Optimization of dewatering schemes for a deep foundation pit near the Yangtze River,China

Yng You,Chnghong Yn,*,Botin Xu,Shi Liu,Cnhui Che

aSchool of Earth Science and Engineering,Nanjing University,Nanjing,210046,China

bNanjing Branch,Anhui Hydrogeology and Engineering Geology Corporation,Nanjing,210019,China

1.Introduction

In recent years,due to the rapid development of urbanization,underground spaces have been widely exploited and utilized.Construction of underground projects,such as the base of high-rise buildings,subway stations,and underground transportation corridors,always encounters the problems of foundation pit excavation(Fu et al.,2014;Ma et al.,2014;Xing et al.,2016).Construction of tunnels or foundation pits usually results in surface settlement(Fattah et al.,2011,2013).However,a high groundwater level will increase the difficulty and reduce the safety of construction.Subsequently,dewatering is vital inpit excavation,especially for a large and deep foundation pit in confined aquifers with high piezometric head(Forth,2004;Li and Pei,2011;Ding et al.,2014;Pujades et al.,2014).

The lowering of the groundwater level outside the pit can lead to ground subsidence,affecting the stability of surrounding buildings.Thus drawdown of the groundwater outside the pit must be strictly controlled(Chen and Xiang,2006;Ding et al.,2011).In sedimentary strata,the horizontal hydraulic conductivity(KH)of the aquifer is usually greater than the vertical value(KV)(Wang et al.,2013a;Wu et al.,2016).Combination of partially penetrating wells and curtains can change the groundwater fl ow from a horizontal direction to an approximately vertical direction and lengthen the flow path(Zhou et al.,2010;Wang et al.,2014,2016).As a consequence,less pumping wells,slower pumping rates and less time are needed,and the drawdown of groundwater level outside the pit could be much smaller than that inside.Water recharge is another effective method to control ground subsidence by injecting water into aquifers to reduce or balance the drawdown of groundwater level(Yuan and Li,2015;Wang et al.,2012).

Numerical simulation has been demonstrated to be a useful approach to evaluate the effect of dewatering in a foundation pit,calculate the ground subsidence and adjust the hydraulic parameters of aquifers(Su et al.,1998;Shen et al.,2006;Luo et al.,2008;Kaneda and Yamazaki,2009;Zhang et al.,2013).Some threedimensional(3D)numerical simulation packages have been widely used by many researchers,such as Visual MODFLOW,in which the governing equation is solved by the finite difference solution(Painter et al.,2008;Wang et al.,2013b;Xu et al.,2016a).

In August 2014,the Second Youth Olympic Games was held in Nanjing,Jiangsu Province,China.To ease traffic pressure,a crossriver tunnel was constructed by shield method under the Yangtze River,to link the playing venues on both sides of the river.The construction site is located in the southwestern part of the city.As the southeast entrance of the tunnel,an underground transportation hub,which is the largest underground interchange at present in China,was constructed by open cut method.Thus excavation and dewatering design of the foundation pit was inevitable.To meet the dewatering requirement and reduce the deformation of the nearby buildings caused by dewatering,partially penetrating wells,in combination with waterproof curtains,were adopted.However,the aquiclude between two confined aquifers is partially missing,and some parts of the aquifers were not isolated by the curtains.The dewatering design should consider the different hydrogeological conditions of each area of the foundation pit.A 3D numerical simulationwas adopted to evaluate the effect of the dewatering schemes.This paper aims to propose the optimal dewatering scheme according to the simulation results and provide reference to similar engineering projects.

2.Background

2.1.Deep foundation pit

The foundation pit is located in the southeastern bank of the Yangtze River,only 120 m away from the river(Fig.1).The foundation pit consists of three ramps(A,B and C)and a main road connecting the cross-river tunnel.Ramps A and B connect the Yangzijiang Road on the ground and ramp C connects the Youth Olympic Center,which will be constructed in the future.The length and width of the foundation pit for the main road are 213 m and 110 m,respectively.The width of the ramp pits is approximately 12 m.Unlike normal rectangular-shaped excavation,the shape of this foundation pit is irregular and the depth is not uniform.The bottom elevation of the main road foundation pit reduces gradually from the southeast to the northwest,ranging from-11.88 m to-19.45 m,downward toward the tunnel,while those of the three ramp pits increase from the main road to the ground,ranging from-9.66 m to-5.76 m for ramp A,-11.46 m to-1.95 m for ramp B,and-9.92 m to-2.82 m for ramp C(Table 1).The elevation of the ground surface is approximately 7.5 m,so that the maximum depth of the foundationpit is up to-27 m at the joint between the pit and the cross-river tunnel.

