influence of large scale inhomogeneities on a construction dewatering system in chalk - bevan et al

7/27/2019 Influence of Large Scale Inhomogeneities on a Construction Dewatering System in Chalk - Bevan et al

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information did not specifically identify such a zone and inany case it was considered that, even if present, the high-permeability zone would probably be cut off by the dia-phragm wall.

PROJECT DESCRIPTION AND EXCAVATIONGEOMETRYHS1 is the UKs first high-speed railway line, connecting

London to the Channel tunnel with a journey time ofapproximately 35 min. Construction of the Thames tunnel,as part of Section 2 of the line, involved boring twin 8.15 mdiameter tunnels up to 25 m below the bed of the Thamesfrom Swanscombe, Kent to West Thurrock, Essex. Eachtunnel drive used a separate tunnel-boring machine (TBM)attached to a 200 m long drive train, and commenced from asouthern launch chamber on the Swanscombe marshes. Thesouthern launch chamber was part of a longer excavation forthe tunnels southern approach. The southern approach struc-ture is 445 m long and 26 to 29 m wide, and was con-structed within diaphragm walls as either a cut-and-covertunnel or a retained cut. Construction dewatering was re-quired to lower groundwater levels to 1 m below formationlevel. As this varied along the length, the southern approachstructure was divided into four separate dewatering cells bymeans of cement/bentonite slurry cross-wall cut-offs. Theexcavation geometry and target drawdowns are summarisedin Table 1 and Figs 1 and 2. An aerial view of the southernapproach during construction is shown in Fig. 3.

GROUND CONDITIONSThe main water bearing strata at the site are the terrace

gravels and the underlying upper chalk, which are generallyin hydraulic connection. The chalk was eroded and weath-ered prior to the deposition of the terrace gravels duringQuaternary interglacials as sea levels rose. Superficial allu-vial deposits consisting of soft silty clay with peat horizonsact as a confining layer over much of the site (Fig. 4).

Detailed descriptions of the Lower Thames geological suc-cession are given by Marsland (1986) and Gibbard (1994).The depth profile shown in Fig. 2 is typical of much of

the site, with ground level at approximately +2 m OD. Theconfining alluvial layer is generally 910 m thick, withapproximately 7 m of terrace gravels below. The thickness ofboth the alluvium and terrace gravel decreases towards theshallow end of the excavation, furthest away from theThames, as shown in Fig. 1.

The chalk outcrops to the south of the site, close to thesouthern end of the excavation, as shown in Fig. 4. Theextent of the chalk outcrop was determined from boreholelogs and is consistent with the geological map (BGS, 1997).An outline of the chalk geology associated with the CTRLproject is given by Warren & Mortimore (2003).

The upper chalk encountered during construction was ofthe Seaford chalk formation. Typically there was 1 to 2 m ofstructureless chalk at the interface with the terrace gravels.Below this the chalk was generally classified as grade B2 orB3 using the Construction Industry Research and Informa-tion Association (CIRIA) grading scheme (Lord et al.,2002), indicating that discontinuity apertures are less than3 mm and the discontinuity spacing is between 60 and

Table 1. Design specifications for the four dewatering cells

SLC + SCC1 SCC2 SCC3 + SRC1 SRC2 + SRC3

Description Launch chamber + cut and cover Cut and cover Cut and cover + 25 m retained cut Retained cutLength: m 75 135 115 120Width: m 2926 2628 28 29Ground level: m OD +1.3 +1.3 +1.5 to +2.0 +2.0 to +3.2Toe of d-wall: m OD 29 25.5 to 21 21 to 19.0 17 to 12Formation level: m OD 17 to 14 14 to 11 11 to 7.25 7.25 to 4.5Target drawdown: m OD 18 to 15 15 to 12 12 to 8.25 8.25 to 5.5

SL SCC1C SCC2 SCC3 SRC1 SRC2 SRC3

120 m115 m135 m75 m

m OD

5

0

5

10

15

20

25

30

Tunnel portal Toe of diaphragmwall

Formation level Slurry bentonitecross cut-off wall Made ground

Alluvial deposits

Terrace gravels

Upper chalk

m OD

5

0

5

10

15

20

25

30

Fig. 1. Longitudinal profile of the approach structure and geological strata derived from borehole logs

636 BEVAN, POWRIE AND ROBERTS


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200 mm. This corresponds to Mundford grade III (Spink,2002), which suggests a bulk hydraulic conductivity of 105

to 103 m/s (Roberts & Preene, 1990).The dewatering system was designed using estimates of

hydraulic conductivity based on limited data from a pumpingtest carried out some distance away from the actual excava-tion. Analysis of the pumping test data from a single wellindicated an equivalent uniform horizontal hydraulic conduc-tivity of 1.73 104 m/s (14.7 m/day) for the gravel and thechalk above a level of 65 m OD. The level of 65 m ODwas taken as the aquifer base because the productive zoneof the chalk aquifer is generally assumed to be the top 5060 m, see Price et al. (1993). Packer tests carried out inboreholes at the southern approach site indicated chalkpermeabilities of 2.03 106 to 1.03 104 m/s (0.28.6 m/day), although packer tests are likely to underestimate thehydraulic conductivity because they tend not to intersectmajor discontinuities.

After the dewatering system had been designed, moredetailed pumping test data, including tests in piezometerswith defined response zones and borehole packer tests,became available from the site of the northern tunnel ap-

proach on the opposite bank of the River Thames. Thesetests draw water from a limited and reasonably well-definedhorizon, and may therefore be used to provide an indicationof the variation in hydraulic conductivity with depth. The

stratigraphy and formative geological processes for the twosites are almost identical, so it would seem reasonable toassume that the data (shown in Fig. 5) are also representativeof conditions on the south side of the river. The data indicatea clear decrease in the hydraulic conductivity of the chalkwith depth. Simple statistical analysis of the chalk perme-abilities revealed that there was a significant boundary atapproximately 25 m OD, marking the interface between themore weathered surface chalk and the base chalk. Table 2shows the mean hydraulic conductivity of the surface chalkto be 3.063 104 m/s, compared with 4.713 106 m/s forthe base chalk. The hydraulic conductivity of the surfacechalk appears to be more spatially variable, which mayreflect increased localised fissuring. The decrease in chalk

hydraulic conductivity with depth was confirmed by theresults of geophysical flow logging of screened wells at thenorthern approach site.

