CA1237904A

CA1237904A - Drainage mat

Info

Publication number: CA1237904A
Application number: CA000451021A
Authority: CA
Inventors: Barry J. Dempsey
Original assignee: Monsanto Co
Current assignee: Solutia Inc
Priority date: 1983-03-31
Filing date: 1984-03-30
Publication date: 1988-06-14
Also published as: US4662778A; JPS59233013A

Abstract

APPLICATION FOR LETTERS PATENT FOR DRAINAGE MAT ABSTRACT OF THE INVENTION Drainage mat comprising three-dimensional openwork covered on at least a major surface with a water permeable fabric having a permittivity from 0.2 seconds-1 to 2.0 seconds-1 and exhibiting a dynamic permeability after 106 loadings of at least 10-4 centimeters per second.

Description

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BACKGROUND OF THE INVENTION

This invention relates to multidirectional drainage mats which are useful and effective, for instance as a highway edge drain for toe detouring of highway pavement systems.
The problem of water in pavement has been of concern to engineers for a considerable period of time. As early as 1823 Madam reported to the London (~nyland) Board of Agriculture on the importance of keeping the pavement sub grade dry in order to carry heavy loads without distress. He discussed the importance of maintaining an impermeable surface over the sub grade in order to keep water out of the sub grade.
The types of pavement distresses caused by water are guile numerous. Smith eta in the Highway Pavement Distress Identification Manual"
(1979) prepared for the Federal Highway Administration of the United states Department of Transportation identifies most of the common types of dusters.
moisture in pavement systems can come from several sources. Moisture may permeate the sides, particularly where coarse~grained layers are present or where surface drainage facilities within the vicinity are inadequate. The water table may rise this can be expected in the winter and spring seasons. Surface water may enter joints and cracks in the pavement, penetrate at the edges of the surfacing, or percolate through the uxfacing and shoulders.
3Q Water may move vertically it capillaries or interconnected water films. Moisture may move in vapor form, depending upon adequate temperature gradients and air void space. Moreover, the problem of water in pavement systems often becomes more severe 35 in areas where frost action or freeze-thaw cycles - - - - - --- - -

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occur, as well as in areas of swelling soils and shales.
The type of pavement dotters caused by water are quite numerous and vary depending on the type of pavement system. Fox flexible pavement systems some ox the distresses related to water either alone or in combination with temperature include:
potholes, loss of aggregates, raveling, weathering, alligator cracking, reflective cracking, shrinkage cracking, shoving, and heaves from frost or swelling soils). For rigid pavement systems, some of the distresses include faulting, joint failure, pumping, cortex cracking, diagonal cracking, transverse cracking, longitudinal cracking, shrinkage cracking, blowup or buckling, curling, D-cracking, surface spelling, steel corrosion and heaving (from frost or swelling soils).
Similar types of distresses occur in Tess and runways ox airfields.
Numerous of these joint and slab distresses are related to water pumping and erosion of pavement base materials used in rigid pavement construction. Water plopping and erosion of pavement base materials have been observed to cause detrimental effects on shoulder performance as well. Also, many of the distresses observed in asphalt concrete pavements are caused or accelerated my water.
For instance, faulting at the transverse joints is a normal manifestation of distress of I unreinforced concrete pavement without load transfer.
Faulting can occur under the following conditions:
1. The pavement slab must haze a slight purl with the individual slab ends raised slightly off the underlying stabilized layer (thermal gradients and differential drying within the I

slab create this condition).
2. Free water must be present.
I heavy loads must cross the transverse joint first depressing thy approach side of the joint, then allowing a sudden rebound, while instantaneously impacting the leave side of the joint causing a violent pumping action of free water.
4. ~umpable fine must be present (untreated base material, the surface of the stabilized vase or sub grade, and foreign material entering the joints can be classified as pump able fines.
Faulting of 1/4 in. or more adversely affected the riding quality of the pavement system.
Methods for predicting and controlling water contents in pavement systems are well documented by Dempsey in "Climatic Effects on Airport Pavement ; Systems State of the Art", Report No. FURROWED
~1976), United States department of Defense and United States Department of Transportation. Methods for controlling moisture in pavement systems can generally be classified in terms of protection through the use of waterproofing membranes and anti capillary courses, the utilization of materials which are incitive to moisture reneges an water evacuation by means of subdrai~age.
Field investigations indicate what evacuation by mean of a sub drainage system it often the preferred method for controlling water in pavement systems. In this regard proper selection, Dunn, end construction of the sub drainage system is important to the long-term performance of a pavement. A highway subsurface drainage system should, among other functions, intercept or cut off the seepage above an "I

