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Geotechnical and GeologicalEngineeringAn International Journal ISSN 0960-3182 Geotech Geol EngDOI 10.1007/s10706-016-0071-1

Evaluation of Flow Reduction due toHydraulic Barrier Engineering Structure:Case of Urban Area Flood, Contaminationand Pollution Risk Assessment

Yohannes Yihdego

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ORIGINAL PAPER

Evaluation of Flow Reduction due to Hydraulic BarrierEngineering Structure: Case of Urban Area Flood,Contamination and Pollution Risk Assessment

Yohannes Yihdego

Received: 27 March 2016 / Accepted: 8 August 2016

� Springer International Publishing Switzerland 2016

Abstract In this study a vertical barrier forming the

exclusion system in relation to partials extending into

an impermeable stratum was analysed using a 3-D

numerical modelling used to quantify the effect of a

hydraulic barrier on flow which allows taking the

anisotropy and heterogeneity of the site in a complex

hydrogeological context and hydraulic barrier into

account. A simulation in this study shows that for 0 %

cut off the % reduction in flow is 0 and for 100 % cut

off the % reduction in flow is 96, 94 and 92 % at 5, 10

and 15 days respectively due to leakage through the

sheet piles, even with 100 % of the aquifer cutoff, the

% of groundwater inflow impounded never reaches the

100 %.Also the change in trendwhere the% reduction

in flow increases significantly with % cut off occurs at

around 60 % cut off. That is, the reduction in flow

through the aquifer only becomes significant after

60 % cut off by the sheet piles. The sensitivity analysis

allows determining the factors of influence. A sensi-

tivity analysis indicates the relationship appears rela-

tively less sensitivity to varying the hydraulic

conductivity, but very sensitive to the % cut-off.

Therefore the effect of the sheet piles start to be

significant after cut off exceeds 80 % and that the total

profile length matters, i.e. the 60 % cut off must be

applied to the whole width of the aquifer and not a

portion of the aquifer, i.e. the minimum required to

adequately reduce flow under the levee is 80 % cut off.

From this study it can be derived that lesswater flows to

a levee structure surrounded by sheet piles, depending

on the depth of the sheet piles in proportion to the depth

of the water bearing layer. The relationship is inde-

pendent for the hydraulic conductivity but dependent

on the ratio between the installation depth of the sheet

piles beneath the Piezometric level and the depth of the

bottom of the water bearing layer beneath the same

Piezometric level. This study demonstrated the major

influence of the technical design of the barrier on the

simulated flow disturbances. The current approach can

be applied elsewhere in related field for variety of

application including formulating a resource manage-

ment strategy, contamination containment and settle-

ment risk. Overall, the capacity of decision makers to

understand flow systems, how they function and

respond to the placement of hydraulic barriers in the

area theymanagewill form the basis for the operational

management of the resources and infrastructure.

Keywords Cut-off walls � Sheet-piling �Engineering � Excavation � Leakage � Seepage �Contamination � Infrastructure � Flood � Pollution �Urban

1 Introduction

Barriers to the flow of groundwater may be required as

part of the permanent works, to protect buildings

Y. Yihdego (&)

Snowy Mountains Engineering Corporation (SMEC),

Sydney, NSW 2060, Australia

e-mail: yohannesyihdego@gmail.com

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DOI 10.1007/s10706-016-0071-1

Author's personal copy

adjacent to excavations, to reduce flow to a dewatering

scheme for a deep excavation adjacent to the sea or river,

to reduce rate of flow through a ground-freezing zone, to

reduce uplift pressure, to reduce the area required for

side slopes to excavations and control migration of

pollutants and gases. A vertical engineered barrier is a

wall built below ground to control the flow of ground-

water. Engineered barriers may be used to divert the

direction of contaminated groundwater flow to keep it

from reaching drinking water wells, wetlands, or

streams. They also may be used to contain and isolate

contaminated soil and groundwater to keep them from

mixing with clean groundwater. Engineered barriers

differ from permeable reactive barriers in that they do

not clean up contaminated groundwater (Chen et al.

