GEOSYNTHETIC DRAINAGE DESIGN TOOL
To create a detailed design of landfill and liquid containment drainage layers, you need to know the performance characteristics of the materials you want to use. While project specifications will list design requirements in tender documents, along with references to the test methods that must be followed, they often do not include design calculations.
This online drainage design manual provides project engineers with:
The material properties for drainage geonets and geocomposites
The design procedures for using these materials in solid waste and liquid containment applications
Calculation equation sheets and interactive design examples.
BASIC DESCRIPTION AND FUNCTION OF GEONETS AND GEOCOMPOSITES
Geocomposite drainage material comprises a geonet and a geotextile, with the geotextile heat-laminated to one or both sides of the geonet. The geonet, made of an extruded high-density polyethylene (HDPE), has an open structure that allows the in-plane conveyance of liquids and/or gases. The primary purpose of the geotextile is to act as a filter and separator between the surrounding soil and the geonet, preventing the intrusion of soil particles into the geonet structure. The geotextile also improves the interface shear strength of the drainage system, as a “geonet-to-geomembrane” interface is usually much weaker than a “geotextile-to-textured HDPE geomembrane” interface.
There are three basic geonet structures that are widely used in the industry:
FIGURE 1.1 – BIPLANAR GEONET
A biplanar geonet comprises two equally-sized sets of extruded parallel ribs set at various angles to the machine direction, as illustrated in Figure 1.1.
FIGURE 1.2 – TRIPLANAR GEONET
A triplanar geonet comprises centralized middle HDPE strands that provide channelized flow, and diagonally placed top and bottom strands that minimize geotextile intrusion.
FIGURE 1.3a – T-SHAPED TRIAXIAL
A triaxial geonet consisting of vertical central HDPE strands with horizontal top strands (T-shaped) for improved performance.
FIGURE 1.3b – BOX-SHAPED TRIAXIAL
A triaxial geonet consisting of vertical central HDPE strands with horizontal top and bottom strands (box-shaped) for improved performance.
In addition to standard geonets and geocomposites, Solmax has developed highly-engineered, specialized drainage systems for applications that demand superior flow, high compressive strength and filtering performance. Our specialty systems include CoalDrain for coal ash containment, MineDrain for heap leach pads, BioDrain for leachate recirculation landfills, and SolFlow GeoNet for landfill gas collection.
OF DRAINAGE GEONETS & GEOCOMPOSITES IN SOLID WASTE CONTAINMENT FACILITIES
One of the major applications of drainage geocomposites is in solid waste containment. In these facilities, geonets and geocomposites can be used for surface water collection, gas venting, leachate collection, and leak detection.
FIGURE 1.4 – SOLID WASTE CONTAINMENT ILLUSTRATION
Municipal landfills are commonly closed with final covers that must contend with broad, gently sloping tops and side slopes as steep as 3H:1V. The final cover must minimize, to the maximum extent practical, post-closure infiltration of liquids into the waste. Designs also need to include any measures necessary to ensure major slope stability, and account for any condition that may cause the final cover system not to perform as intended. The final cover must remain stable relative to surface erosion. The hydraulic design of the drainage geocomposite in landfill closure is quite straightforward: the water percolating through the vegetative layer must be drained without exceeding the flow capacity of the drainage layer. This capacity must be reduced in the design to accommodate future degradation of the drain due to root intrusion, long-term intrusion, creep, etc., and to include an adequate factor of safety.
SURFACE WATER DRAINAGE LAYER
In a landfill final cover system, gas is released when the underlying waste mass degrades. If gas pressure is allowed to build underneath the geomembrane, it can cause bubbles to form, the geomembrane to burst, and slopes to fail. To effectively reduce volumes of gas build-up, a drainage geocomposite must be properly designed to collect and discharge gas to a gas venting system or gas collection system.
