Patent Application
20120020745
The present invention is directed to a self-supporting earth facing module and method for constructing mechanically stabilized earth walls and slopes using a fabric, tubular sock module filled with compost or fill material and having integrated geogrid extensions for reinforcement of a geosynthetically stabilized earth zone.
Soil reinforcement with layers of man-made inclusions, such as steel strips, steel grids, geotextile fabrics, and polymeric geogrids, has been used by geotechnical engineers for the past 40 years. The use of planar, horizontal reinforcing elements in a compacted soil backfill allows for the construction of mechanically stabilized earth (MSE) structures that include steepened slopes (known as reinforced soil slopes, or RSS) and near-vertical walls.
Large precast concrete panels and smaller modular block dry-cast concrete units are the most common hard facing units (examples (4) and (8) shown in
Wall facings also can be comprised of welded wire panels or gabions (wire baskets filled with stones) connected to grid reinforcement elements in the backfill zone. Wrapped geosynthetic faced MSE walls and slopes also can be constructed, but they require temporary bracing (falsework) at the face to support the soil being compacted immediately behind the geosynthetic facing, as illustrated in the steps depicted in
In recent years, tubular geofabric socks filled with soil, growing media, organic fibers, and/or compost have been available commercially for various applications in erosion and sediment control. For example, the FilterSoxx™ tubular sock product is commercially available from Filtrexx International, LCC, of Grafton, Ohio, and another product known as the BioSock™ system is commercially available from EnviroTech BioSolutions, Inc., of Honolulu, Hi. These types of socks can be used as wall or slope facings when wrapped with geosynthetic layers to construct the MSE structures. The combination of the tubular sock facing with geosynthetic layers can provide a wrapped, erosion-preventive surface treatment that contains a growth medium for vegetation that can be seeded or sprigged to form a vegetated slope facing. However, the combined structures require a wrapped geosynthetic protocol to tie the socks to the reinforcing layers in the backfill in order to stabilize them in position. They can eliminate the need of temporary forms or falsework, but still require using cable anchors, steel rods, or wooden stakes to anchor each lift of sock to the slope face.
Thus, regardless of the type of materials or construction method used conventionally for wrapped geosynthetic MSE structures, the soil-compaction process in the reinforced backfill zone near the final slope face requires that the face (or facing elements) be supported with temporary bracing (falsework) or by using cable anchors or stakes. In addition, these wrapped MSE structures are limited to linear alignments due to the manner in which the geosynthetic fabric is wrapped back over itself with each backfill lift. It would be highly desirable to provide a self-supporting facing module and method of installing each lift of a mechanically stabilized earth (MSE) structure without the need for supporting each lift with temporary bracing (falsework) or by using cable anchors or stakes.
In accordance with the present invention, a self-supporting earth facing module comprises: (a) a tubular sock made of a specified fabric material having a hollow interior volume which is to be filled with a compost or fill material so as to form a semi-rigid sock facing of a given sock diameter and a given sock length extending in a widthwise direction of the module; and (b) a geogrid sheet made of a geosynthetic sheet material and having a sheet width in the widthwise direction of the module and a sheet length sufficient to extend around the circumference of the tubular sock with upper and lower extensions of given extension lengths extending in a lengthwise direction of the module to one side of the tubular sock, wherein the sheet material of said geogrid sheet is joined by a high-strength seam to the sock material of the tubular sock at a juncture point where at least the lower extension of the geogrid sheet adjoins a circumferential point of the tubular sock, whereby said module forms an integrated unit that can be used to form a sequenced horizontal earth layer of a plurality of layers and serve as facing for a geosynthetically stabilized earth wall or slope.
In a preferred embodiment, the tubular sock containing the compost or fill material has a double layer of two different high-strength fabrics that combine to form a strong containment system for the infill. An inner fabric is a porous nonwoven material that has small enough openings to contain organic fines, yet is coarse enough to allow germinated seedlings to grow through the material. An outer fabric is a high-strength netting that provides tensile constraint to maintain module roundness as the compost or fill material is tightly packed into the sock to form a dense infilling.