Analysis of constant flow rate pumping tests carried out

CLDischarge450 mm diameterdischarge main

Standing water

level

Dewatering

well

Roof

slab

Baseslab

Internal

wall

FormationlevelTarget water level

Pump level

Upperchalk

Terra

cegravels

Allumialdeposits

MG

2

0

5

10

15

20

25

30

Level:mOD

Diaphragm wall

Fig. 2. Idealised cross-section through the cut and cover section of the approach

INFLUENCE OF LARGE-SCALE INHOMOGENEITIES ON A CONSTRUCTION DEWATERING SYSTEM 637


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on both banks of the River Thames suggested that thehydraulic conductivity of the terrace gravels was generally inthe range 5.83 104 to 4.63 103 m/s (50400 m/day).This is broadly consistent with an estimate based on the D10particle size and Hazens formula

k 0:01(D10)2 (1)

(where k is the hydraulic conductivity in m/s and D10 is inmm), which with 0.2 mm


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tidal range of the Thames is up to approximately 5 m, withfluctuations generally between +2 m OD and 3 m OD.

SRA5945 was situated in the confined part of the aquiferand comprised a dual installation with two separate piezo-

meters screened and sealed in the terrace gravels and in theupper chalk. The monitoring data sets from the two piezo-meters are virtually identical in Fig. 6; both show a tidalpeak-to-peak amplitude of approximately 0.5 m. The data

from each stratum were similar throughout the confined area,suggesting good hydraulic connection between the gravelsand the chalk. The maximum recorded tidal amplitude was2.50 m at the piezometer SR1232, 20 m from the shoreline.Fig. 6 shows amplitudes of approximately 1.5 m for thepiezometer SA5981, approximately 300 m from the shore.

The methods described by White & Roberts (1993), basedon the theory set out by Ferris (1951), were used to interpret

the attenuation of tidal amplitudes across the confined area.The attenuation is related to the aquifer transmissivity (T)and storage coefficient (S) by

h2=h1 exp ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

S=to T p

x2 x1

h i(2)

where to is the tidal period (12.5 h), x2 x1 is the horizontaldistance between two standpipe piezometers and h2/h1 is theratio of the mean tidal amplitudes. At this site, for piezo-meters located along lines approximately perpendicular tothe centre-line of the Thames channel, this equation ade-quately describes the attenuation with T 1.313 102 m2/s(1134 m2/day) and S 0.005, for an effective aquifer basedepth of 65 m OD. According to White & Roberts (1993)the resulting value of T/S 2.65 indicates partly confinedconditions. The analysis suggests that, in general, the effec-tive source of the tidal fluctuations is close to the centre ofthe Thames channel, rather than at the bank; boreholerecords confirm that the alluvial seal on the Thames bed isonly present close to the shore.

At the boundary between the confined and unconfinedareas the tidal amplitude is reduced to ,0.2 m, as shown bythe data for AC1919 and NBH5 (Fig. 6). The fluctuationsfor the unconfined standpipe piezometers further south, awayfrom the river, are negligible.

DEWATERING SYSTEM DESIGN

In view of the high flow rates anticipated, a deep-wellsystem was installed to lower the groundwater levels withinthe tunnel approach structure. The proposed design incorpo-rated 42 wells (W01 to W42), located as indicated in Table 3and Fig. 8. The additional wells W43 to W54 will bediscussed later.

The design provided increased pumping capacity at thedeeper end of the excavation, where the required drawdownwas greatest. Either a 15 kW or 9.2 kW electrical submers-ible pump, with flow capacities of 20 and 12 l/s respectively,was installed in each well. Wells were installed to a depth ofbetween 29 and 24 m OD using the cable percussionmethod, and screened from 1 m below formation level to thewell base. Well installation, pumping and excavation started

at the deep end of the tunnel approach (cell SLC SCC1)to enable the TBM to be prepared at the earliest possiblestage. Dewatering and excavation then progressed towardsthe shallow end.

05

00

05

10

15

20

07-Nov

08-Nov

09-Nov

10-Nov

11-Nov

12-Nov

13-Nov

14-Nov

15-Nov

16-Nov

17-Nov

18-Nov

Date

Waterlevel:mOD SA5945 chalk

SA5945 gravel

AC1919 chalk

NBH5 chalk

SA5981 gravel

G11 alluvium

Fig. 6. Example of tidal influence on water levels in piezo-meters. Data were logged at 15 min intervals. Piezometerlocations are shown in Fig. 7

SR5945

SR5958

AC1919

NBH1

NBH2

NBH3

NBH4

NBH5

G09

NBH7

NBH6

G10

G11

G12

G13

G14

G15G16

G17

SA5981

Thames

Chalk piezometerGravel piezometer

Gravel and chalk piezometers

SR5957

m

Thames

NBH8

NBH6

Thames

0

ConfinedUnconfined

500

SR1232

Fig. 7. Location of remote standpipe piezometers around theSwanscombe Peninsula (not all piezometers are labelled)

Table 3. Distribution of pump capacity within the excavation

Dewatering cell Length: m Number of wells Pump size: l/s Total pump capacity: l/s

20 12

SLC + SCC1 75 16 16 320SCC2 135 14 4 10 200SCC3 + SRC1 115 6 6 72SRC2 + SRC2 120 6 6 72Totals 445 42 20 22 664



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INSTRUMENTATIONThe dewatering system was monitored with standpipe

piezometers installed both inside the excavation (the PIseries), and outside but close to the diaphragm walls (the PEseries). The PE series included dual installations into thegravel and the chalk strata. Pore pressures in the alluvialclay layer were monitored using pressure cells (VE series).All of these piezometers (PI, PE and VE) incorporated avibrating wire transducer wired to a datalogger to recordwater level readings hourly. The instrument locations areshown in Fig. 8. The instruments were calibrated againstmanual dip readings and corrected for the effects of changesin atmospheric pressure. Piezometers and cables that becamedamaged during construction were repaired where possible;cases of instrument breakdown and repair are indicated by agap in the data record.

Remote standpipe piezometers in the chalk and gravel, atthe locations shown in Fig. 7, were monitored manuallyusing a dip meter. In addition to a selection of the siteinvestigation piezometers, the monitoring programme re-quired the installation of standpipe piezometers in the chalk(NBH series) and in the gravel (G series). Pore pressures in

the alluvial clay were monitored by pressure cells installedat the G series locations. In general, the remote standpipepiezometers were monitored once every two to five daysduring commissioning of the dewatering system, reducing toonce every seven to 14 days once the system was fullyoperational and flows had stabilised. Baseline data of in situgroundwater levels were obtained by monitoring each remotepiezometer for at least 24 h, using a stand-alone pressuretransducer and datalogger, to establish the tidal range (asshown in Fig. 6).

