GROUND PENETRATING RADAR (GPR) SCANNING IN GEOLOGICAL AND GEOTECHNICAL RECOGNITION OF MOUNTAIN SITE FOR POLISH OIL & GAS COMPANY.

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1 GROUND PENETRATING RADAR (GPR) SCANNING IN GEOLOGICAL AND GEOTECHNICAL RECOGNITION OF MOUNTAIN SITE FOR POLISH OIL & GAS COMPANY. Zbigniew Bednarczyk, Poltegor-Institute, Wroclaw, Poland Adam Szynkiewicz, University of Wroclaw, Wroclaw, Poland Abstract The results of GPR scanning made together with geological and geotechnical investigations on site in Carpathian Mts. for Polish Oil & Gas Company are discussed. At one site, depleted gas field Strachocina was located near Sanok, the underground gas storage is developed. In the project the new buildings and compressors are situated on the steep slope which built by flysch deposits. Flysch is built of many thin layers of claystone intercalated by sandstone and is also covered by mixture of rocks with soil. These layers were difficult for in-situ and laboratory tests and are prone to creep processes. In the preliminary investigations the investor used only hand auger, but due to bed recognition of site, the geotechnical investigation report was ordered. New report contains data of: core drillings, different types of geotechnical monitoring methods, laboratory tests and GPR transects. All investigations detected complicated folded geological structure which was cut by the faults. The differences between upper part of the hill (built by rocks) and its lower part (with very low geotechnical conditions) were recognized. Results of GPR scanning and detail geological and geotechnical researches was used for project modification and checked by slope stability analysis to locate constructions in save area. Introduction Underground gas storage Strachocina was developed in a depleted gas field located near Sanok. By the year 212, the Polish Oil and Gas Company plans to increase the existing storage capacity by.15 bcm, i.e., up to the level of.33 bcm. The civil engineering project included foundations of technical facilities and buildings on the slopes built from soft-layered deposits inclined in some places up to 2%. The research area was 2 m wide and over 6 m long. It was characterized by flat upper section, and steep slopes to the south and north directions. To the north, along both sides of the hill were situated two streams valley (Figure 1). Geotechnical engineering site investigations included 1 core boreholes, to the depth of 2 m. Slope instrumentation included inclinometer, pore pressure, groundwater level measurements and different types of laboratory tests. This delivered many very important data but due to complicated geology and many faults, it was difficult to predict internal geological stratification and tectonic setting of the investigated area. During geotechnical investigation it was recognized that gas storage surface area is characterized by flat upper part built from stiff shale s and claystons, strength of over 4 MPa and soft, very highly saturated clays on the slopes to the depth of 1116 m bellow the natural terrain level. Precise localization of borders, between these two zones, were very difficult due to the vertical inclination of investigated layers. To solve this problem and obtain more precise data about internal stratification and faults data over 2 km of Ground Penetrating Radar (GPR) scanning were performed. GPR surveys with 1 Mhz antennas were found to be a very useful method for geotechnical and engineering geology site investigations. 731

2 5 w ieœ: P akoszwi eów ka gm i na: San ok L s A /2 A1 Str achoci na gmi na : Sanok 6(1) L s B C1 5(18/18) 6 K Ps 1 5P 3(2) X1 4 L s Y1 4 2P 183 L sv 2(2/12) 183 V 3 VX 1(1) A Y 1/1(14) B Ps 4P 4(18/18) 11 C K K 9 K 1 LEGEND GPR profiles boreholes Inclinometers piezometers/pore pressure transducers landslide borders 13 landslide depth geotechnical cross-sections faults 1 m 12 Fig.1. Strachocina site investigation map Geology of investigated area Research area was localized on Strachocina anticline, in the eastern part of Carpathian Depression. Marine soils and rocks from Paleogene period (Tertiary) were represented by flysch type shale s mudstones and sandstones, consisting of many thin layers. These deposits were elevated during Alpine orogenesis and formed nappe type structure with two directions of faults. Structure was characterized by older, relatively stiff shale s mudstones (Eocene) in the middle of the structure and younger calaystones, clays and sandstones (Oligocene) on the borders with thickness of over 3 m. Region is represented by main NW-SE faults (along the anticline) and secondary SW-NE faults. Natural terrain relief was partly from Tertiary and Quaternary periods. Erosion and mass movements processes characterized by thick weathering zones formed two deep valleys on the borders of investigated area. Field investigations Ground Penetration Radar (RAMAC/GPR) scanning was performed for recognition of geological stratification and suspected mass movements. It was calibrated by diamond impregnated 6 core drillings diameter of 132 mm (up to depth 2 m). Eight profiles, totally 213 meters of GPR scanning with 1 MHz unshielded antenna were included in engineering geology site investigations. Two longitudinal A-A1 and A-A2 profiles had directions with slope inclination to the north (from proposed storage area into the valleys). Cross section A-A1 was started at the south side of the hill to 732