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impervious boundary, draw down or lower the water table, anywhere collect the flow from other drainage 8y8tem5 .
Existing highway drains include a 5 multitude ox designs. Among the simplest are those which comprise a perforated pipe installed at the bottom of an excavated trench backfilled with sand or coarse aggregate. For instance, a standard drain specified by the Stat of Illinois requires a 4-inch diameter perforated pipe be placed in the bottom of a trench 8 inches (20.3 cm) wide by 30 inches (76 cm) deep. The trench is then backfilled with coarse sand meeting the State of Illinois standard Foal or FAX.
Such drains are costly to fabricate in terms of labor and materials. For instance the material excavated from the trench must be hauled to a disposal site, and sand backfill must be purchased and hauled to the drain construction site.
Other types of drains have attempted to avoid the use of the perforated pipe by utilizing a syrlth2tic textile fabric as a trench liner. The fabric lined trench is filled with a coarse aggregate which provide a support for the fabric. The void space within the combined aggregate serves as a conduit for collected water which permeates the fabric. Such drains are costly to install, for instance in terns of labor to lay in and fold the fabric as well as in terms of haulage of excavated and backfill material. Moreover, there it considerable fabric area blocked by contact with the aggregate surface. This results in an increased hydraulic resistance through the fabric areas contacting the aggregate surface.
Other modifications to drainage material 35 include fabric covered perforated conduit, such as corrugated pipe a disclosed by Siesta eta in United States Patent 3,~30,373 or raised surface pipe as ~3'79~f~
I, disclosed by Urea eta in United States Patent 4,182,5~1. A disadvantage is that the planar surface area available for intercepting subsurface water it limited to approximately the pipe diameter unless the fabric covered perforated conduit is installed at the bottom of an interceptor trench filled, Jay, with coarse sand. A further disadvantage is that much of the fabric surface, say about 50 percent, is in contact with the conduit, thwacks reducing the effective collection area.
The problem of limited planar surface area for intercepting subsurface water is addressed by drainage products disclosed by Heavy eta in United States Patents Nos. 3,563,038 and 3,654,765~ Heavy eta generally disclose a planar extended surface core cowered with a filter fabric which serves as a water collector One edge of the core terminates in an pipeline conduit or transporting collected water.
Among the configurations for the planar extended core are a sguare-corrugated sheet and an expanded metal sheet. A major disadvantage of designs proposed by mealy eta is that the drains are rigid and not bendable; this requires excavation of sufficiently long trenches that an entire length of drain can be installed. The pipeline conduit require a wider trench than might otherwise be needed. Moreover, the expanded metal sheet core does not provide adequate support to the fabric which can readily collapse against the opposing fabric surface, thereby greatly reducing the flow capacity within the core. Al o the square corrugated sheet core is limited in that at least 50 percent of the fabric surface arc is occluded by the core, thereby reducing water collection area.
A related dx~inage material with e~t2nded surface is a Tyler composite of polyester non-woven filter fabric heat bonded to an expanded nylon non oven matting such as ENKADRAINr foundation I
I, drainage material available from American Enka Company of Enka, North Carolina. The drainage material which can be rolled ha filter fabric on one side of the nylon non-woven matting. The drainage material serves as a collector only and requires installation of a conduit at the lower edge. This necessitates costly excavation of wide trenches, in addition to cost of conduit.
Another related drainage material with extended surface comprises a filter fabric covered core of cuspated polymeric sheet, such as STRIP DRAIN*
drainage product available from Nylex Corporation Limited of Victoria, Australia. The impervious cuspated polymeric sheet divides the core into two isolated opposing sections which keeps water collected on one side on that side. Moreover, in order that the drainage material be flexible, the core must be contained in a loose fabric envelope, which being unsupported on the core can collapse due to soil loading into the core thereby blocking flow channels.
The cuspated polymeric sheet is bendable only along two perpendicular axes in the plane of the sheet.
This makes installation somewhat difficult, for instance whole lengths must be inserted at once in an I excavated trench.
A still further similar polymeric drainage product comprises a perforated sheet attached to flat surfaces of truncated cones extending from an impervious sheet, such as CULDRAIN*bo~rd-shaped draining material available from Mets Petrochemical Industries, Ltd. The perorated sheet has holes in the range of 0.5 to 2.0 millimeters in diameter and allows fine and small particles to be leached from the subsurface toil.
The drainage materials available have one or more significant disadvantages, including economic disadvantages of requiring extensive amounts of labor * Trade Mark I
I