2013; Ervin and Morgan 2001; Janssen 2001; Wood-

ward 2015). Cut-off walls are used to exclude ground-

water from an excavation, to minimize the requirement

for dewatering pumping. Typically, the method

involves installing a very low permeability physical

cut-off wall or barrier around the perimeter of the

excavation to prevent groundwater from entering the

working area. Most commonly, the cut-off is vertical

and ideally penetrates down to a very low permeability

stratum (such as a clay or unfractured bedrock) that

forms a basal seal for the excavation. Several methods

are available to form cut-off walls or barriers around

excavations, including Steel sheet-piling, Slurry trench

walls, Concrete diaphragm walls, Bored pile walls,

Grout barriers, Mix-in-place barriers and Artificial

ground freezing (Wu et al. 2015a, b). The selection of

a given exclusion method used to form a cut-off barrier

will depend on the conditions and constraints on a given

project. Primary constraints are desired depth of wall,

ground conditions, geometry ofwall (somemethods can

be used horizontally or inclined to the vertical, while

others are limited to vertical applications), and whether

the barrier is intended to be permanent or temporary

(Bray et al. 2016; Janssen 2001).

In an environment with complexmore or less united

subsurface structures the effect of these barriers for

groundwater flow cannot be disregarded if there is

significant groundwater flow and a difference in

potential of the site, especially in shallow thin phreatic

aquifers (Janssen 2001). The modification of the

groundwater flow system induced by an underground

structure (in this case the sheet piles) might face issues

with sewer leakage and contamination. Sometimes the

soil-bentonite mixture is not able to withstand attack by

chemicals such as strong acids, bases, salt solutions, and

certain organic chemicals. This hastens deterioration of

the wall. As a result, the wall material should be tested

prior to construction. In addition, the wall should be

monitored for leakagewhen it is installed, and as it ages.

Physical factors, such as seismic activity and pressure

build-up, may degrade or deteriorate slurry walls over

time, causing them to lose their containment capacity.

Thorough characterization of the subsurface is required

because settling or unstable ground can limit effective-

ness. This is another reason that the wall should be

monitored for leakage as it ages. The benefits of slurry

walls rely on their ability to create impermeable barriers

togroundwater flow.Therefore, they shouldbedesigned

so groundwater does not flow underneath the wall.

Because slurry walls have been used for decades, the

equipment and methodology are readily available and

well-known. However, the process of designing the

proper mix of wall materials to contain specific

contaminants is less well developed. Excavation and

backfilling of the slurry trench is critical and requires

experienced contractors (Vilarrasa et al. 2011; Wang

et al. 2013;Wu et al. 2015a, b). Due to the underground

structure, damage to buildings including flooding of

lower levels, excessive hydrostatic stress and corrosion

of foundationsmayoccur.Questions on the risk of rising

groundwater levels have been raised and solutions have

been proposed that permit underground infrastructures

to remain in placewith coexistence of groundwater flow

(Attard et al. 2016; Xu et al. 2009). Understanding the

effect of a hydraulic barrier of the flow system makes it

possible to predict the risks inherent to the groundwater

surface and evaluate the contribution of the obstacle to

its vulnerability, under worst climatic condition and

imminent climate change.

Cutoff Walls offer a cost-effective solution to

groundwater control problems. These barriers are

constructed to intercept and impede the flow of fluids

(groundwater, contaminants) underground and can be

effective for site dewatering, underground pollution

containment and seepage barriers under dams and

levees. Constructed by excavating a narrow trench

under an engineered fluid, the slurry trench technique

permits the installation of deep barriers in all types of

soil and groundwater conditions. Containment tech-

nologies such as slurry walls can eliminate the need for

excavation and disposal of impacted soils and con-

taminants as well as costly shoring and dewatering

associated with deeper removals.

