Post-Earthquake Damage Assessed Nondestructively

 

By the time southern California settled (literally and figuratively) back to normal from the 1994 Northridge earthquake, 25,000 dwellings were uninhabitable and the state faced rebuilding bills of $500 million for freeways and $700 million for schools (Time, Jan. 31, 1994). And there was death: a 20-year-old hospital patient whose respirator cut off with the loss of electricity, a police officer whose motorcycle hurtled off the edge of a twisted and broken highway, a young girl crushed by a collapsing building, and 52 others.

One hopeful result of the quake has been a refocusing on architectural designs and materials that can make buildings safer in future earthquakes (Time, Feb. 14, 1994). For example, the Warner Center Marriott in Woodland Hills decided not only to repair the damage, but also to reinforce the 473-room luxury hotel against tremors to come.

The reinforcement required anchoring the structure to two reinforced concrete columns framing the main (front) entrance. The poured concrete columns, 500 mm (20 in.) in diameter, are sunk 4.5 m (15 ft) into the ground and extend 6 m (20 ft) up to the bottom of the second floor.

The general contractor decided to drill horizontally through the columns to anchor the 15-story building more securely to them, but this presented a challenge: how to drill into the columns without damaging the steel reinforcing spiral wrap. The steel strips are spun clockwise and counterclockwise, top to bottom, within the columns.

The inspection team could not drill into the columns to locate the reinforcing wrap within because that would have compromised their structural integrity. The term "nondestructive testing" implies its own benefit. You locate your subsurface targets - in this case the steel wrap - without damaging the overall structure.

The contractor hired an NDT inspection company to pinpoint the steel reinforcement so he could drill safely. The subsurface scan used a proprietary radar system.

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For this project, however, the higher resolution and deeper penetration of radar was needed.

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Many Nondestructive Testing Alternatives
The first decision was which NDT technology to use: ground penetrating radar, magnetometer, or X-ray. Each technology has its own strengths and drawbacks, which must be considered in light of the conditions at a given site. No one of these options is always best, but one or the other is usually most appropriate for the task at hand.

Magnetometers are less expensive than radar systems and easier to use. In some applications, a magnetometer will perform adequately at locating embedded objects, permitting the user to take advantage of its low cost and ease of use. For this project, however, the higher resolution and deeper penetration of radar was needed. The team knew that the spiral wrap was in the columns, but needed to identify the precise location to avoid hitting it with drill bits.
Magnetometers can determine quickly if a site houses metallic anomalies, such as rebars, tendons, or conduit, and place the surveyor in the general area of those anomalies. The surveyor can then use a radar system, which has superior spatial resolution, to pinpoint the number and location of the targets.

The team selected a radar system over X-ray testers for this job because of two peculiarities of the site:

• Extended exposure of the front of the building to X-rays would have required clearing the area for days. Guests and employees would have had to avoid the main lobby, other public rooms, the valet parking area, part of the parking lot, and several dozen sleeping rooms. In addition, the hotel would have had to close the Lobby Bar, a popular gathering place in the San Fernando Valley. Rental of the Marriott’s 1,000 seat ballroom and patronage of its Parkside and Pearl’s restaurants would have been reduced.
• X-ray testing would have required placing film behind the columns, then transmitting rays through the columns onto to film to produce an image of the columns’ contents. There was not enough space between the building and the columns to position the film easily.

Because of the characteristics of this assignment, the radar system was chosen as the best NDT method to use. The survey team scanned 6 m (20 ft) of both columns in one eight hour day. X-ray testing would have required about two weeks, along with a large amount of expensive X-ray film. With radar, the bill to the client was about five percent of what X-ray testing would have cost.

With ladders and scaffolding, they hoisted the radar antenna to the top of the pillars, then lowered it to scan the subsurface. They were able to tell the contractor exactly where to drill to avoid the reinforcing steel within the columns and penetrate the building at the planned locations.

As they lowered the antenna along the column, the system transmitted electromagnetic impulse energy through the antenna into the column, and received reflections from the subsurface back through the antenna. A central processing unit analyzed the data in real time and displayed it on a thermal printer.

A relatively high frequency antenna (1 GHz) was chosen for clear resolution; frequencies range from 16 MHz to 2 GHz. Low frequency antennas penetrate more deeply into the subsurface, but provide less clear resolution. In this case, the signal had to penetrate only a few feet into the columns, so the higher resolution was of more value than deep penetration.

 

Governing Factors
Several characteristics of any job affect the applicability of radar systems; the conductivity of the media (soil or, in this case, concrete) to be scanned is important because it governs how deeply the electromagnetic impulse signal can penetrate toward its target. The signal penetrates to greater depths in highly resistive (low conductivity) media; in general, media resistivity should be above 100 ohm meters (or below 10 milli-ohms per meter conductivity) to achieve best results. A geophysicist can measure this media characteristic on site with a resistivity meter.

Dry, sandy media are best for ground penetrating radar systems, and so concrete is ideal. Moist, claylike soils make application of this technique the most difficult.

The dielectric properties of the targets of a search and their surrounding media — concrete, soil, rock — also affect application of this technology on a job. The term "reflection coefficient" refers to the difference between the dielectric properties of a target and media; as the coefficient increases, so does radar effectiveness. The difference between the dielectric properties of steel and concrete is extremely high, so the radar system located the reinforcing wrap effectively.

