GEO-RADAR & NO DIG TECHNOLOGIES –

SOME APPLICATIONS

Complimentary data from INSET s.r.o. (1) (1) INSET s.r.o., Novakovych 6, 180 00, Praha 8,Czech Republic, www.inset.cc

Ground Penetration Radar (GPR) has since long been used in various fields. But its range of applications is growing bigger with the emergence of new techniques and requests for larger investigations. A) Inspection of tunnels and pipelines

A frequent task is to detect voids and zones of weakness around tunnels. Progress of the cavities behind tunnel walls can lead not only to the collapse of the tunnel body itself but also to subsidence of soil above the tunnel, surface sinking, hazardous surface destruction and even loss of life. Preventive inspection of the tunnel construction and soil condition around it is therefore very important. Geo-radar is a tool that

allows inspection through and behind walls by detecting voids, zones of weakness and other inhomogenities before they become a serious problem.

Case history: In the summer 1997, after heavy rainfalls, the fast developing crater above a collapsed sewer appeared in a busy residential area of Prague. Within a few days a large open void of 15 m in diameter and more than 11 m in depth endangered a nearby family house. Inspection of the sewage system revealed two other large cavities close to the crater with a high risk of collapsing. It was decided to inspect immediately the rest of the tunnel with GPR to detect further zones of weakness and cavities hidden behind the walls of the tunnel. The basic survey consisted of a surface radar measurements in three lines parallel to the tunnel with 100 MHz antenna, and a radar inspection from inside the tunnel with 2 parallel lines positioned along each side wall, one in the ceiling and one towards the floor, with 200 and 400 MHz antennas. At the problematic parts the detailed geo-radar measurements were complemented with other geophysical methods – shallow seismic, seismic tomography and detailed micro-gravity surveys. Left Fig. An example of an interpretation of radar data from the GPR surface and underground surveys.

Right Fig. Schematic cross section as a result

from the detailed complex geophysical measurements above detected zone of weakness.

Fig. Tunnel inspection with Malå GeoScience RAMAC MC-4 multichannel radar and two 500 MHz shielded antennas

Since 1997 great improvements in survey techniques and instrumentation have been made. The shielded antennas bring better data resolution with decrease in air reflections; also a multi-channel RAMAC system from Malå GeoScience increased survey production. The radar measurement from inside the tunnels is still the best method of inspection, although it may be often combined with other techniques.

Case history: One of the main sewer lines in Prague (circular in shape and 2.5 m in diameter) was inspected in the year 2000. A combination of geo-radar measurements and concrete wall integrity testing techniques were used to detect voids behind the tunnel walls, to determine their thickness and approximate volume of empty space. Ordinary video camera inspection did not reveal any cracks or breaks in the tunnel walls, so it was expected that the condition of ground behind the walls was good as well. As detailed bellow, the contrary was the true. When core drills were placed into major GPR anomalies a layer of void, up to 60 cm thick, was found behind the wall. Significant deterioration of ground conditions was also detected close to where the sewer was crossing the subway line.

B) Wall inspection In the similar way the tunnel is inspected, the supporting walls and embankments can be surveyed with geo-radar for voids, zones of weakness and other inhomogenities. From the radar sections the thickness of individual construction layers and the condition of material behind the walls can be estimated.

Case history: During a planned general repair of the embankment walls of the Vltava river in Prague INSET was engaged to perform a GPR survey of the walls. The aim of the measurement was to specify the thickness of construction layers and detect the inhomogenities like zones of weakness, voids, cracks etc. During the measurements the Malå GeoScience RAMAC 250 MHz antenna was moved up and down along the wall along profiles spaced 1 metre. The interpreted radar sections were then converted into schematic block diagrams (see bellow). The major anomalies were verified with the core drillings. Fig. Interpretation of radar data from measurement of the embankment walls of the Vltava River, in Prague.