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In reality the impact of groundwater barriers will be

modest for most engineering structures. The exception

is where very long linear structures (such as metro

stations or cutting for roads or railways) are contained

within low-permeability walls. If such structures are

located across the direction of natural groundwater

flow, then groundwater flow will be diverted around

the sides of the structure. This can reduce the supply to

nearby groundwater sources or cause flooding of

adjacent basements upstream of the structure. Where

impacts are a potential concern, it may be appropriate

to use numerical groundwater modelling to assess

changes in groundwater level. If impacts are assessed

to be significant, then consideration should be given to

modifying the cutoff wall or piled foundation design to

limit the depth of piles or cutoff walls or to use cutoff

walls that are temporary in nature (such as artificial

groundwater freezing or steel piles removed at the end

of construction). Structures with deep basements or

below ground spaces may also provide potential for

discharges to groundwater in the long term. If the

structures are not water tight and penetrate confining

beds over aquifers, leaks, spill-ages, or surface water

flooding may percolate more freely into the ground-

water. Individually, such leakage may be small, but

their combined effect may lead to significant ground-

water contamination (Shaqour and Hasan 2008;

Preene 2012; Pujades et al. 2012a, b; Pujades et al.

2014; Tan and Wang 2013; Wang et al. 2009).

A sheet piling causes a discontinuity in the

groundwater head. For the computation, this beha-

viour is represented in different way with analytical

features, together with the presence, position and

hydraulic property of the sheet piles. The theoretical

assumption is that the extent to which the groundwater

is influenced by a barrier (sheet piles), largely depends

on the proportion to which the barrier cuts off the

water bearing layers/aquifers (Huang and Han 2009;

Kimura et al. 2007; Knight et al. 1996; Lin et al. 2010;

Ma et al. 2014; Peng et al. 2011). This paper assesses

the optimum amount of sheet pile cut off, based on the

depth of the sheet piles in proportion to the depth of the

water bearing layer for the hypothetical Levee work to

control groundwater flow and contaminant migration

timing. Such analysis is found to be useful, because

groundwater control for excavations can be achieved

by exclusion or partial exclusion in conditions where

removal is not feasible or economic. Also, without

proper understanding of the barrier effect, drainage

can also cause the collapse of the impounding

structure, possibly leading to a modification of engi-

neering structure and a costly interruption of con-

struction. This study demonstrates the influences of

the technical design of the barrier on the simulated

flow disturbance and its implications.

2 Hypothetical Site Conceptualization

Vertical barriers are frequently used with surface caps

to produce an essentially complete containment

structure. These vertical barriers must reach down to

an impermeable natural horizontal barrier, such as a

clay zone, to effectively impede groundwater flow.

Vertical cutoff walls frequently key into a stratum of

naturally low hydraulic conductivity. A key is not

necessary or cost effective when completed cutoff-

wall. Cutoffs can be constructed using other materials,

such as sheet pile driven to provide cutoff in the

subsurface (Attard et al. 2016; Daniel 1993). Sheet

piles (Fig. 1) are subsurface barriers that impede or

stop groundwater flow. Sheet piles are used to contain

contaminated groundwater, divert uncontaminated

groundwater flow, and/or provide barriers for ground-

water treatment systems.

As shown in Fig. 1, a schematic conceptual model

of the area is located in the first stratigraphy units. To

the east of the aquifer, the river is assigned with

hydraulic head (range of 100 year Recurrence Interval

Average flooding (0.8 m to 1.9 m)). The elevations

and horizontal are illustrated in Fig. 1.

2.1 Approach/Modelling

To estimate the optimum sheet pile length, modelling

was undertaken using MODFLOW-SURFACT code

(HydroGeoLogic Inc 2002), an advancedMODFLOW

based code developed by HydroGeoLogic Inc. that

handles complete desaturation and resaturation of grid

cells), within the framework of Visual MODFLOW

Version 4.6. The aquifer (sand layer) is simulated using

a grid of 5 layers, 30 columns and 30 rows (Figs. 2 and

3). All layers have the same type 3: confined/uncon-

fined (variable transmissivity). The cut-off wall is

modelled by using the Horizontal-Flow-Barriers pack-

age. The model allows computing discharges per unit

length of levee/sheet pile in a hypothetical case

(Fig. 1). The model was simulated in a steady state

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condition by varying the cut-off wall to a depth of up to

4 m depth as shown in the model cross section.