for installation and performance disadvantages such as requiring separate conduit for removing collected water. A further performance disadvantage it what the drainage materials utilize ~c~bric which, depending on 5 the adjoining toil, may become blinded with soil particles or may allow too much material to pays through resulting in loss of sub grade support.
This invention overcomes most if not all of the major disadvantages of engineering fabric utilized in previously known drainage materials.
Among the useful parameters for characterizing fabric useful in the drainage ma of this invention is the coefficient of permeability which indicates the rate of water flow through a fcibric material under a differential pressure between the two fabric surfaces expressed in terms of velocity, e.g., centimeters per second. Such coefficients of permeability can be determined in accordance with American Society for Testing and Materials (AUTUMN) Standard D-737. Because of difficulties in determining the thickness of a fabric for use in determining a coefficient of permissibility, it is often moxie convenient and meaningful to characterize fcibric in terms of permittivity which is a ratio of the coefficient of permissibility to fabric thickness, expressed in terms of velocity per thickness, which reduces to inverse time, e.g., seconds I. Permittivity can be determined in accordance with a procedure defined in appendix A of Transportation earache Report ~0~2, available from the United State Department of Transportation, Federal Highway Administration.
Engineering fabrics used with drainage mats can be guile effective in protecting isle from erosion while permitting water to pass through the fabric to the conduit part of the drainage mat.
However, the fabric must not clog or in any way significantly decrease the rate of flow. At the tame time the fabric must not let too much material pass through, or clogging of the drainage mat could occur.
However, lows of sub grade support could also occur.
When considering the actual soil-filter fabric interaction, a rather complex bridging or arching occurs in the soil next to the fabric that permits particles much smaller than the openings in the fabric to be retained. Failure of the soil-fabric system can result from either excessive piping of soil particle through the fabric or from substantial decrease in permeability through the fabric and adjacent soil The use of engineering fabrics in highway drainage mats requires the consideration of an additional factor. A highway is subjected to repeated dynamic loading by traffic. Such loading can lead to substantial pore pressure pulse in a saturated pavement system. During and after heavy rain a soil-filter fabric at the pavement edge may be subjected not only to a static hydraulic gradient, but also to a Dominique ~xadient caused by the highway traffic loading.
In this regard another useful parameter for characterizing fabric useful in the drainage mat of this invention it "dynamic permeability which indicates the rate of water flow through a column of ~ecifically graduated soil over a layer of fabric material under a combined static and dynamic hydraulic gradient. "Dynamic permeability' characterizes fabric performance in resisting blinding and luggage under conditions which duplicate thy effects of repeated traffic loading. The method for determining "dynamic permeability" is disclosed in Example II, herein.

