GPR Investigations to Assess the State of Damage of a Concrete Water Tunne

7/27/2019 GPR Investigations to Assess the State of Damage of a Concrete Water Tunne

1/11

GPR Investigations to Assess the State of Damage of a Concrete Water Tunnel

Diego Arosio1, Stefano Munda1, Luigi Zanzi1, Laura Longoni2 and Monica Papini2

1Dip. di Ing. Strutturale, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy2Dip. di Ing. Idraulica, Ambientale, Infrastrutture Viarie, Rilevamento, Politecnico di Milano,

Piazza Leonardo da Vinci 32, 20133 Milano, Italy

ABSTRACT

Erosional voids developing around concrete-lined tunnels can compromise the safety of

the surrounding areas, as well as of the tunnels themselves. In this study, ground penetrating

radar (GPR) was used to assess the condition of a water tunnel built to channel a river under a

mountain road. The tunnel is lined with 6080 cm thick concrete and has a semicircular cross-

section with a diameter that varies between 3 m and 4 m. The concrete structure has been

damaged from erosion beneath the concrete floor, creating a sequence of pools and waterfalls,

which further extend the erosive action below the floor and side walls.

After the collapse of a section of the tunnel

running below a nearby parking lot, a GPR investiga-

tion was initiated to assess the extent of the erosive

action behind the tunnel walls and below the concrete

floor. Most GPR measurements were performed from

inside the tunnel with a 200-MHz antenna, which was

selected as the best trade-off between penetration and

resolution. GPR results, integrated with a priori

information and geological investigations, indicated a

highly permeable soil consisting of a thin layer of

alluvial sediments that covers an altered limestone layer

strongly affected by erosion and karst phenomena.Fortunately, GPR inspections on the parking lot surface

were able to exclude the presence of large cavities above

the tunnel vault. On the contrary, GPR inspections

performed inside the tunnel detected many voids

forming behind the walls, especially near the concrete-

rock contact. GPR inspections performed on the tunnel

floor confirm that water erosion is active below the

concrete paving. Overall, the survey was useful for

identifying the damaged tunnel segments where repair

interventions are most urgent.

Introduction

Tunnels are important underground structures

used in the transportation of vehicles, water, electricity,

and other items. Many of these structures have been

built decades earlier, and it is of paramount importance

to assess the integrity of these underground construc-

tions to ensure safety and long-term viability. Towards

this end, several empirical (Schmidt, 1974; Attwell,

1978), analytical (Verruijt and Booker, 1996; Bobet,

2001) and numerical analyses (Leca and Clough, 1992;

Augarde and Burd, 2001; Menguid and Dang, 2009)

have been conducted to evaluate tunnel stability. In

particular, Meguid and Dang (2009) used numerical

models to evaluate the negative effect of erosional voids

developing in close vicinity of tunnels. They studied the

effects of voids on the circumferential stresses in the

lining and the change in lining response caused by the

introduction of voids behind lateral walls (i.e., at the

springline) and below the paving (i.e., at the invert).

While they assumed a homogeneous hosting medium

and a theoretical 2-D geometry of the voids, their workhelped to improve the knowledge of the stress state of

these structures.

In this work, we attempt to identify the erosional

voids in a water-filled tunnel using geophysical methods,

particularly ground penetrating radar (GPR). The

geophysical investigation was part of a broader study

focused on understanding the hydrogeological condi-

tions that potentially could contribute to the deteriora-

tion of the water-carrying infrastructure along a

national road that runs through a narrow valley. The

road is periodically affected by service interruptions

caused by small landslides or partial road collapses

induced by water erosion and intense precipitationevents. The risk of road collapses increases where the

road runs close to the river or where the river crosses the

road by flowing into concrete-lined tunnels. The con-

crete tunnels were built at the beginning of the 20th

century and were not properly maintained over the past

decades. Rapid deterioration induced by water erosion

and lack of maintenance are a major safety concern. For

example, a large sinkhole triggered by a partial collapse

159

JEEG, September 2012, Volume 17, Issue 3, pp. 159169


2/11

of one of these underground infrastructures occurred in

2007 in a parking area near the road. No injuries were

reported, but the incident could have been much worse if

the concrete tunnel had collapsed a few meters ahead,

where it passes under the road, or if the sinkhole had

occurred during a weekend when that parking lot is

often crowded.

