the impact of wire length on the compaction and porosity of tire

THE IMPACT OF WIRE LENGTH ON THE COMPACTION AND POROSITY OF TIRE CHIPS IN ON-SITE SEWAGE SYSTEM

NITRIFICATION TRENCHES Prepared for: I&E Committee Review C/o Ms. Tricia Angoli, EIT On-Site Wastewater Section 1642 Mail Service Center Raleigh, NC 27699-1642 Phone: (919) 715-3272

Report Prepared By: Larry F. Graham, P.E. Environmental Engineering Services P.O. Box 426 Aberdeen, N.C. 28315 Phone: (910) 944-1648, Fax: (910) 944-1652 And

Dr. Michael Hoover, Ph.D. N.C. State University

Department of Soil Science Box 7619 Raleigh, N.C. 27695-7619 Phone: (919) 515-7305

Report Completion Date: July 23, 2008 Latest Revision: August 5, 2008

Tire Chip Testing Final Report

TABLE OF CONTENTS

NOTE TO THE READER ...........................................................................................................1

EXECUTIVE SUMMARY ..........................................................................................................1

INTRODUCTION AND BACKGROUND ...................................................................................2

PROJECT OBJECTIVES ...........................................................................................................4

TIRE CHIP BACKGROUND INVESTIGATION........................................................................4

Purpose - ................................................................................................................................................4 Historical Regulation Development - ....................................................................................................4 Existing Tire Chip Research And Opinions - ........................................................................................5

EXPERIMENTAL PROCEDURE...............................................................................................7

Equipment..............................................................................................................................................7 Test Media Definitions ..........................................................................................................................7 Test Procedures......................................................................................................................................7

TEST RESULTS .........................................................................................................................8

A Note About The Test Results.............................................................................................................8 Measured Constants...............................................................................................................................8 General Compaction Test Results: ........................................................................................................8 Porosity Test Results: ..........................................................................................................................10 Conclusions: ........................................................................................................................................10

WIRE CHANGE RECOMMENDATIONS................................................................................11

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ACKNOWLEDGEMENTS

Mr. Larry F. Graham, PE and Dr. Michael Hoover were assisted in this research by Dr. Sushama Pradhan, PhD, and Mr. Craig Callis. Their help in gathering data, performing tests, and analyzing the data was invaluable. We would also like to acknowledge Dr. Barbara Grimes for her willingness to provide information, literature references, and opinions on the use of tire chips as nitrification field aggregate.

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THE IMPACT OF TIRE CHIP WIRE LENGTH ON CERTAIN PHYSICAL PARAMETERS ASSOCIATED WITH ITS USE IN ON-SITE SEWAGE SYSTEM NITRIFICATION

TRENCHES

NOTE TO THE READER

1. This report is a summary of a study performed on the effect of tire chip wire length on the physical properties of the chips once installed inside a nitrification trench. An earlier “Introductory Report” was submitted to the I&E Committee to give background on the project as of that time. The earlier report was dated July 23, 2008. Today’s report contains some of the same information as mentioned in the 7/23/08 report in order to provide a complete explanation of the whole scope of work, but it also contains updated data and conclusions, especially about the impact of wire length on tire chip compaction.

EXECUTIVE SUMMARY

1. Recycled tire chips, as historically used, have proven to work as a suitable nitrification trench aggregate. However, the restriction of using ½ inch wire protrusions as a manufacturing standard has more to do with tradition than demonstrated science.

2. Tire chips with few or no wires protruding from the rubber are becoming increasingly expensive and may be difficult to obtain in the near future. At one NC tire chip manufacturing facility about 60 tons per day of DFM tire chips (chips with the majority of wires exceeding ½ inch) go into a landfill. Many of these discarded tire chips could be used as nitrification field aggregate.

3. Based on test results, we have concluded that the trench settling that is often observed in the field within the first 2 or 3 years is most likely due to soil compaction rather than tire chip compaction. We believe the soil fill within the trench accounts for between 60% and 80% of all observed settling during the first few years after installation.

