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Pavement Evaluation 2002, Roanoke, VA, 21-25 Oct 2002

Void Detection Under Airfield Pavements

L.J. Malvar, NFESC, 1100 23rd Avenue, Port Hueneme, CA 93003, (805) 982-1447, malvarlj@nfesc.navy.mil G.D. Cline, NFESC, 1100 23rd Avenue, Port Hueneme, CA 93003, (805) 982-3655, clinegd@nfesc.navy.mil

ABSTRACT

Several recent accidents involving aircraft punching through pavements prompted the Naval Facilities Engineering Command (NAVFAC) to task the Naval Facilities Engineering Service Center (NFESC) to address this problem. Initially, NFESC and NAVFAC’s Southern Division completed a successful void detection survey at Naval Air Station (NAS) Pensacola, where several voids were generated by leakage of large underground drain pipes. Various methods were used such as internal video taping of the pipes, heavy weight deflectometer (HWD) testing, ground penetrating radar (GPR), and dynamic cone penetrometer (DCP) testing. From the developed void detection methodology, a draft Interim Policy and Technical Guidance (IP&TG) was prepared and issued by NAVFAC on 23 March 2000. NFESC was also tasked to assess all existing technology applicable to void detection under pavements, and completed a state-of-the-art review. However, the optimum technology determined (visual inspection, HWD, and DCP) still presented limitations in terms of availability and speed of data acquisition, requiring prioritization of the work. A risk analysis was completed, establishing work prioritization within each airfield and providing a prioritization of all U.S. Navy and Marine Corps airfield pavements. The void detection methodology has been successfully used at several airfields and Naval stations, detecting voids or loose layers, and providing repair recommendations.

BACKGROUND

In May 1999, the front gear of a T-34C aircraft fell into a 12-inch deep hole that formed in a taxiway at NAS Pensacola. This pavement failure was due to local base and subgrade erosion from a leaking drain pipe under the taxiway. NFESC was tasked by NAVFAC to determine the extent of potential voids near all drain pipes under the runways, taxiways, and aprons [1]. In addition, NFESC was tasked to assess current technology for void detection applications under pavements, determine the optimum methodology, and prepare a guidance for proactive detection of voids to prevent future similar failures [2]. To reduce the cost of the assessments, a risk analysis was completed, establishing work prioritization within each airfield, and providing a prioritization of all U.S. Navy and Marine Corps airfield pavements [3]. This paper details the development of the optimum void detection methodology.

VOID DETECTION AT NAS PENSACOLA Several non-destructive techniques (NDT) were used to assess the potential for voids under the pavements. These included:

Visual Observations

Depressions in asphalt pavements, as well as excessive cracking and faulting in concrete slabs, were early indicators of subgrade deterioration at some locations.

Heavy Weight Deflectometer Evaluation

An NDT evaluation was completed using an HWD. HWD testing was performed by NAVFAC’s Southern Division in cooperation with NFESC (Figure 1). The HWD is an impact load device, which applies a single-pulse transient load of about 20 to 30 milliseconds of duration. This trailer-mounted device applies a dynamic force to the pavement surface by dropping a weight onto a set of rubber cushions which in turn transfer the load to the pavement through a 17.7-inch (45-cm) diameter plate (Figure 2). The drop height can be varied from 0 to 15.7 inches (40 cm) to produce forces from 9,000 to 60,000 lbf (40 to 267 kN). Load is measured with a load cell at the center of the plate. Typically, seven velocity gages are used to measure pavement velocities and determine corresponding

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deflections. These deflection gages (D1 through D7) are located at 0, 15, 24, 36, 48, 60, and 72 inches (0, 38, 61, 91, 122, 152, and 183 cm) from the load point to capture the deflection basin (some can be seen in Figure 2).

The advantage of this method is that the results can be directly related to a load carrying capacity for the pavement, e.g. using the aircraft classification number / pavement classification number (ACN/PCN) approach [4, 5]. The disadvantage of the method is that tests can only be completed at a limited number of discrete locations, and each test covers only an area about 5 feet (1.5 m) in radius, or less. Hence in the search for voids or pavement weaknesses, tests must be conducted on 10-foot (3-m) grids, which becomes very labor intensive. A secondary testing method, such as ECP (electronic cone penetrometer), DCP (dynamic cone penetrometer), or SPT (standard penetration test) must be used to further pinpoint the origin of the weakness and its depth. This method will detect not only actual voids but also weak areas in the subgrade or the base, which can weaken the pavement like a void. In this paper no distinction is made between an actual void or a weak area, since only their effect on load-carrying capacity is of interest. Weaknesses that do not affect the current or future load-carrying capacity are of secondary interest.

