Applications :: Engineering Applications Page 3

The geophysical applications in engineering include those in geotechnical engineering, civil engineering, offshore drilling, offshore cable and pipeline route selecting, tunneling, transportation, and construction.

Roadway subsidence is caused by sinkhole developing and soil compacting. Several methods can be used to locate cavities. These include gravity, resistivity (and conductivity), seismic methods, and Ground Penetrating Radar (GPR). The methods that should be used depend significantly on the host geology, and the depth and configuration of the voids. EM31 and tow array resistivity methods are used to detect clay underneath roadbed.

Vibrations from blasting, traffic, and construction can impact three areas of concern: human perception, harmful effects, and structural damage. Special care must be taken to mitigate their effects.

Shafts for deep foundations must be tested to ensure their integrity. During construction, the drilled shafts can be imaged using various testing methods for integrity assessments. This is important since defects in the shaft may result in structural instability and/or safety concerns. These tests are called Nondestructive Tests (NDT) and involve measuring various physical properties of the shafts. Commonly used techniques include crosshole sonic logging, gamma-gamma density logging, sonic echo/impulse response, single hole sonic logging and spectral analysis of surface wave.

Scour has been linked to nearly 95% of all severely damaged and failed highway bridges and port piers constructed over waterways in the United States. There are two issues associated with such scour-induced damage to bridge/port pier footings. The first effect is the loss of foundation material, which exposes the footing and lowers its factor of safety with regard to sliding or lateral deformation. The greatest loss of sediment to scour occurs at high water velocities, such as during floods. Secondly, pier movement may occur because of material loss beside and beneath the base of the footing, which produces undesired stresses in the bridge structure and ultimately results in structural collapse.

Current methods for measuring scour include visual underwater bridge inspection and various sonic devices such as sonar, sounding rods, and buried or driven rods, along with other buried devices. Methods used to detect scour include Time Domain Reflectometry (TDR), Parallel Seismic, Ground Penetrating Radar (GPR), Continuous Seismic Profiling (CSP), and Fathometer.

1. QA/QC of new deck QC programs intended to address construction quality of decks routinely include (a) visually inspecting the forms and deck reinforcement at regular intervals during construction (and carefully repairing nicks and scratches found in epoxy-coated bars within the reinforcing cage with fresh epoxy); (b) careful field and laboratory testing of the quality of materials used in its construction; and (c) testing and inspection activities, both in the field and laboratory, associated with placing and curing the concrete. Results are carefully measured and archived as a permanent part of the job record and/or used to modify field (construction and inspection) practices and take corrective action while construction is still underway.
   
   Quality control of construction practices and materials selection contributes to a deck that can last at least as long as its design life, from meticulous construction and inspection of the reinforcing cage to care in designing, mixing, placing, finishing and curing the concrete, along with proper monitoring during each of these processes. This quality-control series of activities is consistently practiced, in general, throughout most of the industry.
   
   However, the quality assurance component, particularly as it relates to inspection of the internal condition of the deck after construction, generally receives much less attention and care, because it seems to be physically more difficult to do. In addition, when accuracy or reliability of QA inspections come into doubt, QA can often be viewed as an added construction cost with low, or indeterminate, perceived value. Well-written QA verification specifications in terms of the desired inspection outcomes and consequences related to noncompliance often become seemingly meaningless when inspection capabilities appear to fall short of expected results. This statement is particularly true when the inspection methods cannot be effectively critiqued or corrected, resulting in an inherent ineffectiveness in their ability to be used for enforcement or improvement of QA policies.
   
   GPR, spectral analysis of surface wave (SASW), and impact echo are used for QA.
   
   
2. Condition evaluation of existing bridge deck As a reinforced concrete bridge deck deteriorates with time and exposure, a proper diagnosis and preventive maintenance schedule, whereby smaller and less expensive repairs are made at properly timed intervals throughout the life of the structure, significantly extends its life and reduces the costs associated with its ownership.
   
   GPR can be used to determine the rebar density. The combination of GPR and half-cell corrosion potential can be used to determine the corrosion activity of rebar. The combination of GPR, Impact echo, and SASW can be used to test deck condition/integrity and detect incipient spalling.

Integrated bathymetry, side scan sonar, and high resolution seismic reflection profiling are used for bedrock topography mapping, and for pre and post-dredging comparison.

Bridge foundations can be divided into shallow footings or deep foundations. Footings are mostly square or rectangular in shape. They may also be pedestal masonry stone footings or massive cofferdam footings in shape. Piles might be present with or without pile caps and may be battered or vertical. Piles can be made of concrete (round, square, or octagonal), steel (H-piles or round pipe sections), or timber. Deep foundations can be pre-cast concrete piles, or more recently, drilled shafts and auger-cast concrete piles. The top of footings or pile caps may be buried underneath riprap, backfill mud and/or channel soils.

Nondestructive tests (NDT) that are used for the determination of the unknown depth of foundations usually involve seismic methods and are often called small strain tests. The term "small strain test" is used to describe tests where a small seismic energy source, such as a hammer, is used to generate the seismic waves.

These tests can be divided into two groups: Surface NDT, if access is required only at the surface of a foundation, or Borehole NDT, if a borehole is drilled close to the foundation structure and extends along its length.

Five potential surface NDT methods are used to determine foundation depths. These are Sonic Echo, Bending Waves, Ultraseismic Vertical Profiling, Seismic Wave Reflection Survey, and Transient Forced Vibration Survey. Borehole NDT methods include Parallel Seismic, Borehole Radar, Magnetic Methods, Dynamic Foundation Response, Borehole Sonic, cross-borehole tomography, and, where the substructures contain steel, Induction Field.