The geogrid sheet is preferably a single, rectangular piece with biaxial tensile strength and with a width approximately equal to the length of the assembled tubular module. The overall sheet length may typically be equal to about 10 to 15 times the diameter of the sock. The geogrid material is centered and sewn to one hemispherical side of the sock such that the free extension ends of the geogrid sheet extend in the lengthwise direction at the top and bottom of the tubular sock as it lies horizontal. The hemispherical side of the sock wrapped with the geogrid sheet serves as the outward facing for the earth layer filled using the modular unit. The extension ends of geogrid sheet extend along the top and bottom of the earth layer into the reinforced soil zone behind the facing.
This new technology provides an engineered and erosion-preventive facing that can be pre-seeded within the compost or fill material prior to installation, hydroseeded after installation, and/or sprigged with live cuttings of plants in-between consecutive layers of the facing modules during installation.
The modular sock is preferably manufactured such that the sewing of the geogrid sheet to the sock wall is accomplished to ensure that the wide-width tensile strength of the stitched seam exceeds the wide-width tensile strength of the geogrid sheet itself. Thus, if an installed module were to experience an over-stressed condition, the geogrid extensions and/or hemispherical wrap would rupture before the seam would break, meaning that the known design strength of the geogrid sheet serves as the critical element in the geotechnical design analysis for the MSE structure.
The construction of an MSE earth wall or slope involves installation of a plurality of the self-supporting earth facing module units in successive horizontal lifts, with each lift comprised of modular units installed by butting them end-to-end, wherein the top and bottom geogrid extensions of adjacent modules are locked into the backfill zone in horizontal soil layers compacted to density specifications from the design engineer. To assure optimal interlock between the backfill and the geogrid sheets, the backfill material shall preferably consist of granular, well-graded sand or sandy gravel with less than 15 percent by weight passing the No. 200 sieve, and with a maximum particle size of 0.75 inch (19 mm). The extension length of this granular interlock zone behind the facing modules is preferably at least twice the diameter of the tubular sock modules.
Other objects, features, and advantages of the present invention will be explained in the following detailed description of the invention having reference to the appended drawings.
In the following detailed description of the invention, certain preferred embodiments are illustrated providing certain specific details of their implementation. However, it will be recognized by one skilled in the art that many other variations and modifications may be made given the disclosed principles of the invention.
Referring to
The compost or fill material used in the tubular sock modules may consist of various materials or mixtures of materials, such as: a) compost derived from green waste or other organic materials, such as grass clippings, chipped tree branches, shredded wood, shredded brush and other plant debris, chipped bark, wood sawdust and shavings, peat moss, hay or straw, seed hulls, rice hulls, manure, poultry dung, sea-bird or bat guano, and sea kelp; b) soil mixtures with or without organic constituents, such as loam, silt, sand, gravel, crushed aggregate, crushed coral, gypsum, lime, dolomite, oyster shell, vermiculite, perlite, cinder, pumice; c) plant and animal fibers, such as virgin cellulose fiber, coir, bagasse, corn fiber, flax fiber, cotton fiber, hemp fiber, wool fiber, and hair; d) synthetic materials such as recycled plastic, recycled glass, recycled paper and cardboard, recycled foam, plastic beads, Styrofoam, synthetic fibers, and polymers; e) nutrients, soil amendments and fertilizers, such as fish emulsion, flax seed oil, enzymes, soil bacteria and microorganisms, fungi, bio-stimulants, microbial inoculants, worm castings, bone meal, feather meal, humate, humic acid, organic fertilizers, synthetic fertilizers, and nutrient-rich plant food; and f) plant seeds and live cuttings, such as live seed and roots, spores, vines, sprigs, stolons, and rhizomes.
In a preferred embodiment, the tubular sock containing the compost or fill material has a double layer of two different high-strength, geosynthetic fabrics that combine to form a strong containment system for the infill. An outer fabric S1 is a high-strength netting that provides tensile constraint to maintain module roundness as the compost or fill material is tightly packed into the sock to form a dense infilling. An inner fabric S2 is a porous nonwoven material that has small enough openings to contain organic fines, yet is coarse enough to allow germinated seedlings to grow through the material.