Video Taping Video taping of all the pipes was contracted by NAS Pensacola and provided an independent verification of the HWD predictions. Video taping the interior of drain pipes under airfield pavements can help pinpoint the location of potential problem areas. Leaks in the pipes, or at pipe joints, indicate suspicious locations. Accumulations of fines near the leaks are a good indicator of a loss of subgrade material, and possibly subgrade strength. In some cases, joint and pipe breaks, and even actual voids beyond them have been observed [1]. However, it has been shown that this method may not identify all weak pavement areas [1], and it cannot determine the loss of load carrying capacity at the identified locations.

Ground Penetrating Radar Three sets of GPR data were gathered at NAS Pensacola. The first set was completed by the US Army Waterways Experiment Station, Geotechnical Laboratory. The second set was contracted out by NAS Pensacola. The third set was completed by the Air Force Research Laboratory (AFRL), Tyndall Air Force Base, FL.

GPR works by transmitting a short radar pulse, typically from the transient voltage pulse from an overloaded avalanche transistor. The pulse reflection is measured, and is dependent on the soil electrical conductivity. Objects or areas in the ground with different electrical properties will reflect the pulse differently, and appear as anomalies. Soil penetration depends on soil type and antenna type. Soil moisture, as well as high clay soils, will quickly attenuate the radar signal and decrease its performance (i.e. dry sandy soils are best). High frequency antennas, in the order of 1 to 2 GHz, produce the best resolution (e.g. can find small objects), but can only penetrate a few inches. Low frequency antennas, in the order or 10 to 200 MHz, can penetrate tens to hundreds of feet, depending on soil conditions, but may not be able to locate small objects, such as small diameter pipes. For airfield pavements, where the depths or interest vary from 1 to 20 feet, two antennas may be required, one around 900 MHz and one around 200 MHz. Although the GPR is supposed to be able to operate at a single frequency, it typically generates a broad band signal (e.g. from 75 to 300 MHz for a center frequency of 150 MHz). As a result it may not be able to differentiate between closely spaced objects (for example two or more pipes).

GPR systems can be mounted on vans or carts, and generate a continuous record of soil cross sections, with a given depth and width. Real time soil cross sections can be obtained for immediate analysis, or they can be stored on videotape for later evaluation. The raw signal can also be postprocessed to better identify anomalies. In any case, proper interpretation of GPR output requires considerable operator experience and is an art as well as a science.

GPR results, however, cannot be directly related to load carrying capacity, as is done with the HWD. The GPR can identify areas that are different, i.e. anomalies in the pavement, but cannot quantify their impact on the load capacity. Also, the GPR will not penetrate significantly through clays or wet soils, limiting its applicability. A survey of GPR users indicated that it was a useful tool only between 25% and 75% of the time. On the other hand, the advantage of the GPR resides in its ability to process a large amount of data quickly (covering a large area while producing a real time output) and in its ability to pinpoint drain pipe locations.

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Soil Penetration Techniques

Various techniques exist whereby rods with a conical tip are pushed or pounded into the subgrade to determine the subgrade bearing strength at various depths. These are destructive techniques, since they require drilling or coring through the pavement to reach the base and subgrade. The advantages of these techniques are that they can provide an independent verification of the existence of a subgrade weakness or void, they can also indicate the relative strength loss, and finally the depth and vertical extent of the weakness.

Three soil penetration techniques are identified below:

• The Standard Penetration Test, also called Split-Spoon test because of the split-barrel, is used for soil sampling (ASTM D 1586). It consists in driving a split-barrel sampler to both obtain a representative soil sample and a measure of the soil resistance to penetration. The sampler is driven by dropping a 140-lb (64-kg) mass from a 30-inch (76-cm) height. The sampler is driven at 6-inch (15-cm) increments into the ground. For each increment the number of blows is recorded and is assumed to be representative of the soil strength.

• The Electronic Cone Penetrometer uses a rod that is pushed at constant velocity into the soil and readings of the resistance to penetration. The Air Force Civil Engineer Support Agency (AFCESA) owns a truck-mounted ECP that is routinely used in their pavement evaluations. This system can perform quick evaluations down to 8 feet (2.4 m) or more, but requires additional personnel.

• The Dynamic Cone Penetrometer uses a rod that is pounded down using a calibrated weight dropped from a constant height. This system is portable, and its most recent version only needs a single operator. This system is designed to reach a depth of only 4 feet (1.2 m), but in testing weak areas for voids, this is often sufficient.

