Top Geophysical Techniques for Bridge Site Investigations

Dr. Sanjay Rana, Director, PARSAN Overseas Pvt. Ltd
sanjay@parsan.biz

INTRODUCTION

Modern major construction is inconceivable without high-level engineering explorations, which play a major role in increasing the economic efficiency of capital investments. For the design of structures it is indispensable to procure comprehensive high-quality information about the subsurface, within very short periods. The study of diverse natural conditions predetermines a variety of methods and technical means which can be used for carrying out exploratory work.

The geological aspects of the civil engineering site, surface and subsurface, have to be studied in details before commencement of the project. The civil engineer, engineering geologist and geophysicist work in collaboration at various stages to complete tasks like mapping, site exploration, groundwater studies, slope stability, seismicity etc.

Any major construction work like power plants, high rise buildings, bridges etc., calls for a well-considered approach in view of the restricted timeframe and finance. In planning and development of project adequate knowledge of the geotechnical conditions at site is important besides the other factors that are involved. Application of tools and techniques that are helpful in enhancing efficiency of the geotechnical evaluation study is therefore preferable.

Engineering geophysics is an efficient means of subsurface investigation. The merit of application of this low cost aid lies in its ease of deployment and rapidity in providing a reliable knowledge of the underground over a large area, substantiating the requisite geotechnical evaluation studies thereby. Technological advancements and development of portable digital data acquisition instrument systems have increased the versatility in evaluating underground conditions and site characterization.

The state-of-the-art non-destructive subsurface geophysical investigations are helpful towards minimizing involvement of the conventional direct invasive exploration methods, aiding in accelerated and economical development of the construction projects.

WHAT IS ENGINEERING GEOPHYSICS?

Engineering geophysics is the application of geophysics to geotechnical engineering problems; such investigations normally extend to a total depth of less than several hundred feet but can be extended to thousands of feet in some instances. Geotechnical geophysical surveys are performed on the ground surface, within boreholes, and from the water and air. Continuing improvements in survey equipment performance and automation have made large area surveys with a high data sample density possible. Advances in processing and imaging software have made it possible to detect, display, and interpret small geological features with great accuracy.

Engineering geophysics is routinely used for many types of civil engineering investigations, including:

1. Subsurface characterization: bedrock depth, rock type, layer boundaries, water table, groundwater flow, locating fractures, weak zones, expansive clays, etc.

2. Engineering properties of Earth materials: stiffness, density, electrical resistivity, porosity, etc.

3. Highway subsidence: detecting cavities beneath roadways caused by sinkholes, abandoned mines, etc.

4. Locating buried manmade objects buried utilities, underground storage tanks, etc.

WHY USE ENGINEERING GEOPHYSICS?

A geotechnical geophysical survey is often the most cost-effective and rapid means of obtaining subsurface information, especially over large study areas. Geotechnical geophysics can be used to select borehole locations and can provide reliable information about the nature and variability of the subsurface between existing boreholes. An isolated geologic structure such as a limestone pinnacle might not be detected by a routine drilling program (Figure 1). An effective geophysical survey however, could detect the presence of the pinnacle and map the height and aerial extent of the same.

Figure 1: An isolated geologic structure such as a limestone pinnacle might not be detected by a routine drilling program.

Other advantages of geotechnical geophysics are related to site accessibility, portability of instruments, non-invasiveness, and operator safety. These methods can also provide temporal measurements (detecting changes in conditions with time). Geophysical equipments can often be deployed beneath bridges and power lines, in heavily forested areas, at contaminated sites, in urban areas, on steeply dipping slopes, in marshy terrain, on pavement or rock, and in other areas that might not be easily accessible to drill rigs or cone penetration test (CPT) rigs. Also, most surface-based or airborne geophysical tools are noninvasive and, unlike boring or trenching, leave little, if any, imprint on the environment. These considerations can be crucial when working in environmentally sensitive areas, on contaminated ground, or on private property. In addition, geophysical surveys are generally considered less dangerous than drilling since there are fewer risks associated with utility encounters and operations. Lastly, geophysical surveys can enable engineers to reduce the number of required boreholes.