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g SPRY OF THE INVENTION

This invention provides a drainage mat comprising a ~hree-dimensional openwork cowered on at least a major surface with a water permeable fabric, having a permittivity from 0.2 seconds 1 to 2.0 seconds 1 and exhibiting a Dominique permeability after 106 loadings of at least 10 4 centimeters per second.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 schematically illustrates an embodiment of the drainage mat of this invention.
FIGURE 2 schematically illustrates a synthetic grass-like material useful as the three-dimensional openwork of the drainage mat of this invention.
FIGURE 3 is a sectional view of a triaxial cell apparatus useful in determining dynamic permeability.
FIGURE 4 is a schematic illustration of triaxial cell apparatus and ancillary equipment as used in determining dynamic permeability.
FIGURE 5 is a plot of particle size analysis of a soil mixture used in determining dynamic permeability .
FIGURES 6, 7 and 8 are plots of dynamic permeability for accumulated loadings Or various engineering fabric The drainage mat of this invention comprises a three dimensional openwork covered on at 30 least a major surface with a water pinball fabric.
A drainage mat is generally planar shaped with its icons being ~uh~tantially smaller than its other dimensions. The dimensions of the drainage mat correspond closely to the dominoes of the three-dimensional openwork, which provides support for the fabric and has a substantial void volume to allow for multi-directisn water flow within the open work.
The openwork can comprise a variety of configurations and Motorola useful configuration for some applications is a synthetic grass-like 10 material as described by Coleman eta in U.S. Patent ~,507,010, incorporated herein by reference. In this regard Figure 1 schematically illustrates such a drainage mat where fabric 1 envelops a synthetic grasslike material 2. Figure 2 illustrates such 15 synthetic grass-like material. Other configurations include any of those planar-shaped openwork known in the art which do not block substantial area of the fabric covering.
Useful materials for openwork include 20 polymeric materials such as polyethylene, polypropylene, polyamide, polyesters and polyacrylonitriles. It has been found that hydrophobic materials, such as polyethylene, are generally preferred to hydrophillic materials, such as 25 polyamides. Fine particles which wash through the fabric may contain charges or have some other chemical or electro-chemical affinity, for hydrophillic materials, resulting in material buildup, and possible luggage, within the openwork.
Polymeric materials are generally preferred since they are lightweight, easy to handle and fabricate and are generally environmentally resistant. however, depending on the application, otter materials could be u~edt for instance metal, 35 such as aluminum expanded metal sheet.
The enveloping water permeable fabric can coup A ye a wide variety of materials. Among the preferred fabrics are those made from polymeric materials such as polyethylene, polypropylene, polyamides, polyesters and polyacrylics. In most instances it is preferred that the fabric comprise a hydrophobic material such as polypropylene or polyester. Such fabric should be sufficiently water permeable what it exhibits a water permittivity in thy range of from about 0.2 seconds 1 to 2.0 seconds 1, More preferred fabrics are those having a permittivity in the range of from about 0.5 seconds 1 to about 1.0 seconds 1. The fabric can either be of a woven or non-woven manufacture; however non-woven fabrics are often generally preferred.
Such permittivity indicates that the lo fabric allows adequate water flow through the fabric to the conduit part of the drainage mat. Such water flow is not so great as to allow so much suspended material to pass through the fabric that would result either in loss of sub grade support or clogging of the drainage mat.
The fabric should also exhibit substantial resistance to blinding and luggage, for instance as may be caused by bridging or arching of soil particles next to the fabric. Since the fabric in many installations, for instance in highway edge drains, is subjected to both static and dynamic hydraulic gradient due to repeated traffic loading, dynamic permeability is an essential characteristic of the drainage mat of this invention. In general, ye fabric should exhibit a dynamic permeability after 106 loadings, as described in the procedure of Example II
below, of at least 10 centimeters per second. A
more preferred fabric will exhibit a dynamic permeability after 106 loadings of at least 10 3 centimeters per second, for instance in the range of 10 2 to 10 3 centimeters per second. In some instances, a fabric which exhibits a dynamic I

