Fighting off bottom feeders

By: Michael J. Ganas, P.E.

Scouring of bottom material around bridge foundations is often the most common cause of damage to bridges spanning waterways, particularly since approximately 86% of all bridges in the National Bridge Inventory (NBI) are built over waterways. For this reason, scour evaluations are conducted to determine if a bridge is scour critical—that is, if it has an abutment or pier foundation which is rated as being unstable, or potentially unstable based on observations or from a scour evaluation study.

In assessing the potential for scour, inspectors need access to site-specific information if they are to determine if a bridge is scour critical, particularly waterway bottom substrata and bottom of footing or pile elevations with respect to the mud line. The material comprising a waterway bottom is an important consideration because different materials scour at different rates. Underaccelerated water flow, loose granular soils will erode more rapidly than cohesive or cemented soils which are more scour resistant. However, maximum scour in cohesive or cemented soils can be as deep or deeper than scour in sand. And while it may take days for a cohesive riverbed material exposed to heavy flow to reach its maximum depth, it may take only hours for a riverbed composed of sand and gravel to reach its ultimate depth under the same flow.

Quite often, as-built plans of a water-spanning bridge are non-existent, with pile driving records or actual installed lengths unavailable. When such a situation occurs, it is therefore impossible to make a viable assessment of the susceptibility to scour a bridge has since footing or pile embedment lengths are unknown, as is the subsurface soil composition. In order to determine embedment lengths, a nondestructive testing method referred to Parallel Seismic can be used.

In Parallel Seismic (PS) tests, a case borehole (usually 4 in. in diameter and capped at the bottom) must be installed adjacent to an existing pile or footing, typically within 3 - 5 ft, and placed to a depth at least 10 ft below the anticipated bottom of the embedded member. At the time of testing, the borehole casing is filled with water before a hydrophone receiver (i.e., transducer sensitive to changes in water pressure) is lowered to the bottom.

The PS method involves hitting any part of the bridge structure that is connected to the foundation with an instrumented hammer, causing low strain compression or shear waves to travel down the foundation into the surrounding subsoil. These waves ultimately trigger the hydrophone, which identifies direct arrival times of compression and shear waves at the receiver’s location. During testing, the receiver is raised in intervals of 1 - 2 ft, with hammer strikes being made at the same location on the structure.

At each elevation in the borehole casing, the hydrophone measures the difference in time between the hammer strike and the initial arrival of the wave which is plotted against the hydro-phone’s corresponding depth in the borehole. The slope of this curve indicates the speed of the sound wave through the material. The elevation of the pile tip or bottom of footing is indicated when a change in wave speed occurs between the foundation material and the soil. This determination is possible when the speed of sound in the foundation is sufficiently different from the sound speed in the soil medium, and works well for purely columnar foundation elements such as piles embedded in fully saturated soil (i.e., soil below groundwater level) where the surrounding soil wave speed is constant.

The PS method can be applied on concrete, timber, masonry and steel foundations. In addition, it can provide valuable information about the substrate material surrounding and below the bridge foundation. Leading researchers and practitioners consider the borehole parallel seismic method to be more accurate and versatile than other nondestructive testing methods for determining unknown depths of embedded structure foundations, including spread footings, driven piles and drilled shafts.

Crosshole Sonic Logging (CSL) tests are performed to evaluate the condition of concrete within footings by propagating ultrasonic waves between two access tubes to measure the velocity of the waves. The easiest application of the CSL method involves strapping water-filled access tubes such as PVC pipes to the exterior sides of concrete columns and foundations. By acting as guides for sound waves, the tubes allow signals to be transmitted through the outside of the concrete into the interior where they are recorded and analyzed.

Often, however, the concrete exterior may have undergone degradation either during the original construction due to aggregate separation or sometime after, or the outside of the footings are not fully exposed due to the soil embedment design or siltation which occurred after placement.

In such circumstances, a minimum of four drilled coreholes through the entire depth of the rectangular footings will allow testing along the edges and through the center in order to assess the integrity of the concrete. Upon drilling the coreholes, the extracted concrete cores can be laboratory analyzed for assessing compressive strength and petrographic characteristics of the concrete, therefore becoming another tool for evaluating the condition of the concrete.

In a CSL test, a source transducer is lowered to the bottom of one of the tubes while a hydrophone receiver, located at the same elevation as the source, is lowered to the bottom of another tube. The passage of ultrasonic pulses or compression wave energy is then measured between the source in one tube to the receiver in the other tube. The test typically involves raising both the source and receiver in equal increments prior to measuring the horizontal wave pulse at each level through the concrete medium.

