Pennsylvania Handbook of Best Management Practices for Developing Areas

Appendix E -- Approach for Developing Material Specifications

1. Introduction

Physical properties of construction materials used in the BMPs vary significantly based on their natural origin (in the case of natural materials such as sand or rock) or the materials and process used to manufacture them (in the case of geosynthetic materials). The purpose of the construction material will also vary significantly depending on its application within a specific BMP. Because both the properties and purposes of the construction materials are variable, it is critical to the success of the BMP that the a materials possessing the correct properties for the desired application be specified. This appendix provides guidance for developing specifications for various construction materials used in the BMPs.

2. Development of Specifications

2.1 Specifications Through Design-by-Function

The primary functions of the BMP construction materials are summarized below:

• Separation
• Protection
• Reinforcement
• Filtration
• Drainage
• Barrier

Once the primary function of the material is identified, the required property values of the construction materials can be quantified. For many engineering applications, the quantified required property is then compared to available material properties to select an appropriate material at an allowable factor-of-safety (FS) for the design:

FS = Material Property/Required Property

A FS greater than 1.0 indicates that the material property exceeds the requirement; however, it provides no allowances for incorrect design assumptions or material variability. An allowable FS greater than one should be selected based on the actual application, available design information, the length of time the material is expected to perform, and the consequences of a failure.

For the BMPs provided in this manual, an allowable FS of 1.5 is a reasonable starting point for analysis of temporary applications. An FS of 2.0 is a reasonable starting point for analysis of permanent applications. These factors should be adjusted based on the application-specific details discussed above.

Some other analyses, such as filter design or rip rap sizing, are based on empirical data or incorporate an FS. For these types of analyses, the required criteria is that derived directly from the analysis and no adjustments for FS are required.

2.2 Standard Specifications

Because BMP construction materials are extensively used by public agencies (particularly Departments of Transportation), many standard specifications are available for the more common applications. The most readily available resource to users of this manual is probably the Pennsylvania Department of Transportation (PennDOT) Standard Specifications (Publication 408). This manual provides specifications for both natural and geosynthetic construction materials. Other useful specifications for geosynthetics (specifically geotextiles) were developed by Task Force #25, a Joint Committee of the American Association of State Highway and Transportation Officials (AASHTO), American Building Contractors (ABC), and American Road Builders and Transportation.

Caution must be used when using standard specifications to ensure that they are appropriate for the application considered. In particular, the uniqueness of the application and consequences of a failure should be considered. For specialty applications or applications where failure could result in significant loss, the specifications should be developed by design.

3. Construction Materials

3.1 Natural Materials

Granular Filters

Granular filters are used to prevent the movement of fine particles out of soils and other natural materials through which seepage occurs. A granular filter can contain several different layers, each being coarser grained than the previous upgradient layer. The individual filter layers are fine enough to prevent the movement of particles from the upgradient layer, while being pervious enough to offer minimal resistance to seepage. While there are several accepted design methods for design of granular filters, some of the more commonly used criteria were developed by Terzaghi and Peck (1948):

D15(c)/D85(f) < 4

and

D15(c)/D15(f) > 4

where:

The subscripts "c" and "f" denote the layers with coarser and finer material. In most applications, the filter will be a layer of coarser material that is used to confine finer grained soils that are experiencing seepage flow. D15 is the sieve opening size through which 15 percent of the layer material will pass. D85 is the sieve opening size through which 85 percent of the layer material will pass. Filters should be uniformly graded to provide adequate permeability and to prevent segregation during processing, placing, and handling.

A "choker" course is a filter layer of finer material that is installed over a coarse road base material. The purpose of the choker course is to provide a stable foundation of fine-grained aggregate for the construction of a pavement.

Soil filters should not be confused with sediment traps, which employ rock-fill embankments to reduce the velocity of runoff and give fine particles an opportunity to settle.

Aggregate

PennDOT defines aggregate as natural or manufactured hard, durable, uncoated inert particles, reasonably free from deleterious substances. Substances such as reactive chert, gypsum, iron sulfide, or amorphous silica are considered deleterious because they reduce the durability of the materials. Fine aggregates are essentially sand materials and coarse aggregates are essentially stone or gravel.

Section 703 of the PennDOT specifications provides grain sizes and durability specifications for several grades of coarse and fine aggregates, including the AASHTO specifications referenced in the BMPs.

Rock Lining (Rip Rap)

Rock lining or rip rap is a constructed layer or facing of stone, placed to prevent erosion, scour or sloughing of a structure or embankment. The term "rip rap" also is frequently defined as the stone used to construct such a lining.