Groundwater abstraction flow rates from the dewateringsystem were monitored using in-line flowmeters installed ineach of the two discharge mains. Each discharge main wascapable of delivering 300 l/s of water from the excavation to

the discharge outfall into the Thames. The flowmeters werelogged by the datalogger at hourly intervals. Initially, air inthe discharge main, which was drawn in when the pumpsdrew down to the pump intake level, resulted in erratic andincorrect measurement. This was resolved by trimming backindividual wells using the valve at the well-head, and byinstalling air escape valves on the discharge main.

DEWATERING SYSTEM PERFORMANCEThe dewatering system was commissioned in mid-Novem-

ber 2001 with all 16 wells in SLC+SCC1 pumped briefly, togive an initial flow in excess of 200 l/s (Fig. 9). It was

immediately evident that this cell could be dewatered to thetarget level (approximately 18 m OD) by pumping fromjust two of the 16 wells. Dewatering in SCC2 began on1 January 2002, with the pumps being switched on progres-

sively. By the end of January the discharge flow wasapproximately 200 l/s, but two additional wells, W43 andW44 (Fig. 8) were required to achieve the target drawdown.

Dewatering of SCC3+SRC1 began in March 2002, in-creasing the total extraction flow rate (from all cells) toabout 370 l/s. The required drawdown in the retained cutsections (SRC2+SRC3) could not be achieved with thedesigned pumped capacity, and nine additional wells (W45to W49 and W51 to W54) had to be installed in this area.The required drawdowns were achieved across the entiresouthern approach structure with a total abstraction rate ofjust under 600 l/s.

EVIDENCE FOR LARGE-SCALE INHOMOGENEITIESDuring the commissioning of wells in the retained cut

section, it became clear that locally the ground was far morepermeable than expected. This is indicated by the variationin the specific capacities of the wells (defined as the yieldper unit drawdown) across the excavation as a whole (Fig.10). The specific capacity depends both on well depth anddiameter and on the hydraulic conductivity of the ground. Inthis case the well depth and diameter did not vary greatlyacross the site, so the distribution of specific capacityprovides a good indication of zones of increased hydraulicconductivity. It is apparent from Fig. 10 that there is a zoneof elevated hydraulic conductivity present diagonally acrossthe excavation between 300 and 400 m from the tunnelportal.

As discussed previously, the upper chalk is predominantly

of the CIRIA grade B2 or B3. In the highly permeable zone,the chalk was more difficult to classify but is described aspossibly grade C4C5 for the top 5 m. This implies adiscontinuity aperture greater than 3 mm and a discontinuity

W03

W07W

01

W02

W10

W14

W11

W13

W16

W15

W17

W18

W19

W20

W21

W22

W23

W24

W25

W26

W43

W44

W27

W28

W29

W30

W31

W32

W33

W34

W35

W36

W45

W53

W54

W46

W47 W

49

W38

W37

W48

W51

W39A

W52

W40

W41

W42

PI09

PI08

PI23

PI22

PE13

VE13

PE14VE

12

PE12

PI21

PI07

PI20PE

11

PI06

PE21

PI19P

I05

PI18

PE20

VE09PE

09

PI17

PE19

PI15

PI16

PE07

PI04

PI03

PI14

PE18

PI13

VE06PE

06

VE05P

E05

PE16

PI12

PI10

PE17P

E15

PI11

PE02

VE02

PE03V

E03

Designed dewatering well

Additional dewatering well

Piezometer

N

SLC SCC1 SCC2 SCC3 SRC1 SRC2 SRC3

0 50

m

W06

Fig. 8. Layout of wells and piezometers

Nov-01 Feb-02 May-02 Aug-02 Nov-02 Feb-03 Aug-03

qona c d e h

0

100

200

300

400

500

600

700

May-03

Date

Dischargeflow:l/s

Recorded flowRepresentative flow for period

b f i j k l m p r s

Dewatering periods

Fig. 9. Total abstraction of groundwater by the dewateringsystem. A description of the dewatering schedule is provided inTable 4



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spacing less than 60 mm, suggesting a more permeablematerial.

Further evidence of an increased hydraulic conductivity inthe chalk at the retained cut end of the approach excavationwas provided by the core samples taken during the pre-construction site investigation. Fig. 11 compares the qualityof the core samples for two rotary drilled boreholes:SR5957, located within the supposed highly permeable zone;and SR5958, located on the exposed chalk outcrop to thesouth of the site. The locations of the boreholes are shownin Fig. 7. Caution must be exercised in assessing the state ofrock cores, because they are largely a function of the drillingmethod and the care taken by the driller during boring andextraction (Clayton et al., 1995). However, the total corerecovery (TCR) gives an indication of the presence ofnatural voids and is expressed as a percentage of the rockrecovered during a single coring run. The solid core recov-

ery (SCR), which is the percentage of full diameter corerecovered during a single coring run, gives an indication ofthe fracture state; a low SCR suggests a high degree offracturing.

Figure 11 suggests a high degree of fracturing in the top5 m of the chalk at SR5957. At this location the SCRgenerally appeared to increase, suggesting a decrease in thedegree of fracturing, with depth. Overall, the profiles suggesta greater degree of fracturing at the highly permeable zone(SR5957), particularly above 22 m OD, than at the chalkoutcrop (SR5958). This evidence was only assembled retro-spectively once the precise location of the high-permeabilityzone in the excavation had been identified and following adetailed review of the borehole logs. Borehole SR5957

proved to be the only borehole which was known to passthrough the high-permeability zone.

The highly permeable chalk lies at the extremity of theThames floodplain in the old river valley. An increase intransmissivity of chalk in valleys has been widely observed,most notably by Ineson (1962). Price (1987) (after Rhoadesand Sinacori (1941)) identified the influence of convergingflowlines towards the area of groundwater discharge and thedevelopment of secondary fissures due to dissolution bywater containing carbon dioxide as explanations for elevatedchalk hydraulic conductivity in valleys.

