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. N Ch on gm in g Di S str ict Jiad Di ing Beijing st ric 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].