permeability of as low as 10 5 centimeters per second may be acceptable.
Dynamic permeability readings may vary over the course of repeated loadings, for instance 5 over 106 loadings. It is generally desired that variations in dynamic permeability be within an acceptable range based on the highest reading of dynamic permeability. For instance, the ratio of the highest reading of dynamic permeability to the lowest reading of dynamic permeability over 106 loadings (a million loading dynamic permeability ratio) should no exceed 100. It is more preferred that the million loading dynamic permeability ratio be about 50 or less.
The water permeable fabric need not envelop the entire openwork. The fabric should however totally cover at least a major surface which is intended to intercept ground water.
The drainage mat of this invention is useful in any number of applications where it is desirable to remove water from an area. It is particularly useful in subsurface applications where ground water removal is desired.
A large surface area available for drainage is provided by the rectangular transverse cross section of the drainage mat. This is particularly advantageous in those installations where the drainage mat is installed such that the larger of its transverse cros~-~ectional dimensions is normal to the surface of an area to be drained. Such an advantageous installation is in a highway system where the drainage mat is installed parallel to a road for instance in a vertical orientation under a highway shoulder joint. In such an installation water infiltrating in a vertical direction through the highway shoulder joint can be intercepted by the narrow transverse cross-~ectional area at the top of ~23~
~13-the drainage mat and water present under the highway can be intercepted by the large transverse cro~s-sectional area which is normal to the highway support Ted, and the opposing large transverse cross sectional area can intercept ground water approaching the highway from the outside. All such intercepted water can be carried away as soon as it is collected by the drainage mat.
In other installations where it is desired to maintain a moisture level in a highway support bed, a drainage mat with an impervious layer can be installed with the impervious layer in contact with the vertical edge of the support bed preventing flow of water either into or out of the support bed.
The drainage mat can intercept and carry away water which could otherwise enter the support bed.
This invention is further illustrated by, but not limited to, the following examples.

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EXAMPLE I

Three varieties of engineering fabric were obtained. These three fabricfi and their 5 equivalent opening size the equivalent U.S. Sieve No, as determined by Test Method QUEUE) are identified in Table 1. The three fabrics were subjected to permittivity analysis. The results of the permittivity analysis based on ten random specimens for each fabric and ten test runs on each specimen are shown in Table 2.

Equivalent Opening Fabric No. Description Size _ 1. Non-woven spun bonded polyp 140^170 propylene fabric, obtained from ELI. Dupont de Numerous & Co. as TOPPER pun bonded polypropylene, style 3601 2. Woven polypropylene fabric obtained from Advanced Construction Specialties Company designated as Type II

3. Non-woven polypropylene (minimum) fabric, obtained from Amoco Fabrics Company, as PROPER 4545 Soil Filtration Fabric, calendered Fabric No. Permittivity 1. 0.094 seconds 2. 1.~0 seconds 3. 0.75 second aye E LYE II

This example illustrates the test procedure for determining "dynamic permeability" of a fabric. The three varieties of engineering fabric identified in Example I were subjected to "dynamic permeability" analysis using the triaxial cell apparatus schematically illustrated in Figure 3. The triaxial cell apparatus comprises a metal base plate 1, having a central raised boss 4 of 8 inches (20 cm) in diameter and an annular groove to accept cylinder 2. The metal base plate has a fluid port from the center of the raised boss 4 to the periphery. A
flexible outer confining membrane 3 of 1/32 inch (0.8 mm) thick neoprene rubber is secured to the periphery of the central raised boss 4. Silicone grease is applied to the interface of the outer confining membrane and the central raised boss to provide a water tight seal. A porous Carborundum stone 5, 8 inches (20 cm) in diameter, is placed on the central raised boys 4. Four perforated rigid plastic discs 6, 8 inches (20 cm) in diameter, are placed on Carborundum stone 5. A piezometric pressure tap tubing 7 it installed in a hole in the outer confining membrane 3, just below the top of thy plastic discs 6.
A single layer of lass spheres I, 0.625 inch (1.5 cm) in diameter, is placed on the top plastic disc.
A flexible inner membrane 9, having inches ~20 cm) diameter engineering fabric disc 10 secured to the bottom edge of flexible inner membrane 9, it inserted within the flexible outer membrane 3, such that the engineering fabric disc 10 rests on the layer of glass spheres 8. A coating of silicone grease at the interface of flexible inner membrane 9 and flexible outer membrane 3 provides a water tight seal between the two membranes.