3 investigate colluviums at the south. Another two cross sections B-B1 and C-C1 were perpendicular to the slope inclination. In the longitudinal cross-section, geological stratification of Strachocina anticline were very visible. For flat part, stiff shale s area precise localization, also additional cross sections X-X1 and Y-Y1 were performed. Scanning profiles are listed in a table 1 and presented on the Figures: 3, 4, 5. Table 1. GPR scanning profiles No. Profile Length [m] 1 A-A1(1) 25 2 A-A1 (2) 52 3 A-A B-B C-C X-X Y-Y Along the road 28 Total 213 GPR Scanning results Scanning to the north and south of stiff shale s area allowed detection of faults and landslides. Mass movements were recognized to the south and to the north directions from stiff shale s area. Results indicated that nearly 65% preliminary designed gas storage infrastructure (based only on hand auger geological data) was localized on landslide area with very low geotechnical parameters. GPR detected layers were to the depth of 2 m below the natural terrain level. Survey profiles were interpreted, scaling and filtered using good quality core drilling results. There were two directions of faults recognized in GPR scanning that was also detected in previous research in this area. Fig. 2. Ground Penetrating Radar (GPR) scanning at Strachocina site 733

4 Fig. 3. GPR longitudinal cross-section A A1. Fig. 4. GPR longitudinal cross-section A-A2. 734

Fig. 5. GPR transverse cross-section B B1. After borings were completed the analysis concludes: colluviums deposits covered claystone (black shale) and mudstone (fine sandstone) rocks.

5 Fig. 5. GPR transverse cross-section B B1. After borings were completed the analysis concludes: colluviums deposits covered claystone (black shale) and mudstone (fine sandstone) rocks. The calibration of the depth scale was calculated after boreholes data: thickness of colluviums and type of rock below. It was calculated for colluviums, built from soil loamy wet deposits, the attenuation (dbm -1 ) as 6, relative permittivity range as 3 and relative permeability 3. For claystone/shale dry, the attenuation (dbm -1 ) as 1, relative permittivity range as 9 and relative permeability 9. For mudstone (fine sandstone), the attenuation (dbm -1 ) as 1, relative permittivity range as 5 and relative permeability 1 (see: Daniels 24). The GPR raw data was processed in GroundVision program. On printed radargamms the mudstone/fine sandstone was marked in yellow and claystone/shales was marked gray. The topographysation of cross-sections was made in CorelDraw program. In this program on cross-sections was marked: geographical directions, control point, boreholes, faults, limits of colluviums and describe type of rocks below. Georadar (GPR) crosssection show, that on research area the colluviums deposits can be up to depth 1-7 m. Below colluviums the claystone and mudstone are very strong deeping (? vertical). In this rocks were found faults. Comparison with geotechnical methods GPR profiling was essential for engineering geology cross-section construction and recognition of landslide colluviums. Obtained results were corrected for slopes morphology and calibrated by boreholes. Geotechnical investigations covered different types of in-situ and laboratory tests. It included core drillings and sampling, in-situ vane sounding, laboratory tests (index tests, oedometer, direct shear 735

tests), ground movements inclinometer measurements and groundwater level and pore pressure monitoring. GPR surveys were calibrated and compared with other types of field and laboratory investigations.

6 tests), ground movements inclinometer measurements and groundwater level and pore pressure monitoring. GPR surveys were calibrated and compared with other types of field and laboratory investigations. Caution was paid to careful interpretation of data and scaling. Detected vertical inclinations of layers were prone to water infiltration into deeper geological layers. Pore pressure measurements indicated that reported values of pore pressure at the depth of 5 m rose was from 28 kpa to 42 kpa after high precipitation in July 28. Also groundwater level was very shallow and varied mainly between.8-1 m below the natural terrain level. Two landslide areas were detected. Geodynamic processes were recognized on slopes in both north and south directions with a thickness of 1-14 m. In the valleys at the distance of 4 m from the upper flat area the colluviums were 6 m thick. Geotechnical monitoring detected that movements up 5 mm in 5 months periods occurred also at the depth of 1-16 m and they have different magnitudes and depths mainly due to the flysch sediments lithology nature (Fig. 7, 8, 9). Geotechnical monitoring interpretations shows that it has quite good comparison with GPR scanning results (Fig. 3, 4, 5, 6). Fig. 6. Geotechnical cross-section A-A1 based on the GPR results and boreholes with ground movements monitoring localization Fig. 7. Landslide monitoring measurements 736

