Compute the undetermined coefficients in the expression with the observation data.

J. Mar. Sci. Eng. 2019, 7, 58 15 of 20

Step 5: Compute the undetermined coefﬁcients in the expression with the observation data. According to the aforementioned, the expression for ground settlement prediction can be described as follows (details can be found in the AppendixA):

n n γw H 2.3Qi 2.25Tt 22.3Qi ∆s = ∑ lg − lg ri = ∑ Ji lg t − Ki lg ri + Li (1) Es i=1 4πT S 4πT i=1 where ∆s is the ground settlement [L] at distance r [L] and time t [T]; Es is the compressing modulus −1 −2 −2 −2 [ML T ]; H is the thickness [L] of the pumping aquifer; γw the bulk density [ML T ] of water; Q is the discharge rate [L3T−1]; T is the transmissivity [L2T−1], and S is the storage coefficient [dimensionless]. Besides, Ji, Ki and Li are undetermined coefficients and can be expressed as Ji = 2.3γw HQi 2·2.3γw HQi 2.3γw HQi , Ki = and Li = ·lg(2.25T/S). The undetermined coefficients follow 4πTEs 4πTEs 4πTEs the relationships: J1 : J2 : ··· : Jn = K1 : K2 : ··· : Kn = L1 : L2 : ··· : Ln = Q1 : Q2 : ··· : Qn, Ki = 2Ji, and can be determined using the nonlinear curve fitting function of the Origin software based on the ground settlement observation data.

5.2.3. Validation and Analyses During the test, there were four pumping wells employed. Thus, the total number of the undetermined coefficient was 12. Subsequently, these undermined coefficients were determined by performing the nonlinear fitting according to Equation (1) based on the ground settlement observation data of D1~D23 from Day 4 to Day 7. The results are shown as: J1 = 0.00128, J2 = 0.00275, J3 = 0.00205, J4 = 0.00191, K1 = 0.00256, K2 = 0.00550, K3 = 0.00410, K4 = 0.00382, L1 = 0.00228, L2 = 0.00490, L3 = 0.00366, L4 = 0.00340, and the goodness of fitting is 0.870. Further, the predicting formula is employed to predict the ground settlement on Day 8, and the predictive values and observation values, J.as Mar. well Sci. as Eng. the 2019 error, 7, x analyses, FOR PEER are REVIEW depicted in Figure 11. 17 of 21

0.032 80 70.5 Percentage Error

0.028 53.2 53.6 Measured value 60 Predicted value 39.4 42.7 0.024 40 39.3 ）

29.5 22.0 24.6 22.7 ）

m 18.8 0.020 19.3 20 % （

9.06 18.3 20.5 16.0 （ 11.1 20.1 16.9 14.6 19.3 0.016 6.13 0 0.428 0.012 -20

0.008 -40 Error Percentage

Ground Settlement

0.004 -60

0.000 -80 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16 D17 D18 D19 D20 D21 D22 D23 Ground settlement monitoring points Figure 11. Comparison of the observation value and prediction value.