2.2 Assumptions

• It is assumed that the cross section of the aquifer to

be cut off is uniform over its length and that flow is

generally perpendicular to the levee to simulate

flow in the vertical plane perpendicular to the

levee.

• The flooding lasts up to 2 weeks’ time (range of

flooding duration). The maximum probable flood

stage is assumed to be 1.9 m.

• The flood in rise and recession is instantaneous.

The infiltration of recharge to the aquifer/layers is

instantaneous (no delay between the time precip-

itation infiltrates the surface until it reaches the

water table).

• The thickness of the aquifer (sand layer) is a

uniform 4 m.

• The horizontal hydraulic conductivity of the sand

layers is assigned 15 m/day. The vertical hydraulic

conductivities are assumed to be a tenth of the

horizontal hydraulic conductivity (i.e. 1.5 m/day).

• The specific storage and specific yield of the

aquifer are 0.000007/m and 0.18 respectively.

• A value of 0.00008 m/day is assigned for the sheet

pile conductance.

3 Result and Analysis

The simulated inflow past sheet pile per meter width,

with a hydraulic resistance varying from 5, 10 and

15 days are shown in Fig. 4 and is summarized in

Table 1. The sheet pile has a resistance proportional to

the width. Therefore, the total inflow will need to

multiply inflow per width (Table 1), by the length of the

sheet pile which is assumed 200 m for this study. Zone

budget, comprising the inflow and outflow component

for 5, 10 and 15 days is shown in Fig. 5.

3.1 Impact of Flow Obstacles in the Barrier

Structure Area

A simulation was carried out for the obstacle effect

due to the hydraulic barrier, of the cut-off wall, in

3

2 River

1

0

Shee

t Pile

Ground

Surface

-1

-2 SAND

-3

-4

-5 clay

-6

-7

-8

-9

-10 Sand

-11

-12

Fig. 1 conceptual sketch of the engineering barrier/sheet pile

Fig. 2 Model grid. The red and grey shaded areas at the

boundary lines represent the flood stage (constant head) and

sheet pile (drain) respectively

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comparison to the natural flow of the groundwater.

Figure 7 shows the relationship between the reduction

in flow (through the aquifer or the impoundment on the

flood side) and % cut-off for the modelled flooding

duration (in this case 15 days). The % reduction in

flow after 5 and 10 days (of the 15 day flood duration)

is also plotted. It can be seen that for 0 % cut off the %

reduction in flow is 0 and for 100 % cut off the %

reduction in flow is 96, 94 and 92 % at 5, 10 and

15 days respectively due to leakage through the sheet

piles, even with 100 % of the aquifer cutoff, the % of

groundwater inflow impounded never reaches the

100 % as shown in Fig. 6; the inflow and outflow past

the cutoff wall has a slight difference, after 10 years

period of simulation. This is in agreement with

previous works (such as Janssen 2001) where by the

simulation shows that, due to the leakage, even with

100 % cut-off, the drive-up of the water never reaches

the 100 %, unless the aquitards have a very high

resistant for vertical flow.

From Fig. 7 the change in trend where the %

reduction in flow increases significantly with % cut off

occurs at around 60 % cut off. That is the reduction in

flow through the aquifer only becomes significant after

60 % cut off by the sheet piles (Figs. 7 and 8).

The sensitivity analysis allows determining the

factors of influence. A sensitivity analysis indicates

the relation appears relatively less sensitivity to

varying the hydraulic conductivity, but very sensitive

to the % cut-off. Therefore the effect of the sheet piles

start to be significant after cut off exceeds 60 % and

that the total profile length matters, i.e. the 60 % cut

off must be applied to the whole width of the aquifer

and not a portion of the aquifer. Also upward driving

of groundwater due to the existence of sheet piles will

not be significant unless:

1. There is a significant groundwater flow and

significant difference in potential between both

sides of the sheet pile structure.

2. The cut-off % (the extent in which the sheet pile

structure cuts in the aquifer) has to exceed 60 %; a

barrier of significant width is placed perpendicular

to the direction of flow (however, the groundwater

flow pattern is not known);

3. The aquitard above and below the aquifer (if any)

have a resistance to vertical groundwater flow i.e.

very low permeability thus preventing vertical

leakage.