A team of geologists, geophysicists, hydrologists,

and civil engineers from Politecnico di Milano were

involved to assess the risk of similar events along two

segments of the concrete tunnel responsible for the 2007

sinkhole. The most severely damaged section of the

tunnel runs parallel to the valley for 300 m before

intersecting the road, where the valley makes a

pronounced left turn. No traces of regular maintenance

operations or previous assessment studies were found in

the archives of the local environmental agency in charge

for the management of the water infrastructures. An

exception was a partial visual inspection performed

inside the tunnel in the year 2004, which reported asevere deterioration of the concrete structure and

suggested further investigations to be performed with

the georadar method. Although the archives do not

report any repair operation, the visual inspection of the

tunnel assessed the existence of some provisional

reinforcements that were installed in two points of the

tunnel where the concrete structure is highly fractured.

These reinforcements, consisting of timber and metal

elements, were likely installed in the 1990s and already

had presented signs of deterioration. We mention these

details because it is likely that other tunnels of similar

vintage have incomplete inspection records or less-than-

effective maintenance.The team of Politecnico di Milano planned to

perform a visual inspection of the entire tunnel to assess

the present condition (concrete fracturing, water infil-

trations, floor erosion, etc.) and to explore the feasibility

of non-destructive investigations testing (NDT) of the

tunnel. One of the more common NDT methods that

have been used to perform investigations on concrete

liners of tunnels, sewers, and railway or road tunnels is

GPR. For example, Maekawa and Fenner (1994)

reported the use of GPR to assess the extent of cavities

behind concrete tunnel linings. Cardarelli et al. (2003)

used GPR to evaluate the quality of the contact between

concrete lining and massive rock from inside a potablewater supply tunnel. Davis et al. (2005) reported the use

of GPR to examine the efficiency of tunnel lining

groutings and showed examples from investigations

performed on water supply and sewer tunnels. Parkin-

son and Ekes (2008) showed the use of GPR to map

tunnel lining condition, to locate concrete deterioration,

and to detect voids developed between the concrete liner

and rock surface caused by water flowing either in or

out via defects in the liner. Zhang et al. (2008) used GPR

to evaluate the lining quality of a railway tunnel and to

detect the hidden flaws in the lining. The investigation

results were used to direct subsequent grouting, with a

follow-up survey to assess the quality of the grouting.

Finally, Zhang et al. (2010) described a similar applica-

tion where the use of GPR was proposed to detect the

grout thickness behind the lining segments of metro lines

in Shanghai, China.

In the literature that focused on GPR investigations

of underground and concrete-lined tunnels, most of the

focus was on the inspection of the upper part of the

tunnels, i.e., walls and vaults, where overburden stresses

were the highest. However, in the case of water-filled

tunnels, the floor has been given little attention, likely

because of the difficulties of performing NDT without

service interruptions. However, it is important to mention

that the lower part of a water tunnel can be as critical or

even more so than the upper part, depending on the

geometry of the tunnel cross-section (circular or semicir-cular) and on flow regime and water pressure.

GPR is a method that is often proposed to perform

near-surface geophysical investigations on karst areas

where the risk of sinkhole formation is high. Carpenter

et al. (1998) and Batayneh et al. (2002) reported examples

where GPR and other geophysical methods were tested to

locate buried sinkholes as a means of inferring the

existence of hydraulically-active karst features. Leucci

et al. (2004) applied GPR and ERT for mapping

karstified zones. Beres et al. (2001) combined GPR and

microgravimetric methods to achieve the same objective.

Conroy and Guy (2005) and Conroy and Daniels (2006)

described GPR surveys conducted along a section ofhighway that had collapsed into underground coal mine

workings to detect other locations of mine-related

disruptions. Miller et al. (2008) showed GPR data

collected at two Vermont highway sites exhibiting

pavement subsidence. The GPR data were useful to

study the soil properties and to understand the subsidence

mechanism, in one case related with water erosion and

resulting in fine particle migration. Pueyo-Anchuela et al.

(2009) tested the GPR method to study the internal

structure of sediments in search of indicators of active

karst processes. They performed a classification of karst

hazard problems in terms of cavities, evidences of

subsidence and paleocollapses. In a following paper thesame authors propose a combination of geophysical

techniques that can be successfully applied in alluvial

karst regions to locate areas of high sinkhole risk (Pueyo-

Anchuela et al., 2010).