The geogrid sheet is preferably a single, rectangular piece with biaxial tensile strength and with a width approximately equal to the length of the assembled tubular module. The overall sheet length may typically be equal to about 10 to 15 times the diameter of the sock. The geogrid material is centered and sewn to at least one hemispherical side of the sock such that the free extension ends to of the geogrid sheet extend in the lengthwise direction at the top and bottom of the tubular sock as it lies horizontal. The hemispherical side of the sock wrapped with the geogrid sheet serves as the outward facing for the earth layer filled using the modular unit. The extension ends of the geogrid sheet extend on the top and bottom of the earth layer into the reinforced soil zone behind the facing.
The modular sock is preferably manufactured such that the sewing of the geogrid sheet to the sock wall is accomplished to ensure that the wide-width tensile strength of the stitched seam exceeds the wide-width tensile strength of the geogrid sheet itself. Thus, if an installed module were to experience an over-stressed condition, the geogrid extensions and/or hemispherical wrap would rupture before the seam would break, meaning that the known design strength of the geogrid sheet serves as the critical element in the geotechnical design analysis for the MSE structure.
The tubular sock is preferably of the type that is in modular sock units with built-in irrigation tubing that can be connected serially end-to-end so that a long continuous row of facing can be formed in each horizontal earth layer, such as the product known as the Wiki-Garden™ system, commercially available from EnviroTech BioSolutions, Inc., of Honolulu, Hi. The modular sock units each have a pre-determined modular length and can be pre-filled with growing media for plants therein. A plastic/nylon irrigation tube is installed lengthwise through the growing sock with opposite ends thereof projecting through apertures formed in the sock material, wherein a male coupling fitting is attached to one end of the tube and a female coupling fitting to the opposite end. A plurality of tubular sock module sections can be coupled together in series with each section having its male and female coupling fitting attached to an opposite coupling fitting of an adjoining section.
Referring to
The modular length of tube 12 has a number of emitter holes 12c distributed over its length. The number of emitter holes and their orifice size are designed to deliver a pre-determined volume of water by drip irrigation into the length of the growing sock. As described in detail below, the pre-determined volume of water delivered by each modular section is selected to facilitate easy computation of the number and types of sections that can be coupled together in a series for a water source of given water pressure and water delivery volume. For example, for a tube of 3-foot length the same as the growing sock, there may be 3 emitter orifices spaced from 8″ up to 18″ apart.
For a more detailed description of this modular sock system, reference is made to U.S. patent application Ser. No. 12/604,132, of Alan Joaquin (who is a co-inventor herein), filed Oct. 22, 2009, entitled Modular Tubular-Sock Garden Growing System, which is incorporated by reference herein.
Construction of a MSE wall or slope using the tubular sock modules and soil-reinforced zone provides a coherent gravity mass to resist overturning and sliding forces that result from the active earth pressure applied by the retained soil in the slope. The system relies on the synergy between the facing module, its attached geogrid extensions, and the granular interlock zone. This interlock zone, wherein the modules are connected to each other and to the reinforced soil backfill, is adjacent to the back of the modules and typically has a width of at least two times the diameter of the tubular facing modules.
Referring to
(1) Prepare and compact a gravel leveling pad to receive the first row of sock modules. Lay first row of modules end-to-end along the prescribed wall alignment, with the lower geogrid extension on each module pulled out flat on top of the gravel pad, and with the upper geogrid extension laid loosely back over the module in the opposite direction. Where the tied-off end of each module (that is, the tied-off end of the sock that results from filling the sock with compost or fill material) butts against the end of the adjacent module, the excess geogrid flap (approximately 5 inches wide, which results from cutting the geogrid as required to tie-off the sock after filling) is overlapped onto the lower geogrid of the adjacent module after spreading a thin layer of granular fill onto that lower geogrid. This layer of granular fill prevents the two geogrids from being in direct contact with each other, which could form a weak shear surface.
