FULL-SCALE BURIAL TESTING OF PIPES AND STORM CHAMBERS

Transcription

1 FULL-SCALE BURIAL TESTING OF PIPES AND STORM CHAMBERS James E. Sprague, CPESC 1, C. Joel Sprague, P.E., M.ASCE 2 1 TRI/Environmental, 4915 Hwy 76, Anderson, SC 29621; jesprague@tri-env.com 2 TRI/Environmental, PO Box 9192, Greenville, SC 29604; jsprague@tri-env.com ABSTRACT A complex soil-structure interaction is involved when buried flexible plastic pipe or storm chamber systems are placed under load, and the stability of these buried systems must be demonstrated under worst-case live and/or dead load conditions. While it has become common place to demonstrate buried structure stability through analytical methods, including empirical equations or finite element modeling, full-scale burial tests have been recognized as necessary to, at least, calibrate these analytical methods. Two types of field tests have been used for this purpose: conventional excavation/backfill construction and compact burial chambers. This paper describes these full-scale burial testing facilities and procedures and how they have been used to develop deflection and strain data, along with a visual record, for use in correlating results of small-scale laboratory tests and calibrating design models. Included is an example of each type of field test one evaluating storm chambers and another looking at corrugated pipe. OVERVIEW OF IN-SOIL BURIAL OF PIPE AND STORM CHAMBERS Flexible (corrugated) plastic pipe and corrugated wall stormwater collection chambers, or storm chambers, are now commonly used for water transmission and detention, respectively, often buried beneath critical transportation facilities. While corrugated pipe is a familiar commodity and standard design procedures exist, storm chambers are a more recent innovation. The chambers are designed to detain stormwater runoff and to facilitate the seepage/recharge of the detained runoff into the ground. This reduces both the water quantity and the water quality impacts of precipitation on surface waters. Frequently these buried stormwater detention chambers replace surface detention ponds and are often located beneath parking and roadway areas. In this way, developments can manage stormwater impacts without the space, safety, and maintenance issues associated with surface ponds. A complex soil-structure interaction is involved when these flexible systems are buried and placed under load. Thus, in order to be confidently used beneath parking and roadway areas, both pipes and chambers must be shown to be structurally stable enough to sustain anticipated backfill and vehicle loadings without excessive deformation. While conventional pipe structures have been thoroughly studied, and it has become common place to demonstrate the structure stability of these systems through analytical methods, including empirical equations or finite element modeling, full-scale burial tests have been recognized as necessary to assess the structural integrity of innovative systems, such as pipe that includes recycled plastic and storm chamber systems. To this end, full-scale testing has been used to expose fully-instrumented pipes and chambers to relevant live and dead loading conditions while monitoring the wall strains and crown and toe deformations of the buried units. The objective of this testing is to show that pipe and storm chambers are structurally stable enough to sustain anticipated backfill and vehicle loadings without excessive deformation, stress cracking, or some other premature or unexpected response. The conventional excavation/backfill approach imposes a worst case load condition to a buried 1

2 pipe and is suited for evaluating pipe structural performance under shallow burial conditions and, with much more difficulty and cost, deep burial conditions. The compact burial chamber imposes a uniform loading condition and creates an idealized installation. The compact burial chamber also presents the capability to economically expose multiple replicates to buried conditions for extended periods. These two test approaches are contrasted in terms of their capabilities and therefore complement each other well. PIPE AND STORM CHAMBER INSTRUMENTATION Instrumented pipes and chambers typically include strain gauges spaced around the inside circumference of the corrugation located below the load path and in the adjacent valley. The gauges are positioned symmetrically at mulitple elevations along the arch profile. For storm chambers, strain gauges are also mounted on the associated end cap. Additionally, displacement gauges (potentiometers) are placed on each instrumented corrugation. At least one extends horizontally at mid-height of the pipe or between the opposite toes of the chamber to measure spreading/squeezing, and one extended from the chamber crown down to the supporting aggregate base to measure the upward/downward deflection of the crown. The instrumentation is hard wired and plugged into a data collection server that can be hooked up to a laptop computer for data collection at any time. Microstrains and deflections are recorded for analysis and plotting. Figures 1 and 2 show typically instrumented pipe and chambers. Figure 1. Typical Instrumented Pipe Figure 2. Typical Instrumented Chamber CONVENTIONAL EXCAVATION/BACKFILL BURIAL TESTING Test Setup. Pipes and chambers can be buried in both shallow and deep excavations using standard construction equipment for excavation, backfill, and compaction. While pipe is typically backfilled with a well graded, compactable soil, the aggregate used to surround storm chambers is a coarse free draining aggregate with little compaction. For a typical conventional excavation/backfill burial test two test pits are excavated one for live load testing and one for dead load testing. While for pipe testing, the excavation is typically an appropriate sized trench, for storm chambers a typical test pit is large enough to accommodate at least a 3 x 2 array of chambers below grade. Each chamber manufacturer has chamber-specific installation instructions, but generally they include minimizing compaction of the exposed native soils, using a nonwoven geotextile between the aggregate/chamber system and the excavation bottom, walls, and overlying backfill soils. Commonly, pipe installation is in accordance with the requirements of ASTM D 2321, Standard Practice for Installation of Thermoplastic Pipe for Underground Applications. 2