From a summary of the collected works, it is clear

that GPR is a robust method to effectively determine

trouble spots along the water-filled tunnel at the test

site. Therefore, this paper addresses the geophysical

160

Journal of Environmental and Engineering Geophysics


3/11

investigation in three different areas to determine those

needing further investigation: a) inside the tunnel, b) in

the parking lot near the road, and c) along the segment

of the road that runs close to or above the tunnel.

Particularly, the investigations inside the tunnel were

aimed to assess the extent of water erosion behind the

tunnel walls and below the segments of the tunnel floor

that are still preserved. Since the mechanism of collapse

of the structure was likely triggered by the erosion of the

wall foundations, the investigations on the preserved

concrete floor, especially near the wall-floor contact, are

particularly important (Meguid and Dang, 2009).

Furthermore, these studies represent an interestingapplication that is not well documented in the GPR

literature. To conduct these measurements, the GPR

antennae were placed inside a PVC box and the

surveying of the concrete floor was performed by

preserving the contact between the antenna and the

floor. Investigations in the parking lot and along the

national road were aimed to detect GPR anomalies that

could be related to cavity formation caused by erosional

activity above the tunnel vault, whose depth varies from

1 m in the parking lot to about 8 m at the end of the

second segment of the tunnel.

Site Description

The geophysical investigations were conducted in

the area of the waterfalls located in the Valganna valley

(Fig. 1), 60-km north of Milan (Italy). Geological

mapping of the site was performed at the beginning of

the study and it was determined that two different

carbonate facies, pertaining to a dolomitic-limestone

sedimentary succession of Mesozoic age, outcrop in thearea of the Valganna waterfalls. The travertine is highly

karstified, whereas the dolomite rock is heavily frac-

tured with the presence of thick folds. The tunnel was

built along the contact between alluvial deposits of the

Olona River and the strongly karstified travertine. At

shallow depth, close to the tunnel, alluvial deposits are

characterized by loose, unconsolidated soil in a sandy

matrix with centimeter-sized clasts. The deposit shows a

Figure 1. Location of the GPR investigations and map of the tunnel. The area is located in the Pre-alpine region, 60 km

north of Milan (Italy). The drawing illustrates the section of the national S.S.233 road where the Olona River is forced to

enter the first segment of a concrete-lined tunnel to underpass both the parking area of the Valganna Waterfalls and the

road. A second segment of the tunnel runs parallel to the road before crossing it again. On top right, a drawing of the

tunnel cross-section shows the position of the antenna for the GPR profiles undertaken inside the tunnel.

161

Arosio et al.: GPR Assessment of a Water Tunnel


4/11

variable thickness across the area, with a maximum

depth of 1.5 m. Beneath the alluvial deposits, the

bioconstructed limestone bedrock is heavily weatheredbecause of karst processes in the first 2 m (Fig. 2), while

its mechanical properties tend to improve at greater

depth.

From a hydrogeological point of view, this site is

subjected to a double erosion process: the first is directly

related to water flowing inside the tunnel and a second is

caused by water infiltration in the heavily fractured and

karstified hosting rock mass. The Mesozoic sedimentary

succession shows a significant secondary permeability

(i.e., permeability developed in a rock after its deposi-

tion), mostly because of karst processes, giving rise to the

second erosional process. However, it is worth noting

that permeability of dolomites is mainly related toextensive fracturing, while limestones are heavily affected

by karst phenomena that give rise to subsurface water

flow.

As shown in Fig. 1, the Olona River was diverted

into an underground concrete chamber below the hotel

of the Valganna Waterfalls, after which it enters a

concrete-lined tunnel to underpass the parking area of

the waterfalls and the national road (S.S.233). Splitting

the study site effectively into two segments, the first

segment ends on the south side of the parking area,

highlighted in Fig. 1. Within 50 m, the river enters a

second segment of the tunnel that runs parallel to theroad for about 300 m. At the end of the second segment

of the tunnel the road turns left in the direction of a

brewery.

