Framework for LTPP Forensic Investigations - Final

Appendix B - Example SMP Forensic Plan

Introduction

LTPP GPS-1 test section 091803, located in Groton Connecticut, is scheduled for rehabilitation. This test section was included in the Seasonal Monitoring Program (SMP) phase 1 study. Prior to rehabilitation of the test section Connecticut DOT has offered to perform additional sampling and testing at the site to supplement the previously collected information. An overview of supplemental data collection needs and field procedures are presented in this memorandum. This data collection activity is in addition to the standard LTPP pavement performance measurements to be performed prior to the rehabilitation construction event.

Data Needs

• Specific gravity measurements are needed on the unbound granular materials surrounding the Time Domain Reflectometry (TDR) probes. This information is useful for estimation of moisture contents from the TDR measurements.
• In-situ dry density of material adjacent to the TDR probes. Although difficult to measure, this value is used to convert volumetric moisture contents estimated from the TDR measurements to gravimetric contents that can be compared to laboratory determined values.
• "Ground truth" moisture contents at TDR probe locations coupled with TDR measurements. A matched set of TDR measurements and gravimetric moisture contents from the material surrounding the TDR probes will permit evaluation of the accuracy of the estimated moisture contents from the TDR measurements. Also during installation, a complete set of matching TDR and gravimetric moisture content measurements was not obtained.
• Materials characterization of the subgrade layer(s). During the standard GPS materials sampling and testing performed under SHRP, because of the presence of rocks and cobbles in the subbase material, material samples of the subgrade layer(s) were not obtained. The only information available on these layers is the visual classification performed during site verification and instrumentation placement. (Note there appears to be two distinct layers of material beneath the subbase in which TDR probes are located.)
• Distress mechanism investigation. As a test section go out of service, in some instances, it is useful to perform different types of measurements to investigate the distress formation mechanism. For example, if a test section has rutting, then a transverse trench could be used to identify the layers contributing to the permanent deformation.
• Within section layer thicknesses. One unknown for GPS test sections is the variation in layer thicknesses along the test section. This information can be used to refine FWD backcalculation and as a measure of construction variability.

Pavement Structure

The pavement layer structure and location of the TDR probes are shown in Figure 1, which was obtained from the SMP Instrumentation Installation Report prepared for this site.

SMP Instrumentation Measurements

Unlike many of the other supplemental data collection to be performed during this exercise, the data collected from these measurements can and should be entered into the IMS. Just prior to beginning excavation of the materials at the instrumentation hole location, we recommend that the following SMP instrumentation measurements be performed:

1. Three sets of automated TDR measurements using the LTPP mobile data acquisition unit. These measurements can be performed in sequential order at the start of the work day.
2. As part of the standard LTPP mobile data acquisition, automated resistivity measurements should also be obtained. Although it is not anticipated that the site will contain frost lenses when this work is performed, the purpose of these measurements are to provide comparative data to the first set of measurements in order to evaluate the performance of the resistivity probe. Manual electrical resistivity measurements are not proposed.
3. One set of manual TDR measurements. Since the interpretation methods between automated and manual TDR measurements differ, the interpretation of the manually collected will provide a good indicator of change and comparison of the difference between the two methods.
4. Water table depth measurement.

These measurements should be possible since, except for the water table depth, they are all performed using the mobile data acquisition unit. Since this SMP site was previously deactivated, issues concerning air, rainfall and pavement gradient temperature measurements, measured by the instrumentation previously installed on-site are not considered an issue.

Materials Sampling at SMP Instrumentation

In order to obtain the material samples and perform material tests to provide supplemental data for the TDR measurements, an excavation will be required next to the instrumentation location located at station 5+21. There are two approaches to this excavation. The first approach would be to use a backhoe to excavate a small test pit adjacent to the instrumentation hole large enough for a person to stand in order to obtain material samples from around the TDR probes and perform measurements. The second approach utilizes an auger boring adjacent to the instrumentation.

During the excavation and sampling process, efforts should be made to remove some of the TDR and other SMP probes to examine them, note their general condition and take pictures. The intent of this procedure is to provide indications of the likely longevity of these types of probes and the likely corrosion mechanisms that affect their performance. Particular attention should be given to sensors, which have failed in order to discover why they failed.