4. Tire chips with long wires were compared to the same tire chips without wires in an effort to see what would happen to compaction after the wires corroded. Test data shows that the difference between the longer wire chips just after installation and the no-wire chips is an additional 7% to 8% settling. This would be a worse case situation. If the initial thickness of tire chips was 12 inches, the thickness of the chips might compress down to 11 inches in 5 to 10 years or once the wires are dissolved. Even this amount of settling might not occur since roots and biomass may help prop up the tire chips after years of service.

5. When tire chips are compacted, their porosity or void space is reduced, but the magnitude of reduction is relatively small. Even when the tire chips are compacted to a very high degree (400 psf to 500 psf), their porosity still exceeds that of gravel. For example, highly compacted tire chips without wires have a void space of between 53% and 56%, while gravel has a void space of between 40% and 47%.

6. Increasing the wire length on tire chips from ¾ inch to 1 inch should not have any significant adverse impact on nitrification trench performance.

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INTRODUCTION AND BACKGROUND

1. With more than 250 million tires being discarded annually, there has been a multitude of efforts over the last two decades to find uses for this byproduct of the modern age. Since about 1985 uses for discarded tires and recycled tire products have been on the increase, howbeit a slow start for the first 10 to 15 years.

2. For over a decade recycled tires, in the form of tire chips, have been found to be an effective, and in some circles, a preferred substitute for rock aggregate in on-site sewage nitrification trenches. In fact tire chips have found their way into a host of innovative value added uses such as landfill drainage aggregate, road construction, embankment stabilization, tire derived fuel (TDF), and molded rubber products. New markets for the tire chips are emerging rapidly.

3. With the cost of fossil fuels at all time record highs, the markets which utilize tire chips and crumb rubber for fuel has exploded. This has added much pressure on the manufacturer’s of tire chips to recycle their tires for fuel related sales. The TDF chips can have only minimum amounts of protruding wire in order to meet the mechanical requirements of boilers and energy consuming equipment. The protruding wires come from the steel belts used in the manufacturing of modern day tires.

4. Tire chips that meet the fuel grade requirements, i.e. TDF chips, are sold at a premium. There are several factors which cause these chips to demand a high price, such as: 1) TDF chips with short wires are difficult to make and only account for a moderate percentage of the tire chips made on a day-to-day basis, 2) they are very competitive with fossil fuels, and 3) they are in high demand. While these chips will meet the current standards for nitrification trench aggregate, they cost considerably more than tire chips with longer protruding wires. The TDF chips are typically 2 to 3 times more expensive than chips just under TDF quality.

5. The N.C. Department of Environment and Natural Resources (NCDENR), Division of Environmental Health (DEH), On-Site Waste Water Section (OSWWS) has been allowing tire chips to be used as a replacement media for stone aggregate (i.e. gravel) in on-site nitrification trenches for a number of years. This Innovative Wastewater System Approval can be seen as IWWS-2002-03-R2. A copy of this approval is attached to this report. In short, DEH has adopted a set of standard guidelines which the tire chips must meet in order to be used for stone replacement in ground absorption nitrification trenches. The North Carolina regulations for tire chips dictate the chips must meet the following physical criteria (in summary form):

• At least 98% of the chips (by weight) shall be free of balls of wire and fine rubber particles. • At least 98% of the chips (by weight) shall be clean and free of fines and soil particles either

adhering to the chips or floating loose within the chips. • At least 95% of the chips (by weight) shall be two inches in nominal size and may range from 1/2

inch to 4 inches in any one direction. • The chips shall pass grading according to the ASTM D-448 (standard sizes of coarse aggregate). • At least 95% of the chips (by weight) shall have wire protrusions 3/4 inch or less from the sides of

the chips.

• Plus a few additional requirements.

6. In North Carolina, the acceptance and resulting demand for tire chips in sewage nitrification trench construction has grown considerably over the last 5 to 8 years. Tire chips tend to afford

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considerably more profit for the installer, allow more volume to be carried for less cost per truck load - as compared to gravel, provide more void space within the aggregate – as compared to gravel, and many investigators have found that the desirable microbe populations within tire chip nitrification trenches exceeds that of gravel.

