Application of electrical resistivity tomography to leak detectionin a geomembrane at A55 Conwy Tunnel, North Wales

D. Nichol NEWTRA Geotechnical Group, Wrexham, UK

J.K. Ferris & J.M. Reynolds Reynolds Geo-Sciences Limited, Mold, UK

Keywords: case history, electrical resistivity, geomembranes, geophysics, highways, leak detection, tunnels

ABSTRACT: At Conwy Tunnel on the A55 North Wales Coast Road, leaks developed in a high-density polyethylene (HDPE) geomembrane surrounding the cut-and-cover approach to the western portal of the tun-nel. The geomembrane forms an impermeable barrier some 1-7 m below the ground surface that protects the highway from ingress of saline water from the tidal estuary of the River Conwy. The subsurface leaksprompted concerns about the extent of the problem and the potential to trigger instability of the cutting slopesand so electrical resistivity tomography was employed to determine the distribution of the leaks. A total of 7traverse lines, up to 73 m long and 2.5-5.0 m apart were deployed along the northern cutting slope. Resistivitymeasurements were made using a Geopulse Tigre 64 resistivity meter controlled by Campus ImagerPro 2000software. An inter-electrode spacing of 1 m and a roll-on of between 10 m and 22 m were adopted to provide geoelectric profiles to nominal depths of 7.7 m. The apparent resistivity pseudosections were processed usingan inversion software package to generate stable models of true depths and formation resistivities. The finalpseudosections confirmed that the geomembrane behaves as a laterally consistent resistor (resistivity 190,000

m) and the stacked models of true resistivity revealed sharp anomalies associated with the geomembranealong certain traverse lines. The anomalies feature relatively low resistivity values of around 600-1000 mthat appear in marked contrast to the very high resistivity of the geomembrane. They also differ in signaturefrom the membrane cover-materials which have typical resistivities of 100-300 m. The anomalies were in-terpreted to correspond to possible breaches in the hydraulic integrity of the geomembrane. In this way, thegeophysical survey pinpointed 3 leaks beneath the lower slope of the cutting. Photographs taken during con-struction revealed that leak positions coincide with patch points where crease-induced sagging of the ge-omembrane had caused stretching. This case history illustrates the benefits of using electrical resistivity to-mography to gain an understanding of leak distribution for geomembranes in engineered areas where non-intrusive investigations are required to provide reassurances in relation to public safety.

1 INTRODUCTION

Crossing the estuary of the River Conwy presented a formidable challenge along the route of the new A55 North Wales Coast Road and a tunnel solution was adopted (Fig. 1). Opened in 1991, the Conwy Tunnel measures 1090 m in length and has the distinction of being the first immersed tube tunnel constructed in the UK (Nichol, 2001). The immersed tube segments were prefabricated as six 118 m long elements each containing both carriageways in a single monolithic structure with a single dividing wall. Approximately 3.5 Mm

3 of alluvium was dredged from a trench

across the estuary bed. The giant reinforced concrete culverts were floated into position, ballasted down into the dredged trench and mated together at their gasket joint faces. Sand jetting beneath the units en-sured a permanent foundation and then graded back-

fill was placed around the sides and top. The tunnel is approached by cut-and-cover sections to the east and west (Fig. 2).

At the west portal area (National Grid Reference SH 776785), the cut-and-cover and the approach cut-ting incorporate an impermeable HDPE geomem-brane (Carbofol CHD2). Around 1997, seepages and ponding of water behind the retaining wall were first noticed but thought to be associated with blocked drains. A monitoring programme was instigated to determine the source of the problem but, by 2001, uncontrolled water flows onto the carriageways be-came evident and prompted concerns about the in-tegrity of the membrane, the possibility of slope in-stability and the potential threat posed to the highway (Fig. 3).

The cutting slopes are planted with a wide variety of trees, shrubs and wildflowers and the site has won

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Proceedings ISC-2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)© 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

Figure 1. Location of Conwy Tunnel in North Wales, UK.

awards from the Welsh Assembly Government for its landscaping. The geophysical surveys were car-ried out in areas of dense growth that include black thorn, hawthorn, hazel, gorse, willow and pine and so field operations were confined to narrow path-ways cleared by landscaping contractors.

The use of electrical resistivity tomography (ERT)techniques has become well established for leak de-tection in geomembranes associated with landfill sites (eg Reynolds and Taylor, 1993). However, as far as can be determined, the use of resistivity tech-niques for highway engineering purposes appears exceedingly rare. In this paper, non-invasive ERT

investigations were carried out to determine the ex-tent and distribution of geomembrane leaks within a major road cutting and also to gain a better under-standing of the potential for leak-induced ground movements.