Rip rap is a special class of very large aggregate. Rip rap gradations range in diameter from 2 inches to 42 inches. Because rip rap is subject to significant energy, it is important that it be sound and free from defects or entrained substances such soil shale or organic materials.

The resistance of rip rap to displacement of moving water is a function of the weight, size, and shape of the stone, the geometry of the channel or bank it is protecting, and the filter blanket over which the rip rap is placed. Sizing or design of rip rap is discussed with the specific BMPs.

Section 805 of the PennDOT specifications gives the size, gradation, and durability specifications for several grades of rip rap.

3.2 Gabion Systems

Wire-enclosed rip rap or gabion systems consist of mats or baskets fabricated from wire mesh, filled with small rip rap, and anchored to a slope. Wrapping the rip rap enables using smaller rip rap for the same resistance to displacement by water energy as larger unwrapped rip rap. This is particularly advantageous when constructing rock lining in areas inaccessible to trucks or large construction equipment. The wire baskets also allow steeper (i.e., vertical) channel linings to be constructed. Gabion baskets or mattresses can be constructed from commercially available wire units or from available wire fencing material. Wire enclosures should be fabricated from No. 9 or No. 12 galvanized wire.

According to FHA guidance (i.e., HEC-11), the thickness of wire mattresses used for channel linings is determined by three factors: the erodibility of the bank soil, the maximum velocity of the water, and the bank slope. Guidance for minimum thickness is in Table E-1.

Suppliers of prefabricated gabions generally provide extensive criteria for using their products. If prefabricated systems are used, manufacturers’ guidelines must be followed.

3.3 Geosynthetics

Construction materials consisting of synthetic components made for use with or within earth materials generally are referred to as geosynthetics. Geosynthetics used in the BMPs can be further categorized into the following components:

• Geotextiles
• Geomembranes
• Geonets and Geocomposites

Geotextiles

Geotextiles are the most common geosynthetics, and consist of woven or nonwoven fabric made from polymeric materials such as polyester or polypropylene.

Using geotextile for separation, filtration, and in-plane drainage is discussed briefly below. The discussions address the main function of a geotextile for a specific application; however, rarely will a geotextile only serve one function in a given application. For example, geotextiles specified for separation alternatives will generally have to function as filters and those specified as filters generally will serve a separation function. In fact, if natural materials are substituted for geotextiles in either separation or filtration applications, a soil filter will have to be designed. Design of all functions for a specific application must be addressed regardless of the main function of the geotextile.

Furthermore, geotextiles frequently are used for applications other than those described here, including reinforcement and stabilization. For more information about other applications, publications provided in the Resources section should be referenced.

In addition to possessing parameters suitable for the function in the specific application, candidate geotextiles must be able to withstand installation stresses. A discussion of survivability requirements is provided at the end of the section.

Separation

Separation applications for geotextiles are those where the geotextile is placed between dissimilar materials to prevent migration of one of the materials into the other. An example of this application would be a geotextile placed between native subgrade material and rock placed for a construction entrance.

Strength parameters for separation geotextiles should be designed to exceed burst, tensile, puncture, and impact stresses. Equations for quantitative analysis of these parameters are available. PennDOT Standard Specifications for separation applications are provided in Table E-2.

Table E-2 (landscape, end of this document)

Geotextiles in BMP applications may be called upon to discourage root growth into underlying drain systems. The degree to which the root growth can be discouraged is a function of the type of vegetation and the weave or construction of the geotextile. While the plethora of operating field applications attests to the ability of the systems to discourage root growth, there is also documentation of roots growing through or into a geotextile matrix. While no literature specific to the subject was identified, a more tightly woven or non woven fabric would be expected to be more discouraging to root growth than a loose or open weave.

Filtration

Geotextile would be used for filtration in applications similar to those in which a natural soil filter would be employed. They prevent the movement of fine particles from soil through which seepage occurs.

PennDOT divides the filtration category into three separate categories:

• Subsurface Drainage—Subsurface drainage geotextiles, as defined by PennDOT, are those employed in subsurface drainage applications, such as filters around underdrains or edge drains.
• Erosion Control—Erosion control geotextiles are those employed to protect cut slopes or drainage features. They would be used in conjunction with a stone lining or rip rap, and as such, would serve a secondary function of separation. Erosion control, as used by PennDOT, should not be confused with "slope erosion protection materials," as discussed in that section.
• Sediment Control—Sediment control geotextiles are exclusively those used for silt fence applications. While they serve the purpose of "filtering" runoff, the mechanism by which they function is different than subsurface drainage or erosion control applications. Accordingly, silt fence applications are discussed separately.