DEVELOPMENT OF THE NUMERICAL MODELA numerical model had been used in the dewatering

system design and was further used to develop the schemeas information became available following start-up. It was

recognised in the course of this process that the inhomo-geneities and extent of the variation in permeability withdepth identified were rather greater than anticipated at de-sign stage. The programme constraints for the works did notallow time for these conditions to be fully explored duringthe construction phase. However, the data set collected wasconsidered to be sufficiently comprehensive that there wasvalue in subsequently further developing the numerical mod-el to help understand the hydrogeology at the site and toinvestigate in more general terms the level of complexityrequired in analysis for the design of a large-scale dewater-ing system. In particular, the influence and effect of threelarge-scale features were investigated. These were

(a) a high degree of anisotropy of the surface chalk,thought to be responsible for unexpectedly low flowrates in the SLC+SCC1 cell

(b) a generally highly permeable zone, leading to unfore-seen high flow rates in the retained cut sections

(c) a transition zone between the highly permeable zoneand the anisotropic surface chalk, which led to the needto install the additional wells (W43 and W44) in SCC2.

Table 5 shows the four steady-state models investigated.Model 1 includes all three features and models 2 to 4 eachhave one feature omitted. For each model the hydraulicconductivities of the hydrostratigraphic units were varied,within the limits of the expected hydraulic conductivityranges, to find the best calibration (i.e. the best fit betweencalculated and observed piezometer levels).

The models were developed using Groundwater Vistas, aWindows interface for processing input and output files for

the three-dimensional finite difference code Modflow(McDonald and Harbaugh, 1988). The steady-state modelrepresented the dewatering period l (Fig. 9 and Table 4),when the dewatering system was yielding its maximum flow.This was a relatively stable period in terms of both flowsand drawdowns, as shown in Fig. 9.

Each model represented an area 2.5 km 3 2.5 km on planwith the excavation situated close to the centre. The gridwas oriented so that the diaphragm walls around the excava-tion were aligned with the x and y axes. Cells of varyingsize were used to allow more closely spaced nodal pointsinside the excavation (4 m by 4 m) than at the modelboundaries (33.33 m by 33.33 m) (Fig. 12). This gave anincreased sensitivity in the zone where the hydraulic gradi-

ents were greatest without using an impractically large num-ber of nodal points. The widths of adjacent cells did notdiffer by a factor of more than 1.5.

The vertical spacing of the layers of the three-dimensional

40035030025020015010050

SRC2 SRC3SCC3 SRC1

0

30

Distance from tunnel portal: m

25

2550

50 100

100 200

200 400

400 800

800

SCC2

Specific capacity: m /day2

0

Width:m

SLC SCC1

Fig. 10. Distribution of specific capacity inside the southern approach excavation, where dataare available (not to scale). Shaded area indicates the estimated extent of the highly permeablezone



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model was dictated by the need to be able to simulateapproximately the variable depth of the diaphragm walls andthe levels of the interfaces between each of the hydrostrati-graphic units. The base of the model, which was assumed to

be impermeable, was set at 65 m OD. Fig. 13 shows thediscretisation into horizontal layers, together with thestepped diaphragm wall profile. The diaphragm walls andcut-off cross walls were incorporated into the model using

the Modflow wall facility, as elements of thickness 1.2 mand hydraulic conductivity 1.13 107 m/s. Zones of poten-tially different hydraulic conductivity were incorporated intothe grid as indicated in Fig. 14. The chalk from 17 downto 26 m OD was split into an anisotropic surface chalkzone and a transition zone, as shown. It was assumed thatthe highly permeable zone followed the line of the chalkoutcrop and the old river valley. The effect of varying the

depth of the highly permeable zone was investigated duringthe modelling.The model boundary positions, shown in Fig. 12, should

be sufficiently far away from the excavation to exceed theestimated distance of influence (Lo), calculated usingSichardts empirical formula

Lo C hffiffiffiffiffiffiffi

k p

(3)

where h is the drawdown at the excavation and C is afactor of 3000 for radial flow. Assuming a drawdown of18 m and a Darcy hydraulic conductivity (k) o f 33 104

m/s, the estimated distance of influence (Lo) should be inthe order of 1000 m. However, measured drawdowns atpiezometers within this range suggested that the distance ofinfluence was rather greater than this. Therefore, the bound-ary heads in the gravel and chalk were set to 2 m OD or3 m OD, as shown in Fig. 12. As drawdown was notobserved, or expected, in the confining alluvium, the bound-aries surrounding this material (in model layers 1 and 2)were set at a head of 0.5 m OD. The Thames shoreline wasused to define the boundary position around the upper andright sides of the model.

Each boundary cell was assigned a hydraulic conductivityequal to kh of the surrounding aquifer material, althoughparametric analysis suggested that the model was insensitiveto changes in the boundary conductance.

Each dewatering well was represented by an analyticalwell in the model, with the well coordinates and the depthof well screen specified in accordance with reality. Fig. 15shows the recorded abstraction flow rates which were appliedto the wells and remained unchanged for each model run.The total system abstraction rate was 51 149 m3/day, or592 l/s for the model. The model was used to calculate thedrawdowns at 59 target locations, corresponding to theresponse zones of 11 internal, 16 external and 32 remotepiezometers installed in the field, and were compared withthose measured on site. Fig. 16 shows the mean measuredwater levels during the steady-state period for three examplepiezometers. The modelled and measured drawdown datasets were compared using the statistical methods shown inTable 6.

RESULTSModel 1, with all the large-scale features, gives a close fit

between the measured and modelled internal drawdowns(Fig. 17), and this model represented the overall hydrogeol-ogy convincingly (Fig. 18, and see Table 8 later in thissection). The modelling suggested that the hydraulic conduc-tivity of the terrace gravels (150 m/day) (Table 7) wasslightly lower than expected based on pumping tests (190400 m/day), but within the estimated range based on particlesize distribution (34306 m/day). Pumping tests are gener-ally considered to provide a more accurate estimate. In thiscase analysis of the pumping test data was complicated by

the fact that the gravel was in good hydraulic connectionwith chalk below.

To achieve the measured drawdown in the cellSLC + SCC1, an anisotropy ratio kh/kv 350 had to be

10080604020

36

32

28

24

20

16

12

0

Percentage

(a)

Depth:mOD

TCR SCR

30

26

22

18

14

10

6

2

0

Percentage

(b)

Depth:mOD

TCR SCR

10080604020

Fig. 11. The quality of chalk cores from boreholes (a) SR5957,in the highly permeable zone, and (b) SR5958, on the outcropchalk. Total core recovery (TCR) and solid core recovery (SCR)are shown



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applied to the surface chalk. Assuming a low anisotropyratio of just 1.5 for the surface chalk, a kh in the order of1.25 m/day was required to achieve the same drawdown inSLC + SCC1, as shown for model 4. Such a low horizontalhydraulic conductivity in the top 9 m of the upper chalk

could be explained by the presence of putty chalk (definedas structureless chalk that is devoid of factures) but little ifany thickness of this material was observed at this site. Toavoid a significant overestimation of drawdown along SCC2

in model 4, the hydraulic conductivity of the transition zonewas increased.