7. When the tire chip manufacturer’s tire chipping equipment has new or recently sharpened blades, the tire chips are cut clean with only a few wires protruding from the rubber part of the chip. Today these clean cut chips are in high demand, especially for fuel - and as said earlier, they bring a high price on the open market. As the chipper blades dull, the exposed wires become longer. The cost of replacing blades is very expensive and they are time consuming to change. One manufacturer reports it costs his operation around $24,000 per month just to change blades 3 times (about every 10 days). The manufacturer cannot afford to change the blades every day or two, thus they produce a blend of chips, some with none or few wire protrusions and some with moderately short wires. If the chipper blades are allowed to get too dull, often just before the blades are changed, the wire protrusions become excessively long and stringy.

8. Tire chip manufacturers are faced with the added cost of producing more chips with fewer protruding wires and selling them at higher prices, or making a blend of no-wire chips and chips with relatively short wires for non TDF purposes. Because of the NC standards for tire chips used as nitrification trench aggregate, most of the septic system installers have been using the Tire Derived Fuel (TDF) chips for their on-site sewage installations. In order to help this customer base, the NC manufactures have been selling the TDF chips to the installers at below market prices, but it is only a matter of time before they are forced to charge the full TDF prices to installers. Most of the tire chips that contain slightly longer wires (longer than ½ inch) have been going to landfills. One NC tire chip manufacturer sends about 60 tons of chips to the landfill per day. The tire chips that are just outside of the NC on-site septic system criteria for wire length are called Drain Field Material or DFM chips.

9. There has been considerable promotion given to the use of tire chips for drainfield aggregate in North Carolina – and this promotional campaign has been successful. However, if the fuel demand placed on recycled tires and tire chips continue, there will be fewer fuel grade tire chips for nitrification fields - either due to availability or due to price.

10. Some people have reasoned that if something other than TDF quality tire chips (i.e. those chips with ½ inch or longer wires) are used as nitrification field aggregate, the wires would eventually corrode – resulting in excessive compaction within the trench. Excessive compaction of wireless tire chips, it is thought, would lead to less than adequate void space within the chip strata. In addition, there is speculation that if these wires eroded over time, the trench tops would settle, allowing surface water to find its way into the trench and hydraulically overload the soils. It should be pointed out that long term trench settling because of tire chip wire length is more of a theory than a proven fact.

11. Tire chips with excessively long wires are difficult to install and tend to string together instead of flowing into the trench bottom. Excessively long wires are considered to be perhaps 2 inches outside the sides of the rubber edges of the tire chips. While the excessively long wires may in fact work as a nitrification trench media, their installation difficulties would likely prohibit them from installer acceptance and use.

12. There is reason to believe that tire chip wire lengths greater than ½ inch and less than 2 inches will not change the overall effectiveness of the sewage effluent treatment in the nitrification trench.

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13. The purpose of this report is to explain the research performed by Mr. Larry Graham, PE, and Dr. Michael Hoover, PhD that looks into the performance of the TDF chips and the DFM chips and how they compare under various compressive situations. If the DFM tire chips perform comparably with the TDF chips, then it may be prudent to change the NC Standards on tire chip wire length.

PROJECT OBJECTIVES

1. To give the reader a background into the more recent issues related to tire chip availability, costs, and concerns over wire length as it relates to tire chip use as nitrification trench aggregate.

2. To address the question about tire chip compaction vs. wire length.

3. To investigate what happens to tire chip compaction and porosity once the steel wires erode.

4. To try and evaluate the cause of trench “cupping” when tire chips are used as a gravel substitute.

TIRE CHIP BACKGROUND INVESTIGATION

Purpose -

1. Environmental Engineering Services (EES) (Larry F. Graham, PE) performed a background investigation into tire chip manufacturing and especially their use as nitrification aggregate. This background work was done by reviewing literature, interviewing professionals, interviewing regulators, and interviewing researchers. The objective was to obtain a historical perspective on tire chip use in nitrification drainlines. Most of the published research on tire chips was done outside of North Carolina. During this investigation EES reviewed dozens of publications and interviewed at least 20 people, inside and outside of North Carolina, all in an effort to get a history of tire chip specification development and to find out what is working around the country. In other words, why are the specifications for the use of tire chips for nitrification trench aggregate written to use ½ or ¾ inch wire lengths as a criteria?