LANDFILL GAS COLLECTION LAYER
A leachate collection and removal layer removes leachate from the lining system to maintain less than or equal to 1 ft head of liquid as required by Environmental Protection Agency (EPA) regulations. Using drainage geocomposites in place of natural granular soils for this layer offers significant advantage over using traditional materials, such as soil to provide granular drainage layers on side slopes. While natural soils consume valuable airspace (typically one or more feet of soil) and require extra control to retain adequate permeability, geosynthetic drainage products are manufactured, quality-controlled products that occupy far less airspace than natural drainage soils (0.20 in to 0.30 in), and can be installed by simply unrolling the product. The geonet leachate collection and removal layer must withstand heavy compressive loads as well as maintain the flow capacity necessary to limit the head within the drainage layer. Properly designed and selected drainage geocomposites provide excellent long-term hydraulic performance and creep resistance to ensure the leachate collection system will continue working over the life of the project.
LEACHATE COLLECTION & REMOVAL LAYER
EPA regulations pertaining to hazardous waste landfills (EPA Subtitle C) specify use of a leak detection system comprising a double liner (two HDPE geomembranes) with a geonet or geocomposite in the middle. This system warns of any failures in the primary liner system. Properly designed and selected drainage geonet provides the most efficient material for rapid detection of leaks in the overlying liner systems. The drainage geonet limits the head on the secondary liner to the thickness of the drainage geonet. Geonets have very limited storage capacity and high transmissivity. The combination of these characteristics means that the geonet will not store fluids, but move them quickly through the net to detection locations, minimizing the time between leak occurrence and detection to ensure unconfined flow and provide a reliable and accurate Action Leakage Rate (ALR). Using a box-shaped triaxial geonet as a leak detection layer under a geomembrane or geocomposite liner helps to maintain the integrity of overlying geosynthetic clay liners (GCLs), and minimizes geotextile intrusion.
LEAKAGE DETECTION LAYER
OF DRAINAGE GEONETS & GEOCOMPOSITES IN LIQUID CONTAINMENT FACILITIES
FIGURE 1.5 – DRAINAGE GEONET AND GEOCOMPOSITE IN LIQUID CONTAINMENT FACILITIES
There are more than 200,000 surface impoundments storing hazardous and nonhazardous liquids in the USA. The hydraulic head on the geomembrane liner is project-specific but can be quite high compared to the landfill maximum allowed, namely 30 cm (12 inches). However, the greater the hydraulic head, the greater the amount of leakage through leakage points. The Action Leakage Rate (ALR) regulations for lined liquid containment facilities vary from state to state. The very first use of drainage geonet was as a leakage detection layer for this application. A key characteristic of the design of the geonet leak detection layer is that it ensure unconfined flow, taking into consideration the potential leakage quantity from the overlying liner.
FIGURE 1.6 – GEOMEMBRANE 'HIPPOS' OR 'WHALES' IN A GEOMEMBRANE-LINED WASTE WATER POND
Liner failure in the form of geomembrane whales and hippos can occur, as shown in Figure 1.6, due to gas pressure on the bottom of geomembranes. The use of a geonet drainage layer beneath the geomembrane can effectively capture and laterally transmit the gases to and up the side slopes, venting them out. Solmax’s triaxial geonet, a double-sided geocomposite not only provides rapid gas/water transmissivity thanks to its open structure, but also additional geomembrane protection.
TRANSMISSIVITY & MODIFICATION FACTORS
The primary function of geonets and geocomposites is to convey or transmit fluid within the planar direction of a drainage layer. Transmissivity is defined as the flow rate of water transmitted through a unit width of the product under a specific hydraulic gradient as measured in a laboratory test. Specifically, the transmissivity of a material is related to the flow rate by the following equation:
θ = transmissivity (m /sec)
Q = flow rate (m /sec)
w = width (m)
i = hydraulic gradient (dimensionless), which is the ratio of head loss to flow length.
In equation 1.1, it should be noted that transmissivity is equal to flow rate per unit width only at a gradient of 1. At gradients other than 1, either transmissivity or flow rate should be considered throughout design calculations as well as project specifications.