In model 1, the degree of anisotropy in the transition zoneis low (1.5), and the horizontal hydraulic conductivityslightly greater than the surface chalk. Omitting the transi-tion zone, as in model 2, overestimates the internal draw-downs in the cell SCC2 by up to 12.5 m at P104 (Fig. 17),and overestimates some external drawdowns. However, thefit to the remote drawdowns remains largely unaffected (Fig.18, Table 8), suggesting that anisotropy only has a signifi-cant influence on drawdowns in and immediately around theexcavation where the cut-off walls restrict horizontal flow.

In addition to the four steady-state models, a study of thedewatering of section SLC + SCC1 was used to confirm theanisotropy of the surface chalk unit. A transient model, withthe same mesh and conceptual hydostratigraphy as model 1,was constructed to simulate a period of dewatering inDecember 2001 when only this section was being dewatered.Records showed that a pumping rate of 20 l/s, evenlydistributed between wells W04 and W12, lowered the waterlevel in the standpipe piezometer PI01 to 9.2 m OD.Doubling the pumping rate to 40 l/s reduced the water levelin the piezometer to 18.5 m OD. A series of transientmodel runs was carried out with different degrees of aniso-tropy, where kh was constant at 35 m/day. All other hydro-stratigraphic units were assigned the hydraulic conductivities

of model 1, as listed in Table 7. Fig. 19 shows that thedrawdown inside the cell is highly sensitive to the degree ofanisotropy at ratios kh/kv between 10 and 1000. Interpolationof the data suggests that a ratio kh/kv of 350 to 750 is

Table 4. Description of the dewatering schedule and total system flow rates

Period Start date Description Approximatetotal flow: l/s

a 26-Nov-01 Wells in SLC + SCC1 commissioned 220b 27-Nov-01 Only wells W04 + W12 operational 20c 01-Jan-02 First pumping in SCC2 using wells W19 + W20 40d 11-Jan-02 Wells W21 to W30 started 200

e 18-Feb-02 Additional wells W43 + W44 started 235f 08-Mar-02 First pumping in SCC3 + SRC1 using wells W32 to W34 290g 19-Mar-02 All wells in SCC3 + SRC1 operational 370h 24-Mar-02 All wells in SCC3 + SRC1 stopped 235i 01-Apr-02 All wells in SCC3 + SRC1 operational 370

j 01-May-02 First pumping in SRC2 + SRC3 using wells W37 + W38 430k 21-May-02 Wells W39 to W42 started. Wells W41 + W42 then permanently decommissioned 470l 20-Jul-02 Additional wells W45 to W54 started 590m 13-Sep-02 Wells W47 to W54 stopped, except W52 500n 18-Oct-02 Wells W45 and W46 stopped 430o 07-Nov-02 Additional wells W55 to W58 commissioned 500

p 17-Nov-02 All wells in SRC2 + SRC3 operational 580q 12-Mar-03 Gradual decline in flow rate from SRC2 + SRC3 as dewatering operations are scaled down to allow

drawdown recovery580 to 490

r 10-May-03 Increase in pumping in SRC2 + SRC3, to prevent temporary leakage of base slab 550s 22-May-03 Decline in pumping rate and start of gradual system decommissioning 550 to 370

Table 5. Features included in the groundwater models

Strong anisotropy ofsurface chalk

Isotropic highlypermeable zone

Transition zone

Model 1 4 4 4Model 2 4 4 3Model 3 4 3 4Model 4 3 4 4

5 10 15 20 25 30 35 40 455055 60 65 70 75 80

110

105

100

95

90

858075706560555045403530

25

20

15

10

5

Boundary head2 m OD

Boundary head2 m OD

Reduced rowspacing

Reduced columnspacing

Boundary head3 m OD

Fig. 12. The reduced spacing of cells around the excavation andthe boundaries for the model layers 3 to 12



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appropriate for the chalk in this location, giving

0.047< kv < 0.1 m/day. This is consistent with the level ofanisotropy applied in model 1. The models were largelyinsensitive to any anisotropy of the base chalk over the samerange.

It is clear from the analysis that anisotropy can have animportant influence on drawdowns inside excavationsbounded by cut-off walls. In this case the effect is particu-larly pronounced as the diaphragm walls extend into the lesspermeable base chalk, so that horizontal flow below the toe,as well as flow around the toe, is limited. It is possible thatthe anisotropy is largely determined by the orientation offractures, as suggested by Toynton (1983) for Norfolk chalk.Bedding-parallel fractures, related to the deposition of thechalk, occur at an angle approximately equal to the mean

plane of dip, and trace lengths can persist across the fullextent of the formation. In contrast, the trace length ofbedding-normal fractures is typically less than 1 m (Younger& Elliot, 1995). Warren & Mortimore (2003) show that thebedding planes at this location are near horizontal. Solutiondevelopment of such fractures may make anisotropy morepronounced, particularly if the connectivity between bed-ding-parallel fractures (by way of intersecting bedding-nor-mal fractures) is low.

The remote drawdowns are highly sensitive to the hydrau-lic conductivity of the highly permeable zone, which dom-inates most of the hydrogeological system. In model 1 theisotropic hydraulic conductivity of this feature is estimatedto be approximately 4800 m/day. Model 3, which omits the

highly permeable feature completely, significantly overesti-mates drawdowns centred on the retained cut sections by upto 16 m at PI08. Excessive drawdowns in this area causeincreased drawdowns throughout the excavation (Fig. 17).

While it is possible to determine the approximate width of

the highly permeable zone using the specific capacity datafrom individual wells (Fig. 10), the depth of the zone is lessclear. The base of the highly permeable zone must extend toabout 20 m OD because it was not cut off by the dia-phragm wall that reached to 19 m OD at SRC1 and 17 mOD at SRC2. There is some indication that the chalk corequality at borehole SR5957 improves below 30 m OD (Fig.11), implying that this may signify the base of the high-permeability zone at this location. Fig. 20 shows that varyingthe thickness of highly permeable material while maintainingthe same overall transmissivity has a minimal impact on thefit of the model. For these model runs the vertical hydraulicconductivities were unchanged from those listed in Table 6for model 1. The effect on the overall model fit is shown in

terms of the sum of squares of residuals Sr, as defined inTable 6. If a residual of 0.5 m is acceptable for each of the60 piezometers, Sr should not exceed 15 for a close modelfit, although this assumes that the residuals are distributedevenly. For the base case (model 1) Sr 9.95 and for eachof the model runs in Fig. 20 Sr, 15. The best fit wasachieved using a depth of 23 m OD for the highly per-meable zone, but this was inevitable given that the hydraulicconductivies of the other zones were calibrated using thisconfiguration; improved fits for the other configurationscould be achieved if small changes were made to thehydraulic conductivity of the terrace gravels, for example.The internal drawdowns at standpipe piezometers PI07 andPI08 show a maximum variation of 0.9 m for the different

zone configurations, and the mean residual for all the 60modelled piezometers varies by just 0.38 m.