2. The reader should note that the investigation and conclusions mentioned in this background section ARE NOT all encompassing. There are many reports and research projects that have been conducted on tire chips and their many uses, which the engineer identified in his examination - but were not reviewed for time reasons. In addition, certain reports found referenced could not be located.

3. The reader should also note that much of the information in this report was obtained from telephone interviews with persons familiar with the use of tire chips for sewage applications. Many of these people have opinions based on their experience, but often do not have documented research to back up those opinions. EES has attempted to use both written documentation and oral communications to draw conclusions within this section of the report.

Historical Regulation Development -

1. EES began the investigation by reviewing the existing North Carolina tire chip specifications and interviewing persons at the NC Division of Environmental Health (DEH). Dr. Barbara Hartley Grimes, Ph.D. was the primary contact person with the NC On-Site Wastewater Section of DEH. Her responsibility, for a number of years, has been largely related to the permitting, use, and quality control of tire chips going into nitrification fields. Dr. Grimes has extensive knowledge of the

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performance of tire chips and their treatment effectiveness with regards to on-site sewage. She is a real proponent of tire chip use where applicable.

2. According to Dr. Grimes, the N.C. tire chip rules were developed using the State of Virginia as a model. An interview with Mr. Allan Lassiter with the Virginia Department of Environmental Quality revealed that Virginia developed their tire chip specifications based on the South Carolina model. In fact, after interviews with numerous state regulatory agencies, most, if not all of the different state regulations on tire chip manufacturing and their use for soil absorption nitrification trenches were based on the South Carolina model. South Carolina was one of the first, if not the first, state with a tire chip program for on-site sewage systems.

3. Mr. Clifton Roberts was one of the pioneers in using tire chips for nitrification trench aggregate. Mr. Roberts started this program in South Carolina and is currently retired from public work. EES interviewed several persons in the South Carolina Department of Health and Environmental Control (DHEC). Mr. Leonard Gordon, director of the on-site wastewater treatment program with DHEC told EES that originally S.C. developed the tire chip specifications without any specific scientific research and that their objective was to replace gravel with chips in a more-or-less equal ratio. The development and use of protruding wire length specifications at 1/2 inch was more or less a “best guess” as to what a tire chip should be. Any subsequent S.C. research with tire chips used this same wire length specifiction as a safe standard.

4. In conclusion, most, if not all state specification development for tire chip use as a gravel aggregiate substitute in nitrification trenches appears to have been derived from the groundbreaking S.C. work in this area. Indirectly, the North Carolina specifications for tire chips sizes and wire length came from South Carolina. Thus the current tire chip wire length specified in North Carolina is based more on historical experience and less on research data. In the background investigation performed by EES, we did not find research that specifically addressed “optimal” wire length criteria. This of course does not reflect poorly on the regulators in North Carolina, it just happens to be the way this standard has evolved.

5. A few states have recently started to alter their more traditional tire chip manufacturing criteria for nitrification trench use. An example would be New Jersey. Recently New Jersey altered their regulations on tire chip size and wire length. Specifically the new regulations on wire length state that 75% (by weight) of the chips must have exposed wire length no greater than 1 inch. This decision was made by a cooperative effort between the tire chip manufactures, researchers, installers, and state regulators. EES could not find any research on how well the new criteria was working in the field. Anecdotal reports indicate the longer wired chips are working fine.

Existing Tire Chip Research And Opinions -

1. There is a large amount of published research on using tire chips in nitrification trenches, under roadways, and in landfills. The vast majority of the research in on-site sewage systems has studied lechate quality and wastewater treatment performance. EES found only a very small amount of research on the short or long term physical qualities of tire chips inside nitrification trenches.

