Evaluation of a prototype.
By Ruth Roberson, John Stormont, and Karen Henry
Excess moisture coupled with inadequate drainage are believed to be the primary causes of roadway distress and failure. Manifestations of moisture-related distresses such as rutting, potholes, longitudinal and shrinkage cracking are commonly observed in bituminous pavements.
In concrete pavements, moisture-related distresses are manifested as pumping, faulting, corner breaks, and longitudinal cracking. These distresses diminish the structural integrity of the pavement and reduce pavement life. To address moisture-related distresses, pavement engineers typically construct subsurface pavement drainage systems.
A pavement research project, evaluating the effectiveness of an innovative pavement drainage product, a Geocomposite Capillary Barrier Drain (GCBD), is now under way at the Minnesota Road Research Facility (MnROAD) in Albertville, Minn. The project is funded by the NCHRP Highway IDEA Program and by a FHWA Pooled Fund Project/TPF-5 (126).* It is a collaboration among three organizations: the University of New Mexico (UNM), the U.S. Army Engineer Research and Development Center’s Cold Regions Research and Engineering Laboratory (ERDC-CRREL), and the Minnesota Department of Transportation (MnDOT).
The overall purpose of the project is to facilitate the transfer of the new pavement drainage technology to engineering practice. This is being accomplished through construction of research test sections at MnROAD, demonstration and documentation of GCBD prototype assembly, field installation and performance, and the development of the capability to incorporate the GCBD in subsequent designs.
GCBD: Geocomposite Capillary Barrier Drain
The geocomposite capillary barrier drain (GCBD) is a three-layer composite, from top to bottom: a transport layer (a specially designed geotextile), a capillary barrier (a geonet), and a separator (geotextile). When placed between a base and subgrade, it can drain the unsaturated base and reduce its water content as well as prevent water from reaching the subgrade. The principal function of the GCBD is illustrated in Figure 1. Water infiltrating through the base is prevented from moving into the underlying subgrade by the capillary barrier formed by the geonet. The transport layer (a special geotextile) becomes increasingly hydraulically conductive as it gets wet. If the GCBD is on a slope, water will drain along the slope in the transport layer. If the transport layer does not become saturated, no water will break through into the capillary barrier. The bottom separator protects the geonet from becoming filled with subgrade soil. The GCBD also cuts off capillary rise of water in the underlying soil, and if the overlying base and transport layer become saturated due to an excessive infiltration, the geonet provides saturated drainage.
The GCBD resembles a conventional drainage geocomposite; however, the transport layer is designed to transmit water under negative water pressures. In contrast to the GCBD, conventional drainage is designed for saturated flow, even though the positive pore water pressures required for saturated flow reduce strength and lead to rutting, heaving, and failure when present in the soil layers of a road or airfield.
Minnesota Road Research Facility (MnROAD)
The Minnesota Department of Transportation constructed the Minnesota Road Research Project (MnROAD) between 1990-1994. MnROAD is located 40 miles northwest of Minneapolis/St. Paul and is an extensive pavement research facility consisting of two separate roadway segments containing 51, 500-foot pavement test sections (Figure 2). The 3.5-mile Mainline Test Roadway (Mainline) is part of westbound Interstate-94 and it contains 31 test sections and carries an average of 20,000 vehicles daily. Adjacent to the Mainline is a Low Volume Roadway (LVR) that is a 2.5-mile/closed loop containing 19 test sections. Traffic on the LVR is restricted to an 18-wheel, 5-axle, tractor/trailer with two loading configurations of 102kips and 80kips.
Test section design
Approximately 1,000 ft. on the MnROAD low-volume loop was dedicated to this project. Figure 3 is a cross section of the pavement section design, which included a GCBD test section and a control section. The structural design of the pavement sections is identical. Each pavement section consists of 4-in. of HMA over 6-in. of MnDOT Class 5 aggregate base, over a silty clay subgrade. The sections were constructed with 13-ft. lanes, 4-ft. shoulders, and a lateral grade of 1-2% (Figure 3). Water draining laterally from the test section will be collected in an edge drain and routed to tipping buckets for measurement. Figure 4 shows pavement section dimensions, GCBD placements, and sensor locations. The 400–ft. test section containing the prototype GCBD lies between two control sections, 100-ft. and 400-ft. long.
Instrumentation for monitoring the soil/water content, water potential (soil suction), temperature, and water-table depth were installed in the pavement subsurface during construction. There are 10 sensor arrays totaling 80 sensors on the project. In each array, sensors were placed at the top and bottom of the base course, just above and below the base-subgrade interface, and at 11 in. and 14 in. below the top of the subgrade. Two longitudinal edge drains (Figure 5) terminating at the headwall of the tipping bucket enclosures were installed on the section containing the GCBD. All instrumentation is connected to an automated data-acquisition system, loaded and stored in a geospatial database for future analysis.
Drain pipe segments
Schedule 40 PVC pipe was used for a drainpipe. Drainpipe segments with the GCBD terminating in the pipe were prefabricated at the MnROAD site for ease of installation. Prefabrication consisted of cutting a 1-in. slot the length of the pipe, fitting all three layers of the GCBD into the pipe, and attaching the GCBD to the pipe.
Pipe segments were laid end-to-end in the trench (Figures 7 and 8). The bottom portion of the geocomposite, consisting of the separator and geonet bonded together as a single product, was rolled out longitudinally to form a butt joint at the trench and centerline (Figure 9). Sections of the transport layer were then rolled out and overlapped (Figure 10). After the GCBD was in place, end dumps of aggregate base were spread using a bulldozer and front-end loader (Figure 11).
Pavement drainage modeling and design
Based on previous laboratory testing and field experience, the GCBD is expected to increase the rate and amount of drainage from the pavement system and reduce the equilibrium water content in the pavement subsurface layers. Lower equilibrium water contents are expected in the pavement foundation layers when compared to typical pavement.
This has implications for mechanistic pavement design. Thus, there is a need for a design tool that can predict drainage and water content in pavement systems that include a GCBD. A design tool will incorporate numerical simulations of unsaturated flow within a pavement section, which will be calibrated against a robust set of experimental data collected from the MnROAD field test.
Initial conditions (measured moisture contents and temperatures of the sub-surface layers) and climatic conditions will be used as input to the simulations. The calibrated model will be used to investigate the impact of design parameters on GCBD performance. In this way, the design tool will allow site-specific conditions to be used in other designs. The principal output from the model will be drainage quantity and water content within the pavement section, both with and without the GCBD. The calibrated design tool will be used to produce guidance for applications with different climate, materials properties, and pavement sections.
Ruth Roberson (Minnesota DOT), John Stormont (University of New Mexico), and Karen Henry (U.S. Army Corps of Engineers Cold Regions Research Laboratory) are principal investigators on this GCBD project.
The authors would like to acknowledge the support of the NCHRP Highway Innovations Deserving Exploratory Analysis (IDEA) program—especially Inam Jawed, who has been very supportive in the development of the GCBD, the New York Department of Transportation (Robert Burnett), the Michigan Department of Transportation (Michael Eacker), the MnROAD operations personnel, and the Minnesota Local Road Research Board. A special thanks to engineering students Cassandra O’Neal and Felipe Camargo who were crucial in devising successful means to install the GCBD drain pipe and were very helpful in installing the sensor arrays; and to Larry Salzer from Tenax Corp. who provided critical guidance during the installation of the GCBD.
For more information, go to: www.mrr.dot.state.mn.us/research/construction/2006geocomposite.asp
*Further information about active and proposed FHWA Pooled Fund Projects is available at: www.pooled fund.org