Fig.1.Location and plane layout of the foundation pit.

Table 1Parameters of the foundation pit and piezometric head requirements of two confined aquifers.

According to the requirements of foundation pit support and dewatering,underground diaphragmwalls are installed around the foundation pit of the main road.In the pit,two sets of waterproof curtains are installed,dividing the pit into three regions(I,II and III)(Fig.2).The upper part of the curtains above the pit bottom will be removed when the pit is excavated.Bored piles with waterproof curtains are mainly used around the ramp pits.The bottom elevations of these support and waterproof structures are listed in Table 1.

An existing levee above the ground was built between the foundation pit and the Yangtze River approximately 10 years ago(Fig.1),of which the top elevation is 12 m,and the width is 6 m.The levee is used to prevent water from rushing into the urban district during the flood season.Subsequently,deformation of the levee caused by dewatering,including the accumulated and differential settlements,should be strictly controlled.

2.2.Geological conditions and soil properties

According to the geotechnical investigation report,the soils in the site are Quaternary sediments.A thick fl uvial sedimentary formation overlies mudstone,and a miscellaneous fill layer lies on the top.Due to long-term sorting and deposition by the Yangtze River,the upper strata are mainly composed of clay and muddy silty clay,while the lower strata are mainly composed of silty fine sand and gravel sand,forming the typical dual-structure strata(Li et al.,2012).The site stratigraphy is listed in Table 2 and the strata distribution is shown in Fig.2.Most strata are continuously distributed in the construction site,while layer②1is only locally distributed in region II.Layer④is partially missing in region III,where layer⑤1is distributed instead.Before the excavation of the foundation pit,64 boreholes were drilled around the construction site for soil sampling.Several laboratory and in situ tests were performed to obtain the soil properties.The average values of these soil properties are listed in Table 2.The results of the grain-size distribution of the soils are given in Table 3.

2.3.Hydrogeological conditions

The hydrogeological parameters of the strata in the construction site are listed in Table 4.Layer①is the unconfined aquifer with the water table of 6.5 m,overlying an aquiclude composed of layers②1and②2.Below layer①,there are two confined aquifers separated by the aquiclude of layer④.The upper confined aquifer consists of layers③1and③2,while the lower aquifer consists of layers⑤1and⑤2.The piezometric heads of both confined aquifers are 5.5 m.Layer⑥contains mudstone,which can be regarded as an aquiclude.

Fig.2.Profile of the strata and foundation pit in section 1-1′shown in Fig.1.

Table 2Geotechnical properties of the soil.

Because the bottom of the pits is constructed in the upper confined aquifer and the piezometric heads of the two confined aquifers are relatively high,the water table in the two confined aquifers should be dewatered for the convenience and safety of excavation.If the bottoms of the diaphragm wall around region I and the waterproof curtain between regions I and II are installed at an elevation of-45 m,and inserted into layer⑥,the two confined aquifers as well as the unconfined aquifers in this region will be completely isolated.As the top elevation of the mudstone layer reduces in regions II and III,installation of waterproof structures into layer⑥to cut off the hydraulic connection to the lower aquifer is not economical.Instead,if the bottoms of the waterproof structures of the two regions are only installed into layer④at an elevation of-38 m,the upper confined aquifer can be isolated.However,it is noteworthy that layer④is missing in region III,thus the two confined aquifers have a very close hydraulic connection with each other,which brings more difficulties to the dewatering design.In addition,because the bottom elevations of the waterproof curtains around the three ramp pits are only-7.65 m to-19.4 m,the curtains cannot isolate the upper confined aquifer.

Table 3Grain-size distribution of the soil.

Table 4Hydrogeological parameters of the strata.