2 GEOLOGICAL SETTING

The valley of the River Conwy cuts into bedrock of Ordovician and Silurian strata. It also contains het-erogeneous glacial deposits associated with the last glaciation affecting the area during Late Pleistocene (Devensian) time. Characterization of the materials

Figure 2. Western cut-and-cover approach to Conwy Tunnel.

Figure 3. North cutting slope on the western approach to Conwy Tunnel. Seepage onto the road is indicated by the ar-row.

across the river channel at the tunnel site disclosed extensive soft, interstratified alluvial deposits over-lying heterogeneous glacial tills (Davies et al, 1989). Although soft alluvial deposits and mixed glacial tills underlie much of the tunnel area, made ground predominates at the western cut-and-cover approach.

The made ground consists of engineered fill placed under controlled conditions. The sequence of construction involved excavation of natural ground and then placement of Class 1 fill to form the cutting slopes. Layers of sand were used to facilitate laying of the HDPE geomembrane; the under-layer to pro-vide a smooth and even surface for positioning and manipulating the membrane itself and the over-layer to protect the membrane from piercing during sub-sequent filling operations. Class 1A fill was placed on the top until the final profile was attained (gradi-

1148 © 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

ent 1:3). The fill materials were variable and in-cluded clays, sands and gravels. Finally, the slopes were topsoiled, planted and seeded.

3 ELECTRICAL RESISTIVITY TOMOGRAPHY SURVEY

In ERT applications, an array of 50 or more elec-trodes is implanted on the ground surface. Each elec-trode is connected via a multi-core cable to a switch box, resistivity meter and controlling computer. Each set of 4 adjacent electrodes for a given elec-trode separation is scanned automatically and a value of resistance obtained. The data are plotted us-ing special software as a pseudosection which is then processed to form a 2-dimensional electrical image. This depicts the lateral and vertical variation in true resistivity as a function of depth against posi-tion along a survey line. Where correlation of mate-rial types and true resistivity are justified, pseudo-sections can be used to define the spatial limits of specific materials. Consequently, anomalous ground will be evident on the corresponding pseudosection. Reynolds (1997) describes this technique in greater detail.

The ERT survey employed a Geopulse Tigre 64 resistivity meter, which was controlled using Cam-pus ImagerPro 2000 software. While the meter re-cords values of resistance, the software produces values in terms of resistivity in units of Ohm.m ( m).

Seven closely spaced lines were set out with ropes along the strike of the slope (Fig. 4). The spac-ing between the lines was 2.5 m on the lower slope in order to achieve a higher resolution and 5.0 m on the upper slope where water was less likely to in-gress. An electrode separation of 1 m applied throughout and each line comprised a single traverse covering 63 m and a roll-on of between 10 m and 22 m. Line 1 was extended by 10 m. Resistivities were measured down to 14 levels on most lines but only

Figure 4. Section through the cutting slope showing the posi-tions of the survey lines.

12 levels on Line 1. The corresponding nominal depths for 14 and 12 levels are 7.7 m and 6.4 m re-spectively. These exceed the depth to the membrane.

Before each traverse or roll-on was carried out, an electrode test was performed to ensure that the con-tact resistances were acceptable. During this process, electrodes on lines near the top of the slope dis-played the highest contact resistances, typically ex-ceeding 2500 . In order to reduce all the contact re-sistances to below 1500 , saline water was used to wet the immediate area surrounding the electrodes. For consistency, the same watered-in procedure was adopted for lines on the mid-slope, although here, contact resistances typically fell below the 1500 threshold. Following data acquisition on each line, contact resistances were re-checked to verify that they were unaffected by drying-out.

3.1 Data analysis

Over 98% of the data points sampled have repeat-ability errors of less than 1%. Of the remaining data, less than 0.1% have repeatability errors larger than 1.5%. However, the majority of the data with repeat-ability errors larger than 1% originated from near surface readings where the presence of roots and variable watering conditions were the probable causes.

Setting-out errors accrued along certain lines where tape measurements along straight lines were impeded by vegetation. Such errors are few in num-ber and are manifested on the pseudosections by un-dulations on otherwise flat resistivity contours. In addition, survey lines were laid out using the con-crete wall at the bottom of the slopes as a baseline and this method produced a slight skew to the lines in relation to the slope. This resulted in the resistive effect associated with the membrane appearing slightly deeper in the west and shallower in the east.