Subsurface Drainage and Erosion Control

The two major geotextile properties that should be analyzed when designing for filtration are apparent opening size (AOS) and permittivity (cross-plane permeability). The apparent opening size of a geotextile controls its ability to retain soil. The permittivity of a geotextile describes its ability to pass water through or across it.

As with filtration, equations for quantitative analysis of these parameters are available (See "Resources"). PennDOT standard specifications for separation applications are in Table E-2. Note that even with these standard specifications, knowledge of soil grain size and permeability and minor design is required.

For example, for erosion control geotextiles, PennDOT requires that the AOS of the fabric be design specified. Task Force 25 recommends that the specified opening size be based on the following criteria:

1. For soil £  50 percent passing the No. 200 sieve, AOS of the fabric ³ No. 30 sieve (i.e., 095 < 0.59 mm)
1. For soil > 50 percent passing the No. 200 sieve, AOS of the fabric ³ No. 50 sieve (i.e., 095 < 0.297 mm)

Several other more conservative AOS criteria have been published and should be considered for critical application.

The permittivity of the fabric must be sufficient to allow the unimpeded flow of water through it. The permittivity of the geotextile is defined as follows:

y = kn/t

where: y = the permittivity (units of 1/time)

kn = cross plane permeability (length/time)

t = thickness (length)

Most geotextile manufacturers will provide the cross-plane flow capacity of a fabric in terms of permittivity. PennDOT specifications provide a minimum permeability of 0.01 cm/sec in addition to the requirement that the permeability of the fabric be at least 10 times that of the soil. Accordingly, permittivity values must be converted to permeability values to evaluate the suitability of the fabric relative to the specifications.

Silt Fences

Sediment control geotextiles are used for construction of silt fences. Silt fences consist of fabrics placed vertically on posts to prevent sediment carrying water from entering into downstream creeks or sewer systems. Silt fences are commercially available or may be constructed with and without wire support backing. Silt fences function first as true filters, but gradually convert to sedimentation reservoirs as the lower fabric clogs with soil particles. Water impounds behind the silt fence, and as particles settle out, clear water will rise over the clogged section and pass through the fabric.

Silt fence design methodology considers the height to which water will rise behind a silt fence based on its geotextile permeability and the tensile strength the flow will mobilize as it impounds behind the fence. As Table E-2 shows, however, PennDOT provides specifications for both Type A and Type B sediment control fences independent of permittivity and AOS. Instead PennDOT considers two additional tests to determine the effectiveness of silt fences: slurry flow rate and retention efficiency. Task Force 25, however, recommends that geotextiles used for silt fences have a minimum permittivity of 0.1 sec^-1 and an AOS of £  0.84 mm.

In-Plane Drainage

Certain types of geotextiles, in particular thick-needled nonwoven geotextiles, have sufficient in-plane flow capacity for use as flow conduits in certain applications. Geotextiles used in these applications must be selected on the basis of the hydraulic designs. The in-plane flow capacity of a geotextile is sometimes described in terms of its transmissivity.

q = Kp t

where: q = the transmissivity (length²/time)

Kp = in-plane permeability coefficient

t = thickness

The minimum allowable transmissivity of a geotextile would be calculated based on Darcy’s law with an appropriate factor of safety applied:

q _reqd = q/(i x w)

q _all = q _reqd (FS)

where: q _reqd = required transmissivity

q _all = allowable transmissivity

FS = factor of safety

q = required flow capacity (length³/time)

i = gradient (length/length)

w = width of geotextile (length)

Application of normal stresses to a geotextile will compress the void spaces between the filaments and alter the flow characteristics. It is, therefore, important that transmissivity values specific to the anticipated normal pressures of the application be used in the design (Table E-3).

Survivability

Regardless of the required design parameters identified for the geotextile, the adequacy of the physical strength parameters of the geosynthetic to survive installation should be reviewed. Task Force 25 provide the guidelines presented in Table E-4 for survivability of the fabric. These values should be the minimum specified for a given application.

Geomembranes

Geomembranes are continuous polymeric sheets that are, for all practical purposes, impermeable. There are many varieties of geomembranes in use today; however, the most frequently used geomembranes for ground applications are thermoplastic products manufactured from high-density polyethylene (HDPE) and polyvinyl chloride (PVC). Ethylene propylene diene monomer (EPDM) is a thermoset polymer frequently used for membrane roofing applications.