HWD PREDICTION METHODOLOGY

The following procedure using the heavy weight deflectometer was developed by NFESC for void detection in limited critical areas [1, 2, 3]. In general data are collected above drain pipes (where most problems arise), and the data collection procedure for asphalt pavements was as follows: • Follow each drain pipe and test every 10 feet (3 m) (line 1) • Follow each pipe every 10 ft again but offset to right by 10 ft (3 m) (line 2) • Follow each pipe again but offset to left by 10 ft (3 m) (line 3)

Hence, three sets of readings are obtained for each distance along the pipe. The 10-foot (3-m) distance was chosen because it is expected that the HWD cannot sense pavement deficiencies beyond a 5-foot (1.5-m) radius. For concrete pavements, the procedure is similar: • Follow each drain pipe and test at the center of each slab above the pipe • Test at the center of each slab on either side of the slab above the pipe.

Because Navy slabs are usually 12.5 by 15 feet (3.8 by 4.6 m), a slightly different grid is obtained. To allow for comparison between slabs, tests are only carried out at the center of each slab.

At each test location a set of seven deflections is obtained, D1 though D7, where D1 is under the load point. Once the data is gathered, the impact stiffness modulus (ISM) can be used to assess the pavement’s relative strength at each drop location. The ISM reflects the local pavement stiffness under the load point, and is found by dividing the load by D1. Similarly, the load can be divided by the other deflections, to give ISM2 = Load/D2, and so on up to ISM7 = Load/D7. This is of interest since D1 usually reflects mostly the state of the pavement itself, whereas D7 reflects mostly the state of the subgrade. Using D1 alone is not sufficient to successfully detect voids under the pavement. The ISM1 through ISM7 values along the drainpipes can be plotted and analyzed. They can also be normalized (by dividing each value by the highest value along the drain pipe) to determine relative effects of pavement weaknesses on each sensor.

Once the plots are completed, the following rules can be followed to determine potentially weak areas: • An absolute ISM value below 500 kips/in (87.5 kN/mm) is of concern. • A relative ISM decay indicates an unexpected weakness. • A weakness in ISM 1 indicates it is shallow. • A weakness in ISM 7 indicates it is deep (from 3 to 20 feet, or about 1 to 6 m). • A weakness in both ISM 1 and ISM 7 indicates a general lack of support.

The HWD data does indicate if the potential void, or subgrade weakness, is shallow or deep. It can be seen in Figure 3 that at station 1,060 feet (323 m), the value of ISM1 is not affected, but the values of ISM4 to ISM7 show a more significant loss of stiffness, when compared with the rest of the data above that drain pipe – this is an

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example of a void being created near the pipe. Figure 4 shows data for sets of three concrete slabs along another drain pipe (slabs above and to the left and right of the pipe). Two of the slabs in particular show a significant loss of ISM1 around station 1,150 feet (350 m). The normalized plot shows that all 7 sensors exhibit a significant relative loss, indicating a general lack of support for those slabs. This could be due to a sinkhole with weak subgrade for several feet below the pavement, or a settlement of the subgrade under most of the slab - in this case the latter happened, leaving a 2-inch (5-cm) deep void under most of the slab. Figure 5 shows the case where a shallow weakness is found by the first sensor around station 10 feet (3 m) – this is confirmed in the normalized plot where ISM2 is also relatively lower, but ISM 4 to ISM7 do not appear affected. Also Figure 3 shows an ISM1 always above 3000 kips/in (525 kN/mm) that indicates a strong pavement (the void may take a long time to affect the pavement load carrying capacity) whereas Figure 5 shows a somewhat low ISM1 which may impact the traffic.

These guidelines have proven very successful in determining voids and weak or loose soils under pavements using the HWD [1]. Other void detection methodologies using the HWD have been attempted [2], but they either are more complex or do not appear as reliable.

VERIFICATION OF HWD PREDICTION METHODOLOGY

The HWD void predictions were checked using independent methods. For example coring one of the faulty slabs (completed by AFCESA) from Figure 4 resulted in the core falling into the 2-inch (5-cm) deep void under the slab (Figure 6). Figure 7 shows an independent confirmation using video taping of another HWD predicted void. In this case the pipe joints had deteriorated, and Figure 7b shows water flowing into the drain pipe and depositing fines taken away from the adjacent subgrade. Figure 8 shows a GPR verification (completed by AFRL) of an HWD detected void. Figure 9 shows an ECP confirmation (completed by AFCESA) of a HWD predicted sinkhole – the California Bearing Ratio (CBR) is less than 2 for almost 8 feet (2.4 m). SPT data was also used to confirm HWD predictions [1], and DCP testing is routinely completed as a verification. In summary, the HWD is considered a reliable tool to detect void or weak layers, but a DCP (or ECP or SPT) is recommended to further determine the extent and severity of the weakness.