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Waxer is allowed to flow into the confining membrane 3 from the port in the base plate to a level above the fabric disc to remove any trapped six. The water is when drained to the level of the fabric disc loo A dry soil mixture of 90 percent by weight Clays X concrete sand (no minus number 200 sieve material) and lo percent my weight Reaction silt is prepared. The dry soil has a gradation analysis as shown in Figure 5. 30 pounds (13.6 kg) of dry soil is thoroughly mixed with 2 livers of water to produce a mixture at close to 100 percent water saturation. The mixture M is loaded into the flexible inner membrane 9 to a height of about 9.4 inches (24 cm) above the fabric disc lo As the mixture M is loaded into the membrane, excess water is allowed to drain from mixture M by maintaining the open end of tubing 7 at a level about 0.4 inch (1 cm) above the fabric disc 10 .
After all excess water has drained from the mixture M, a porous Carborundum stone 11, 20 cm (8 inches) in diameter, is placed on the mixture M.
metal cap 12, 8 inches (20 cm) in diameter, is placed over the stone 11. Silicone grease is applied to the interface between the cap 12 and the flexible inner membrane 9. Bands (not shown) are used to secure the membranes to the cap 12. The cap 12 has two ports and a raised center boss. A transparent cylinder 2 is placed over the assembly with the bottom edge of the cylinder 2 fitting into the annular groove of the base 1. A metal cell top 13 is placed over the cylinder 2 with the top edge of the cylinder fitting into an annular groove in the cell top 13. The cell top 13 and the base plate 1 ore held against the cylinder 2 by bolts (not shown.
The cell top 13 has four partisan port is connected to tubing 14 which provides cell pressurizing water; another port is connected to I
~17-tubing 15 which runs through the cell top 13 to a port on the cap 12 which can be used to provide flush water Jo the confined mixture I; another port it connected to lobbying 16 which suns through the cell top 13 to a port on the cap 12 which provides water flow fox analysis; the fourth port is connected to tubing 7 which is used to monitor pressure below the fabric disc lo. The cell top 13 has a bore through the raised boys 17. The bore allows loading rod 18 to lo pass through the cell top lo to the top of metal cap 12~ The bottom surface of the loading rod 18 and the top surface of the metal cap 12 have spherical indentations to receive metal sphere lo which allows a point load to be transmitted. O-rings (not shown) provide a seal between the loading rod 18 and the bore through the cell top 13.
The triaxial cell apparatus is prepared for operation by filling the annular space between the cylinder 2 and the membranes with water to the level of the cap I Tubes 15 and 16 are conrlected from ports on the cap 12 to ports on the cell top 13.
Water is allowed to enter the membrane containing mixture M from the bottom up to saturate mixture M.
Valve 20 on tubing 15 can be operated to vent air.
Water is allowed to fill tubing 16 connected to a pair of pressurizable reservoirs of darted water. The pressure within the membraIles (the "internal pressure") can lye adjusted through tubing 16 connected to the pressurizable reservoir which is loaded with air pressure. The pressure yin space Surrounding the membranes (the "confining pressure") can be adjusted through tubing 14~
Wrier now to Figure 4 which is a simplified schematic illustration of the apparatus illustrated in Figure 3 together with one of the pressurizable decorated water reservoirs 22, mercury manometer 23 and water manometer 24. The ~23~79~

pressurizable reservoir 22 is located above the triaxial cell 25, for instance a convenient distance between the average height of water in the reservoir and the level of water I in the triaxial cell 25 is lo cm.
It it desirable to operate with the air pressure on the reservoir 22 at about 220 ~N/m2 (32 psi) while maintaining a "net confining pressuxel' ox 12 . 1 kN/m2 ( 1 . 75 psi). Net confining pressure, P, can be calculated from the following equation:
P = 1.33 (H HOWE), where P is the net confining pressure, expressed in terms of kN~m2;
is the pressure difference, measured by mercury manometer 23, of the excess air pressure at tubing 14 over air pressure at tubing 27; and HO is the average distance between the level of water in reservoir 22 and the level of water 26 in the triaxial cell 25.
For instance, when En is about lo cm, it is desirable to slowly increase the confining pressure measured at tubing 14 to at least 15 cm Hug (6 itches Hug) greater than the pressure at tubing 27. Then both pressures are slowly raised until the air pressure on the reservoir I is about 220 kN~m2 (32 swig. The confining pressure should be adjusted such that the mercury manometer 23 indicate that the air pressure at tubing 14 it 16.5 cm Hug (6.5 inches Hug) greater than the air pressure at tubing 27. This should provide a net confining pressure of about 12.1 kN/m2 (1.7S psi).
Flow is initiated by opening bleeder valve 28. The rate of flow is adjusted to generate a pressure drop measured at water manometer 24 in the ,..