This analysis suggests that the hydraulic conductivity ofthe highly permeable zone is in the range 23005850 m/day

220210200190180170160150140130120110100908070600 10 20 30 40

12

123

45

6

7

8

910

11

123

4

56789

10

11

Modellayer

Bottom elevation:m OD4404304204104003903803703603503403303203103002902802702602502402302202102009121719

212324252629

35

65

SCC3 SRC1SRC2 SRC3

Distance from headwall: m

9121719

21

23

24

25

2629

35

65

SLC SCC1 SCC2

Modellayer

Distance from headwall: m

Bottom elevation:m OD 50

12

Fig. 13. Layer discretisation and the stepped diaphragm wall profile



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2500 m

(b)

AA65

262317

129

3

Depth: m ODExcavation (445 m)

Alluvium

Thamesboundary

Base chalk

Outcrop chalk

Surface chalk

HPZ

Cut-off walls

Tunnel portal

Transition zone

Terrace gravels

Surface chalk

Thames

A

Surface chalk

Tunnel portalHPZ

Transitionzone

Thames

A

(a)

HPZ

Surface chalk

0 500

m

Fig. 14. (a) Plan (217 to 223 m OD) and (b) cross-section(AA) showing the hydrostratigraphic zones of the model(HPZ denotes the highly permeable zone)

Jul-03

Apr-03

Jan-03

Oct-02

Aug-02

May-02

Feb-02

8

6

4

2

0

2

Nov

01

Date

Waterlevel:mOD

0

100

200

300

400

500

600

700

Totalflow:l/s

SA5945 chalk

SA5945 gravel

SA5981 gravelTotal flow

Mean waterlevels formodelledperiod

Fig. 16. Example piezometer hydrographs. Target drawdownsare shown for the steady-state period from 10 August 2001 to 13September 2002

0

5

10

15

20

25

30

0

Measured well flow: l/s

Modelledwellflow:l/s

SLC SCC1

SCC2

SCC3 SRC1

SRC2 SCR3

30252015105

Fig. 15. Comparison of modelled well flows with those applied

in all four models

Table 6. Statistical parameters used to compare the modelled and measured drawdown data sets, where xi is the measured and yi isthe modelled drawdown

Statistical parameter Calculation Comments

Mean residual error, MM

1

n

Xni1

(xi yi)Ideal value is 0.0 m. Negative values indicatethat model underestimates water levels;

positive values indicate an overestimation.Negative and positive residuals can canceleach other.

Absolute mean residual error,MA

MA 1

n

Xni1

jxi yijIdeal value is 0.0 m.Givesindication of modelfit irrespective of negative and positive signs.

Gradient, b, from linearregression analysis

b

XxyXx2

Used to plot trendline on scattergram in theform y bx, where b 1 is the ideal value.

Product moment correlationcoefficient, R

R nX

xy X

xX

y

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffinXx2 Xx 2 nXy2 Xy z2h isR +1 is a perfect positive correlationbetween modelled and measured water levels.

Sum of squares of residuals, SrSr

Xni1

(xi yi)2 Idealvalueis 0.0. Usedfor sensitivityanalysis.

Not influenced by sign of errors.



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(2.663 102 to 6.773 102 m/s), implying that the zoneconsists of karstic chalk or a network of enlarged fissures.Karstic behaviour of the chalk is characterised by high-velocity flow through dissolution pipes, which provide path-ways for the rapid transport of water (Banks et al., 1995;MacDonald et al., 1998). Significant conduits are commonin the area; during a regional hydrogeological study of thechalk, such features were observed in a chalk quarry that

lies 2 km to the southwest of the site. Reeves (1979) definedenlarged fissures as having an aperture greater than 3 mmand a spacing from 2 to 20 m, which can give a bulkhydraulic conductivity of the order of 1000 m/day(13 102 m/s). Examination of chalk cores from boreholeSR5957 suggested a fissure spacing less than 60 mm withapertures greater than 3 mm, which is consistent with theeven greater hydraulic conductivities apparently identifiedhere by the modelling studies.

A formal sensitivity analysis for model 1 confirms thefindings already discussed. For the model 1 base case, theparameter values in Table 7 were used. The impact of each

4003503002502001501005030

25

20

15

10

5

0

0

Distance from tunnel portal: m

Waterlevel:mOD

Recorded level

Model 1

Model 2

Model 3

Model 4

W18

Piezometer

W14

W01

PI04a

PI15

W31

PI06

PI21

PI07

PI22

PI08

PI09

Fig. 17. Drawdown profile inside the excavation for each of themodels. The recorded levels are shown with error bars of 60.5 m

0510152025

30

25

20

15

10

5

030

Measured water level: m OD

Modelledwaterlevel:mOD

Internal piezometers

External piezometers

Remote piezometers

051015202530



30

25

20

15

10

5

0

Internal piezometers


Remote piezometers

051015202530



051015202530



(c) (d)

30

25

20

15

10

5

0Internal piezometers


Remote piezometers

30

25

20

15

10

5

0Internal piezometers


Remote piezometers

(a) (b)

Fig. 18. Comparison of measured and modelled piezometric heads for each model. The solid line indicates the ideal correlation andthe dashed lines represent an error of60.5 m. (a) Model 1; (b) model 2; (c) model 3; (d) model 4



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hydraulic conductivity zone was investigated in turn byvarying the hydraulic conductivity parameter, while main-

taining the other parameters unchanged. The relative sensi-tivity of the model to each of the parameters is shown bythe gradient of the sensitivity curve (Fig. 21); the drawdownis influenced mainly by the horizontal hydraulic conductivity

of the highly permeable zone but is insensitive to thehorizontal hydraulic conductivity of the base chalk. The

anisotropy of the surface chalk, transition and base chalkzones influence the model fit, as explained previously inrelation to the internal drawdown. The apparent insensitivityof the model to the hydraulic conductivity of the outcrop