• In way of summary, here is a brief list of conclusions drawn by persons interviewed by EES.

• Tire chips do an excellent job at replacing stone aggregate in nitrification trenches. Tire chips typically exceed gravel in terms of effluent treatment, especially after an initial start-up time period. The percent pore space within tire chips exceeds that of gravel, at least in a loose state.

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• Tire chips are considerably less expensive than gravel for use as nitrification trench aggregate. In many states, there is a tight supply of tire chips, even being unavailable in some states. Alternate more lucritive markets for the short wire tire chips are making them less available for nitrification trench applications.

• Most of the scientific research reviewed by EES concluded that the leachate from tire chips is not an environmental threat. Semivolatile and volatile organic compounds were not found in tire chip leachate. A few of the secondary groundwater standards were exceeded in several tire chip lechate studies. These secondary groundwater standard metals typically included iron, manganese, and occasionally zinc. Metal concentrateions in tire chip leachate tended to rise early in many of the studies only to fall later, perhaps after the exposed wire had rusted away. Wire corrosion and disentergration is thought to be the cause of the rise in metals loadings. There was no link to environmental or groundwater damage with any of the leachage studies reviewed by EES.

• In one study (Amoozegar & Robarge, NCSU), tire chips which were dug up 13 months after being in service within a nitrification trench did not show signs of serious degredagation. Some exposed wire corrosion was noted on the 13 month old tire chip wires, but it seemed most wires were still in tact. The closer the chips are to the bottom of the trench, the more rapid is the wire corrosion (S. Robinson, Masters Thesis, February 2000).

• The average bulk density of loosely packed tire chip pieces was reported by Amoozegar & Robarge to be approximately 29 lbs/cu.ft. The total porosity of loosely packed tire chips with wire protrusions was estimated to be around 80% (Envirologic, Inc, May 22, 1990). The total porosity of loosely packed tire chips without wire protrusions were estimated to be about 70%. The total porosity of compacted tire chips with wire protrusions was about 60%. Gravel has a porosity of about 40 % (all based on Envirologic, Inc, May 22, 1990).

• In one study (Envirologic, Inc, May 22, 1990), tire chips were compacted with hand tamping to observe settling. Tire chips with wires settled 36% while tire chips without wire protrusions settled 21%. This same study concluded that even if exposed wires corroded, the settlement of the chips would not be of a significant magnitude.

• The vertical and horizontal permeability of uncompacted tire chips typically exceeds that of gravel. • Several regulators interviewed (states other than North Carolina) did not think exposed wire length

in tire chips (within reason) was a significant factor in the effectiveness of nitrification trench performance or in the overall trench structural integrety. Some concern was expressed that if the wire lengths were allowed to be too long, there could be problems with transferring the media into the nitrification trenches by the installers, thus resulting in non-uniform filling.

• Dr. Dana N. Humphrey, Ph.D, P.E. is widely know for his research on tire chips in civil engineering applications. In an EES telephone interview with Dr. Humphrey, he stated that in his opinion a “universal standard” for tire chip wire lengths would be: 1) 90% of the chips (by weight) should have exposed wire lengths 2 inches or less, and 2) 50% of the tire chips (by weight) would have exposed wire lengths of 1 inch or less. While not having direct access to nitrification trench research, Dr. Humphrey felt as though nitrification aggregate tire chips with 1 inch exposed wire should not adversely impact trench performance or long term aggregate compaction.

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EXPERIMENTAL PROCEDURE

Equipment

1. Two test cells were set up in which to perform compaction tests on various aggregate and soil types. These were heavy duty PVC cylinders with flat bottoms. The cylinders were set-up so that a measured force could be applied to the media inside each cell while measuring the change in depth of the media. Each cell had a drain port at the bottom terminal end that would allow liquids to escape. Figure 1 shows a photograph of this test set-up.

Test Media Definitions

A1 Chips = Tire Derived Fuel (TDF) chips. These typically meet or exceed the current DEH standards for nitrification field aggregate. These chips come directly from the manufacturer and were not modified for our tests. There are only a few wires protruding out of the rubber on these chips.