However, engineering geophysics is not a substitute for boring and direct physical testing. Rather it complements a well-planned, cost-effective drilling and testing program, and provides a volumetric image of the subsurface rather than a point measurement. Geophysicists refer to borehole information and field geologic maps as ground truth, and rely on ground truth to constrain and verify all geophysical interpretations.

GEOPHYSICAL METHODS OF SITE INVESTIGATION AND CONDITION MONITORING FOR BRIDGES

A bridge is defined as a structure which provides a passage over a gap without closing the way beneath for speeding transportation of men and material. While a bridge is a structure that connects two or more points separated by a natural phenomenon such as a river, valley, sea or any other water body, a flyover is a structure that connects two or more points already accessible and is really flying over and/or across man made features such as roads, railways.

Before selecting a site for a bridge, it is necessary to study the requirements of the bridge, volume of traffic, nature and extent of river system, hydro meteorological factors, maximum flood levels, geological conditions, technical feasibility, seismicity of the region, economic factors etc. Also, few of the bridge design considerations include the following:

� At bridge site, reach of stream should be straight

� It should be away from the fault zone, having unyielding, non-erodible foundation for abutments and piers

� The stream should be narrow with well defined and firm banks at the site of construction; river flow should be without whirls and crosscurrents

� There should be suitable high banks above high flood level on each sides.

For effective site investigation and characterization few of the obvious geological factors taken into consideration are:

� The type of the rock and their strength and deformation behavior i.e., igneous, sedimentary or metamorphic

� Depth of bedrock

� Soil profile

� Geological discontinuities and associated strength and deformation behavior i.e., folds, faults, joints and unconformities

� Groundwater conditions

� Squeezing and swelling rock conditions

� Running Ground

� Gases in rocks

� Rock temperature

� Topographic conditions and structural dispositions

A variety of remote sensing, surface geophysical, borehole geophysical and other non-destructive methods can be used for site characterization prior to construction, during the construction phase to test the bridge foundation integrity or capacity and stresses of a driven pile or monitor vibrations and post construction quality control and assessment of bridges. Satellite data, aerial photography and airborne geophysical measurements are used to provide reconnaissance level data over large areas. While surface geophysical methods yield much less spatial coverage per unit time than the airborne methods, they significantly improve resolution while providing subsurface information upto a few 100 feet. Downhole geophysical methods are used to provide very localized details down a borehole, corehole in a concrete footing or pile or a well. Unlike surface methods, the resolution of downhole logging is independent of depth. Also, measurements between boreholes increase the volume being sampled. Unlike direct sampling, these methods provide in-situ measurements of some physical, electrical or chemical property of the soil, rock and pore space fluids or some property of the subbase, asphalt or concrete.

SITE INVESTIGATION FOR NEW BRIDGES

Seismic Refraction Surveys

Seismic technique is one of the most developed geophysical techniques, providing vital information on subsurface, crucial for most of the engineering projects. Seismic Refraction surveys are routinely carried out for assessment of subsurface conditions prior to engineering projects. The recent development has been use of seismic refraction tomography which provides much more realistic and accurate subsurface velocity model compared to typical layered models. An example gradient velocity model, with conventional layered model superimposed has been presented in Fig-1 hereunder:

Fig-2: Example Seismic Refraction Result

Based on the velocity model, thickness and topography of overburden, weathered rock and bedrock are easily obtained based on P-wave velocities.

Key features of seismic refraction survey are:

� Precise determination of soil thickness.

� Precise determination of seismic velocities.

� Precise determination of water table in overburden.

� Localization and identification of geological units.

� Detailed analysis of soil.

� Great accessibility to rough terrain and remote regions.

Key advantages of seismic refraction survey are:

� Rapid ground coverage.

� Only option in rough remote terrains.

� Provides continuous profile of subsurface, critical for engineering projects.

� Estimation of Dynamic Elastic Moduli like Poissons Ration, Youngs Modulus, Shear Modulus.

Key applications of seismic refraction survey are:

� Bedrock profile, rock quality and depth.