The investigations were planned after the formation

of the 2007 sinkhole. The sinkhole was caused by the

collapse of the tunnel, which created a void 13-m long

and 3.5-m high. Figure 3 shows the construction site

during the repair of the tunnel. The picture shows that the

internal section of the tunnel is semicircular with a

diameter of about 3 m and a height of about 2 m. The

thickness of the concrete wall varies from 60 to 80 cm and

the vault is about 1 m below the parking level.

The internal inspection of the tunnel revealed anextreme deterioration of the concrete structure. Figures

4(a)(b) show large areas where the concrete floor of the

tunnel partially collapsed as a result of bedrock erosion.

Bedrock erosion caused the most dangerous situations

at the base of the lateral walls of the tunnel, as depicted

on the right side of Fig. 4(b). As a result of the erosion

action, the water flow was quite irregular with pools and

waterfalls existing throughout (see Figs. 4(c)(d)), espe-

cially in the second segment of the tunnel. Temporary

bridges were assembled to perform inspections and

geophysical investigations. The second segment also

showed evidence of water seeping through the concrete

ceiling (Fig. 4(e)). Finally, Fig. 4(f) was taken near theend of the second segment and showed timber and metal

reinforcements that were used in the past for repair.

The internal inspection revealed a major conse-

quence of water erosion and subsequent collapse of the

concrete slabs that forms the tunnels pavement. That

is, the erosional activity can progress laterally below

the tunnel walls at a much higher rate than would be

expected from water infiltration through secondary

Figure 2. The lapideous rock layer consists of weathered

limestone highly affected by karst phenomena.

Figure 3. The construction site that followed the tunnel

collapse in 2007. The tunnel section is semicircular (about

3-m across and 2-m high) with a concrete wall thickness of

about 6080 cm.

162



5/11

Figure 4. Inspection of the tunnel. a) and b) Examples of damages of the concrete paving created by water erosion below the

tunnel floor. Note also the very dangerous erosive actions of the water below the lateral walls of the tunnel. c) and d)

Examples of waterfalls and water pools caused by collapsed portions of the concrete floor induced by water erosion.

Temporary bridges were needed to bypass the larger pools with the GPR equipment. e) Signs of water seeping through the

concrete vault in the second segment of the tunnel. f) Remains of an old repair intervention in the second segment of the tunnel.

163



6/11

permeability features (fractures and karst) alone. This

creates a very dangerous situation, with the potential

to trigger new tunnel collapses. Thus, the results of

the internal inspections underline the importance of

further diagnostic investigations to assess the extent

of water erosion activity beyond the tunnel walls and

especially below the still preserved slabs of the concrete

floor.

Data Acquisition

Despite the adverse conditions created by floor

collapses and water pools, an extensive GPR survey was

undertaken from inside the tunnel. The late summer

(September 2008) was chosen for operation inside the

tunnel because of the reduced rate of water flow. After

some preliminary testing performed with two different

antennae frequencies (200 MHz and 600 MHz), the

lower frequency was selected as the best tradeoff

between resolution and penetration. Both the antennaswere able to detect areas of detachment between the

concrete liner and the rock, but the 200 MHz antenna

was offering higher penetration thus extending the

ability to detect cavities and karst features a few meters

beyond the tunnel wall. The 200 MHz antenna was also

selected for the radar survey of the parking lot and for

the radar profiles along the national road to ensure a

penetration of several meters. Thus, the equipment for

all the investigations consists of a 200 MHz shielded

antenna with a TX-RX separation of 19 cm controlled

by a radar unit from IDS S.p.A.

GPR data were initially collected on the surface by

exploring the parking area of the hotel near the waterfallsand the road that runs close to the river. The aim of these

surface measurements was to evaluate GPR penetration

in the specific area and to assess the ground properties

above the tunnel with special attention to detect karst

features and cavities. The parking area was explored by

selecting seven zones above the tunnel route that were

scanned in 3-D mode by executing parallel profiles spaced

50-cm apart.

Several long profiles were collected along the

national road, which runs near the underground tunnel.

GPR data acquisition was particularly focused on the

road segments that cross over the tunnel. GPR measure-

ments were also collected inside the tunnel, which tookseveral days to complete, and were performed to achieve

the following goals: a) detect possible voids excavated by

water behind the lateral walls of the tunnel either

proximal to the concrete-rock contact area or within a

3-m distance from the wall; and b) explore the apparently

undamaged segments of the tunnel floor to assess the

extent of the water erosion below the concrete paving.