3.Dewatering requirement

For the convenience of excavation,the water levels of the aquifers,where the foundation pit is excavated,should be generally dewatered to 1 m below the pit bottom(Yao et al.,2006).If there is a confined aquifer under the pit bottom and its piezometric head is relativelyhigh,this maycause an uplift of the bottomplane,and the water table should be dewatered to a safe level(Sun et al.,2012).Moreover,excessive ground subsidence caused by dewatering may threaten the stability of buildings near the pit;thus,drawdown of groundwater outside the pit should be strictly controlled(Yu et al.,2011).

3.1.Uplift of the pit bottom

The piezometric head of the lower confined aquifer is relatively high,up to 5.5 m,thus the possibility of uplift of the pit bottom should be taken into consideration.During the construction of the foundation pit,the gravity of the soil from the pit bottom to the top of the lower confined aquifer must be greater than the water pressure produced by the confined water.The stability of the pit bottom can be calculated as(Ministry of Housing and Urban-Rural Development of the People’s Republic of China(MOHURD),2011):

whereHis the distance from the bottom of the foundation pit to the top of the confined aquifer(m);γsis the unit weight of soils in the calculation domain(kN/m3);Hwis the distance from the piezometric head of the confined water to the top plane of the confined aquifer(m);γwis the unit weight of water(kN/m3);andFsis the factor of safety,given as 1.1 in this calculation.

The safe piezometric heads calculated by Eq.(1)are listed in Table 1.The results show that the safe piezometric heads of the lower confined aquifer in regions I and II are lower than the initial head,and the uplift of the pit bottom may occur.However,in this calculation,the strength of the soil is not considered,and the stress state and failure behavior of the soil differ from that in practical situations(Sun et al.,2012).

It is argued that the aquiclude beneath the pit bottom maycrack due to the tensile stress(Liang,1996;Ma et al.,2004).However,in their analysis models,the pit bottom was constructed in the aquiclude;while in this study,the pit bottom was constructed in the upper confined aquifer which consists of silty fine sand.Once the aquiclude layer④cracks due to the high upward water pressure,the groundwater in the lower confined aquifer will rush into the upper sand layer.As a result,soil flow or piping may emerge at the pit bottom(Xu et al.,2016a).Hence the stability of the aquiclude must be ensured.The aquiclude can be simplified as a beam with both ends fixed(Ma et al.,2004).The stress diagram of the aquiclude is shown in Fig.3a.Under the resultant force of the upward confined water pressure and the downward soil gravity,the largest tensile stress occurs at the top of the middle section.On the other hand,both ends of the beam are subjected to the soil lateral pressure,resulting in compressive stress in the beam.

The bending moment diagram is shown in Fig.3b.According to the structural mechanics calculation,the bending moment at the middle section can be calculated as(Guo,2012):

whereMis defined as the bending moment at the middle section(kN m);pis the difference of the upward confined water pressure and the soil gravity(kN/m);uis defined as the confined water pressure(kN/m);lis the length of the pit(m);γ1and γ2are the unit weights of layers③2and④,respectively(kN/m3);andh1andh2are the thicknesses of layers③2and④,respectively,beneath the pit bottom(m).

Fig.3.Calculation sketches of the aquiclude.(a)Stress diagram of the aquiclude;and(b)Bending moment diagram.

The tensile stress at the top of the middle section can be calculated by

whereσtis defined as the tensile stress at the top of the middle section(kPa),andWis the bending section coefficient(m3).For the beam with rectangular cross-section(Sun et al.,2009),we have

wherebis the width of the rectangular cross-section(m),equal to 1 m in this two-dimensional model.

Substituting Eqs.(2)and(4)into Eq.(3)results in

The lateral pressure at the top of the middle section can be calculated as follows:

whereσcis defined as the lateral pressure at the top of the middle section(kPa),andk0is the lateral pressure coefficient of layer④.

In general,the tensile strength of soil is negligible,thus the lateral pressure must be greater than the tensile stress at the top of the middle section.The following equation should be satisfied to ensure the stability of the aquiclude:

whereFsis also given as 1.1 in this calculation(MOHURD,2011).

According to the given parameters,the calculated required piezometric head of the second confined aquifer in region I is lower than 1.77 m,and that in region II is lower than 4.74 m.Both are greater than the values calculated by Eq.(1),indicating that Eq.(7)may be more economical and Eq.(1)is conservative but safe for pit excavation.