3 Chamber installation is much less standardized and typically follows the manufacturer s installation instructions. Following are some typical installation steps along with descriptive pictures in Figures 3 through 10. Live Load / Shallow Fill Test Setup: 1. Location of a 3 x 2 chamber array was marked on the base stone surface. 2. Chambers were positioned with the appropriate separation between rows (toe-to-toe). 3. End caps, feed connectors, and inspection ports were placed on/between the chambers. 4. A pipe opening was cut in one end cap of the center row to accommodate an access pipe. 5. Surface stakes were driven on either side of the excavation to locate the desired wheel path over the instrumented chamber corrugation (i.e. load arch). 6. Data recording was then turned on during backfilling operations. 7. The chambers were backfilled with embedment stone in lifts, evenly distributing the stone. 8. Lifts were leveled and hand tamped until sufficiently above the top of the chambers. 9. A nonwoven geotextile was deployed over the aggregate and general backfill soil was placed and compacted in lifts to achieve 95% standard Proctor density. Dead Load / Deep Burial Chamber Test Setup: 1. Follow the same steps 1 thru 7 above. 2. When the embedment stone layer reached the top of the chambers, a section of pipe was positioned vertically at an inspection port to act as a viewing port once the fill was placed. 3. Proceeded with the same steps 8 and 9 above. 4. Finally, an additional 15.2 ft (1.95 x 8.33 ft) [4.63 m (1.95 x 2.54 m)] of general fill was then placed in lifts, compacting as necessary to construct a stable fill. Figure 3. Initial Excavation Figure 4. Aggregate Base Construction 3

4 Figure 5. Chambers Being Backfilled Figure 6. Fabric and Cover Soil Figure 7. Compaction of Cover Soil Figure 8. Rolling Live Load Figure 9. Static Loading over Frame Figure 10. Completion of Backfill for Dead Load Testing. Test Procedures. As noted earlier, conventional pipe structures have been thoroughly studied and rarely require further field evaluation. A few exceptions to this include extending the use of flexible corrugated pipe to applications typically reserved for rigid pipe, such as under railroads or airport taxiways, or modifying the composition of the pipe, such as by including recycled plastic. In these cases, the field evaluation is on a case-by-case basis and typically requires the specific loading condition under questions. Evaluation of storm chambers, on the other hand, is more standardized and follows ASTM F Standard Specification for Polypropylene Corrugated Wall Stormwater Collection Chambers. F2418 has the requirement that The manufacturer shall verify the 4

