Identification on Unknown Bridge Foundations Using Geophysical Inspecting Methods

Transcription

1 More Info at Open Access Database Identification on Unknown Bridge Foundations Using Geophysical Inspecting Methods Helsin WANG 1, Chih-Hsin HU 2 1 Institute of Bridge Engineering, China Engineering Consultants, Inc.; Taipei, Taiwan; Phone: ext.1218, Fax: ; herschel39@gmail.com 2 HCK Geophysical Company; Taipei, Taiwan; sunhu88@ms29.hinet.net Abstract In order to evaluate the foundation stability on old in-service bridges, non-destructive testing (NDT) is required to inspect their unknown depths in advance. In this paper, two proposed NDT methods, including electrical resistivity tomography (ERT) and ultra-seismic (US) technique, are introduced to detect the foundation depths on two river-crossing pre-stressed concrete bridges in northern Taiwan. The bridge agency also requests proof excavation and borehole sampling in order to re-verify the reliability of NDT inspection results. Comparing all information, the ERT reasonably identifies the foundation type and US presents an accurate inspection ability to determine their foundation depths. Keywords: Unknown foundation, electrical resistivity tomography, ultra-seismic, parallel seismic, excavation, borehole sampling 1. Introduction Current condition of bridge foundations is crucial information for bridge engineers to evaluate their stability. For example, lacking design information on foundations could impede of rating the flood or earthquake resistance on old bridges. Furthermore, materials deterioration, scour variation, or severe cracking could also undermine the foundation capacity of bridges as well. Non-destructive testing (NDT) has been developed to assess the integrity of foundations for many years. Currently, both the surface reflection technique, including impulse response and ultra-seismic (US) methods, is commonly used to evaluate the integrity of concrete piles, drilled shafts, and caissons in field [1, 3, 5-7]. Besides wave-based inspection, the geophysical inspection methods are introduced to subsurface and substructure investigation for bridges in past few years. The most useful technique electrical resistivity tomography (ERT) provides an outstanding identification on the interface between foundations and surrounding soils/rocks [4, 6-8]. In addition, since site conditions usually vary from dry riverbed, floodplain, to flowing water, the type of NDT inspection is associated with site situation. Onsite construction record or post-excavation is strongly suggested to re-verify the reliability of NDT inspection results [2]. 2. Geophysical inspecting methods 2.1 Electrical Resistivity Tomography The principle of electrical resistivity tomography is to develop an artificial potential field by installing a pair of current electrodes (A and B) into the ground surface around a target zone (Figure 1). Another pair of electrodes (M and N) are used to detect the ground potential difference within a specific zone. Traditionally, the measured resistivity image illustrates the resistivity intensity distributions in space by visualizing a display format of color scale, corresponding to resistivity values from over 1,000 to less than 1 Ω-m [4]. This technique is based on how differences in field mineral composition, grain size, mineral formation, water content, and ion concentration affect the detected apparent electric resistivity. Hence, higher