I

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range of 24 to 26 cm water (about 9.5 to 10.25 inches water). Reading of flow rate, time and water manometer differential are recorded until permeability is stabilized, for instance usually lo to lo minutes.
Axial loading via loading rod 18 it then parted. An air actuated diaphragm air cylinder (not shown) is connected to the loading rod 18. A load pulse of 17.5 kN/m2 ~2.5 psi) is applied Jo the cap 12 and transmitted to mixture M at a frequency of once every lo two seconds (0.5 hertz). This loading simulates stress within the mixture M similar to sub grade stress from truck loading on a highway system.
Readings axe taken after 1, 10, 100 and 500 loads and thereafter generally at six hour intervals.
Dynamic permeability of the engineering fabric is calculated from the following equation:
K = QL/~T
where K is dynamic permeability, expressed in terms of cm/sec;
Q is water flow volume, expressed in terms of cm3, collected over time, T;
1. is the height of soil mixture M, expresses in terms of cm:
H it the hydraulic gradient over the mixture as measured on water manometer 24, expressed it terms of cm;
A it the cross-sectional area of the fabric disc 10, expressed in terms of cm2; and T is the time to ~0112ct a volume Q, expressed in terms of sec.
Dynamic permeability for the engineering fabrics identified in Example I is shown in Figures I, ~L23~

7, and 8, which are plots of dynamic permeability versus loadings.
Figure 6 it a plot of dynamic permeability, recorded for Fabric No. 1, which decreases to less than 10 4 cm/sec after about 450,000 loadings.
Figure 7 is a plot of dynamic permeability, recorded for Fabric No. 2, which decreases gradually but remains above 10 4 cm/sec even after one million loadings Figure 8 is a plot of dynamic permeability, which remains between 10 3 and 10 2 cm/sec over the application of one million loadings.
In view of the results of dynamic permeability analysis, Fabric No. 1 would be unacceptable for use with the drainage mat of this invention, while Fabric No. 2 and Fabric No. 3 would be acceptable for use with the drainage mat of this invention. Fabric No. 3 is exemplary of a more preferred fabric.
While the invention has been described herein with regard to certain specific embodiments, it is not so limited. It is to be understood that variations and modifications thereof may be made by those skilled in the art without departing from the spirit and shape of the invention.

I.,

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A drainage mat comprising a three-dimensional openwork covered on at least a major surface with a water permeable fabric having a permittivity from 0.2 seconds -1 to 2.0 seconds to 2.0 seconds -1 and exhibiting a dynamic permeability after 106 loadings of at least 10-4 centimeters per second such that said mat is resistant to soil pluggage from pulsating water flow.

2. The drainage mat of claim 1 wherein said fabric has a permittivity from 0.5 seconds -1 to 1.0 seconds -1.

3. The drainage mat of claim 2 wherein said three-dimensional openwork comprises a polymeric core having a plurality of fingers extending from a layer.

4. The drainage mat of claim 3 wherein the fingers are grass like fingers.

5. The drainage mat of claim 4 wherein said fabric substantially envelops the core.

6. The drainage mat of claim 2 which after from 1 to 106 loadings exhibits a dynamic permeability in the range of 10-4 to 10-2 centimeters per second.

7. A highway edge drain comprising a generally-planar three-dimensional openwork covered on at least a major water-intercepting surface with a water permeable fabric having a permittivity from about 0.2 seconds -1 to about 2.0 seconds -1 and exhibiting a dynamic permeability after 106 loadings of at least about 10-4 centimeters per second such that the mat is resistant to pluggage from pulsing water flow.