3.2.Stability of the river levee

The Yangtze River levee along the river bank is approximately 60 m away from the foundation pits.It can be seriously threatened by any dewatering,because the aquifers in the foundation pits are not completely isolated by waterproof structures.The pore-water pressure in the aquifers decreases as the groundwater level decreases,which will lead the effective stress in the soil to increase and in turn cause ground settlement(Tian and Hua,2007;Roy and Robinson,2009;Ding and Meng,2010).The settlement caused by dewatering can be calculated by a layer-wise summation method(MOHURD,2011):

wheresis the total additional settlement caused by dewatering(m);φ is the empirical coefficient,defined as 1 in this calculation;ndenotes the number of soil layers in the calculation range;ΔPirepresents the additional load of the calculated soil layer caused by dewatering(kPa);Esiis the compression modulus of the calculated soil layer(kPa);andHiis the thickness of the calculated soil layer(m).

The additional load of the soil layer caused by dewatering can be calculated as

wherehbandhaare the piezometric heads in the calculated soil layer before and after dewatering,respectively(m).

The river levee is vital to prevent land and buildings from being destroyed by floods,but the excessive ground subsidence and differential settlement around the levee can trigger cracks(Jin et al.,2014).As a consequence,the deformation of the levee should be strictly controlled during dewatering of the foundation pit.According to previous projects constructed adjacent to the Qiantang River levee,the accumulated and differential settlements of the river levee should not exceed 30 mm and 2‰,respectively(Xie and Wu,2011).

4.Pumping test and parameter calculation

4.1.Pumping test

To obtain the hydraulic conductivity of the upper confined aquifer and water production of a single well,a pumping test was conducted by one pumping well(P)and two observation wells(OB1 and OB2).The three wells were set up in one line,and the two observation wells were 18 m and 28.6 m away from the pumping well,respectively(Fig.1).The structures of the wells are shown in Fig.4.The filter tubes of the three wells are all located in the upper confined aquifer.Because layer③1is relatively thin and the hydraulic conductivity is close to that of layer③2,the two layers were considered as one aquifer in the calculation.The test consisted of three stages with different pumping rates.After the start of pumping in every stage,the water level in each observation well was recorded at 1 min,3 min,5 min,10 min,15 min,20 min,25 min and 30 min during the first 30 min,and then every half hour until the next stage began.The time and average pumping rate in each stage are listed in Table 5 and the recorded water levels are shown in Fig.5.The highest pumping rate was approximately 1300 m3/d.

To obtain the hydraulic conductivities of aquifers,analytical calculation and numerical simulation are the most frequently used methods based on pumping test results(Xu et al.,2016b).

Fig.4.Structures of the wells in different foundation pits.

Table 5Pumping rates and drawdown of wells in three stages.

4.2.Analytical calculation

When the water levels in the three wells reach a relatively steady state in each stage,Dupuit equations,which are applied for steady flow of groundwater,could be used for the hydraulic conductivity calculation of the upper confined aquifer.Three different combinations of wells,i.e.(a)one pumping well,(b)one pumping well and one observation well,and(c)two observation wells,are adopted and the corresponding equations are shown as follows(Yao et al.,2006):

whereKis the hydraulic conductivity of the aquifer(m/d);Qis the pumping rate(m3/d);Ris the influence radius(m);sw,s1ands2are the steady drawdowns of the pumping well,and the two observation wells,respectively(m);rwis the radius of the pumping well(m);r1andr2are the horizontal distances between the pumping well and two observation wells,respectively(m);Mais the thickness of the aquifer(m);and ζ0,ζ1and ζ2are the resistance coefficients of the partially penetrating pumping well and the two partially penetrating observation wells,respectively.

According to Dai(2011)’s study performed at the same construction site with similar strata distribution,groundwater depth and surrounding environment,the influence radiusRequal to 400 m is reasonable for the dewatering scheme design.The coefficientsζ0,ζ1and ζ2are 67.7,2.69 and 0.45,respectively(Yao et al.,2006).The thicknessMais 32 m,which is the sum of the thicknesses of layers③1and③2.The values of the other parameters used in Eqs.(10)-(12)are listed in Table 5.The calculated results are listed in Table 6.The results show that the hydraulic conductivity of the upper confined aquifer ranges from 13.98 m/d to 38.07 m/d,and the average value is 20.45 m/d.However,layer④is missing at the pumping area,and groundwater in the lower confined aquifer can recharge the upper one.Therefore,the analytical results will be greater than the actual values.