2 conductivity values on resistivity images would indicate the locations of steel-embedded structures, anomalies, or pollutants. Figure 1. Schematic representation of the electrical resistivity tomography method Figure 2. Schematic representation of the ultraseismic method 2.2 Ultra-Seismic Method The US method is performed by aligning several receivers on one side of a partially exposed foundation (Figure 2). Strikes from a heavy hammer on the surface of the foundation are then used to generate small stress waves [5]. As the shock waves are transmitted through the foundation components, their reflection waves are generated at the interface of the foundation and the stratum (sand/clay/grave/rock and so forth). Due to variations in their acoustic impedances, the direct and reflection seismic waves can be directly recorded using multiplechannel seismograms, and used to correlate the travel times of the artificial elastic waves in the target foundation. Contrasting this data with waveform imaging can yield a more accurate estimate of the interfaces of the various foundation components and determine its reflection depths. 3. Investigation cases 3.1 A Bridge Supported by Shallow Foundations A single-lane pre-stressed concrete bridge consists of 6 spans with a total length of 94 meters and crosses over the Nanshi Stream in northern Taiwan (Figure 3). This bridge constructed in 1981 is repeatedly found with a foundation scour problem during its routine inspection. The bridge agency decides to evaluate its flood resistant capacity and its stability for further improvement, if needed. Unfortunately, its original substructure design information cannot be found in the Taiwan Bridge Management System. Therefore, the ERT and US inspections are required to determine the foundation information on Pier P5. (1). A 70-m long ERT layout line is selected to set 5 meters parallel to P5 pier wall using an electrode spacing of 2 meters (Figure 3(b)). The position of the target pier is located at the position of 32~39 meters on the probing line (Figure 4). The black dashed line represents the interface between sand/gravel and bedrock. Foundations, usually consisting of reinforced bars, could increase the conductivity value and also form a low resistivitycontent zone. The ERT image reveals a pretty low resistivity zone, as dark blue, at the position of 31~40 meters, which overlaps the P5 position. The step-like image is marked

3 with a red dashed line and identified as a footing and, overall, the substructure height is 7.8±1 meters. P5 P5 P4 P3 P2 (a) Figure 3. (a) Panorama of bridge site (after TBMS) and (b) ERT inspecting around Pier P5 (b) Figure 4. Electrical resistivity tomography image surrounding Pier P5 (a) (b) P5 Figure 5. (a) Ultra-seismic inspection conducted and (b) US inspecting outcome on Pier P5

4 (2). The exposed height of the substructure is around 3~3.5 meters above the sand/gravelcovering floodplain. Twelve sensors are aligned 0.5 meters below top surface in an equidistant linear fashion (0.25 meters) on one side of P5 downward to ground surface (Figure 5(a)). A hammer is used to strike P5 surface, thereby generating elastic waves. As the impact waves are travelling through the foundation components, reflection waves are generated at the interface of the foundation and soil layer. The reflected waveform imaging indicates a depth of 7.6±0.25 meters as the reflection interface from foundation bottom (Figure 5(b)). Proof excavation is chosen to confirm the substructure and subsurface situation on P5. An open excavation lowered the excavation elevation down to 4.1 meters from the ground surface as shown in Figure 6. Two step concrete footings are found to be connected right below the exposed pier wall. Figure 7 shows the measured geometric dimensions of substructure profile on P5. Accordingly, the estimated heights of the substructure are 7.8±1 meters and 7.6±0.25 meters for ERT and US inspection, respectively. The excavation results positively confirm the predictions on the foundation depth from the ERT and US inspection. Figure 6. Site proof excavation on Pier P5 Figure 7. Substructure profile of Pier P5

5 4.2 A Bridge Supported by Caissons A 3-span 94-m length twin-lane vehicular pre-stressed concrete bridge constructed in 1999 crosses the Dahu Stream in northern Taiwan (Figure 8(a)). Due to channel variations, the unknown effective embedment depth of this bridge arises as the key issue for assessing its possible stability during storms or typhoon-induced floods. Therefore, NDT inspection is conducted to collect the essential information on the foundation on Pier located in the main channel. P2 (a) Figure 8. (a) Panorama of bridge site (after TBMS) and (b) ERT inspecting around Pier (b) Figure 9. Electrical resistivity tomography image surrounding Pier The ERT inspection provides resistivity distribution over a wider area of formation characteristics and preliminarily identifies the interface between the foundation and surrounding soil layer. A 70-m long ERT layout line is selected and set from the upstream side of by using an electrode spacing of 2 meters (shown in Figures. 8(b) and 9). The position of the target pier was located at 33~37 meters on the ERT inspection image. The reinforced concrete foundation presents as an extremely low resistivity closed contour. The ERT image significantly reveals a relatively low resistivity zone, marked as a black dashed rectangle. The bottom position of the black dotted rectangle indicates the interface between