6. ConclusionAs illustrated in Figure 11, the percentage errors vary from 0.428% to 70.5% with an average value of 25.57%. The prediction method functioned well for a majority of the monitoring points with a percentageIn this errorpaper, of field less thanpumping 25%. well However, tests were for the performed points at thein Pudong far-ﬁeld New (such Aera as D9 to and investigate D15) of the responsestest site, the of errorthe groundwater was much larger level and and stratum even exceeded deformation 50%. due At the to dewatering far-ﬁeld, the in ground the MCA. settlement On this basis, practical methods for hydrological parameter estimation and ground settlement prediction were proposed and discussed. Following conclusions can be drawn: 1) Both the single‐well and multi‐well pumping tests indicate that there is a close hydraulic connection between MCA and AqⅠ. Hence, even if dewatering measurements are only performed in MCA, the groundwater drawdowns and stratum deformation in AqⅠ should be considered to avoid underestimating water inflow and ground settlement. 2) The distributions and development of ground settlement are similar to those of groundwater drawdowns. It is tenable for most areas except the immediate vicinity of the pumping wells, where the subsidence is larger and cannot rebound sufficiently with groundwater recovery due to ground collapse induced by high‐speed groundwater flow. 3) During the pumping well test, soil settlement and stratum compression can be observed not only in the pumping aquifer but also in its underlying aquifers and their distributions and development show correlation with those of groundwater fluctuations, which also indicates there is a close hydraulic connection among each aquifer. 4) During the pumping well test, because a stress arch is formed in the top layer due to uneven deformation and surrounding constraint, the top layer is expanded, and the largest subsidence usually occurs at the top of the pumping aquifer instead of the ground surface. 5) For the parameter estimation under complex hydrogeology conditions, especially when the micro‐confined aquifer is directly connected with the deep confined aquifer, the proposed numerical method can consider the effect of hydraulic connection, and the results are more reliable and accurate compared with those of the conventional analytical methods. 6) The proposed prediction method for the dewatering‐induced ground settlement functions well at most parts of the test site except at the far‐field and the central parts, indicating its feasibility. Moreover, the parameters used in the method can be obtained by performing fitting with observation data, avoiding the dependence on precise hydrogeological parameters.

Author Contributions: J.L. analyzed the experimental data and wrote the original draft paper; M.‐G.L. provided analytical procedures and conducted the formal analysis; H.C. conducted the series of pumping well tests and

J. Mar. Sci. Eng. 2019, 7, 58 16 of 20 induced by groundwater extraction was very small, usually 2~3 mm. For this reason, the error caused by human activities and measurements was inevitable and considerable. In addition, at the central part of the test site, the measured value of subsidence was usually larger than the prediction value. The possible reason may be the aforementioned ground collapse due to high-speed groundwater ﬂow. In general, the prediction values match well with the observation data, indicating that the prediction method proposed in this paper is feasible.

6. Conclusions In this paper, field pumping well tests were performed in Pudong New Aera to investigate the responses of the groundwater level and stratum deformation due to dewatering in the MCA. On this basis, practical methods for hydrological parameter estimation and ground settlement prediction were proposed and discussed. Following conclusions can be drawn: (1) Both the single-well and multi-well pumping tests indicate that there is a close hydraulic connection between MCA and Aq I. Hence, even if dewatering measurements are only performed in MCA, the groundwater drawdowns and stratum deformation in Aq I should be considered to avoid underestimating water inflow and ground settlement. (2) The distributions and development of ground settlement are similar to those of groundwater drawdowns. It is tenable for most areas except the immediate vicinity of the pumping wells, where the subsidence is larger and cannot rebound sufficiently with groundwater recovery due to ground collapse induced by high-speed groundwater flow. (3) During the pumping well test, soil settlement and stratum compression can be observed not only in the pumping aquifer but also in its underlying aquifers and their distributions and development show correlation with those of groundwater fluctuations, which also indicates there is a close hydraulic connection among each aquifer. (4) During the pumping well test, because a stress arch is formed in the top layer due to uneven deformation and surrounding constraint, the top layer is expanded, and the largest subsidence usually occurs at the top of the pumping aquifer instead of the ground surface. (5) For the parameter estimation under complex hydrogeology conditions, especially when the micro-confined aquifer is directly connected with the deep confined aquifer, the proposed numerical method can consider the effect of hydraulic connection, and the results are more reliable and accurate compared with those of the conventional analytical methods. (6) The proposed prediction method for the dewatering-induced ground settlement functions well at most parts of the test site except at the far-field and the central parts, indicating its feasibility. Moreover, the parameters used in the method can be obtained by performing fitting with observation data, avoiding the dependence on precise hydrogeological parameters.