A2 Chips = Tire Derived Fuel (TDF) chips – (A1 chips modified). These typically meet or exceed the current DEH standards for nitrification field aggregate. These chips had all wires trimmed more or less even with the edges of the rubber prior to testing.

B1 Chips = Drain-Field Material (DFM) chips. These typically do not meet the current DEH standards for nitrification field aggregate due to excessive wire length. These chips come directly from the manufacturer and were not modified for our tests. According to a laboratory tire chip analysis performed by Trigon/Kleinfelder, Inc. only 86.2% of the DFM chips passed the ½ inch wire test. A copy of the DFM tire chip analysis is attached as Exhibit 1.

B2 Chips = Drain-Field Material (DFM) chips – (modified B1 chips). These typically do not meet the current DEH standards for nitrification field aggregate due to excessive wire length as they come from the manufacturer. These chips had all wires trimmed more or less even with the edges of the rubber prior to testing. B2 Chips would resemble B1 Chips after all wires are corroded away.

Filter Sand = This media is sand typically used in recirculating sand filters. It is uniform in consistency and in grain size.

Mortar Sand = This media is sand typically used to mix with concrete or mortar. It has a bright tan /orange color.

Stockpiled Clay Soil = This media is a red/tan clay soil which was stockpiled at the Lake Wheeler Training center. It contained a few clumps or clods of soil and a few roots. It is representative of soil from the Wake County area.

Freshly Dug Clay Soil = This media is a red clay soil which was dug at the Lake Wheeler Training Facility. It contains small rocks, clods of soil, some roots and light amounts of vegetation.

# 57 Gravel = Typical nitrification stone aggregate.

# 5 Gravel = Typical nitrification stone aggregate.

Cell 1 and Cell 2 = Cylindrical tire chip test cells.

Test Procedures

1. For brevity, the step-by-step test procedures will not be given herein. Suffice it to say that all of the different test media were tested in like fashion under the same conditions. Many of the tests

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had multiple repetitions in order to develop prediction curves. Most all of the data will be reported in Percent Change vs. Applied Force.

TEST RESULTS

A Note About The Test Results

1. Below is a brief outline of the conclusions we obtained from our testing. Only the most important conclusions will be discussed for brevity.

Measured Constants

The following density measurements were collected on the various test media.

Bulk Density of A1 Tire Chips = 30 pounds per cubic foot +/-.

Bulk Density of A2 Tire Chips = 31.7 pounds per cubic foot +/-.

Bulk Density of B1 Tire Chips = 26.6 pounds per cubic foot +/-.

Bulk Density of B2 Tire Chips = 36.4 pounds per cubic foot +/-

Bulk Density of # 57 stone = 97.5 pounds per cubic foot +/-.

Bulk Density of # 5 stone = 91.3 pounds per cubic foot +/-.

Bulk Density of Filter Sand = 87.2 pounds per cubic foot +/-.

Bulk Density of Mortar Sand = 92 pounds per cubic foot +/-.

Bulk Density of Stockpiled Clay Soil = 75 pounds per cubic foot +/-.

Bulk Density of Freshly Dug Clay Soil = 86 pounds per cubic foot +/-.

General Compaction Test Results:

1. Individual repetitions of compaction tests on all A1 tire chips had very similar curves when the change in tire chip depth was measured against increasing force. This shows that our testing was repeatable and predictable. This repeatable trend also held true for A2, B1, and B2 tire chip groups.

2. A1 chips (TDF chips without wires removed) and A2 chips (TDF chips with wires removed) had similar compaction vs. force graphs. The removal of the wires on A1 chips had very little impact on their compaction characteristics.

3. B1 chips (DFM chips without wires removed) had a compaction vs. force graph shaped similar to A1 and A2 chips. However, the percent change in depth of the B1 chips was greater than A1 chips at the same applied force. For example, at 150 psf force, the B1 chips experienced about a 15% change in depth, while the A1 chips at 150 psf force changed about 10% in depth. This follows along with our observations that the longer wire chips are more “spongy”, and due to the wires, the chips tend to compact a little more than the chips without wires. One must keep in mind that a difference of 5% to 7% in overall tire chip depth (or compression) is not as large as one might expect.