6 the foundation and gravels. The estimated depth of the substructure is around 6.5±1.0 meters from the top of the foundation apron. (a) (b) Figure 10. (a) Ultra-seismic inspection conducted and (b) US inspecting outcome on Pier The exposed height of the pier is 5 meters above the caisson top. Twelve sensors are aligned in an equidistant arrangement (0.25 m) along the pier side of, downward to the water surface (Figure 10(a)). A heavy hammer is used to strike the pier surface in order to generate stress waves. As the impact waves passed through the foundation component, their reflection waves are generated at the interface of the caisson and the gravel layer. The reflected waveform imaging indicates the depth of the foundation tip at 6.4±0.25 meters, identified as the reflection source (Figure 10(b)). Borehole exploration is conducted to confirm the actual condition of the substructure on. The borehole sampling depth is up to 8 meters from the caisson top (see Figure 11). The measured depth of the caisson is identified as 6.3 meters for further comparison. Figure 11. Borehole exploration on Pier The estimated heights of the caisson foundation using the ERT and ultra-seismic inspection techniques are 6.5±1.0 meters and 6.4±0.25 meters, respectively. Figure 12 shows the actual geometric dimensions of the substructure profile on. This validates the accuracy of the estimations of the foundation depth by using the ERT and US inspection techniques.

7 Figure 12. Substructure profile of Pier 5. Conclusions Two proposed NDT methods, including ERT and US inspection, are conducted to identify the conditions on two unknown bridge foundations. The ERT provides resistivity distribution over a wider area of formation characteristics and identifies the interface between bridge substructure and surrounding strata. The US method presents a precise ability to determine the depths of substructures. Accordingly, the reliability of these NDT inspection results is reconfirmed with the findings of proof excavation or borehole exploration. The entire investigation also re-verifies that a solid conclusion could be determined if two inspecting methods reach the identical answers [7]. Acknowledgements The authors would appreciate the financial support and execution assistance of the Public Works Department, Miaoli County Government in Taiwan. References 1. Chinese Institute of Civil and Hydraulic Engineering (CICHE) (2010), Inspection Method and Application of Bridge Inspection, Scientific and Technical Publishing Co., Ltd., Taipei, Taiwan, pp. 5-27~ Geo-Institute Deep Foundation Committee (2000), Nondestructive evaluation of drilled shafts, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 126, No. 1, pp

8 3. Hertlein, B.H. and Davis, A.G. (2006), Nondestructive Testing of Deep Foundations, John Wiley & Sons Ltd., Chichester, U.K. 4. Loke, M.H. (2000), Electric Imaging Surveys for Environmental and Engineering Studies A Practical Guide for 2-D and 3-D Surveys, ABEM Instrument AB, Sunbyberg, Sweden. 5. Olson, L.D., Jalinoos, F., and Aouad, M.F. (1998), Determination of Unknown Subsurface Bridge Foundations, NCHRP Project No.E21-5, Transportation Research Board, National Research Council, Washington, D.C., U.S.A. 6. Wang, H., Wang, C.-Y., and Hu, C.-H. (2012), Conditioning inspection and interpretation on bridge foundations, Proceedings of the 36 th National Conference on Theoretical and Applied Mechanics, Chung-Li, Taoyuan, Taiwan, pp. R-009-1~R Wang, H., Hu, C-H., and Wang, C.-Y. (2014), Conditioning Inspection on Unknown Bridge Foundations, The e-journal of Nondestructive Testing, Vol. 19, No. 12, Session NDT Civil Engineering and Concrete Structures, Sub. ID: 2 (total 8 pages). 8. Wightman, W.E., Jalinoos, F., Sirles, P., and Hanna, K. (2003), Application of Geophysical Methods to Highway Related Problems, FHWA Report No.DTFH68-02-P , Central Federal Lands Highway Division of the FHWA, Lakewood, Colorado, U.S.A.