5 installation requirements and design basis with full-scale installation qualification testing of representative chambers under design earth and live loads, in accordance with Practice F2787. F2787, Standard Practice for Structural Design of Thermoplastic Corrugated Wall Stormwater Collection Chambers then provides guidance for determining the appropriate dead loads, live loads, and factored loads to include in testing. Following these guidelines, the following procedures have been employed: Dead Load Testing - Data recording is turned on during backfilling operations and also weekly for subsequent (typically 3) months. Live Load Testing 1. The loading vehicle is positioned with wheel paths perpendicular to the axis of the chambers so that one wheel path will extend over the instrumented corrugation (i.e. load arch). 2. Sensor data recording is initiated prior to applying the live load. 3. The empty vehicle is driven back and forth over the load arch three times (equaling 6 passes). 4. The vehicle is then incrementally loaded with concrete blocks and slowly driven back and forth over the load arch three times (equaling 6 passes). A typical range of rear wheel loadings is 21.4 kn (4800 lbs) when the truck is empty to kn (27,840 lbs) when the truck is fully loaded. The preset tire pressure is maintained, typically at 689 kpa (100 psi). Factored Load Testing 1. The factored load frame is placed over the load arch to create a 51 cm x 25 cm (20 in x 10 in) contact area. 2. The loading vehicle is positioned on the factored load frame and incrementally loaded with concrete blocks until the factored load is achieved. 3. The incremental concentrated loads are held for at least 1 minute at each load level. A typical range of contact area loadings is from 25.8 kn (5800 lbs) when the truck was empty to kn (26920 lbs) when the truck was fully loaded, producing contact pressures ranging from 200 kpa (29.0 psi) to 928 kpa (134.6 psi). Test Results. Figures 11 and 12 present live and dead load responses of one chamber system tested. This graphical presentation of the noted behavior is developed from load-strain and loaddeflection relationships over time. The final system performance assessment is based on postinstallation and total vertical deflections. It should be noted that, generally, a 5% vertical deflection limit applies to buried corrugated plastic structures. Using this criterion, the evaluation focuses on indications of impending failure (i.e. collapse). The horizontal and vertical deflections are calculated as the change in length of the deflection gauges (potentiometers) divided by the original length. The strain gauge readings provide guidance to better understand localized behavior. Typical end-of-test data is summarized in Table 1 and strongly suggests that, under the conditions tested, the tested system is able to withstand short-term rolling and longterm soil embankment dead loads up to those tested herein without excessive deformation.

6 Figure 11. Typical Live Load Response Figure 12. Typical Dead Load Response Table 1. Summary of Results Property AASHTO HL-93 Factored (x 1.75) 16.2 ft (4.94 m) Cover Live Load Load Dead Load Vertical Deflection During Installation 0.75% 0.75% 2.25% Post-installation Vertical Deflection 1.25% 0.85% 0.15% Total Vertical Deflection 2.00% 1.60% 2.40% after 3 mos.

7 COMPACT BURIAL CHAMBERS Still, conventional excavation/backfill approaches to testing can be very time-consuming and costly. To facilitate a highly controlled field study while better controlling cost and improving accessibility to the buried structure, an innovative buried compression load chamber has been developed from a sturdy open-top precast chamber and outfitted with leverage compression devices for applying simulated deep fill loading. Each test assembly installs the sample pipe or chamber with at least one end accessible, thereby enhancing the performance monitoring aspect of the pipe sample. The use of these chambers facilitates cost efficiency for higher frequency testing of pipes or chambers in as buried conditions and enables on-site examination of pipe/chamber performance. Figure 13 is an early schematic of the compact burial chamber concept. Figure 14 shows an actual compact burial chamber. Figure 13. Concept Burial Chamber Figure 14: Actual Burial chamber Test Setup. Pipe/Chamber installation is closely monitored, including backfill soil characterization and lift thickness and compaction (density) documentation. Figures 15 through 18 provide views of construction within the compact burial chamber. Pressure cells are installed in the chambers during the installation process. The pressure cells allow for mapping the soil pressure in the chamber to confirm predicted stresses, as well as, any stress distribution defined by associated finite element modeling or other instrumented installations. Figure 19 shows a typical pressure cell layout and Figure 20 shows typical data collected from the pressure cells. Figure 15: Lubricated HDPE sheeting lines the chamber followed by a well compacted unyielding bedding layer Figure 16: Pressure cells are installed as the backfill advances

8 Pressure (psi) Figure 17: Load plates and loading mechanism are installed over a geogrid-reinforced cover layer. Figure 18: Chambers are Easily Replicated for Additional Tests PC1 PC2 PC3 PC4 PC5 PC6 PC7 PC8 PC9 5 Figure 19. Typical Pressure Cell Layout 0 1/6/15 3/7/15 5/6/15 7/5/15 9/3/15 11/2/15 Figure 20. Typical Pressure Cell Data Test Procedures. Commonly, loading is applied through the leverage compression device to either achieve a specific load condition or, alternatively, to reach a desired deflection. While most state DOT s limit deflections to 5% in the field, many allow up to 7.5% before removal or remediation of the pipe is required. Achieving larger deflections (e.g. 7.5% or higher) is a strategy to accelerate the loading on the pipe. The load to achieve the desired deflection is maintained and circumferential strains are monitored via on-pipe strain gauges and in-pipe potentiometer or laser-assisted radial measurements. This provides load-deflection data to validate that the desired burial condition has been achieved. Long-term load response is periodically monitored via the circumferential strain gauge array and in-pipe radial measurements. A total of four compact burial chambers have been constructed to complete field tests under a current NCHRP research project to examine the effects of recycled plastic on the integrity of corrugated HDPE pipe. The compact burial chambers were employed because the double-lever lever arm assembly with a 12:1 mechanical advantage - could apply and maintain simulated deep-fill loading greater than 170 kpa (25 psi) soil pressure on the pipe. An important additional benefit of the open-top compression chambers was that the chamber base was very stiff and thus could support monitoring and surveying instruments without them being impacted by subgrade settlements. Thus, pipe access and accurate monitoring from either end was facilitated. Multiple compression load chambers were constructed in a matter of weeks and were an economical way to facilitate multiple replicates of the required long-term installations. This allowed the two key elements of the afore-mentioned research to simulate as many different pipe/burial condition scenarios as possible, and to maintain the simulation for as long as possible.