3.2 Data presentation

The measured apparent resistivity pseudosections provided an approximate image of variations in elec-trically resistivity for materials within the plane of the section. Further analysis of the apparent resistiv-ity data was performed using an inversion routine (RES2DINV) which converted the images of appar-ent resistivity into models of true resistivity that are physically diagnostic of the materials present (Fig. 5).

For finite difference modelling purposes, the number of iterations used to attain the stable models varied between two and six. Also, a standard flatness filter ratio of 1.0 was employed to avoid introducing any bias to the models (where 0.5 optimises horizon-tally elongated features and 2.0 gives a higher weight to vertical features). In addition, a fine mesh size was used to cope with the high apparent resis-tivity contrasts.

1149Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)

3.3 Background information

The geomembrane would be expected to behave as

an infinite resistor. Where its physical integrity is in-

tact, current should not leak around it and apparent

resistivities should steadily increase in value with

depth. The Class 1 fills were thought to have a resis-

tivity of >200 m but potentially significantly lower

in the presence of salt water (typical resistivity of

sea water is <0.3 m). The layers of sand on either

side of the membrane were considered unlikely to be

resolve discretely as they have resistivities values

that are intermediate between the membrane and the

overlying Class 1A fill. In addition, the membrane

itself cannot be resolved on resistivity images due to

its thinness (~4 mm). The properties of the overlying Class 1A fill are

difficult to predict due to heterogeneity. However, the material evident in the near surface zone consists predominantly of silty, sandy till (boulder clay). It also appears to vary from clay-rich material at the bottom of the slope to gravels and sands near the top. Accordingly, values of resistivity were expected to lie between 100 m and 300 m, and this is sup-ported by a single measured value of ~120 m.

3.4 Computer model of a leak

A computer model was prepared to attempt to pre-dict the form of a resistivity anomaly that might be expected associated with a leak (Fig. 6). The model consisted of three layers: the uppermost layer repre-sents the Class 1A fill (resistivity 300 m); a middle layer representing the resistive membrane (resistivity 190,000 m); and a lowermost layer representing sand saturated with brackish water (resistivity 30

m). Emergent brackish water above the membrane resulted in a reduction in the resistivity values and a depression formed in the otherwise flat contours that image the effects of the membrane. The observed asymmetry of the anomaly had the strongest gradient over the position of the leak. Accordingly, the pre-cise position of the leak is to be identified on the up-stream side of the asymmetric anomaly.

However, it is noteworthy that the model does not permit the simulation of spontaneous potential ef-fects (cf Line 4).

4 DISCUSSION

4.1 General pattern

The overall pattern comprises variable and low resis-

tivities on the surface due to the effects of the trees

and roots and increasing values with depth, up to

2700 m, due to the resistive nature of the mem-

brane. The modelled depths for this effect of the

membrane vary from 3 m to 6.5 m.

In addition, a general trend exists of decreasing resistivity values from Line 0 at the top of the slope to Line 6 at the bottom. This reflects the distribution of moisture in the slope and the higher proportion of clay within the soils on the lower slopes.

4.2 Individual discrete anomalies

On Line 0, the effects of the resistive membrane are evident at modelled depths of around 1.0 to 1.4 m and appear consistent with the actual depth to the membrane. Lower values below this horizon are taken to indicate current leakage around the upper edge of the membrane at the top of the slope.

On Lines 1-3, no evidence exists of current leak-age. However, at the eastern end of all three lines, the resistivity contours fold over in a nose-shaped pattern that may represent a localised edge-effect.

On Line 3, two discrete high-resistivity anomalies occur within the upper Class 1A fill layer. Based on the size of the anomalies they possibly represent pockets of fill materials of different composition to the bulk of the layer. Another anomaly above the membrane is evident at the westernmost end and this too may be a pocket of different composition. How-ever, its combined effect with the membrane creates the appearance of a resistive hump. Importantly, the profile indicates that the membrane is intact along the entire length of this line. On Line 4, a resistive feature similar but smaller to that on Line 3 is evident at the westernmost end. It also shows the effects of the membrane as a laterally consistent resistor except for a minor reduction in the resistive values at depth towards the eastern end, due possibly either to slight current leakage or to a localized edge-effect. In the west-central part, a sharp low-angle anomaly exists at depth and is paired with a corresponding shallow high. This may be a spontaneous potential effect produced by an upward flow of saline water through a minor leak in the membrane because, within only 5 m laterally there is a drop and rise again of approximately 1000

m, a substantial gradient. More detailed informa-tion on spontaneous potential anomalies is given by Reynolds (1997).

Lines 5 & 6 appear significantly different to the previous lines. Both lines display three major breaks and one minor anomaly affecting the consistency of the resistive geomembrane and good correlation ex-ists from one line to the other.