OPTIMUM VOID DETECTION METHODOLOGY

A review of airfield pavement failures was conducted for the Navy, Army, and Air Force, and several accidents were found where aircraft had punched through pavements [2]. Other Navy pavement failures were reported, which fortunately did not involve any aircraft, although some of them happened in active airfield pavements. Finally some commercial failures were also reviewed. Since many failures are known to have resulted from subsurface voids caused by soil erosion near drain pipes, a survey was conducted of all Navy and Marine Corps airfields in an attempt to quantify the magnitude of the problem. On average, the airfields surveyed have about 15 drainage structures crossing under airfield pavements.

A review of the state-of-the-art non-destructive technologies applicable to void detection under airfield pavements was completed [2]. The techniques addressed were: (1) Electromagnetic (Ground Penetrating Radar, Microwave and Millimeter Wave, Infrared Thermography, Magnetic Fields, Electrical Resistivity/Conductivity, Spontaneous Potential, Visual Inspections), (2) Transient Load (Seismic Waves, Impact-Echo, Heavy Weight Deflectometer, Rolling Deflectometers, Vibratory Loading Systems, Rolling Dynamic Deflectometer, Ultrasound, Acoustic Reflection Sounding, Acoustic Emission, Audible Acoustic Reflection Sounding), and (3) Others (Video Taping, Soil Penetration, Quasi-Static Load-Deflection Devices, Gravitational). The experience of many Government agencies, State Departments of Transportation, academia, and private firms on all available applicable techniques was gathered and summarized.

The following were conclusions derived from their shared knowledge:

1. No single technique is currently capable of providing a complete solution to the void detection problem.

2. A combination of technologies can, however, provide a cost-effective, reliable methodology to minimize the potential for accidental airfield pavement failure due to subsurface voids.

3. The optimum technology combination at the current time is a combination of visual, HWD, and DCP techniques, which can be completed by a single operator. The DCP can be replaced by an ECP or an SPT, but these techniques require additional manpower.

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4. Complementary technologies include video taping and GPR. Video taping detected pipe failures indicative of potential void problems, and even detected actual voids. GPR is very useful for determining pavement layer thickness (for use in HWD structural evaluation), and location of drain pipes (assuming favorable low conductivity subgrade characteristics).

5. Promising technologies include Rolling Weight Deflectometer, High Speed Deflectograph, Rolling Dynamic Deflectometer, GPR, and Infrared Thermography. It is currently not recommended to use these technologies as primary detection tools, but further development may increase their reliability for such application.

6. In some cases, for example when the area to investigate is large, using the HWD to perform a thorough coverage may not be possible. In that case, it is recommended that: (1) the HWD be used at any critical location within the area of concern, (2) the GPR be used to complete coverage of the area in an attempt to identify subsurface anomalies (assuming soil characteristics allow its use), (3) the HWD be used again at the discrete locations where the GPR identifies anomalies, and (4) DCP testing be completed where weaknesses were confirmed by the HWD.

INTERIM POLICY AND TECHNICAL GUIDANCE

From the above information, an Interim Technical Guidance was developed and approved by the Naval Facilities Engineering Command on 23 March 2000. Its recommendations are summarized below:

Visual Inspection

Visual inspection of the airfield pavements should be performed with sufficient frequency to locate potential problem areas and satisfy the airfield manager of its operational safety. Such inspections shall monitor pavements for conditions that may affect aircraft movement (FOD, depressions, pavement deterioration, etc.). Frequency should be determined by local physical conditions and operational tempo as to minimize the hazards. In flexible pavements, depressions are evident after a rainfall, or by the concentric marks left by the evaporated water. In rigid pavements, standard 12.5 by 15-foot (3.8 by 4.6-m) concrete slabs cracked into two or more pieces, as well as slabs that exhibit faulting at joints, may indicate underlying soft spots or voids. In particular, areas above drainpipe crossings should be carefully inspected since most problems appear near these pipes. Problems observed in unpaved areas above a pipe are early warning signs of problems in nearby paved areas above the same pipe. Depressed pavement or shattered slabs surrounding drainage structures (catch basins) indicate infiltration of soil materials into the structure or pipe. Visual inspections can also follow Pavement Condition Index (PCI) guidelines, as detailed in NAVFAC MO-102 manuals, and as detailed in ASTM standards commonly available.