Author Contributions: J.L. analyzed the experimental data and wrote the original draft paper; M.-G.L. provided analytical procedures and conducted the formal analysis; H.C. conducted the series of pumping well tests and provided experimental materials. X.-H.X., L.-L.Z. and J.-J.C. motivated this paper with good ideas and funding support. Funding: This research was funded by the National Natural Science Foundation of China, grant numbers 41602283, 41472250. Conﬂicts of Interest: The authors declare no conﬂicts of interest

Appendix A To develop the prediction method for dewatering-induced ground settlement, several classic theories, including Theis solution and Jacob solution for unsteady ﬂow to a pumping well, superposition principle, theory of effective stress as well as unidirectional compression formula, are employed here. The following steps may be taken to calculate the ground settlement induced by the groundwater extraction: J. Mar. Sci. Eng. 2019, 7, 58 17 of 20

Step 1: Calculate the groundwater drawdown induced by single-well pumping based on Theis Formula. Step 2: Acquire the groundwater drawdown caused by multi-well pumping using superposition principle. Step 3: Obtain the additional effective stress due to groundwater drawdown by theory of effective stress. Step 4: Develop the prediction expression for settlement utilizing unidirectional compression formula. Step 5: Compute the undetermined coefficients in the expression with the observation data. In Step 1, the groundwater drawdown induced by single-well pumping can be obtained by Theis formula, a classic solution for unsteady groundwater flows to a pumping well in a homogeneous, horizontally isotropic, laterally unbounded confined aquifer with a constant discharge rate. And it is expressed as follows: Q Q Z ∞ e−u s(r, t) = W(u) = du (A1) r2S 4πT 4πT 4Tt u where s is the groundwater drawdown [L] at distance r [L] and time t [T], Q is the discharge rate [L3T−1], T is the transmissivity [L2T−1] and S is the storage coefficient [dimensionless], W(u) is the r2S well function, u can be expressed as u = and is a dummy variable of integration [dimensionless]. 4Tt Specially, when u ≤ 0.01, Theis solution can be simplified to Jacob solution and can be expressed as follows: 2.3Q 2.25Tt 2·2.3Q s(r, t) = lg − lgr (A2) 4πT S 4πT In Step 2, the groundwater drawdown caused by multi-well pumping is equal to the sum of drawdown induced by each single-well pumping utilizing the superposition principle and can be calculated as follows: n 2.3Qi 2.25Tt 22.3Qi s(r, t) = ∑ lg − lg ri (A3) i=1 4πT 4πT 4πT 3 −1 where Qi is the discharge rate [L T ] of the ith well and ri is the distance [L] between the monitoring point and the ith well. In step 3, the increment of effective stress due to groundwater drawdown is equal to the decline of pore pressure according to principle of effective stress and the additional effective stress can be calculated as follows: n 2.3Qi 2.25Tt 22.3Qi ∆p = γw·∑ lg − lg ri (A4) i=1 4πT 4πT 4πT −1 −2 −2 −2 where ∆p is the additional effective stress [ML T ] and γw is the water bulk density [ML T ]. In Step 4, the unidirectional compression formula is employed to compute the compression of the dewatered confined aquifer, which is assumed to be as large as the ground settlement, and can be expressed as follows:

n n ∆pH γw H 2.3Qi 2.25Tt 22.3Qi ∆s = = ∑ lg − lg ri = ∑ Ji lg t − Ki lg ri + Li (A5) Es Es i=1 4πT 4πT 4πT i=1

−1 −2 where ∆s is the ground settlement [L], Es is the compressing modulus [ML T ], H is the thickness [L] of pumping aquifer. In Step 5, fitting method is performed on the ground settlement observation data to calculate the undetermined coefficients in Equation (A5). It should be noted that the observation data adopted here should meet the requirement u ≤ 0.01 to reduce the error in the simplification from Theis Formula to Jacob Formula. J. Mar. Sci. Eng. 2019, 7, 58 18 of 20

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