Profiles were collected along both tunnel walls by walking

in the tunnel direction, except for those segments where

the tunnel floor had collapsed creating large and deep

water pools. We were forced to collect profiles at a height

of about 45u on both sides of the tunnel section (top right

in Fig. 1) because at lower angles from the floor metal

reinforcements protrude from the concrete. Floor profiles

were also collected where possible, i.e., on the preserved

concrete slabs, by walking in the tunnel direction with the

GPR antenna placed inside a waterproof PVC box. A

large wheel with encoder was attached to the box to

trigger the radar unit to help with geo-referencing. The

floor profiles run close to the walls rather than in thetunnel center (top right in Fig. 1) because the erosive

action of the water is particularly dangerous for the

stability of the tunnel when it creates large voids below

the lateral walls, as visible in Fig. 4(b).

Data Processing and Interpretation

Data processing was performed with 2-D and 3-D

software developed at Politecnico di Milano. The data

were bandpass filtered and time calibrated before

applying a gain function to compensate for signal loss

caused by divergence and absorption. A function for

background subtraction was applied where needed toenhance very shallow targets. Some data were also

migrated to focus the diffractions produced by cavities

or other scattering targets.

On the whole, all the surface profiles collected in

the parking area or along the national road show good

penetration. Reflections or diffractions from targets 5-m

deep (about 80 ns) are often observed (Fig. 5). This

suggests that the subsoil contains little to no clay and is

Figure 5. Example of a 40-m GPR profile along the

national road. Penetration down to 80 ns (i.e., about 5 m)

is observed. Alluvial sediments (1- or 2-m thick) overlay a

layer of altered limestone. Reflection A comes from the

base of the paved road. Reflection B comes from the

interface that separates the sediments from the limestone,

while reflection C comes from a change of compaction

within the alluvial sediments. Many diffractions and short

discontinuous reflections are observed in the image as a

result of karst phenomena. Some diffractions within the

alluvial sediments might be caused by pipes crossing the

road (e.g., diffraction D).

164



7/11

rather coarse so that minimal water is retained. This

observation is also consistent with the relatively high

velocity obtained from diffraction analysis: a stable

value of about 12 cm/ns, not compatible with saturated

sediments, was found in all the GPR records. Thesediments are typified by alluvial deposits in a sandy

matrix with centimetric clasts, overlaying a travertine

layer characterized by high permeability caused by both

karst phenomena and significant fracturing. The tran-

sition between alluvial deposits and limestone is visible

in Fig. 3 at about 1 m below the parking level (see the

color change of the exposed rock). In Fig. 5, this

transition generates reflection B, gently dipping from

right to left in the 1040 ns range corresponding to a

depth range of 0.52.5 m. Reflection C comes from a

change of compaction within the alluvial deposits.

The profiles collected on the parking lot and alongthe national road generally present images with a greater

number of diffractions than the tunnel profiles. Figure 6

is an example where the diffraction density is particularly

evident. Some diffractions also show strong reverbera-

tions that suggest the possibility of metal pipes or open

voids (e.g., Radzevicius et al., 2000; Kofman et al., 2006).

Given that no buildings exist along this section of the

road to justify a great number of lateral connections to

sewer or other utilities, it is likely that most of these

diffractions are associated with small voids resulting from

intense karst activity. The voids are also visible on the

trench sides in Fig. 3.

The tunnel vault is observed by the surface radarmeasurements, but only where the tunnel depth is less

than 45 m, i.e., in the parking area and below the road

at the first road/tunnel intersection. Here the vault is

often well depicted and both the reflections from the

outer and the inner sides of the concrete ceiling are

distinguished (Fig. 7). The tunnel floor appears with a

non-flat reflection because of the expected image

distortion caused by the circular vault, enhanced by

the much higher velocity of the radar wave within the

air-filled tunnel.

The GPR data collected on the lateral walls were

processed to enhance diffractions and reflections coming

from the concrete-rock discontinuity and from the voids

in the limestone rock created by both erosion and karstphenomena. All the radar signals that were attributed to

voids very close to the concrete tunnel (within 1 m) were

mapped on the radar images with a solid box, whereas a

dash-dot box was used to map signals attributed to voids

at a distance greater than 1 m from the tunnel. To

separate erosional features from metallic infrastructure, a

dashed box was used to highlight the possibility of pipes.