Different types of geomembranes have significantly different properties, including strength, longevity, resistance to ultraviolet light, thermal expansion and contraction, chemical resistance, and ease of installation. The most appropriate geomembrane to use for a given application is dependent on the application and the environment to which the geomembrane will be exposed.

While different membranes have different strength and elongation properties, membranes should generally be designed so as not to be subjected to tensile stresses and should be treated gently during installation and subsequent use.

Applications provided with the BMPs are generally only expected to be subjected to water, and most will be sheltered from aggressive environments. The differentiating factors between candidate geomembranes for BMP applications are expected to be availability (material and experienced contractors) and cost (i.e., material cost and installation cost).

Geocomposite Drains and Geonets

Geocomposites consist of a combination of geosynthetic components. Geocomposites used in the BMP applications are usually sheet or edge drains consisting of a prefabricated core to which a geotextile filter is bonded. The core provides void space to which water can flow in-plane while the geotextile filter keeps soil from filling the voids created by the core. Geocomposite sheet drains are available that allow flow in from one or both faces.

Geonets are a type of geosynthetic that consists of a continuous extrusion of polymeric ribs. The ribs, themselves, form void space through which provide in-plane flow capacity. Geonets are available with or without bonded geotextile filters. Geonets with bonded geotextile filters are sometimes referred to as composite drainage nets (CDNs).

In-plane flow capacity requirements for sheet drains or edge drains are determined similar to those for in-plane flow in geotextiles. Because of the significantly greater void space provided by geocomposite or geonet products, in-plane flow is not laminar at high gradients. Therefore, transmissivity values provided for these products should be used with caution. Actual in-plane flow information at specified gradients and normal pressures is preferable.

As with geotextiles, normal pressures on geocomposites or geonets will alter their flow characteristics. In-plane flow values for specific loading conditions should be used in the design.

Geocells

Geocells are three-dimensional prefabricated polymeric systems ranging from 4 to 8 inches high. The geocell systems are collapsed for delivery to the site. Upon arrival at a site, they are spread open and filled to form a three-dimensional reinforced mattress. Originally developed to rapidly stabilize soft subgrades for mobilization of large equipment, they are now frequently used for protection and stabilization of steep slop surfaces and protective linings for channels. Use of geocells for slope stabilization applications is relatively new and there is little available design guidance beyond that available from the manufacturers of these systems. This guidance should be used when using geocells in the BMP applications.

Slope-Erosion Protection Materials

Materials used for both temporary and permanent erosion protection are available. Temporary materials consist of open mesh polymeric systems, biodegradable mesh system (e.g., jute), or a combination of polymeric and biodegradable mesh. The open mesh systems serve as a semipermanent mulch, anchoring seeds and soil particles subject to erosive flows in channels. Greater flow rates and volumes usually require denser mesh and more durable construction. Accordingly, more aggressive environments require more expensive products. Information required from the manufacture will include Manning's coefficient (n) for material, permissible velocity, and maximum allowable shear stress (or unit tractive force). Allowable shear stress is related to the hydraulic radius of flow through the expression:

t_a = wRS

where: t_a = allowable shear stress

w = unit weight of water

R = hydraulic radius

S = land slope

For broad slopes, the hydraulic radius will be approximately equal to the depth of flow. Because these products are highly variable in both material and geometry, it is difficult to develop design guidelines applicable to all systems. The manufacturer’s project-specific design criteria should be followed and documented for each specific application.

Common permanent turf-reinforcing materials consist of tangled fiber mats constructed from nylon or polyolefin. The mesh is enclosed within the topsoil layer. The polymeric materials must be stabilized against photodegradation. The mesh will permanently support the roots of the vegetative cover. Different ranges of Manning’s coefficient, permissible velocity, and allowable unit tractive force will generally apply to the early post-construction (i.e., bare ground) and fully vegetated conditions.

4. Resources

The following resources have been identified for use in selecting appropriate construction materials for the BMP applications:

Commonwealth of Pennsylvania, Department of Transportation. Specifications. 1990. Provides standard specifications.

IFAI, Geotechnical Fabrics Report, Specifiers Guide (Published annually in December). Provides comprehensive summary of available geosynthetic products and select material properties

Koerner, R.M. Designing with Geosynthetics. 2nd edition. Prentice Hall. 1990. Provides comprehensive design methodology and examples for most geosynthetic applications.

U.S. DOT, Federal Highway Administration (FHWA). Design of rip rap revetment. Hydraulic Engineering Circular No. 11, Publication No. FHWA-IP-89-016. March 1989. Provides comprehensive design methodology and examples for design of rip rap revetments and slope linings.

Table E-2