Table 7. Values of hydraulic conductivity (m/day) for each of the models, where kh ishorizontal hydraulic conductivity and kv is the vertical hydraulic conductivity

Hydrostratigraphicunits

Model 1 Model 2 Model 3 Model 4

kh kv kh kv kh kv kh kv

Alluvium 0.1 0.01 0.1 0.01 0.1 0.01 0.1 0.01Terrace gravels 150 150 150 150 150 150 150 150

Surface chalk 35 0.1 35 0.1 35 0.1 1.25 0.83Transition zone 55 36.5 35 0.1 55 36.5 90 57Outcrop chalk 50 5 50 5 50 5 50 5Highly permeablezone

4800 4800 4800 4800 55 36.5 4800 4800

Base chalk 2 0.02 2 0.02 2 0.02 2 0.02

Table 8. Summary of statistical data for the model simulations (M is mean of residual errors, MA is the mean of absolute residualerrors and R is the correlation coeffeicent, as defined in Table 6)

Internal External Remote Overall

Number of piezometers 12 16 32 60

Model 1 M: m 0.10 0.10 0.07 0.04MA: m 0.31 0.28 0.34 0.32Linear relationship y 0.988x y 1.012x y 1.013x y 1.000x

R +0.99 +0.94 +0.97 +0.99Model 2 M: m 3.92 0.84 0.07 0.90

MA: m 3.93 1.12 0.34 1.20Linear relationship y 1.326x y 1.106x y 0.987x y 1.176x

R +0.86 +0.84 +0.97 +0.94Model 3 M: m 10.73 8.13 3.92 6.40

MA: m 10.73 8.13 3.92 6.40Linear relationship y 1.715x y 1.923x y 1.764x y 1.778x

R 0.28 +0.63 +0.92 +0.82Model 4 M: m 0.21 0.81 0.57 0.55

MA: m 0.50 0.81 0.65 0.67

Linear relationship y 1.001x y 1.090x y 1.104x y 1.053xR +0.97 +0.84 +0.93 +0.96

100000010000010000100025

20

15

10

5

0

100

Anisotropy ratio, /k kh v

WaterlevelinPI01:mOD

25

20

15

10

5

0

WaterlevelinPI01:mOD

20 l/s

40 l/s

Recorded water level at 40 l/s

Recorded water level at 20 l/s

Fig. 19. The drawdown at the standpipe piezometer PI01 fordifferent anisotropy ratios of the surface chalk, when extractinggroundwater at two pumping rates from the cell SLC + SCC1

3529262312

10

8

6

4

2

0

2

4

21

Depth of base of high permeability zone: m OD

W

aterlevel:mOD

0

10

20

30

40

50

60

70

80

Sumof

squaresofresiduals,Sr

Sum of squared residuals

Mean residual (M) for all piezometers

Water level at PI07 (recorded level 89 m OD)

Water level at PI08 (recorded level 88 m OD)

Fig. 20. The model fit and internal drawdown in the retainedcut sections for different thicknesses of the highly permeablezone. The transmissivity of the effective aquifer is constant foreach configuration



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chalk means that the model could be simplified by treatingthis zone as an extension of the surface chalk zone with thesame hydraulic conductivity values. The terrace gravels andhighly permeable zone would be expected to be substantiallyisotropic in reality, and applying an anisotropy ratio of up to50 has a negligible impact on the overall model fit.

In the models presented in this paper it has been assumedthat the large variation in transmissivity through the excava-

tion is due to inhomogeneities in the surface chalk (17 to26 m OD), and that the terrace gravels and base chalkzones are homogeneous. It is possible that a good, or evenbetter, calibration could be achieved by a combination ofincreasing the hydraulic conductivity of the terrace gravelstogether with a counteracting decrease in the horizontalpermeabilities of the underlying chalk zones. In view of thenumber of hydrostratigraphic zones and parameters it is notpossible to find a unique solution, and this represents alimitation of this type of model. However, the relativedifferences in the hydraulic conductivity of the surface chalkmust still apply approximately, as demonstrated by thesimulations summarised in Fig. 22.

CONCLUSIONS AND IMPLICATIONS FOR PRACTICEThe sensitivity analysis presented here suggests that for a

mildly complex scheme, a numerical modelling approach isunlikely to provide a unique solution giving the hydraulicconductivity profile of each stratum zone identified. Whilesensitivity analysis shows that the numerical modelling ap-proach cannot be used to estimate uniquely the hydraulicconductivity of each of the zones, it does provide a usefulframework for assembling and analysing complex data sets.In this case the performance of the dewatering systemcannot be satisfactorily explained without the inclusion in

0

50

100

150

200

250

300

0

Parameter multiplier

(a)

Sum

ofsquaresofresiduals,Sr Terrace gravels

Surface chalkTransition zoneHigh-permability zoneOutcrop chalkBase chalk

Acceptableerror level

0

50

100

150

200

250

300

1

Anisotropy, /k kh v

Terrace gravelsSurface chalkTransition zoneHigh-permeability zoneOutcrop chalkBase chalk

20181614121008060402

(b)

10000100010010

Sumofsq

uaresofresiduals,Sr

Acceptableerror level

Fig. 21. Sensitivity curves for model 1: (a) horizontal hydraulicconductivity (kh) is varied from base case (Table 7) by themultiplication factor; (b) vertical hydraulic conductivity (kv) isvaried while kh is kept at the base case value to give a variableanisotropy ratio

TG SC TZ HPZ

1

TG SC TZ HPZ

2

TG SC TZ HPZ

3

TG SC TZ HPZ

4

TG SC TZ HPZ

5

100

150 2

00 2

50

300

40

35

30

25

20

60 55

50

45

40

5200

4800

4400

4000

3600

1

10

100

1000

10000

Simulation

kh:m/day

0

20

40

60

80

100

Sumofsqu

aresofresiduals,Sr

Sum of squares of residuals

Fig. 22. Horizontal hydraulic conductivities (kh) used to achieve good calibrations of model 1 (i.e. Sr < 15). khof the terrace gravels (TG), surface chalk (SC), the transition zone (TZ) and the highly permeable zone(HPZ) have been varied, while the outcrop chalk, base chalk and alluvium were kept constant at the basecase (Table 5). Anisotropy was applied at the same ratios as used for the base case



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Delivered by ICEVirtualLibrary com to:

the model of large-scale inhomogeneities within a 445 mlong excavation including

(a) a zone of anisotropic chalk with a ratio kh/kv of 350750

(b) a zone of isotropic highly permeable chalk with akh kv of 2.663 10

-2 to 6.773 102 m/s(c) a transition zone between the anisotropic chalk and

highly permeable chalk.