4 Discussion

Geotechnical processes cover the elements of ground

treatment and improvement, from the control of

groundwater, drilling and grouting to ground anchors

and electro-chemical hardening. There has been

experience in groundwater control projects around

the world in relation to the design and installation

service to control groundwater related problems.

Vertical cut-offs are used to prevent groundwater flow

into excavations and seepage and pressure head under

hydraulic structures such as dams, weirs and reser-

voirs. Also groundwater barriers may be formed where

extensive heavy duty foundations are installed into

aquifers that are shallow or limited thickness (Preene

Fig. 3 Model cross section at row 15 with 100 % cut off wall to the sand layer which is 4 m

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2012). Vertical barriers (or cut-offs) forming the

exclusion system should generally extend into an

impermeable stratum, within an economic depth, to

avoid upward seepage into the excavation during

construction. If the cut-off is not sealed into an

impermeable for permanent works such as a basement,

Fig. 4 Head simulation at 5, 10 and 15 days

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Table 1 Total inflow per

unit sheet pile width

(m2/day) versus flood

duration (day)

Inflow past sheet pile per unit width (m2/day)

Time (days) Cut-off (%) 0 % 20 % 40 % 60 % 80 % 100 %

5 3.53 3.53 3.31 2.78 1.98 1.83

10 2.57 2.53 2.37 2.17 1.67 1.37

15 1.83 1.76 1.64 1.62 1.61 0.99

Fig. 5 Zone budget at 5, 10 and 15 days

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then the basement must be designed to resist up-lift for

its serviceable life. Vertical cut-off walls re tradition-

ally considered for environmental applications are

circumferential barriers. Also, they may be used to

affect a vertical barrier for only a portion of the site. A

key is to assess how cost effective when completed or

partial cut-off are suggested (Daniel 1993; Preene

2012). The use of a cut-off wall can be to minimize the

rate on-site migration of uncontaminated groundwater

from the upgradient area. In this case there is a reduced

concern about deficiencies in the cut-off wall and

potential degradation due to the flux contaminated

groundwater through the wall. To minimize the rate of

contaminant transport off-site, the environmental

control system may include a vertical cut-off wall

coupled with a low permeability cover, groundwater

withdrawal system, and treatment systems for the

pumpage. In defining the vertical cu-toff wall objec-

tives, it is important whether the barrier is to act as a

groundwater barrier with low hydraulic conductivity

or a contaminant transport barrier. As a result of

establishing different objectives, the type of vertical

cut-off wall and design criteria for the cut-off wall

depend very much upon the defined objectives of the

cut-off wall (Daniel 1993). Similar studies concluded

Fig. 6 Zone budget for a

100 % cut off after 10 years

(the volumetric flow is

shown in y-axis as m3/day

Fig. 7 Effective % of reduction in flow versus cut-off

Fig. 8 Effective % of reduction in flow versus flood duration

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that driving-up of groundwater due to the existence of

subsurface barriers will not be significant until:

1. There is significant groundwater flow and a

significant difference in potential between both

sides of the location of the subsurface structure;

2. The % cut off has to exceed 60 or 70 %; and with a

barrier of significant width perpendicular on the

direction of flow (more than a few tens of centime-

tres) and with a barrier of significant length in the

direction of flow of several tens of meters;

3. The aquitards above and under the aquifer have a

resistance against vertical groundwater flow of at

least a few 100–1000 days (preventing leakage).

In an environment with complex more or less

united subsurface structures the effect of these

barriers for groundwater flow cannot be disre-

garded if there is significant groundwater flow and

a difference in potential of the site, especially in

shallow thin phreatic aquifers (Janssen 2001).

From this study it can be derived that less water

flows to a levee structure surrounded by sheet

piles, depending on the depth of the sheet piles in

proportion to the depth of the water bearing layer.