4.3 Interpretation

Comparison of the anomalies on Lines 5 and 6 with the findings of the computer model enabled the loca-tions of possible leaks to be pinpointed (Fig. 7).

1150 © 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

Figure 5. Stacked resistivity models of Lines 0 to 6.

1151Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)

Figure 6. Resistivity model.

1152 © 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

Figure 7. Schematic plan of geophysical anomalies and surface features.

The geophysical interpretation of possible leak positions also corresponds well with the known area of ponded water behind the wall and with the loca-tions of seepage in the highway verge.

4.4 Possible association with known geomembrane creases

During installation of the geomembrane, a series of progress photographs were taken to record the se-quence of operations and these were studied to de-termine the background history at the leak positions. In July 1990, at an advanced stage in the operations, a major crease formed in the membrane and is de-picted in one of the site photographs (Fig. 8). Sig-nificantly, the membrane was covered only four days after this photograph was taken. The location of the crease is almost exactly coincident with the in-terpreted location of the leaks. The crease begins ap-proximately midway down the slope and around the same position, a line of patches covers holes which

Figure 8. Crease in geomembrane during construction, July 1990.

Figure 9. Membrane position secured using wooden pegs. The

holes made for the pegs were patched.

were made to accommodate a line of stakes used to hold the geomembrane in place (Fig. 9). Subsequent photographs illustrate the history of the crease as it was covered. It appears likely that the position where the crease meets the horizontal line of patches would be a strong candidate for the development of leaks and to cause the anomalies evident on Lines 5 and 6.

Other photographs show a set of creases to the west of that illustrated in Fig. 8. This second set of creases comprises at least 5 rumples in the mem-brane that appear to form a conjugate set with the one identified on the eastern side. The coincidence of the resistivity anomalies and the approximate lo-cations of the two sets of creases as seen in the pho-tographs reinforces the suggestion that the leaks are associated with the creases.

A track for vehicular access was also constructed across the area of concern (see Fig. 10). Once buried by successive amounts of fill, these compacted tracks create impermeable horizons that encourage water to be diverted laterally rather than flowing di-rectly downhill. Consequently, given a leak through the membrane, emergent water is considered likely to spread laterally as well as flowing down the slope towards the concrete wall.

Figure 10. Geomembrane almost covered and access track across the surface.

1153Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)

5 CONCLUSIONS

Seven electrical resistivity profiles were obtained over the northern slopes above the eastbound lane at the west portal area of Conwy Tunnel. The objective of the investigation was to locate the presence of any leaks through an impermeable HDPE geomembrane associated with the western approach to the tunnel. The results demonstrate that the integrity of the ge-omembrane appears intact throughout the upper parts of the slope. However, profiles over the lower slope exhibited at least three anomalies representa-tive of leaks. Photographs taken during construction in 1990 reveal two sets of creases in the geomem-brane were buried without being rectified. These creases sag and spread out from foci on a horizontal line of patches and provide an obvious pre-existing weakness. The patches cover a series of holes made in the membrane to accommodate wooden stakes used to hold the membrane in place. These areas are virtually coincident with the locations of the resistiv-ity anomalies. The patch points have probably stretched due to the sagging membrane and failed resulting in the hydraulic integrity of the geomem-brane being compromised.

The understanding of the leak distribution and cover-material conditions at the geomembrane site provided by the geophysical investigation aided the deliberations on remedial measures and provided a major influence on the final design process.

ACKNOWLEDGEMENTS

Field geophysical surveying was carried out during dry weather conditions in April 2002 by STATS

Geophysical in accordance with design, supervision and interpretation by Reynolds Geo-Sciences Lim-ited. This paper is published with the permission of the Director of Transport, Welsh Assembly Gov-ernment.

REFERENCES

Davies, G.W., Cramp, G. & Nielson, H.K. 1989. The Conwy-tunnel – scheme development and advance works. In: Im-mersed tunnel techniques, Manchester. Thomas Telford, London, 125-144.

Nichol, D. 2000. Geo-engineering along the A55 North Wales Coast Road. Quarterly Journal of Engineering Geology and Hydrogeology, 34, 51-64.

Reynolds, J.M. 1997. An introduction to applied and environ-mental geophysics, John Wiley and Sons, Chichester & London, 796 pp.

Reynolds, J.M. and Taylor, D.I. 1993. Use of geophysical sur-veys during the planning, construction and remediation of landfills. In, Bentley, S.P. (ed), Engineering Geology of Waste Disposal, Geological Society, London, Engineering Geology Special Publication No 11, 93-98.

1154 © 2004 Millpress, Rotterdam, ISBN 90 5966 009 9