Usually, the source is excited every 2 in. and a measurement is taken until both the source and receiver are pulled to the top of their respective shafts, using a depth wheel, thus giving a complete assessment of the concrete quality between the access tubes. The tubes are typically comprised of 1.5 - 2 in. inside diameter, Schedule 40 PVC plastic or steel, and must extend about 3 - 4 ft above the top of the foundation or shaft to compensate for the water displaced by the source, receiver and cables.

Collected data from CSL measurements will reveal the arrival time (or velocity) of first waves and their respective energy levels at all elevations, therefore allowing both of these parameters to be plotted against depth. A CSL log showing relatively constant travel times and signal amplitudes between vertical equi-distant tubes at all depths will generally be indicative of uniform, good quality concrete. However, the presence of irregularities, such as poor quality concrete, voids, honeycombs and soil intrusions, will delay wave travel times and reduce amplitudes at a given depth. In concrete with severe defects, the signal may be completely lost altogether. In CSL testing, defects can be seen on the LCD screen of a portable PC and in the accompanying hard copy printout, with the data typically archived on magnetic media for long-term data storage. The use of this type of technology make it possible to determine both the size and locations of all defects within the concrete.

Ultrasonic Pulse Velocity (UPV) tests can be performed to evaluate the internal conditions of structural members with two-sided access such as beams, columns and piles. The existence of covert defects such as voids, honeycombs, cracks, delaminations and other damage normally found in concrete, wood, metal, stone and masonry materials can be determined with this method which also can be used to predict the strength of early age concrete.

In a UPV test, a piezoceramic source is electrically pulsed to generate ultrasonic waves which travel through the structural elements to the opposite side of the member, where they are sensed by a matching receiver. Wave velocities are then computed based on distance traversed and travel time. Sources and receivers used in the tests have resonant frequencies from 50 - 150 kHz, with higher frequencies applied to thinner structural members. Although a meter in the basic test often measures arrival times, the full waveform can be recorded by a digital oscilloscope card interfaced with a PC for evaluation of material conditions.

Three parameters are used in the interpretation of data: arrival time of compression waves; signal strength; and distortion of the transmitted signal.

Although compression waves will usually move more slowly through defective material, differences in wave velocities between sound material and defective areas such as honeycombs will sometimes be too small to distinguish. When this happens, the defects can usually be distinguished through a distortion in the signal (i.e., the filtering of high frequencies).

The ultrasonic pulse velocity method can be used as a supplemental tool when inspecting timber piles undergoing hidden shipworm (Teredo or Bankia) attack since areas of the least and most extensive biodeterioration (section loss) may possibly be identified prior to being confirmed by core sampling in those locations. However, ultrasonic transmissions will provide accurate readings only if the material being tested is homogeneous. Because wood is generally the least homogeneous of materials comprising bridge and marine structures, test results should be verified with core sampling.

A recent method of instantly assessing the occurrence of local scour adjacent to a bridge footing at any point in time relies on the use of a scour monitoring system. Mounted at a fixed location on a bridge pier, abutment, pile or column, such a system incorporates instantaneous data acquisition at a remote location, enabling real-time monitoring of the current mud line elevation contiguous with the footing. By saving recordings of the data, a complete history of changes in the soil elevation with respect to the bottom of footing or pile tip can be electronically stored, thus making it possible to determine the maximum depth of scour, backfill of previous scour and present scour activity.

The system primarily consists of an electronic sonar scour tracker, a data acquisition system, a cellular data retrieval system and an electrical power supply. The scour tracker is normally an 8û cone angle transducer which measures the time for a pulse of sound directed at the streambed to return in the form of an echo. The transducer is usually mounted below the waterline on a set of supports and brackets located at a fixed elevation on a substructure and is wired to a sonar electronic board controlled by an interface "smart" card.

The scour tracking unit is provided with a wireless datalink transceiver which transmits sonar readings to the data acquisition system located either on the bridge superstructure or in close proximity to the bridge where it stores data on a portable computer. Because the data acquisition system is provided with the appropriate modem, cellular transceiver, operating software, antenna, mast and mounting hardware, it is capable of sending recorded data to a remote location.

In effect, a real-time scour monitoring system can prevent a disaster from occurring by providing an advance warning of impending critical conditions in which a bridge footing can become unstable due to loss of support material. Should a scour condition occur, the system’s master station will automatically call designated telephone numbers and issue a programmed digitized voice message alert such as:

"Scour monitor warning from Bridge 5720. The current streambed elevation is at the alarm level of 34.2 feet."

The system also can be expanded to monitor and record other information such as water velocities and water surface elevations.

Overall, the aforementioned technologies can greatly enhance the underwater inspection of a bridge substructure. Bridge owners should keep these in mind when developing requests for proposals during the management of their bridge inspection programs.

About The Author: Ganas is director and general manager of Boswell Underwater Engineering, South Hackensack, N.J.