The transmissivity test is performed according to American Society of Testing and Material (ASTM) procedure D 4716. Depending on the manner in which a test is performed, the resulting data may be either “index” data, which is used for quality control and acceptance purposes, or “performance” data, which can be used in solution design.
For performance tests, the following four test parameters should be selected to represent actual site conditions as closely as possible: (i) test specimen top and bottom boundary conditions; (ii) normal stress on the test specimen; (iii) hydraulic gradient at which the test is performed; and (iv) duration during the test when readings are taken. The test should be run with the same adjacent materials that will exist in the design conditions.
Performance transmissivity tests should be conducted at a gradient equal to sinβ, where β is equal to the slope angle of the geonet or geocomposite with the horizontal. It is unconservative to test at a gradient lower than the design condition. Note that transmissivity is related inversely to gradient because the flow regime with water in a geocomposite is typically turbulent. For laminar flow, as may occur for nonwoven geotextiles, the transmissivity would be independent of gradient. It becomes increasingly challenging to ensure the accuracy of transmissivity measurements when gradients become less than about 0.1. With experienced technicians and recently developed transmissivity units that incorporate accurate head-measuring devices, it is possible to measure transmissivity at gradients as low as 0.02 with a reasonable degree of accuracy.
The significance of test duration and boundary conditions for geonet and geocomposite performance transmissivity cannot be overemphasized. Figure 8 illustrates the typical influence of test duration on the transmissivity of geonets and geocomposites. As shown in this figure, there is initially a rapid decrease in transmissivity, which is attributed to the compression of the geonet and intrusion of the geotextile into the geonet caused by overburden pressure. The transmissivity value recorded at the moment where 100 hours have passed--referred to as θ --is used for the purpose of design as recommended in Geosynthetic Research Institute GRI-GC8 guide, Determination of Allowable Flow Rate of Drainage Geocomposites. Regardless of whether any products are used, designers should contact manufacturers for the most up-to-date information on transmissivity, as products can change over time. Also, verification testing should ultimately be used to determine a given product’s ability to meet design specifications. Detailed discussion about how to use the GRI-GC8 guide is available in Section 2 of this online design manual.
FIGURE 1.7 – EFFECT OF TIME ON TRANSMISSIVITY OF GEONETS AND GEOCOMPOSITES
It is acknowledged by the user that there may be additional impacts upon the long-term transmissivity. GRI-GC8 states that “Other possible flow rate reductions and/or concerns such as flow in overlap regions, effect of high or low temperatures, etc., are site-specific and cannot readily be generalized in a guide such as this”. The user further acknowledges that long-term high temperatures could be of particular concern and in the case where these high temperatures are expected, the user must perform additional performance tests.
GEONETS & GEOCOMPOSITES
It is not uncommon for design or regulatory requirements for lateral drainage layers to be formulated in terms of hydraulic conductivity. In order to evaluate geocomposite products in terms of such requirements, the transmissivity value, as discussed in the previous section, must be converted to hydraulic conductivity. Transmissivity is related to hydraulic conductivity through thickness (t) of a layer. Specifically:
θ = k x t
θ = transmissivity (m /sec)
k = hydraulic conductivity along the length of product (m/sec)
t = thickness (m).
To derive the performance-based hydraulic conductivity of a geonet, both the transmissivity and the thickness of the product should be determined from the anticipated field conditions, and long-term reduction factors and overall safety factors must be taken into account in accordance with GRI-GC8. Note that the Hydrologic Evaluation of Landfill Performance (HELP) model software provides default input values of geonet hydraulic conductivity for two geonet types, textures #20 (5 mm geonet) and #34 (6 mm geonet). A 5 mm geonet is shown to have a hydraulic conductivity of 10 centimeters per second (cm/s) and the 6 mm geonet has a hydraulic conductivity of 33 cm/s. Unless you have a clear definition of the product as a geonet core alone or a geocomposite, and you have the testing conditions (applied loads, boundary conditions, hydraulic gradient, and seating time), these default values are often incorrect, and thus should not be used in the analyses.