The reduction appears to be significant not earlier

thanwith a percentage of cut-off exceeding a value

of 60–70 %. Analogous to this the assumption is

that the reduction in groundwater flow will not be

significant until the percentage of cut-off exceeds

the value of 60 or 70 % and with a barrier of

significant length and width. The reason for this is,

that the amount of water that flows from the

upstream to downstreamof the hydraulic barrier, is

dependent on the difference in potential between

both sides, the thickness of the aquifer and the

hydraulic conductivity. With fixed potentials on

the boundary and decreasing thickness of the

aquifer in the direction of flow, this model looks

like flow of groundwater through a confined

aquifer without recharge’’ (Janssen 2001). Else-

where, similar sensitivity analysis was tested for a

range of hydraulic gradient, water table/piezomet-

ric surface to simulate the effect of an imperme-

able barrier with in an aquifer. As an example, the

radius of the gallerywas varied from 10 t and 50 %

of the thickness of the aquifer and the hydraulic

gradients tested were between 0.5 and 10 %. The

2-D numerical solution obtained confirmed the

relation of linearity between the additional loss of

head and the regional hydraulic gradient. In the

case of the confined groundwater, the simulations

gave the maximum zone of influence of the

structure, which was about 3 times its diameter.

Also disturbances of several centimetres in the

regionwas observed, when the hydraulic gradients

were\1 %. For the same gradient the maximum

additional head loss was reached when the summit

of the structure was located at the level of the

groundwater surface (Attard et al. 2016; Deveugh-

ele et al. 2010; Janssen 2001; Marinos and

Kavvadas 1997; Pujades et al. 2012a, b). Both

studies provid a useful information of the barrier

effect phenomenon and provide sensitivity analy-

sis to several parameters (Attard et al. 2016).

However, some points of discussions deserve to be

noted in comparison with the current study. First,

the 2-D modelling approach is a factor of over-

estimation of the barrier effect generated by a

barrier. The results can only be applied in 2D—

geometry problems, whereby the anisotropy and

heterogeneity are not an issue. Secondly, the

simulations have been run with upstream and

downstream Dirichlet boundary conditions (im-

posed potential). This constrains the head over all

the modelled area, unlike the current method

simulated using ‘‘Horizontal-Flow-Barriers pack-

age’’. In addition to that, the head constraints

upstream and downstream area a factor for under-

estimation of the barrier effect because the barrier

effect is bounded by the difference between

upstream and downstream head constraints. In

fact, when the potential is imposed, the inflow is

numerically reduced to respect the potential

boundary condition. It should be more appropriate

to assign upstream and downstream Neumann

boundary conditions (flow boundary condition as

the case of the current study). In this case, the flow

would be imposed and the barrier effect would no

longer be bounded (Attard et al. 2016).

5 Summary and Conclusion

Due to the increasing urbanisation and subsurface

structures and permanent water retaining walls, the

question arises whether these subsurface elements for

a barrier for ground waterflow are effective. In this

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study a vertical barrier/cut-off forming the exclusion

system in relation to partials extending into an

impermeable stratum was analysed using a 3-D

numerical modelling used to quantify the effect of a

hydraulic barrier on flow. The use of a 3-D numerical

model is a main asset for relatively accurate depiction

of flow disturbances. It allows taking the anisotropy

and heterogeneity of the site in a complex hydroge-

ological context and hydraulic barrier into account.

In this study, the effectiveness of the sheet pile

through estimating the groundwater inflow were

assessed by varying the length of the sheet piles. A

simulation in this study shows that for 0 % cut off the

% reduction in flow is 0 and for 100 % cut off the %

reduction in flow is 96, 94 and 92 % at 5, 10 and

15 days respectively due to leakage through the sheet

piles, even with 100 % of the aquifer cutoff, the % of

groundwater inflow impounded never reaches the

100 %. This is in agreement with previous works

where by the simulation shows that, due to the leakage,

even with 100 % cut-off, the drive-up of the water

never reaches the 100 %, unless the aquitards have a

very high resistant for vertical flow. The minimum cut

off of the sand layer to have any significant effect is

60 % and the minimum required to adequately reduce

flow under the levee to minimise groundwater day

lighting on the inside is 80 % cut off. Also the % cut

off should be applied to the whole length to be sheet

piled, i.e. it is not acceptable to provide 100 % cut off

to 80 % of the section and leave 20 % un-piled, if

assumed to be retrofitted to withstand the 100 years

flood. But there is some concern about the influence of

flood stage water flowing into the aquifer, if the

thickness of the sand layer is variable. To be more

precise in the calculations, it requires conducting a

series of small boreholes to determine the depth to the

impervious/clay layer. However, this might not be

feasible if the total length of the construction zone is

quite large and the current analysis becomes a valid

analysis, including in areas where the subsurface layer

is not certain, merely extrapolated from few boreholes.