(2) After all modules are laid for a given row, granular backfill is placed carefully on top of the lower geogrid extensions of the modules to provide a lift as high as the module height, with special care taken to place the granular material against the lower portion of each module so as to eliminate any voids that could develop underneath the lower curved section of the module. On-site soil can be used in the reinforced zone behind the granular interlock zone, provided the soil is approved by the design engineer. The lift of backfill material is compacted within the reinforced zone according to density specifications from the design engineer. The integrated geosynthetic design of the modules allows mechanical compaction of the granular interlock zone immediately adjacent to the modules without the use of temporary bracing (falsework) and without the use of cable anchors or stakes to restrain the modules at the wall face.
(3) The upper geogrid extensions of the modules then are pulled back over the modules and laid out flat on top of the compacted backfill lift, then a thin layer of granular fill is spread across the top of the geogrid.
(4) The next vertical row (lift) of modules is placed slightly behind the modules below to provide a set-back in the wall face, and then is installed according to the previous Steps. Live vegetation in the form of sprigs or live cuttings can be inserted between lifts if desired. Also, if gentle curves are desired in the wall face, the geogrid extensions can be cut perpendicular to the axis of the tubular modules to allow spreading of the geogrid for concave wall faces and to allow geogrid overlapping for convex wall faces.
(5) In similar manner, these Steps are repeated for each successive vertical lift until reaching the final wall height as specified by the design engineer. If the compost or fill material in the modules did not include plant seeds, then the face of the wall is hydroseeded to establish a permanent, vegetated MSE wall or slope.
The granular interlock zone provides two other important features. First, it provides a free-draining zone to allow ground water to flow easily out of the backfill zone, thus preventing the buildup of any de-stabilizing effects due to pore-water pressure. Second, the high shear strength of this densely compacted granular zone, which typical will have friction angles in the range of 34° to 44°, results in a strong, coherent gravity mass near the slope face, which steepens the angle of the estimated maximum stress line and thus reduces the width of the active zone in front of that stress line and allows the longest possible embedment lengths for the geogrid extensions in the resistant zone behind that stress line.
Unlike traditional, wrapped geosynthetic MSE walls and slopes, the tubular sock modules can be installed to form gentle curves in the wall face, in that the geogrid extensions can be cut perpendicular to the axis of the tubular modules to allow spreading of the geogrid for concave wall faces and to allow geogrid overlapping for convex wall faces.
In areas where consecutive geogrid layers directly overlap each other, they must be separated by a thin layer of granular material to assure solid interlock of the geogrid with the backfill and to prevent the possibility of a weak shear plane being formed between two vertically adjacent geogrid layers. Consecutive vertical lifts of the tubular modules should be laid in a running bond manner, so that the end-butt joint of two modules does not directly overlie an end-butt joint of modules below. This prevents the development of potentially weak vertical discontinuities up the face and within the geogrid-reinforced zone, which is critical for both curved and straight MSE walls and slopes.
The geogrid extensions on the tubular sock module can be any length that is appropriate, reasonable, and practical in regard to the manufacturing and production process, provided that length is not less than 3 feet (1 meter). This minimum length is based on recommendations from industry and government sources, which generally agree that a one-meter embedment length is adequate to engage the full wide-width tensile strength of most geogrid for most soils used in MSE wall backfills (for example, refer to the USDOT Publ. No. FHWA-NHI-00-043 and the NCMA 2009 Design Manual). The minimum embedment length required to engage the full wide-width tensile strength of a geogrid is known as the critical embedment length, and it is estimated as follows:
Le
crit
=Fg
stren/[2dγCi tan(φ)]
where:
As an example, for geogrids at the bottom and midway locations in the stabilized earth structure shown in
Le
crit=(12.7 kN/m)/(2·2 m·21.4 kN/m3·0.90·tan(38°)=0.21 m (0.7 ft), and
Le
crit=(12.7 kN/m)/(2·1 m·21.4 kN/m3·0.90·tan(38°)=0.42 m (1.4 ft), respectively,
assuming a typical sand/gravel unit weight of 21.4 kN/m3 (136 lb/ft3) and a geogrid long-term design strength (LTDS) of 12.7 kN/m (870 lb/ft). Thus, the one-meter long geogrid extensions, in conjunction with the granular interlock zone, provide adequate embedment in the resistant zone of the lower half of the structure where lateral earth pressures are the greatest.