Modelling shows that anisotropy (kh/kv) has a significantimpact on groundwater flows into the excavation bounded bycut-off walls and therefore needs to be considered carefullyduring the design of a dewatering system.

It is common practice for modelling studies to be carriedout during the design of a dewatering system. However, thesuccess of the design is strongly dependent on the concep-tual understanding of the geology and the accuracy of theinput parameters. Generally, the input parameters are as-sumed to be representative of the site as a whole, but theimportance of local variations should not be underestimated.Features such as highly permeable zones may be identifiedby more thorough site investigation, including the more

widespread use of pumping tests. However, the influence ofanisotropy is harder to establish, primarily because it has itsmost significant effect once a horizontal flow barrier is inplace, which is generally not the case during site investiga-tion. The cost benefits that result from achieving an im-proved optimisation of a dewatering scheme design must beweighed against the increased costs of more comprehensivesite investigation and interpretation. If there is sufficientflexibility in the construction schedule, modern methods ofdata collection and review make it possible to take anobservational method approach, as implemented at theCTRL Thames tunnel, with success.

ACKNOWLEDGEMENTSThe Thames tunnel permanent works were designed by

Rail Link Engineering for client London and ContinentalRailways. The main contractor was Hochtief Murphy JointVenture. The specialist dewatering contractor was WJGroundwater Limited. The study described in this paper wasfunded through the engineering doctorate programme of theEngineering and Physical Sciences Research Council in col-laboration with WJ Groundwater Limited and the Environ-ment Agency.

NOTATIONC factor in Sichardts formulah2/h1 ratio of mean tidal amplitudes of two piezometers

kh horizontal hydraulic conductivitykv vertical hydraulic conductivity

Lo distance of influenceR correlation coefficientS storage coefficient

Sr sum of squares of residualsT transmissivityto tidal period

x2 x1 horizontal distance between two piezometersxi measured water levelyi modelled water levelh drawdown

REFERENCESBanks, D., Davies, C. & Davies, W. (1995). The chalk as a karstic

aquifer evidence from a tracer test at Stanford-Dingley,Berkshire, UK. Q. J. Engng Geol. 28, No. S1, S31S38.

BGS (British Geological Survey) (1997). Geological map of theLower Thames Valley. London: British Geological Survey.

Clayton, C. R. I., Matthews, M. C. & Simons, N. E. (1995). Siteinvestigation. Cambridge: Blackwell Science.

Ferris, J. G. (1951). Cyclic fluctutaions of water level as a basis for

determining aquifer transmissivity. Int. Assoc. Sci. Hydrol. 33,148155.

Gibbard, P. L. (1994). The Pleistocene history of the Lower Thamesvalley. Cambridge: Cambridge University Press.

Ineson, J. (1962). A hydrogeological study of the permeability ofChalk. J. Inst. Water Eng. 16, 449463.

Leiper, Q., Roberts, T. & Russell, D. (2000). Geotechnical engineer-ing for the Medway Tunnel and approaches. Proc. Instn Civ.

Engrs, Transport 141, No. 1, 3542.Lord, J. A., Clayton, C. R. I. & Mortimore, R. N. (2002). Engineer-

ing in chalk, CIRIA Report C574. London: Construction Indus-try Research and Information Association.

MacDonald, A. M., Brewerton, L. J. & Allen, D. J. (1998).Evidence for rapid groundwater flow and karst-type behaviour inthe Chalk of southern England. In Groundwater pollution,recharge and vulnerability (ed. N. S. Robins), pp. 95 106.

London: Geological Society.McDonald, M. G. & Harbaugh, A. W. (1988). A modular three-

dimensional finite-difference groud-water flow model. Reston,Virginia: United States Geological Survey.

Marsland, A. & Randolph, M. F. (1978). A study of the variationand effects of water pressures in pervious strata underlyingCrayford Marshes. Geotechnique 28, No. 4, 435464.

Marsland, A. (1986). The flood plain deposits of the Lower Thames.Q. J. Engng Geol. Hydrogeol. 19, No. 3, 233247.

Powrie, W. & Roberts, T. O. L. (1995). Case history of a dewateringand recharge system in chalk. Geotechnique 45, No. 4, 599609.

Preene, M., Roberts, T. O. L., Powrie, W. & Dyer, M. R. (2000).Groundwater control: design and practice, CIRIA Report C515.London: Construction Industry Research and Information Asso-ciation.

Price, M., Downing, R. A. & Edmunds, W. M. (1993). The Chalkas an aquifer. In The hydrogeology of the chalk of North-West

Europe (ed. R. A. Downing), pp. 3558. Oxford: ClarendonPress.

Price, M. (1987). Fluid flow in the Chalk of England. In Fluid flowin sedimentary basins and aquifers, Geological Society SpecialPublication No. 34, pp. 141156. London: The GeologicalSociety.

Reeves, M. J. (1979). Recharge and pollution of the English chalk:some possible mechanisms. Engng Geol. 14, No. 4, 231240.

Rhoades, R. G. & Sinacori, B. W. (1941). Patterns of groundwaterflow and solution. J. Geol. 49, No. 8, 785794.

Roberts, T. O. L. & Preene, M. (1990). Case studies of constructiondewatering in chalk. Chalk pp. 571575. London: ThomasTelford.

Spink, T. W. (2002). The CIRIA Chalk description and classificationscheme. Q. J. Engng Geol. Hydrogeol. 35, No. 4, 363369.

Toynton, R. (1983). The relation between fracture patterns andhydraulic anisotropy in the Norfolk Chalk, England. Q. J. EngngGeol. 16, No. 3, 169185.

Warren, C. D. & Mortimore, R. N. (2003). Chalk engineeringgeology Channel Tunnel Rail Link and North Downs Tunnel.Q. J. Engng Geol. Hydrogeol. 36, No. 1, 1734.

White, J. K. & Roberts, T. O. L. (1993). The significance ofgroundwater tidal fluctutations. In Groundwater problems inurban areas (ed. W. B. Wilkinson), pp. 3142. London: ThomasTelford.

Younger, P. L. & Elliot, T. (1995). Chalk fracture system character-istics: implications for flow and solute transport. Q. J. EngngGeol. 28, S39S50.


influence of large scale inhomogeneities on a construction dewatering system in chalk - bevan et al

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