� Thickness of overburden

� Fractures and weak zones

� Topography of ground water

� Rippability assessment in mines

� Slope stability studies

� Pipeline route studies

Key limitations of seismic refraction survey are:

� Velocity increase with depth a pre-requisite

� Hidden layer & Blind Zone anomalies

Electrical Resistivity Imaging

2D Resistivity Imaging uses an array of electrodes (typically 64) connected by multicore cable to provide a linear depth profile, or pseudosection, of the variation in resistivity both along the survey line and with depth. Switching of the current and potential electrode pairs is done automatically using a laptop computer and relay box. The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased and the process repeated in order to provide an increased depth of investigation.

The technique is extremely useful for investigations of important sites to get information on weak zones or buried channels, under the rock interface, which goes undetected in seismic refraction, which terminated at rock interface. Resistivity imaging can also be effectively used to determine rock profile along dam axis across high current shallow rivers where deployment of hydrophones is not possible restricting use of seismic refraction. For deep penetration seismic refraction techniques requires use of explosives, which are not always feasible to deploy especially in sensitive areas. In such cases resistivity imaging can be effectively used to get detailed information of deeper layers.

Following are examples from work of resistivity imaging carried out at HEP sites.

Fig-3: Example Electrical Resistivity Imaging Result

Key applications of Electrical Resistivity Imaging are:

� Determine the underground water resources

� Bedrock quality and depth measurements

� Mineral prospecting

� Dam structure analysis

� Landfill

� Contamination source detection

Key advantages of Electrical Resistivity Imaging are:

� Excellent 2-dimensional display of ground resistivity.

� Delineation of small features like cavity, contamination plumes, weak zones in structures like dams etc.

ReMi (Refraction Micro-Tremor)

Innovative technique of ReMi (Refraction Micro-tremor) has distinct edge over MASW and SASW in terms of logistics, execution and results. ReMi can be performed under the same layout as used for seismic refraction, to obtain excellent shear wave velocity profiles of subsurface. ReMi is a new, proven seismic method for measuring in-situ shear-wave (S-wave) velocity profiles. It is economic both in terms of cost and time. Testing is performed at the surface using the same conventional seismograph and vertical P-wave geophones used for refraction studies. Depending on the material properties of the subsurface, ReMi can determine shear wave velocities down to a minimum of 40 meters (130 feet) and a maximum of 100 meters (300 feet) depth.

The ReMi method offers significant advantages. In contrast to borehole measurements. ReMi tests a much larger volume of the subsurface. The results represent the average shear wave velocity over distances as far as 200 meters (600 feet). Because ReMi is non-invasive and nondestructive, and uses only ambient noise as a seismic source, no permits are required for its use. ReMi seismic lines can be deployed within road medians, at active construction sites, or along highways, without having to disturb work or traffic flow. Unlike other seismic methods for determining shear wave velocity, ReMi will use these ongoing activities as seismic sources. There is no need to close a street or shut down work for the purpose of data acquisition. And a ReMi survey usually takes less than two hours, from setup through breakdown. These advantages sum to substantial savings in time and cost. Moreover the method provides more accurate results compared to conventional effort of picking up shear waves from records which more often than not lead to errors.

Typical 2D ReMi profile constructed using various 1D profiles is shown hereunder showing shear wave velocities upto a depth of 100m:

Fig-4: Example ReMi Result

The shear wave information obtained through ReMi is used for:

� Earthquake site response

� Liquefaction analysis

� Soil compaction control

� Mapping the subsurface and estimating the strength of subsurface materials

� Finding buried cultural features such as dumps and piers

Cross-hole Seismic Surveys

The primary purpose of obtaining cross-hole data is to obtain the most detailed in situ seismic wave velocity profile for site-specific investigations and material Characterization. Cross-hole velocity data are valuable for assessing man-made materials, soil deposits, or rock formations.

The seismic technique determines the compressional (P) and/or shear (S) wave velocity of materials at depths of engineering and environmental concern where the data can be used in problems related to soil mechanics, rock mechanics, foundation studies, and earthquake engineering.

The set-up for cross hole tests include a source hole and 02 receiver holes as shown hereunder:

Fig-5: Typical Cross-hole Setup

The method allows precise determination of P and S wave seismic velocities which leads to determination of Dynamic Elastic Moduli like Poissons Ration, Youngs Modulus, Shear Modulus. Cross-hole method detects even thin anomalous zones in subsurface.