Fig. An example of a survey results from the sewage tunnel „P“ in Prague, where the significant voids behind the walls were revealed.

Fig. The radar measurement of the wall thickness and the condition of material behind the supporting wall of the railroad in the town of Kolin

C) Detection of obstacles and optimalisation of the route for No-Dig technologies

Horizontal drilling is often the best solution to bypass highways, railroads or rivers. It is important to find the easiest way for drilling, to avoid colliding with any hidden obstacles. Sometimes, mainly in large cities, with high density of underground engineering network, it is difficult to find the optimal route for the drilling. Geo-radar is a fast and non-destructive method, which can offer required information about the ground conditions and help to optimise the drilling depth, direction and angle.

D) Road inspection

There are many different applications of geo-radar in this field. An important and frequent task is to determine the thickness of construction layers. Information about location of fractions, zones of weakness and other inhomogenities bellow the road surface is necessary for designers to plan reconstruction of the roads. Small-scale detail radar surveys are made in areas of road surface subsidence. The GPR survey can not only estimated the area and volume of zone of weakness, but also to predict the reason of road deterioration (i.e. broken water or sewage pipeline, geological structure etc.).

Case history: A major road near the town of Stribro suddenly collapsed. It was strongly suspected that the surface deterioration was connected with an old medieval mine beneath the surface and that the collapse could rapidly extend over a larger area. Therefore, the surrounding area was surveyed with GPR and course of mine adits were mapped. The undermined parts of the road were repaired and ad its refilled.

Fig. A damaged road near the town of Stribro. The subsidence happened at the undermined area, where one of the mine ad its shallow to surface collapsed due to heavy traffic.

Fig. A schematic block diagram showing how the geo-radar measurement can be used to optimize the depth for the horisontal drilling.

Fig. Road Inspection with 250 MHz shielded antenna

E) Archaeology Many historical sites (old towns and castles, chateaus) in the Czech Republic hide mysterious labyrinths of underground passages, cellars and halls carved into rocks. Geo-radar measurements and other geophysical surveys are in many instances the only ways to detect the underground voids. Mapping of medieval underground spaces has not only great historical value, but also largely contributes to urban planning. These are few examples of geo-radar measurements done by INSET s.r.o. A two floor medieval cellar was discovered in Brno, under the area, where large construction cranes were about to be raised. Rescued just in time, the ancient wine cellar now serves to its original purpose – an up-scale restaurant. Than, at many other places in Brno, the GPR was used to survey the underground to delineate the voids, before the construction of a cable tunnel. Before a planned reconstruction of the historical centre of the town of Mlada Boleslav a detailed geophysical GPR survey was conducted shallow seismic and micro-gravity measurements focused on delineating underground cellars beneath urban areas of the city. Stability of such shallow cellars can be endangered by road traffic and construction activities. Old medieval silver mining towns like Kasperske Hory or Stribro have serious problems with soil subsidence, and lateral locations of shafts and mine adits had to be established with geo-radar. Case history: The town of Cesky Dub was once up-on-a-time an important trading post for the Czech Kingdom and Germany. Travelling merchants were offered hospitality and protection by the fortified monastery. According to historical sources a large and complicated underground labyrinth was created under the monastery about the XII century. For centuries it has been almost forgotten. At the approximate location of the underground system, a detailed geophysical survey was performed. The radar and micro-gravity measurements revealed locations of complicated system of underground rooms and confirmed their existence.

Fig. The RAMAC radar survey of a chapel floor in the castle of Karlstein. 1

Fig. To obtain better resolution and higher penetration depth the combination of 100 MHz shielded and unshielded antennas were used together with the Mala GeoScience multi-channel MC-4 RAMAC systém. (GPR archaeological survey in the town of Cesky Dub)

Fig. Example of interpretation of gravity measurements and radar profile. Two of the underground rooms were clearly indicated in the depth of about 5 meters. (Archealogical survey in the town of Cesky Dub)