As an example, Fig. 8 shows a 13-m long section of a wall

profile where voids were detected near the rock-concrete

contact and also far from the contact, at a distance of

about 160 cm. The features are particularly evident in the

migrated section, shown in Fig. 8(b). An accurate report

was produced during the GPR survey to monitor theexistence of any construction elements that could disturb

the radar data. This was important to ensure a correct

interpretation of signals like the artifacts shown in Fig. 8

resulting from two pairs of lateral intake pipes. The radar

image in Fig. 9 illustrates another 13-m long section of a

wall profile. Most of this section is affected by a persistent

detachment of the concrete wall from the rock. In

addition, a strong reflection indicates a suspected cavity

Figure 6. Example of another 44-m GPR profile along

the national road. Reflection A comes from the base of the

paved road. Diffractions B and C are superficial pipes

crossing the road. Many other diffractions are observed as

a result of small voids created by water erosion and

karst phenomena.

Figure 7. Example of a vertical section extracted from a

3-D survey in the parking area. Note the reflection from

the upper (U) and lower (L) sides of the concrete vault of

the tunnel. The time difference, about 11 ns, indicates a

concrete thickness of 55 cm assuming a concrete velocityof 10 cm/ns. Note also the distorted reflection from the

tunnel floor (F). This arrives with a delay from L

(measured at the center of the tunnel) of about 13 ns,

which gives a tunnel height of 195 cm assuming a velocity

of 30 cm/ns (air velocity) within the tunnel.

165



8/11

2-m long located at a distance of about 180 cm from the

tunnel.

Similarly, the data from the investigations on the

tunnel floor were interpreted by mapping with solid

boxes all the signals that were attributed to water pools

created by erosion and karst phenomena below the

concrete paving. An example is given in Fig. 10, wherefour water-filled cavities were detected in a 7-m long

segment of the tunnel floor. Since the radar velocity in

water is very low (about 3.3 cm/ns), the thickness of the

suspected pools observed in Fig. 10 varies from 8 cm to

3540 cm. The signals associated with these phenomena

sometimes appear as diffractions (small cavities filled by

water), while at other times they appear as reflections

(wider water pools). In some cases, the reflection

reverberates indicating that the horizontal size of the

water pool is not much larger than its thickness

(Kofman et al., 2006). The radar velocity resulting from

the diffraction analysis on floor profiles (generally lower

than 6 cm/ns) validate the presence of water below theconcrete paving.

To create a synthetic map of the deterioration

progress that might be used by the engineers to evaluate

the residual risk and to plan the most urgent actions, all

the voids that were observed behind the concrete walls

and all the water pools that were detected below the

concrete paving were transferred on a CAD map, as

shown in Fig. 11. According to the synthetic map,

moving from right to left by following the direction of

the river, we note the following results. The parking area

(section C in Fig. 11), before the first river-road

intersection, shows an average density of radar events

with a number of them generated by suspected cavities

located about 2 m behind the tunnel wall. The short

section where no events are reported corresponds to thearea of the 2007 collapse that was under repair during the

GPR investigations. The second segment of the tunnel

apparently shows two safe sections where no radar events

are reported. Actually, they correspond to areas affected

by large and extended pools that prevented the possibility

to walk and investigate the walls. Thus, they are the most

damaged sections of the tunnel where the concrete floor

has been almost totally eroded. From the entrance of the

second segment to the first unexplorable area (section B

in Fig. 11), we note a higher density of events compared

to the parking area, especially related with detachments

between the concrete wall and the hosting rock. In

between the two unexplorable areas, we note a section(section A in Fig. 11) affected by a very high density of

events of various types (detachments, cavities, water

pools). This section and the two neighboring unexplor-

able areas, especially at the intersection with the road,

define the most damaged stretch of the tunnel where local

authorities and engineers should plan renovation works

with the highest priority. The last section of the tunnel

before the final exit shows an average density of events;

Figure 8. Example of a wall profile: a) before migration, b) after migration. Voids were detected behind the concrete wall

near the rock-concrete contact (solid boxes) and at a distance of about 160 cm (dash-dot box). The dashed boxes indicate

artifacts, followed by reverberations, created by metal water pipes that enter the concrete tunnel from the side of the

GPR profile.