4.3.Numerical simulation of the pumping test

Numerical simulation is an effective method to adjust the hydraulic conductivity and has beenwidely used by many researchers(e.g.Wang et al.,2009).A 3D simulation is performed based on the field pumping test results to adjust the hydraulic conductivity of the upper confined aquifer.

4.3.1.Mathematical model

The governing equation for the unsteady flow of the confined aquifer in the numerical simulation is given by(Wu and Xue,2009):

whereKxx,KyyandKzzare the hydraulic conductivities along thex,yandzdirections,respectively(cm/s);his the water table at the point(x,y,z)(m);Wwis the recharge and discharge of the groundwater(d-1);Ssis the speci fi c storage at the point(x,y,z)(m-1);tis the time(h);Ωis the computational domain;h0is the initial water table at the point(x,y,z)(m);Γ1and Γ2are the first and second types of boundary condition,respectively;nx,nyandnzare the unit normal vectors on boundaryΓ2along thex,yandzdirections,respectively;andqis the lateral recharge per unit area on boundaryΓ2(m3/d).In this simulation,we assumed the soil to be isotropic in the horizontal direction,i.e.Kxx=Kyy.

4.3.2.Numerical model

A 3D software program Visual MODFLOW is adopted to simulate the pumping test using the finite difference method(FDM)to solve Eq.(13).The calculation area of the model is defined as 2000 m×2000 m(width×length),including 113 rows and 159 columns with a denser mesh at the location of the foundation pit,and the depth is 55 m(Fig.6).According to the stratum distribution,the model is divided into six layers vertically.The boundary and the Yangtze River are defined as constant head boundaries,while the bottom of the mudstone is defined as a zero- flux boundary.

Fig.5.Drawdown of three wells in the pumping test.(a)The pumping well;and(b)The observation wells.

Table 6Analytical results of three combinations of wells.

Fig.6.3D finite difference model.

4.3.3.Parameter adjustment

Based on the numerical model built,the hydraulic conductivities of the upper confined aquifer are adjusted by a trial-and-error method to fit the calculated piezometric head to the observed values(Xu et al.,2016b).The fitting curves and error analyses are shown in Fig.7.The results show that the calculated values by numerical simulation fit well to the observed values.Moreover,the correlation coefficients of the observation wells OB1 and OB2 are 0.934 and 0.862,respectively,showing the robustness of the adjusted parameters.

The adjustedKHandKVvalues of the upper confined aquifer are 16 m/d and 8 m/d,respectively.However,the value ofKHis twice that ofKV,which illustrates that the aquifer is anisotropic.In the analytical calculation,the aquifer is assumed to be isotropic;however,in the numerical simulation,it can be considered anisotropic.The results obtained by numerical adjustment will be used in the following numerical simulation of dewatering schemes.

5.Dewatering schemes

For the aquifer that is isolated by the waterproof structures,drainage wells are generally used and the dewatering in pit has no influence on the water level outside of the pit.For the aquifer that is not isolated,partially penetrating wells are widely used,and numerical simulation is usually adopted to evaluate the dewatering effect(Wu et al.,2015).In this project,whether the aquifers are isolated or not,drainage wells will be installed in regions I and II,while in region III and the three ramps,partially penetrating wells will be installed.For comparison,two dewatering schemes for region III and the three ramps will be numerically modeled.

5.1.Scheme 1

5.1.1.Dewatering in regions I and II

In the two confined aquifers in region I and the upper confined aquifer in region II,the hydraulic connection outside of the pit is cut off by underground diaphragmwalls and waterproof curtains.If the drainage wells are installed in these two regions,the dewatering requirement could be met.The number of wells can be calculated by(Wu,2003):

wheremis the number of the drainage wells,Ais the total area of dewatering(m2),andais the area of dewatering by a single well.

Fig.7.Fitting curves and error analyses of the parameter adjustment model.(a,c)Fitting curves of OB1 and OB2;and(b)Error analyses of OB1 and OB2.