HWD Testing

If visual inspection suggests concern, further evaluation using a HWD should be performed. The HWD investigation would cover all pipe crossings and additional suspect areas, following the procedure indicated previously. The HWD will establish the effect of any subgrade weakness (or void) on the load-carrying capacity of the pavement. For the Navy, HWD evaluations have been performed by the cognizant NAVFAC Engineering Field Division. Periodic testing with a HWD is recommended at all pipe crossings. This HWD testing can be completed at the same time as the standard PCN structural evaluation cycle, as described in [4].

Soil Penetration

Weak areas revealed by the HWD should be further tested to determine the depth of the weakness in order to determine the type of repair needed. This testing can be completed using either a DCP, ECP, or SPT. Video taping the interior of pipe crossings is recommended when testing and/or visible failure is evident in or around pipe crossings. It will help pinpoint the location of potential problem areas and define the need for maintenance and repair. Special attention should be paid to assessing pipe crossings and joints. Accumulations of fines near joints or other penetrations are good indicators of a loss of subgrade material and possibly subgrade strength. NAVFAC’s Design Manual 21.06 “Airfield Pavement Design for Frost Conditions and Subsurface Drainage” provides discussion on video inspection of subsurface drainage utilities. In some cases, coring of the pavement may be required to confirm presence of voids directly below the pavement surface.

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Complementary Methods

Alternate non-destructive techniques were evaluated, but are not believed to be as effective as the aforementioned tools in determining the existence of voids. GPR cannot be used as a reliable tool to predict weak areas and GPR should not be used by itself for void detection at this time. However, GPR appears successful in locating the actual location of drain pipes and thickness of pavement layers, and could be used to verify the extent of known voids.

When coverage of large areas is required (e.g. where karst formations are prevalent), the current technology may not always be able to provide a cost-efficient solution. A risk analysis study indicated that for runways, two lines (10 feet, or 3 m, on either side of the centerline) can be covered with the HWD and be cost effective. Along each line, for asphalt pavements longitudinal testing should be completed at 10-foot (3-m) spacings (20-foot, or 6-m, spacings for preliminary assessments), and for Portland cement concrete pavements longitudinal testing should be completed at each slab center at 12½ or 15-foot (3.8 or 4.6-m) spacing, for Navy airfields. The remainder of the runway is less likely to be used, and can be assessed using a less reliable but faster technique. Anomalies found with this complementary technique will need to be verified with the HWD and the DCP.

CONCLUSIONS

An optimum methodology was determined to detect voids, or weak zones, under airfield pavements. It includes a combination of visual inspection, HWD, and DCP testing. The methodology was verified using several other techniques, such as ECP, SPT, DCP, GRP, coring, and video taping, and was supported by a state-of-the-art review of all applicable void detection technologies. The optimum technology can currently be applied only to limited, critical areas, such as above drain pipes, and along the runway centerline.

REFERENCES

1. Malvar, L.J., Lesto, J., Cline, G., Beverly, W. Airfield Pavement Void Detection, NAS Pensacola. Site Specific Report SSR-2534-SHR, Naval Facilities Engineering Service Center, Port Hueneme, CA, December 1999 (limited distribution).

2. Malvar, L.J., Cline, G. Airfield Pavement Void Detection Technology. Special Publication SP-2081-SHR, Naval Facilities Engineering Service Center, Port Hueneme, CA, April 2000 (limited distribution).

3. Malvar, L.J., Cline, G. Risk Analysis for Void Detection Under Airfield Pavements. Special Publication SP-2095-SHR, Naval Facilities Engineering Service Center, Port Hueneme, CA, September 2000 (limited distribution).

4. Headquarters, Department of the Army, Air Force and Navy, “Airfield Pavement Evaluation,” Unified Facilities Criteria UFC 3-260-03, Washington, DC, April 2001.

5. Malvar, L.J., “Issues in Pavement Evaluation,” Transportation Systems 2000 (TS2K) Workshop, San Antonio, TX, February 2000, pp. 193-202.

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Figure 1 HWD testing an airfield pavement above drain pipes.

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Figure 2 Close up of the sensors under the HWD

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Figure 6 Coring confirmation of HWD detected void under concrete slab

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(a) General view of the concrete drain pipe and pipe joints

VOIDVOID

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(b) Close up of the void beyond the pipe joint. Figure 7 Video taping confirmation of deep void near the drain pipe

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Figure 8 GPR confirmation of an HWD detected void

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Figure 9 ECP confirmation of HWD detected sinkhole.