Radar can detect a variety of metal objects embedded in concrete. For example, several years ago, a contractor had to drill into concrete floors of a commuter railroad station as part of a structural reinforcement project. The floors were laced with post-tension tendons, which, if struck by a drill bit, could explode the concrete and cause the floors to collapse. The firm used radar to locate the tendons, completing the drilling without incident.

 

Sub-Pavement Voids
Ground penetrating radar systems can locate voids under roads, airport runways, building slabs, and other structures. In New York City, a leaking park fountain eroded soil beneath a Park Avenue apartment building. Once the leak was plugged, radar delineated the void it had caused so a contractor could drill holes in the pavement near the building and pump grout in to fill the void. Such voids are easy to detect and define with radar because the dielectric properties of the void (air) are starkly different from the dielectric properties of soil.

In late 1990 and early 1991, the Nassau County (NY) Department of Public Works identified a sewer line failure beneath a busy boulevard, but could not determine the exact location because sand infiltration had clogged the line. They believed that washouts under the road might have damaged the sewer line and also might have caused structural damage to the roadway subsurface.

They commissioned a radar survey of 3.2 km (2 mi) of roadway directly over the sewer line. The sewer line itself lay below the water table, which created a more conductive environment than the surrounding dry sands. The road surface was 230 mm (9 in.) thick and fortified with rebars, which can cause problems for other NDT technologies and create confusion in the radar data. In spite of these conditions, the radar performed successfully.

The survey team used a 500 MHz antenna pulled by hand at approximately 3.2 km/h (2 mi/h), while the recording equipment was driven 6 m (20 ft) ahead in a van. The survey revealed several areas of apparent voids below the pavement, which were verified by boring. The survey required four days, without performing unnecessary, destructive boring or disrupting traffic.

 

 

Typical profile showing four rebars in a concrete structure.

 

Safer Shuttle Landings
NASA’s Geologic Science Advisor uses radar to search for voids/fissures below desert runways used by the Space Shuttle at Edwards Air Force Base in California’s Mojave Desert. These fissures develop because of the same soil characteristics that make the desert an ideal landing zone. Rogers Dry Lakebed in the Mojave Desert consists in part of pluvial clay deposited during the Pleistocene Period, when the area was a permanent lake. This clay is now covered by a thin capping layer rich in silt. Below the clay lies a layer of gravel containing extensive aquifers. With the end of World War II and the growth of California and the Southwest, farmers began to draw on that aquifer to irrigate alfalfa and other crops. Cities and towns were settled and also drew on the water. As a result, well diggers found groundwater under artesian flow conditions. By 1990, you would have to pump groundwater from more than 21 m (70 ft) below the lakebed surface.

As people drew off the water, the overlying clay compressed, dried, and shrank. These processes caused fissures to develop far below the surface and propagate upwards. But the capping layer of the desert contained more silt and did not shrink as the clay did, so it remained smooth and unbroken, hiding the fissures beneath it.

Wet winters gradually revealed the extent of the problem. As the rain percolated through the capping layer into the fissured clay below, it migrated laterally and vertically through the fractures, seeking a way down to the voids in the clay layer, and on deeper to the sand and gravel aquifers. Shallow conduits developed when water washed silt from the overlaying capping layer down through the fissures. The resulting near-surface voids are a potential hazard when the overlayering soil is heavily loaded, as during a Shuttle landing.

The NASA team ran 10 traverses down each runway spaced at 1.5, 3, 6, 15, and 30 m (5, 10, 20, 50, and 100 ft) from the centerline. They used a radar system to penetrate to 1.5–3 m (5–10 ft) deep. It took one full day to survey each 5,060 m (16,600 ft) runway.

After analyzing the data, the team returned to the locations where the radar had identified subsurface anomalies and tested each with an automated cone penetrometer to a maximum depth of 610 mm (24 in.). Fissures deeper than this pose no threat to the Shuttle because there is enough soil to support the moving Shuttle.

Dangerous fissures were found in 70 percent of the locations pinpointed by the radar. Many of these fissures had been filled in naturally; the team filled remaining voids with natural lakebed materials.

 

Geotechnical Applications
Proprietary radar systems were developed to perform subsurface scans for the military in natural settings, and are still widely used in nature. Environmental engineers, geoscientists, and others use this technology to plot subsurface stratigraphy for a wide variety of geological surveys. Radar surveys are consistently successful in locating underground storage tanks and burial trenches. These systems also help archaeologists find long-buried sites, police detectives find hidden corpses of murder victims, and historians find artifacts.

 

Summary
Ground penetrating radar systems require skill and practice to operate efficiently, although the latest digital systems automate most system setup parameters once chosen by the operator. In addition, the major manufacturers of these systems offer training classes or instructional video kits, or both, and universities with geology or geoscience departments often offer seminars and mini-courses. Many nonscientists and nonengineers have become skilled users.

Used with skill and experience, radar systems can help locate a variety of buried or embedded objects quickly, accurately, and nondestructively.

 

Ground penetrating radar profile showing three voids under a concrete floor.

 

 Profile of subsurface layering and tanks at a commercial site.

 

References
Time, Jan. 31, 1994, pp 26–37.
Time, Feb. 14, 1994, pp 32–33.

* Ron Nisbet Associates, P.O. Box 1186, San Pedro, CA 90723; (310) 328-4733

†Geophysical Survey Systems, Inc., 13 Klein Dr., PO Box 97, North Salem, NH 03073-0097; (603) 893-1109; fax (603) 889-3984.

Copyright © 1996 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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