By calculation,11 drainage wells are needed in region I with the filter tubes set in the upper confined aquifer to drain the upper confined water.Among these wells,the filter tubes of 5 wells are also set in the lower confined aquifer to drain the lower confined water at the same time.In region II,21 drainage wells are used in the upper confined aquifer.Meanwhile,the filter tubes are also installed in the unconfined aquifers to drain groundwater in the shallow strata.The layout and structure of the drainage wells are shown in Figs.1 and 4,respectively.

5.1.2.Dewatering in region III and ramps

To avoid uplift of the pit bottom in region II,the lower confined water in this region should be drained.However,due to the missing layer④in region III and the close hydraulic connection between the lower confined aquifer in regions II and III,the piezometric head of the lower aquifer in region II will decrease when pumping in region III.In response,12 wells with an average spacing of approximately 14 m are set in region III to drain the upper confined water,and simultaneously,to reduce the piezometric head in the lower confined aquifer in region II.For the ramp pits,11,12 and 12 wells are installed in ramps A,B and C,respectively,with the filter tubes in the upper confined aquifer.The well structure is shown in Fig.4,and the layout of the pumping wells is shown in Fig.8a.

Because the confined aquifers in region III and the three ramps are not isolated by waterproof structures,numerical simulation is adopted to evaluate the effect of dewatering scheme.The same model employed in the parameter adjustment is used.

The simulation results are shown in Fig.8,indicating that the water table of the upper confined aquifer in region III and the three ramps will be 1 m below the corresponding pit bottom,and the piezometric head of the lower confined aquifer in region II will be below 0 m,meeting the required safe water level.

Along the Yangtze River levee,10 settlement observation points with spacing of approximately 40 m are created in the simulation model.The accumulated settlement calculated by Eq.(8)and differential settlement of each point are listed in Table 7.The settlements of all the observation points are larger than 50 mm,greatly exceeding the limited settlement of 30 mm.Moreover,the differential settlements of points O2,O5,O7,O9 and O10 will be larger than the limited value of 2‰.The results indicate that the levee will be at high risk of destruction.In addition,the settlements at observation points O5-O10 will exceed 70 mm,larger than that of the rest of the points,as these six points are close to ramp B.Points O1 and O2 are also close to the western end of ramp A,but the pit bottom in this area is shallower and the required drawdown is relatively less such that the settlements in these two points are less.

Based on the aforementioned analysis,although the dewatering requirement in the foundation pit is satisfied,the scheme 1 will result in excessive accumulated and differential settlements along the Yangtze River levee;therefore,it is not acceptable.Thus,an optimization scheme should be conducted.

5.2.Scheme 2

Due to the large drawdown around the levee simulated in the previous scheme,27 rechargewells with a constant rechargerate of 300 m3/d are employed along the levee based on scheme 1.Nineteen wells are installed along the southeastern side of the levee,while 8 wells are installed along the northwestern side near observation points O7-O10,where greater drawdown could occur.The average spacing of the wells which are close to ramps A and B is approximately 20 m,while the well spacing near observation points O3-O6 is approximately 36 m(Fig.9a).In addition,the bottom of the waterproof curtains around ramp B and the northwestern side of ramp A are installed at an elevation of-28 m,deeper than that of the initial plan(-17 m),to reduce the drawdown outside of the pit when pumping inside.

The simulated water table contours of two confined aquifers in scheme 2 are shown in Fig.9,and the accumulated and differential settlements of 10 observation points are shown in Table 7.The results show that this scheme satisfies the dewatering requirement in the foundation pit,and the accumulated and differential settlements will be within the limited values.In addition,only 9 pumping wells will be needed for ramp B,less than that in the former scheme.

Fig.8.Water table contours(in m)of the(a)upper and(b)lower confined aquifers in scheme 1.

5.3.Comparison of dewatering schemes

According to the design and simulated results of the dewatering schemes,both schemes can satisfy the dewatering requirements in the foundation pit and ensure the stability and convenience of the excavation.However,each scheme has its advantages and disadvantages.The optimum scheme is usually chosen considering the construction cost,period and dewatering effect(Fu et al.,2014;Wu et al.,2015).

