Geophysical Investigation Techniques for New Bridges

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

INTRODUCTION

Modern bridge construction is inconceivable without high-level site explorations, which play a major role in optimizing the design and costs. 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. 

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 and is use for following:

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.

WHY USE ENGINEERING GEOPHYSICS?

A geophysical survey is the most cost-effective and rapid tool of obtaining subsurface information.  It can be used to select borehole locations and can provide reliable information about the nature and variability of the subsurface between boreholes. Isolated geologic structures cannot be detected by a routine drilling program (Figure 1). 

Figure 1: Isolated subsyrface features cannot be detected by a routine drilling program.

Other advantages of geotechnical geophysics are site accessibility, lightweight instruments and operator safety. Geophysical equipments can deployed beneath bridges and power lines, in forests, in urban areas, on steep slopes, marshy terrain, on pavements and in other areas not easily accessible to drill rigs. 

Most importantly, geophysical surveys can reduce the number of required boreholes. However, engineering geophysics is not a substitute for boring and direct physical testing. It complements cost-effective drilling. 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 BRIDGE SITE INVESTIGATIONS

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

• The type of the rock i.e., igneous, sedimentary or metamorphic
• Depth of bedrock
• Soil profile
• Geological discontinuities 
• Groundwater conditions 

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. An example gradient velocity model, with conventional layered model superimposed has been presented in Fig-2 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.
• Localization and identification of geological units.
• Great accessibility to rough terrain and remote regions.

Key applications of seismic refraction survey are:

• Bedrock profile, rock quality and depth. 
• Thickness of overburden 
• Fractures and weak zones 
• Topography of ground water 
• 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 provides the variation in resistivity both along the survey line and with depth. 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 bridge axis across high current shallow rivers where deployment of hydrophones is not possible restricting use of seismic refraction. In such cases resistivity imaging (Fig -3) can be effectively used to get detailed information of deeper layers.

Fig-3: Example Electrical Resistivity Imaging Result

Key applications of Electrical Resistivity Imaging are:

• Determine the underground water resources 
• Bedrock quality and depth measurements 
• 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 

Multichannel Analysis of Surface Waves (MASW)

MASW analyses the propagation velocities of those surface waves, and provides shear-wave velocity (Vs) variations below the surveyed area.  Shear-wave velocity (Vs) is one of the elastic constants and closely related to Young’s modulus.  Under most circumstances, Vs is a direct indicator of the ground strength. After a relatively simple procedure, final Vs information is provided in 1-D, 2-D, and 3-D formats. Fig-4 below shows field data acquisition set up.