166



9/11

this is where the remains of old repair interventions are

still present so that the tunnel is partially protected.

Conclusions

The GPR method contributed successfully to the

characterization of damaged sections around a water-

filled tunnel. Surface profiles showed good penetration(up to 45 m) and high velocity (12 cm/ns) suggesting a

rather coarsely textured soil and excluding the presence of

significant clay content. The GPR data indicated a

pseudo-horizontal reflection at about 1 m and a large

number of diffractions commensurate with a thin layer of

alluvial sediments that covers an altered limestone layer

strongly affected by erosion and karst phenomena. This

suggests that water erosion and karst phenomena

represent the main hazards associated with this concrete

tunnel and a collapse in 2007 was most probably caused

by water excavation at the tunnels base. As further

revealed by internal inspections, water erosion tends to

create variably-sized voids below the tunnel paving,

especially where the semicircular concrete structure is

expected to rest on the bedrock, i.e., below the lateral

walls.

The 3-D GPR inspections performed on the

parking lot surface along the trajectory of the tunneldid not locate any large anomaly that could be related

with the formation of a large cavity above the tunnel

vault. On the contrary, the GPR inspections performed

inside the tunnel detected many voids forming behind the

walls, especially near the concrete-rock contact. Although

less frequently, some voids were also detected inside the

limestone layers at distances of 1 or 2 m from the concrete

wall. The amount of these events is remarkably higher

along the second segment of the tunnel, parallel to the

national road, indicating that this area is much more

deteriorated.

The GPR inspections performed on the apparently

undamaged segments of the tunnel floor validated thehypothesis that water erosion is particularly active below

the concrete paving. Where this paving has not collapsed

yet, the radar often detected water pools where the signal

propagates at lower velocity with reverberations (or

ringing). The results of the GPR investigations were re-

ported on a synthetic CAD map of the tunnel to facilitate

local authorities and engineers in defining the areas where

renovation works are required with higher priority.

Figure 9. Example of a wall profile: a) before migration, b) after migration. A long detachment of the concrete wall from

the rock is observed in the solid box. A strong reflection from a suspected cavity was also detected at a distance of about

180 cm (dash-dot box).

Figure 10. Example of a floor profile. Reflections from

water pools created by erosion below the concrete paving

were observed and are marked with solid boxes.

167



10/11

The information gained by means of the GPRsurveys could also be employed in numerical simulations

to improve the knowledge on the state of stress of the

tunnel and to estimate the risk of collapse. This approach

would allow producing a risk erosion map, which may be

an additional tool for evaluating tunnel safety.

Acknowledgements

The authors are grateful to Alberto Bonaldi, Alessia

Thevenet and Eva Zattera that were part of the GPR team

during the acquisition days. The GPR equipment was kindly

supplied by IDS S.p.A.. The authors gratefully acknowledge

Dr. Dale Rucker for the extensive recommendations andcomments which have contributed to making this manuscript

acceptable for publication.

References

Attwell, P.B., 1978, Ground movements caused by tunneling in

soil: in Proceedings: Large Ground Movements and

Structures Conference, University of Wales, Cardiff,

Wales, 812948.

Augarde, C.E., and Burd, H.J., 2001, Three-dimensional finite

element analysis of lined tunnels: International Journal

for Numerical and Analytical Methods in Geomecha-

nics, 25, 243262.

Batayneh, A.T., Abueladas, A.A., and Moumani, K.A., 2002, Use

of ground-penetrating radar for assessment of potential

sinkhole conditions: An Example from Ghor al Haditha

area, Jordan: Environmental Geology, 41, 977983.

Beres, M., Luetscher, M., and Olivier, R., 2001, Integration of

ground-penetrating radar and microgravimetric meth-

ods to map shallow caves: Journal of Applied Geophys-

ics, 46, 249262.

Bobet, A., 2001, Analytical solutions for shallow tunnels in

saturated ground: Journal of Engineering Mechanics-

ASCE, 127(12) 12581266.Cardarelli, E., Marrone, C., and Orlando, L., 2003, Evaluation

of tunnel stability using integrated geophysical methods:

Journal of Applied Geophysics, 52, 93102.

Carpenter, P.J., Doll, W.E., and Kaufmann, R.D., 1998,

Geophysical character of buried sinkholes on the Oak

Ridge Reservation, Tennessee: Journal of Environmen-

tal and Engineering Geophysics, 3, 133145.