In the aspect of cost,recharge wells are not needed in scheme 1 and the bottom elevation of the waterproof structures in ramps A and B is 11 mwhich is less than that in scheme 2.Therefore,scheme 1 is more economical.On the other hand,more working procedures in scheme 2 can result in longer construction periods.

As for the dewatering effect,the simulation results indicate that the accumulated and differential settlements will exceed the limited values in scheme 1;while the scheme 2 will have less impact on the environment around and the deformation of the levee can be well controlled.

Table 7Accumulated and differential settlements of the observation points.

By comparison,for the sake of stability of the levee,scheme 2 is more reasonable.Though scheme 1 is more economical,the deformation of the levee will exceed the limited value.In scheme 2,although a number of recharge wells are installed,the simulation results indicate that the deformation of the levee can be well controlled.Bycomparison,scheme 2 is more reasonable and will be applied to the construction.

During the construction of the foundation pit,six monitoring points M1-M6 were set along the levee(Fig.1).The maximum accumulated and differential settlements of each point are shown in Table 8.The monitoring results indicate that all the data are within the corresponding limited range.Taking monitoring points M1 and M6 for instance,which are located at similar positions as observation points O1 and O9 in the simulation,respectively,the accumulated and differential settlements of the two points are nearly equal to the corresponding simulated values,proving the validity of the numerical simulation.However,the monitoring results are slightly less than the simulated values,as the levee itself has a certain degree of rigidity which can reduce subsidence(Wang,2013).Five years have passed since the project was completed,and the levee has been stable and no cracks appear,suggesting that the scheme 2 is a more practical optimization design.

Fig.9.Water table contours(in m)of the(a)upper and(b)lower confined aquifers in scheme 2.

Table 8Monitoring data of the levee.

5.4.Challenges of the dewatering scheme

The success of this pit construction has corroborated the feasibility of scheme 2 in the construction projects.However,the dewatering scheme has its own challenges when applying in general situations.Different from the common projects,this construction site is located near the Yangtze River,and the typical dualstructure strata and multiple confined aquifers with complicated hydraulic connections increase the difficulty of dewatering significantly(Li et al.,2012).Due to the short distance between the river and the foundation pit,river water can easily recharge the confined aquifers when pumping the groundwater in the pit.In order to isolate the aquifers in region I and facilitate dewatering,deeper waterproof structures should be set,bringing about more cost and construction challenges.Besides,the piezometric head of the two confined aquifers is relatively high.When pumping is performed in the foundationpit,the decrease of the piezometric head outside the pit will be far less than that in the pit,resulting in a great head difference between inside and outside of the pit(Zhou,2016).Thus,additional pumping wells need to be set outside of the pit in case of cracks of waterproof structures.

6.Conclusions

In this paper,based on hydrogeological conditions,the dewatering requirement of a deep foundation pit near the Yangtze River was analyzed,and the hydraulic parameters were obtained from analytical calculation and numerical simulation according to pumping test results.Next,two dewatering schemes for excavation were discussed by numerical simulation.The main conclusions of this research are drawn as follows:

(1)The complicated geological and hydrogeological conditions of the construction site were analyzed.To guarantee the safety of the pit excavation and ensure the stability of the Yangtze River levee,three dewatering requirements were proposed.

(2)Based on the pumping test,the hydraulic conductivity of the upper confined aquifer was obtained from analytical calculation with three well combinations.The average value was 20.45 m/d,larger than the real value for the recharge from the lower confined aquifer.KHandKVvalues adjusted by numerical simulation were 16 m/d and 8 m/d,respectively,and the large difference between the two values indicated anisotropy of the aquifer.

(3)Numerical results show that,in scheme 1,the largest accumulated settlement and differential settlement would be 94.64 mm and 3.3‰,respectively,exceeding the limited values.Scheme 2,in which recharge wells were installed along the levee and the bottoms of the waterproof curtains in ramps A and B were set deeper,could restrict the accumulated and differential settlements within the limited ranges.

(4)During the construction of this project,scheme 2 was adopted in the site.The success of the construction corroborates the validity of scheme 2,which can provide solutions and experience to similar projects.

Conflicts of interest

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgements

This study was financially supported by the doctoral fund of the Ministry of Education of China and the Nature Science Foundation of Jiangsu Province,China(Grant Nos.20130091110020 and BE2015675).

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