4. B2 tire chips (DFM chips with wires removed) had a compaction vs. force graph shaped similar to A1, A2, and B1 chips. The percent change in depth of the B2 chips was less than the B1

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chips at the same applied force. For example, at 150 psf force, the B1 chips experienced about a 15% change in depth, while the B2 chips at 150 psf force changed about 6.5% in depth. This follows along with our observations that the longer wire chips are more “spongy”, and due to the wires, the chips tend to compact a little more than the chips without wires. The B2 chips should resemble the DFM chips with all wires gone due to corrosion.

5. As expected, the compression tests with #57 stone and # 5 stone showed almost no change in depth with increasing applied force. # 57 stone showed the greatest change and it was only 2.13% at an applied pressure of 466 pounds per square foot. However, we did notice that if some form of vibration was applied to the test cells while force was applied to the stone, there was more pronounced settling. We did not attempt to measure this change since we were really not set up to accurately measure this aspect of stone aggregate.

6. The compression tests with Filter Sand and Mortar Sand showed very little change in depth with increasing applied force. The Filter Sand showed the greatest change and it was only 4.3% at an applied pressure of 614 pounds per square foot. However, we did notice, as we did with the gravel, that if some form of vibration was applied to the test cells while force was applied to the sand, additional settling would occur. We did not attempt to measure this change since we were really not set up to accurately measure this aspect of sand settling.

7. With the sands and the clay soils, we first applied about 80 to 100 pounds per square foot of pressure to the media before performing a water addition. All force was removed before applying 6 inches of water to the media. The water addition had a slight impact on the Filter Sand but no measurable effect on the Mortar Sand in terms of settling. After adding the water and allowing most of the water to drain out of the cells, we again applied force. The percent change we could measure due to this force was very small in both the Filter Sand and the Mortar Sand.

8. The clay soil we obtained from a stockpile at the Lake Wheeler Training Facility showed the most dramatic change in depth due to the addition of water and pressure. This particular soil sample contained some clods, roots, and a little vegetation. Still, it was more friable than what would likely be freshly dug from a trench. This soil behaved significantly different from the sands mentioned above. A sample of freshly dug clay was also tested. This soil was somewhat rocky, had hard clods of soil and a small amount of roots. The freshly dug soil was relatively dry.

9. The stockpiled clay and the freshly dug clay soil were tested at different times but in the same fashion as the sands. We first applied between 87 and 100 pounds per square foot of pressure to the media before performing a water addition. The initial down force caused about a 1% reduction in thickness of both clay soils. All force was removed before applying 6 inches of water to the media. The water addition had a significant impact on the clay soil in terms of settling. Just by the water addition, we observed a 13.83% change in media depth for the stockpiled soil and a 8.65% change in soil thickness for the freshly dug soil. This changed occurred without any externally applied force. After adding the water and allowing most of the water to drain out of the cell, we again applied force. The percent change we could measure due to this force yielded a total change in soil depth of 21.81% change for the stockpiled clay soil and a 21.15% change in the freshly dug soil. Maximum applied pressures were in the 620 psf range.

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Porosity Test Results:

1. Table 1 shows the porosity comparisons between media groups. We did not apply down pressure on the gravel samples because they had almost no change in thickness from earlier compression tests.

2. Our porosity test results for loosely placed aggregate are similar to those reported in most literature. For example, and according to literature, tire chips are said to have a porosity of about 60% and 66% and gravel about 40% to 45%.