Conroy, J.P., and Daniels, J.J., 2006, Improved mine subsidence

detection and roadway evaluation using a 3D GPR and

Figure 11. Compiled map of the GPR results, created by reporting all the voids detected outside the tunnel and all the

water pools detected below the tunnel floor. The distribution of the radar events helps the engineers to evaluate the hazard

level and to plan the repair activities.

168



11/11

statistical correlation of data by cross-plotting approach:

in Proceedings: 11th International Conference on GPR,

June 1922, Columbus, Ohio.

Conroy, J.P., and Guy, E.D., 2005, Borehole tomography and

surface 3D radar for coal mine subsidence detection:

Electronic Journal of Geotechnical Engineering, 10,

bundle F.

Davis, A.G., Lim, M.K., and Petersen, C.G., 2005, Rapid andeconomical evaluation of concrete tunnel linings with

impulse response and impulse radar non-destructive

methods: NDT&E International, 38, 181186.

Kofman, L., Ronen, A., and Frydman, S., 2006, Detection of

model voids by identifying reverberation phenomena in

GPR records: Journal of Applied Geophysics, 59,

284299.

Leca, E., and Clough, W., 1992, Preliminary design for NATM

tunnel support in soil: Journal of Geotechnical Engi-

neering, 118(4) 558575.

Leucci, G., Margiotta, S., and Negri, S., 2004, Geophysical

and geological investigations in a karstic environment

(Salice Salentino, Lecce, Italy): Journal of Environmen-

tal and Engineering Geophysics, 9, 2534.Maekawa, S., and Fenner, T.J., 1994, Study of cavity depth

estimation behind concrete tunnel lining using G.P.R.: in

Proceedings: International Conference of G.P.R., Water-

loo Center for Ground Research, Waterloo, Ontario,

895905.

Meguid, M.A., and Dang, H.K., 2009, The effect of erosion

voids on existing tunnel linings: Tunneling and Under-

ground Space Technology, 24, 278286.

Miller, J., Schultz, G., Calkins, D., and Benda, C., 2008,

Assessing nondestructive geophysical methods for in-

vestigating roadway structural health: in Proceedings:

21st Symposium on the Application of Geophysics to

Engineering and Environmental Problems, April 610,

Philadelphia, Pennsylvania.

Parkinson, G., and Ekes, C., 2008, Ground penetrating radar

evaluation of concrete tunnel linings: in Proceedings:

12th International Conference on GPR, June 1619,

Birmingham, UK.

Pueyo-Anchuela, O., Casas-Sainz, A.M., Soriano, M.A., and

Pocov-Juan, A., 2010, A geophysical survey routine forthe detection of doline areas in the surroundings of

Zaragoza (NE Spain): Engineering Geology, 114, 382396.

Pueyo-Anchuela, O., Pocov-Juan, A., Soriano, M.A., and

Casas-Sainz, A.M., 2009, Characterization of karst

hazards from the perspective of the doline triangle using

GPR Examples from Central Ebro Basin (Spain):

Engineering Geology, 108, 225236.

Radzevicius, S.J., Guy, E.D., and Daniels, J.J., 2000, Pitfalls in

GPR data interpretation: Differentiating stratigraphy

and buried objects from periodic antenna and target

effects: Geophysical Research Letters, 27, 33933396.

Schmidt, B., 1974, Prediction of settlements due to tunneling in

soil: Three case histories: in Proceedings: 2nd Rapid

Excavation and Tunneling Conference, San Francisco,vol. 2, 11791199.

Verruijt, A., and Booker, J.R., 1996, Surface settlements due

to deformation of a tunnel in an elastic half plane:

Geotechnique, 46(4) 753754.

Zhang, L., Liu, Z., and Gong, C., 2008, The lining quality

detection and grouting effect evaluation of the Kunlun

Mountain railway tunnel using ground-penetrating

radar: Near Surface Geophysics, 6, 175180.

Zhang, F., Xie, X., and Huang, H., 2010, Application of

ground penetrating radar in grouting evaluation for

shield tunnel construction: Tunneling and Underground

Space Technology, 25, 99107.

169


Documents

GPR Investigations to Assess the State of Damage of a Concrete Water Tunne