Test Pit

Constructing a small test pit directly adjacent to the instrumentation hole, following the general LTPP procedures, is the preferred option. By carefully constructing a hole larger enough for a person to stand in, is the only way to attempt a measurement of the in-situ density and moisture content using a nuclear gauge at the approximate TDR depths as the hole is deepened. By performing in-situ density measurements with a nuclear gauge, material samples from the base and subbase layer for laboratory moisture content-density relations tests would not be needed. It also permits acquisition of a larger volume of material from the subgrade layers which include the relatively thin black loam layer in which TDR 9 is located and the light brown silt layer in which TDR 10 is positioned. It will also improve the ability to extract material samples from around the TDR probes for moisture content tests.

Some concerns over using a test pit excavation include increased pavement repair size, trench safety regulations, and equipment availability as compared to the auger boring option. However, due to the presence of rocks and cobbles in the subbase, the test pit option may require less time since the back hoe can remove the larger size material and afford easier access for hand removal of problem "large" rocks.

The field work associated with the test pit option would include the following general steps:

1. Remove AC material above instrumentation hole and in test pit. It is desired that the removal of the AC layer be performed without the use of a water cooled saw cut. Depending on equipment availability and site conditions, a backhoe may be able to remove the AC material in the vicinity of the instrumentation hole. If a water-cooled saw is used, then consideration should be given to cutting a larger hole than necessary to reduce the impact of the cooling water on the moisture content of the base layer.
2. Perform nuclear gauge measurements on the base material. Since LTPP standard practice is to use the extension rod for these tests, one density test should be performed on the base layer.
3. Hand excavate material from around the TDR probes and obtain samples for laboratory moisture and specific gravity tests.
4. In the test pit, carefully remove base material to the surface of the subbase.
5. Perform nuclear gauge density measurements on the surface of the subbase.
6. Hand excavate the material from the adjacent instrumentation hole down to TDR 3. Capture moisture and specific gravity samples.
7. Within the subbase layer, excavate the hole in 1 foot increments. For each excavation increment, perform a nuclear density measurement on the bottom of the pit and hand excavate and capture material samples from around the TDR probes. (Although some of the probes in this portion of the hole are spaced at 6" intervals, the 1' interval is suggested to increase speed of the excavation. The density profile is also not expected to change dramatically.
8. Once the black loam layer is reached, perform nuclear gauge measurements on its surface.
9. Obtain a bulk sample of material from the black loam layer. Approximately 200 lbs is required for all standard LTPP subgrade material tests.
10. Hand excavate material from the instrumentation hole and obtain moisture and specific gravity samples from around TDR 9.
11. When the light brown silt layer is reached, repeat steps 8-10.
12. After completion of sampling and testing, fill test pit in lifts and compact.

Since this test section is scheduled for overlay, the destructive nature of some of these procedures are not judged to be as critical as if the test section had to be put back into service with a surface patch for a long period of time.

Auger Boring

The auger option is less intrusive since a smaller volume of material is removed and thus requires less pavement repair material. However, it does not afford the ability to attempt in-situ density measurements, makes large rock and cobble removal more difficult, limits the amount of material that can be obtained from the subgrade, and increases the difficulty in obtaining "good" samples of materials from around the TDR probes. In spite of these difficulties, this method is considered viable.

For the auger option, the largest size auger available is desired. A 10" diameter, hollow stem auger was used to excavate the instrumentation hole. Of primary concern is the amount of material that can be obtained from the black loam subgrade layer. Obviously a smaller auger will produce less material. For LTPP standard testing, three auger borings were used to obtain adequate amounts of materials, however, in the general situation, the base layered tended to be the controlling factor in the need for three borings.

The field work associated with the auger option includes the following general steps:

1. Remove AC material above instrumentation hole and in test pit. It is desired that the removal of the AC layer be performed without the use of a water cooled saw cut. Depending on the equipment availability and site conditions, a backhoe may be able to remove the AC material in the vicinity of the instrumentation hole. If a water-cooled saw is used, then consideration should be given to cutting a larger hole than necessary to reduce the impact of the cooling water on the moisture content of the base layer.
2. Obtain an uncontaminated sample of the base material from the auger hole for the moisture- density relation test.
3. Auger into the subbase layer. Obtain one sample from the top of the layer and one near the bottom of the layer for moisture-density relation test.
4. Auger into the black loam layer, and if possible, capture 200 lbs of material for the standard battery of LTPP subgrade tests.
5. Auger into the light brown silt layer, and if possible, capture 200 lbs of material for the standard battery of LTPP subgrade tests.
6. Using a fabricated side hole material sampling device, starting with TDR 10 and progressing upward to TDR 7, obtain samples of material from the instrumentation hole in the general location of each TDR probe.
7. Starting from the surface of the base layer, hand excavate material from the instrumentation hole and obtain material samples for moisture content and specific gravity at each TDR location. Excess material from the instrumentation hole can be deposited into the auger hole.
8. Once the maximum practical extent of hand excavation is reached, use the side-hole material sampling device to obtain moisture and specific gravity samples from the remaining TDR probe locations.
9. After completion of sampling and testing, fill the auger hole lifts and compact.

Subgrade Material Characterization

It is proposed that the material characterization of the subgrade layers be performed on samples obtained only at the SMP instrumentation location. The standard LTPP practice is to obtain samples from each end of the test section. The concern over not sampling the section approach is the amount of time required to complete the excavation at the SMP instrumentation hole, and the difficulty imposed by the presence of rocks and cobbles in the subbase layer. If all of the field material sampling operations were to be performed within a single day, an additional drill rig might be required.

If the auger option is selected for the excavation at the instrumentation hole, then sample size is a concern for the black loam layer. One way to reduce the needed size of this sample is to omit the resilient modulus test. (We are not sure if Connecticut DOT has the necessary equipment to perform the LTPP P-46 test.) From the perspective of what is needed for SMP instrumentation interpretations, although preferred, the resilient modulus test of this relatively thin subsurface layer could be omitted.

The following are the standard battery of material characterization tests on GPS subgrade materials:

Distress Mechanism Investigation

One of the ideas that have been discussed within the LTPP program for many years is the investigation of distress mechanisms. The concept is that after a test section has gone out of service, or is scheduled of rehabilitation, a "forensic" type of investigation should be performed to examine the details surrounding the causes and mechanisms of distress. Due to funding constraints, LTPP has not developed a formal program for these types of investigations.

To perform this type of investigation, we recommend that as a first step a site inspection be performed by LTPP and highway agency representatives. The purpose of this inspection is to note site specific distresses and likely causative factors. The result of this activity is an investigation plan. Some of the types of investigations and field tests that have been suggested for this type of work include:

• Crack cores. By coring through a crack it may be possible to determine the location of the crack starting point and if it extends completely through the bound surface layer.
• Rut trenches. If rutting is the major distress, transverse trenches at selected locations can be used to determine which layer in the pavement structure contributed to the rutting mechanism.
• Drainage evaluation. Depending on the specific features of the site, drainage evaluation can provide very valuable supplemental information on likely distress mechanisms. On sites with in-pavement drainage features, it could also lead to the need for further functional drainage evaluation tests, such as video inspection of edge drains.
• Joint seal evaluation. If a PCC pavement has exhibited suspected moisture related distresses at the joints, such as pumping, depending on nature of the pavement structure, use of the Iowa PCC joint vacuum device might be indicated to evaluate condition of the seals.

While this approach is based on "what failed", equally important and more difficult to addressed is the investigation of "what worked" and why. Similar to diagnosing problems with a car from a manufacturer's viewpoint, knowing what worked requires understanding of what didn't work. Hence, in the case of superior performing pavements, it may be deemed prudent to conduct addition exploratory tests to discover an unknown factor.

Thickness Variation

If a test section is scheduled to go out of service, that is, no longer be monitored as part of the LTPP program, then measurements of the variation in the thickness of the bound surface layers is important. The concept is to obtain cores at the FWD test points within the test section. This reduces coring costs and provides significant information that can be used to improve the results from the backcalculation of FWD basin tests (i.e. non load transfer, corner, and edge tests on rigid pavements) and to quantify construction variability. First priority is to obtain thicknesses in the wheel path locations. Second priority is at the middle lane, basin test locations.

It is our present understanding that this site is proposed for monitoring continuation after rehabilitation. If the site is accepted for monitoring continuation, then this type of destructive sampling within the test section should not be performed. An on-site inspection maybe useful in evaluating the impact of the pipeline buried, in circa 1997, along the edge of the shoulder on the performance of this section. It is also noted that the agency has collected significant WIM traffic data on this site and has committed to reinstall the WIM scale after rehabilitation.