Ground Penetrating Radar

Ground Penetrating Radar, also known as GPR, Georadar, Subsurface Interface Radar, Geoprobing Radar, is a totally non-destructive technique to produce a cross section profile of subsurface without any drilling, trenching or ground disturbances. Ground penetrating radar (GPR) profiles are used for evaluating the location and depth of buried objects and to investigate the presence and continuity of natural subsurface conditions and features.

Key application areas of GPR are:

� Geological and hydro-geological investigations including mapping of bedrock topography, water levels, solution features, glacial structures, soils and aggregates.

� Engineering investigations to evaluate dams, sea walls, tunnels, pavements, roadbeds, railway embankments, piles, bridge decks, river scour, buildings and monuments.

� Location and evaluation of buried structures including utilities, foundations, reinforcing bars, cavities, tombs, archaeological artifacts, and animal burrows.

� Site investigations: location of buried engineering structures and underground storage tanks.

� Subsurface mapping for cables, pipes and other buried structures prior to trench-less operations.

Fig-5: GPR field work

Key advantages of GPR are

� Rapid ground coverage- Antenna towed either by hand or from a vehicle.

� High-resolution coverage of the survey area, detecting even small objects.

� On-site interpretation possible due to instant graphic display.

Key limitations of GPR are:

� Data acquisition may be slow over difficult terrain.

� Depth of penetration is limited in materials with high electrical conductivities, clays.

� Energy may be reflected and recorded from aboveground features, walls, canopies, unless antennae are well shielded.

� Artifacts in the near surface (reinforcing bars, boulders, components of made ground) may scatter the transmitted energy and complicate the received signal and/or reduce depth of penetration.

� Working on principle of reflection, GPR detects the utilities and provides information on depth and location. Classification of utility any further can be done only with availability of background data and is not a deliverable of GPR survey.

Micro-Gravity Survey

Gravimetry is a potential field technique which measures variations in the Earth’s gravitational field. These variations are caused by density contrasts in the near surface rock and sediment. Gravimetric surveys are carried out using extremely sensitive instruments capable of measuring tiny variations in the gravitational field. These surveys are always carried out in conjunction with a precise topographic survey, to enable terrain corrections to be carried out.

Typical applications are:

� Regional geological mapping;

� Oil and gas exploration;

� Mineral exploration;

� Sediment thickness studies;

� Archaeological surveys;

� Void detection.

The technique is extensively used for cavity detection, along with other techniques of electrical imaging, GPR etc.

Gravity measurements define anomalous density within the Earth; in most cases, ground-based gravimeters are used to precisely measure variations in the gravity field at different points. Gravity anomalies are computed by subtracting a regional field from the measured field, which result in gravitational anomalies that correlate with source body density variations. Positive gravity anomalies are associated with shallow high density bodies, whereas gravity lows are associated with shallow low density bodies. Thus, cavities and voids can be readily detected using micro-gravity surveys.

USE OF GROUND PENETRATING RADAR FOR CONCRETE INSPECTION

Ground Penetrating Radar is being routinely used for applications like Utility Detection, Archaeology, Shallow Stratigraphy and Military applications. The use of technology has become popular for concrete inspection with the advent of digital data processing techniques and 3D processing capabilities.

Ground Penetrating Radar, as the name suggests, uses high frequency radio waves (ranging from 20 MHz to 2000 MHz) to obtain high resolution image of subsurface. Any objects having a contract in dielectric constant (also known as relative permittivity) from its surroundings, can be effectively mapped using this technique. Data collection is continuous, generally done at walking speed, making it possible to scan large areas very quickly.

The applications for concrete inspection most commonly include locating spacing and depth of reinforcing steel, post tensioning cables or anchors, measuring rebar cover, mapping voids, and clearing areas prior to cutting, coring and trenching. Structural applications include addressing the integrity of the concrete itself, such as the presence of voids, cracks or chemical alteration.

Using GPR to Locate Rebar

Concrete and construction professionals use ground penetrating radar to safely locate structures within poured concrete prior to drilling, cutting or coring. Take advantage of locating targets in real time, with only single-sided access required.