TABLE 1. AGGREGATE TYPE REP. NUMBER APPLIED PRESSURE

(PSF) PERCENT VOID SPACE WITHIN THIS

SAMPLE A1 Chips 1 15 64%

231 60 % 480 57 %

A2 Chips 1 57 58 % 241 56 % 466 53 %

B1 Chips 1 24 66 % 239 62 % 466 57 %

B1 Chips 2 35 66 % 239 63 % 483 60 %

B2 Chips 1 38 63 % 239 60 % 471 56 %

# 57 Gravel 1 0 47 % # 5 Gravel 1 0 47 %

Conclusions:

1. Recycled tire chips, as historically used, have proven to work as a suitable nitrification trench aggregate. However, the restriction of using ½ inch wire protrusions as a manufacturing standard has more to do with tradition than demonstrated science.

2. Tire chips with few or no wires protruding from the rubber are becoming increasingly expensive and may be difficult to obtain in the near future. At one NC tire chip manufacturing facility about 60 tons per day of DFM tire chips (chips with the majority of wires exceeding ½ inch) goes into a landfill. Many of these discarded tire chips could be used as nitrification field aggregate.

3. The process by which tire chips are graded needs to be improved. The existing grading standard leaves too much to the “opinion” of the testing agency or person. While this is a critical area of concern, it is beyond the scope of the research conducted within this report to debate.

4. The tire chip test data developed under this study shows the test procedures are valid and repeatable. The coefficient of variation between compaction repetitions was shown to be very small.

5. There is a larger percent compression associated with the B1 tire chips (DFM) than with A1 chips for the same applied force - which is no surprise. However, the amount of compression

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difference between the A1 chips (TDF) and the B1 chips (DFM) is only about 5% at the pressures one would expect with 12 to 18 inches of soil cover.

6. The sands tested had only a small percent compaction when down force and/or water was applied. However, these sands were not typical of what one finds in most on-site wastewater disposal fields. The clay soils tested had considerable settling when water and then down force was applied. The application of water and pressure resulted in a 21% to 22% reduction in depth or thickness of the clay soils.

7. Based on test results, we have concluded that the trench settling that is often observed in the field within the first 2 or 3 years is most likely due to soil compaction rather than tire chip compaction. We believe the soil fill accounts for between 60% and 80% of all observed settling during the first few years after installation.

8. As a rule of thumb, we recommend adding 10% more chips for every foot of soil cover if using chips that resemble B2 or DFM quality chips.

9. There was no significant difference between A1 and A2 tire chips in terms of compaction. A2 tire chips would resemble the A1 tire chips with the wires gone due to corrosion.

10. The B2 tire chips should resemble the B1 Chips after the wires are eventually corroded away. Test data shows that the difference between the B1 chips just after installation and the B2 chips (B1 chips with wires completely gone), is an additional 7% to 8% settling. This would be a worse case situation if DFM chips were used in the trench. If the initial thickness of tire chips was 12 inches, the thickness of the chips might compress down to 11 inches in 5 to 10 years or once the wires are all dissolved. Even this amount of settling might not occur since roots and biomass may help prop up the tire chips after years of service.

11. Porosity testing of all the media groups was found to be similar to what other researchers have found.

12. Tire chips with wires have a higher overall porosity than the same tire chips with the wires removed, but the difference was relatively small.

13. When tire chips are compacted, their porosity or void space is reduced, but the magnitude of reduction is relatively small. Even when the tire chips are compacted to a very high degree (400 psf to 500 psf), their porosity still exceeds that of gravel. For example, highly compacted tire chips without wires have a void space of between 53% and 56%, while gravel has a void space of between 40% and 47%.

14. Increasing the wire length on tire chips from ¾ inch to 1 inch should not have any significant adverse impact on the nitrification trench performance. Having 85% of all wire lengths 1 inch or less would actually result in tire chips of a little higher quality than the DFM chips tested.

WIRE CHANGE RECOMMENDATIONS

Based on research, we are proposing, as a minimum, that the Innovative Wastewater System Approval dated October 24, 2002 and July 16, 2008, Part II Specifications, Item 4 be changed to read:

“Shall not contain wire protruding more than one inch from the sides of the chips (85% or better by weight)”.

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**** END OF REPORT ***

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EXHIBITS SECTION

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FIGURES SECTION

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the impact of wire length on the compaction and porosity of tire

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