The other scenario is a completely different approach.

If the depth to clay is around 4 m assuming the width

of the canal is 15 m, and the pile should penetrate the

clay layer, then a safe assumption would be around

8–10 m of depth plus the additional height for the

levee. But this would be on both sides of the canal

which would mean double the amount of piling

needed, or about 16–20 m. One could limit the

quantity of pile used to 15 m by lining the bottom of

the canal instead of trying to block flow under the piles

along the levees. This would effectively isolate the

canal from the aquifer at all times and one could easily

calculate the cost using 15 m plus the height of the

levee on both sides. Alternatives to this scenario would

be lining the canal with other impervious material such

as bentonite (clay) or reinforced plastic. The life

expectancy and cost would need to be evaluated to

determine the best solution. It is plausible exercise to

compare this with the cost of steel piling 8 m below

surface plus the height of the levee, doubled for both

sides of the canal, for the sake of effective measure to

block the major groundwater flow and cost effective

alternative. Additionally, by installing pilings down to

the clay level it will effectively create a no flow zone

and change the flow system in the sand aquifer and

divide the aquifer into two sections. This may not be

desirable if it impacts other dependent uses of this

aquifer.

The sensitivity analysis allows determining the

factors of influence. A sensitivity analysis indicates

the relation appears relatively less sensitivity to

varying the hydraulic conductivity, but very sensitive

to the % cut-off. Therefore the effect of the sheet piles

start to be significant after cut off exceeds 60 % and

that the total profile length matters, i.e. the 60 % cut

off must be applied to the whole width of the aquifer

and not a portion of the aquifer. From this study it can

be derived that less water flows to a levee structure

surrounded by sheet piles, depending on the depth of

the sheet piles in proportion to the depth of the water

bearing layer. The relationship is independent for the

hydraulic conductivity (k) but dependent on the ratio

between the installation depth of the sheet piles

beneath the Piezometric level and the depth of the

bottom of the water bearing layer beneath the same

Piezometric level. The sensitivity analysis results can

assist to classify the impact regarding hydraulic barrier

structures and design technique. It provided a decision

tool for choosing barrier construction technique for the

flood risk mitigation, with effective and optimum

sheet pile length. The approach employed here can be

similarly implemented for other purposes such as for

settlement risk, threat to water supply and formulating

a resource management strategy. As an example, the

relative depths of the cut-off wall and of the wells have

a significant effect on ground surface settlement

during the withdrawal of groundwater. Therefore,

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the appropriate selection of relative depth of both cut-

off wall and pumping well is an effective way of

controlling surface settlement outside the pit. Similar

case study can be computed elsewhere using the

approach in this study. The user can evaluate alterna-

tives with different site and sheet pile schemes for

building pit design very quickly because the input and

calculation options in this method are convenient and

fast. This study demonstrated the major influence of

the technical design of the barrier on the simulated

flow disturbances. Such study is relevant to infras-

tructures crossing a flow system and hence disturbs the

mass balance. Therefore, to attenuate the mechanical

stress on deep structures, one solution is to drain them,

using several scenarios including partial obstruction as

demonstrated on this study. The current approach can

be applied elsewhere in related field for variety of

application including pollution, contamination con-

tainment, infrastructure safety etc. Overall, the capac-

ity of project managers to understand flow systems,

how they function and response to the placement of

hydraulic barriers in the area they manage will form

the basis for the operational management of the

resources and infrastructure.

Acknowledgment The author acknowledges the contribution

from the reviewers who improves the manuscript.

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