The geogrid extensions can be made longer for applications in stabilized-earth walls and slopes taller than that shown in
In certain applications, the facing module units can be combined with supplemental geogrids to build tall composite, reinforced structures like that shown in
Other unique advantages and alternative applications of this tubular sock module include the ability to use an infilling material partly composed of sand or gravel to allow the first several rows of facing modules to be installed at or below seasonal water levels along shorelines and stream banks; the ability to easily install post-hole socket pipes or sleeves in the uppermost three compaction lifts of a stabilized-earth wall or slope by using pre-cut holes in the integrated geogrid extensions on the modules and then slipping the geogrids vertically down over the sleeve (
Stabilized-earth structures built with these tubular sock modules do no require any special construction equipment (bracing forms, falsework panels) or additional add-on pieces such as anchor cables or stakes. They are self-contained units that, when properly placed in conjunction with a compacted backfill zone, provide a durable, green wall that can be installed by anyone with basic construction know-how and skill.
In most applications it would be advantageous to construct the geogrid extensions out of a high-performance geogrid material, which typically consists of high-tenacity knitted polyester yarns with high molecular weight and covered with a polymeric coating to provide excellent durability and engineering properties. Such geosynthetic materials are mechanically and chemically durable to resist both the harsh construction installation phase and long-term contact with aggressive soil environments (pH range from 3-9). For example, a prime candidate for these geogrid extensions on the modules would be Stratagrid® Microgrid™ manufactured by Strata Systems, Inc., which has the following properties/specifications:
Ultimate biaxial tensile strength (ASTM D 6637): 2,000 lb/ft
Creep limited strength (ASTM D 5262 and D 6992): 1,149 lb/ft
Long-term design strength (LTDS): 871 lb/ft
Minimum molecular weight (GRI GG8): 25,000 g/mol
Maximum carboxyl end group count (GRI GG7): 30 meq/kg
Examples of other potential geogrid types for use as module geogrid extensions include Raugrid™ 2/2-20 biaxial geogrid (product of Carthage Mills), Mirafi® BXG biaxial geogrids, and TenCate Miramesh® SG geogrid (products of Koninklijke Ten Cate nv), the latter of which includes a synthetic grass component integrated with the geogrid sheet.
In applications that require durability it would be advantageous to construct the tubular sock portion of the module out of a synthetic fiber material that resists degradation, such as the BioSock Pro™ sock manufactured by EnviroTech BioSolutions, Inc. This sock is fabricated from polyester material and has the following properties/specifications:
150 Denier per filament
Fabric weight (ASTM D 3776): 3.5 oz/yd2
Puncture resistance (ASTM D 4833): 35
Diaphragm bursting strength (ASTM D 3786): 100 psi
Water permittivity (ASTM D 4491): 2.4 s−1
Water flow rate at 3-inches head (ASTM D 4491): 300 gal/ft2/min
Apparent opening size, AOS (ASTM D 4751): U.S. sieve size 30
UV degradation strength retention after 300 hr (ASTM D 4355) 70 percent
The thread used to stitch the tubular sock module and attach the geogrid extensions could be TENARA® Sewing Thread as manufactured by GORE™ and would resist degradation from water, ultraviolet light, salt water exposure, and other outdoor elements. This adjoining of the sock to the geogrid shall be accomplished using a double-stitched seam, with a resultant wide-width tensile strength that shall exceed the wide-width tensile strength of the geogrid. If a large-aperture geogrid is used for the module extensions, then a high-strength backing tape shall be used for stitching the seam.