7 Deflection (mm ) Deflection (m m) -5-2,5 2,5 5 LEG END -5-2,5 2,5 5 Initial 1 V28 3 VI VII VIII28 11 IX X Depth Depth (m) (m) ,9 mm at 17,3 m 4,5 mm at 2,8 m ,8 m m at 5,3 m 5, mm at 2,8 m ,5 2, ,5 2,5 5 Cum ulative Deflection Cum ulative Deflection Direction A Direction B Underground Gas Storage Strachocina, Inclinom eter CC1 PGNIG Sanok Strachocina Fig. 8. Results of inclinometer CC1 (main movements directions and depths - arrows) D eflection (m m ) D eflection (m m ) -5-2,5 2,5 5 LEG EN D -5-2,5 2,5 5 Initial 14 VII28 1 VIII IX X D epth D e pth (m ) (m ) ,4 m m at 4,3 m 1,9 m m at 14,8 m ,8 m m at 15,3 m 3,8 m m at 1,8 m ,5 2, ,5 2,5 5 C u m ulative D eflection C u m ulative D eflection D irection A D irection B U n derground G as S torage, Inclino m eter C C 2 P G N IG S anok Strachocina Fig. 9. Results of inclinometer CC3 (main movements directions and depths - arrows) Over 3 sets of geotechnical laboratory tests included index tests: grain size, moisture content, plastic and liquid limits, unit weight, content of organic material. Very high values of soil moisture and plasticity index up to 5% were recognized to the depth of 1 m. Soils are characterized also by extremely high 1% content of organic/bituminous material. At depth m below the natural terrain level were still high values. Laboratory tests results are in quite good relation with landslide depth detected by monitoring measurements and GPR (Fig. 1). Results of oedeometer consolidation tests detect extremely high expected ground subsidence. Direct shear tests allowed soil mechanical parameters interpretation these were characterized by cohesion values 6-2 kpa and friction angles degree soil-rock type flysch deposits which were prone to creep processes and dangers for direct foundation design. 737

8 Changes of soil moisture and plasticity index with depth 6 Soil moisture,plasticity index[%] y = -,872x + 34,51 2 y = -1,953x + 34, Depth [m] Moisture [%] Plasticity index [%] Landslides depths Fig. 1. Comparison on laboratory test with landslide depths from GPR and monitoring Conclusions Ground Penetration Radar (GPR) method with careful calibration was found an effective and inexpensive way of geotechnical and geological investigations for foundation design on depleted gas field Strachocina. Together with other engineering geology methods it made possible detailed recognition of mass movements areas and description of colluviums and faults. It allowed delivery of detailed engineering geology site investigation report to the depth of 2 m for gas storage designers. One of the main advantages of GPR method was its efficiency. It was found that landslide stabilization work will be necessary on large parts of investigated area to avoid serious threats for designed facilities. It will include piles drilled into the bedrock at the depth of meters and some other types of construction work including building of effective drainage system. Obtained data was also used in slope stability calculations and stabilization project control. GPR allows to recognize many geological and tectonic structures. Results indicate that GPR scanning with proper correlation by engineering geology and geotechnical methods could help in recognition of internal geology for civil engineering project area and to avoid serious problems during construction works in unstable areas (Bednarczyk 25; 27, 28; Bednarczyk & Szynkiewicz 24). Caution however should be paid in obtaining results of interpretation and calibration. References Bednarczyk Z. 28 Landslide geotechnical monitoring network for mitigation measures in chosen locations inside the SOPO Landslide Counteraction Framework Project Carpathian Mountains, Poland The First World Landslide Forum, Tokyo organized by ICL, ONZ, UNESCO, WMO, United Nations University Tokyo (UNU), Kyoto University, Japan Landslide Society, pp Bednarczyk Z. 28 Application of GPR Scanning for Landslide Investigations in Polish Carpathians Near Surface 28, 14 th European Meeting of Environmental and Engineering Geophysics, European Association of Geosientists and Geoengineers, Krakow, Landslides and Geohazards, paper no B-18, CD-edition 4 pp. Bednarczyk Z. 27. Engineering geology investigation of Carpathian flysch landslides in the Gorlice region. Proccedings of III Polish Symposium Recent Problems of engineering geology in Poland. IAEG: Bogucki Publishing. Bednarczyk Z. 25 Geotechnical and geophysical methods for soil and rock design parameters characterization in mass movement s areas. Centre of Excellence Research on Abiotic Environment (REA) under the guidance of European Community, PGI Spec. Papers 2: Krakow. Bednarczyk Z, Szynkiewicz A. 24 Ground penetration radar (GPR) monitoring of flysch landslide in Lachowice (Carpathian Mountains). Górnictwo Odkrywkowe, 7-8/24:37-4. Wroclaw. Daniels D.J. 24 Ground-penetrating radar 2 nd ed. IEE Radar, sonar, navigation & avionics series 15: 726. The Institution of Electrical Engineers, London U.K., MPG Books Limited, Bodmin, Cornvall. 738

APPALACHIAN COLLUVIAL

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