9 It should be noted that in addition to using lubricated double-layers of HDPE sheeting on the chamber walls, the chamber width was designed to help minimize the edge effects of the walls. Previous research on soil cells for pipe testing indicates that the width of the chamber should be around 3 times the OD of the pipe being tested to ensure sidewall effects are minimized. Since the OD of the 30-inch diameter pipes being tested was approximately 35 inches, the width of the chamber should be around at least 105 inches (2.67 m). Thus, 10 ft (3.05 m) wide chambers were used. Deflections on the order of 10 to 15% were determined to be necessary to accelerate failures (i.e. the appearance of stress cracks) under the surcharge of approximately 25 psi. This load simulated an overburden of approximately 30 feet, for the given backfill conditions. The 12- inch bedding layer was compacted with a vibratory plate compactor to simulate a worst-case, unyielding base layer to accelerate deflections in the pipe. The pipe was intentionally backfilled with very poor backfill materials (silt-like pond fines from a gravel crushing operation) to enable the large desired deflections. Additionally, the backfill materials were dumped along the sides of the pipe with no additional mechanical compaction to provide full encapsulation, yet minimal resistance to deflection. Test Results. Pressure cells indicated that the pressures achieved throughout the chamber were within approximately 20% of the predicted pressures. This difference may have been due to some offset loading or from variations in the soils. The soil pressures continued to increase and redistribute slightly as the soils consolidated, but appeared to stabilize after days. There were several rain events that seem to have affected the consolidation of the soils and the resulting pressure readings in the chambers. Continued deflection in the pipe over time caused the loading arms to settle to the point that the hanging weights were touching the ground. This required the weights to be re-hung with shorter chain. As expected, over time the continuing deflection of the pipe caused soil pressure redistribution. Strain gage readings were used to verify that the installation condition created the target mean localized wall stresses. These wall stresses were used to calculate the expected time-tocrack for the range of pipes tested. Figures 21 and 22 show the typical strain gage positioning and associated gage measurements. Projections of time-to-crack have been preliminarily verified in several of the pipes tested to-date, but final test program reporting is pending. Figure 21. Typical Strain Gage Locations

10 Figure 22. Typical Pipe Wall Strain Response The ability to load, deflect, and monitor the test pipe specimens proved invaluable, as the researchers were able to access the inside of the pipe for visual examination in the deformed, stressed state to look for developing stress cracks. As hoped, stress cracks were found at times and locations projected from laboratory index tests and prevailing theory. The compact burial chambers were instrumental in making this rare correlation between full-scale performance and design theory possible. CONCLUSIONS A complex soil-structure interaction is involved when buried flexible plastic pipe or storm chamber systems are placed under load, and the stability of these buried systems must be demonstrated under worst-case live and/or dead load conditions. While it has become common place to demonstrate buried structure stability through analytical methods, including empirical equations or finite element modeling, full-scale burial tests have been recognized as necessary to, at least, calibrate these analytical methods. Two types of field tests - conventional excavation/backfill construction and compact burial chambers - have been used for this purpose. Using conventional excavation/backfill construction, fully instrumented pipe and storm chambers can be installed and then exposed to design vehicle and backfill loading conditions while monitoring the wall strains and crown and toe deformations of the buried units. In the case where long-term dead load conditions needed to be maintained and monitored, compact burial chamber testing provides a more economical and accessible alternative to the conventional excavation/backfill approach. Both testing approaches have been successfully used to develop deflection and strain data, along with a visual record, for use in correlating results of small-scale laboratory tests and calibrating design models. REFERENCES All referenced ASTM standards, ASTM, West Conshohocken, PA.