In certain applications it would be advantageous to treat the tubular, compost-filled fabric sock modules, including the attached geogrid extensions, with a fire retardant treatment. This would prove especially valuable for applications where the geosynthetic module system can be substituted for competing technology that is inherently flame resistant, such as concrete segmental retaining walls, especially when said applications are located in areas prone to the threat of wildfires.
In certain applications it would be advantageous to construct the tubular sock portion of the module out of natural and biodegradable fabric material, such as a coir blanket woven from coir twines made of bristle coir obtained from freshwater cured coconut husks. For example, the Bio-D Mat® manufactured by Rolanka International, Inc., is comprised of such coir twines that have been machine spun to a uniform diameter. This coir blanket has the following properties/specifications:
Using the aforementioned materials to construct the tubular sock module would result in the geogrid extensions being synthetic and the sock structure being natural and biodegradable. The geogrid extensions would resist degradation and would be classified as “permanent” according to industry standards, such as the Erosion Control Technology Council guidelines for permanent turf reinforcement mats. The tubular sock structure itself would slowly and naturally biodegrade, yet the geogrid extensions would endure to act as reinforcement for the backfill soils and for the vegetative root zones of plants established on top and within the tubular sock module. Therefore, in ecologically sensitive applications, such as installation along a particularly critical body of water, the modules can be installed without the risk of synthetic fibers degrading and migrating into the aquatic habitat, which may pose health risks to biota, fish, and wildlife.
The tubular sock module can be filled with an aggregate, such as crushed rock or coral, or a blend of aggregate and growing medium such as compost or a soil/compost mixture. The specific infill mix selected for filling the tubular sock module would be based on job-specific criteria. In certain applications it would be advantageous to fill the sock module with a pre-seeded and pre-fertilized growing medium. The growing medium could consist of organic compost with the following properties:
The fertilizer blended into the growing medium could be derived from dried poultry waste with a nutrient analysis of 4% nitrogen (1.25% water-soluble organic nitrogen and 2.75% water-insoluble organic nitrogen), 3% available phosphate, 3% soluble potash, and 7% calcium. The seeds blended into the growing media could consist of locally specified species of annual or perennial grasses, herbs, or low-growing shrubs.
The tubular sock module system can be filled with growing media on-site using manual means, such as shovels and/or gravity feed chutes. To increase speed and efficiency the tubular sock module could be filled with growing media using a pneumatic blower machine such as a FINNS Bark Blower® or an auger extrusion machine, such as the BioSolutions™ SE1000™. In certain applications, such as long continuous module installations, the tubular sock module could be filled with a high-speed conveyor belt feeding system, such as the AT7 manufactured by Conveyor Application Systems, LLC.
To promote healthy and vigorous growth, care must be taken to install pre-seeded and pre-fertilized modules on the slope face before the seeds begin germinating within the filled tubular sock. If adequate time and/or weather conditions prevent the modules from being installed within a suitable time frame, the tubular sock modules should be filled absent the seeds and fertilizer. The omitted seeds and fertilizer can be applied in the field after module installation by means of hydraulic planting using commercially available hydroseeding equipment, such as FINN® Hydroseeders®. The hydraulic planting operation would place the desired seeds and fertilizer onto the exposed face of the modules, thereby allowing the seeds to germinate and send roots through the geogrid and sock materials and into the encapsulated growing media, whereby said roots would mature and occupy the confines of the encapsulated growing media. A variety of commercially available hydraulic planting products can be used to manipulate the performance characteristics of the hydraulic planting installation, such as bonding strength, functional longevity, elasticity, moisture retention, and organic content. Some of the classifications and/or types of commercially available hydraulic planting products that are suitable for use with the module system are bonded fiber matrices (BFM), stabilized fiber matrices (SFM), flexible growth media (FGM), hydraulic growing media (HGM), cementitious binders, and organic mulches comprised of paper fiber, cellulous fiber, straw fiber, corn fiber, coconut fiber, hemp fiber, or wood fiber.
It is to be understood that many modifications and variations may be devised given the above description of the general principles of the invention. It is intended that all such modifications and variations be considered as within the spirit and scope of this invention, as defined in the following claims.
