Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control

Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control Soil improvement and ground modification methods chapter 8 geosynthetics for filtration drainage, and seepage control

Geosynthetics for Filtration

Drainage, and Seepage Control

While the use of geosynthetics has proliferated throughout a wide range of geotechnical applications, special attention to hydraulic improvements for filtration, drainage, and seepage control seems appropriate for the discussion

of hydraulic modification In fact, the use of geosynthetics for drainage and filtration has often taken the place of conventional applications that had his-torically been engineered with carefully graded earth materials This not only has led to economic savings and ease of construction, but also has been attributed to providing a more uniform and safer solution by averting natural variability in materials and workmanship, less potential for segregation of materials under hydraulic gradients, and reduced exposure to piping For other applications where the result is to improve the ground by reducing

or eliminating seepage, a few types of geosynthetics can provide “hydraulic barriers” to prevent any significant flow, where naturally occurring soils may

be insufficient or impractical A third category of applications with geosyn-thetics combines different types of materials and serves multiple functions These are referred to as geocomposites While geosynthetics have provided sig-nificant economic, construction, and performance advantages, there can be intrinsic problems, including the loss of ability to "self-heal" after rupture, chemical and/or biological degradation of the geosynthetic materials, and long-term flow compatibility Other primary functions, including separa-tion and soil reinforcement, are addressed inChapters 14,16, and17

A wealth of information and resources on geosynthetics can be found through organizations such as: Industrial Fabrics Association International (IFAI;www.ifai.com), the North American Geosynthetics Society (NAGS;

www.geosyntheticssociety.org), the Geosynthetics Institute (GSI; www geosynthetic-institute.org), or from comprehensive texts such as Designing With Geosynthetics by Koerner (2005, 2012) and Geosynthetic Engineering

byHoltz et al (1997)

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Soil Improvement and Ground Modification © 2015 Elsevier Inc.

8.1 GEOTEXTILES FOR FILTRATION AND DRAINAGE

Geotextiles are continuous sheets of woven, nonwoven, knitted, or stitched fibers (Figure 8.1) They may be made of a variety of types of fibers, includ-ing natural yarns or polymeric fibers Geotextiles are the principle geosyn-thetic materials used for separation (between dissimilar soil types/gradations) and filtration of flowing water (into, through, or exiting a soil mass) Geotex-tiles may also provide significant reinforcement by providing tensile strength and shear resistance, and certain types are used for erosion control The rein-forcing function of geotextiles will be discussed later inChapter 14 In some applications, they may also provide a limited drainage function as described in

Section 8.2

Some newer geotextiles not only provide separation and filtering, but also claim to provide a substantial in-plane drainage capacity based on the in-plane permeability of the geotextile fabric (seeSection 8.2)

8.1.1 Filtering and Geosynthetic Filtering Criteria

Historically, traditional filters were designed by placing carefully graded soils

in “zones” so that soils would be successively coarser in the direction of flow

If engineered correctly in this manner, each successive soil filter would satisfy the basic filter criteria: (1) the filter should have small enough openings

Figure 8.1 Photograph of a range of typical geotextiles used in geotechnical applications Courtesy of Geosynthetic Institute.

(voids or pore spaces) to prevent migration of the soil being filtered (i.e., the

“upstream” soil from which the flow is entering the filter), and (2) the filter should be coarse enough (e.g., have large enough openings/voids) to adequately allow the flow to pass through (without generating excess pore water pressures or seepage forces) Simple criteria were initially developed

by Terzaghi and Peck (Terzaghi et al., 1996) after experimentation on numerous soil filters based on the soil gradations, and subsequently modified

as described inChapter 7

With the same basic philosophy and concepts used for soil filters, design criteria have been developed for geotextile filters Fundamentally similar to those for soil filters, criteria for geotextile filters include (1) soil retention, (2) adequate flow, and (3) long-term flow compatibility (clogging resistance) Geotextiles are permeable fabrics, typically made from polypropylene or polyester They may be woven or bonded by other means (i.e., needle-punched, heat bonded) The fundamental parameters important to the suc-cess, performance, and functionality of geotextiles for filtration closely mimic the criteria for soil filters These generally include the ability to allow adequate flow from a soil across the plane of the geotextile with limited soil loss over a design service life of the soil-geotextile system Specific filter cri-teria are discussed in the following section

In the case of geotextiles, soil retention is addressed by making the geo-textile voids small enough to initially only retain the coarser soil fraction While this might at first seem counterintuitive, the coarser fraction is tar-geted in these designs as research has shown that a process called “bridging” causes the buildup of coarser-sized particles to eventually block the finer-sized grains On the other hand, the retention criteria for soil filter design described in Chapter 7 has similar characteristics, as it targets the D85 of the soil being filtered The design formulas for geotextiles typically use soil particle size characteristics and compare them to the size(s) of the openings in the geotextile fabric The most common geotextile opening size used, defined as O95, refers to the opening size that will retain 95% of uniform-sized spheres by dry sieving In the United States, this is called the apparent opening size (AOS) obtained by dry sieving different sized uniform spheres ASTM D4751 provides guidance for obtaining AOS In Europe and Can-ada, testing is done by wet or hydrodynamic sieving to derive the filtration opening size Similar to the principles of wet sieving for soil gradation, these processes may be preferable (Koerner, 2005), as they are believed to provide more accurate results and more closely represent the actual flow and filtering conditions

The simplest soil retention design procedures are based on the percent-age of soil finer than the #200 sieve (0.074 mm) For example, an AASHTO guideline recommends (Koerner, 2005):

• O95<0.60 mm, (AOS
#30 sieve) for soil with 50% passing #200 sieve

• O95<0.30 mm, (AOS
#30 sieve) for soil with >50% passing #200 sieve Several other researchers have made recommendations based on more complete grain size distributions A simple, but widely used relationship was outlined byCarroll (1983):

• O95<(2-3) D85(where D85is the grain size for which 85% of the soil

is finer)

More detailed design includes information on soil hydraulic conductivity, plasticity (PI), percent clay, and undrained shear strength, such as described

To address the criteria for adequate flow, many designs simply require

a geotextile to have a permeability that exceeds a certain multiple of the soil permeability These multiples typically range from about 4 up to 10 times (or more), depending on the critical nature and severity of the application Because of this, the geotextile permeability (hydraulic conductivity)

is needed When flow is perpendicular to the plane of the fabric ("cross-plane"), the geotextile permeability is typically computed by the term permittivity, often used by the geotextile industry, which includes the thickness of the geotextile This mitigates any discrepancies with variability common to relatively thick and compressible fabrics Permittivity is defined as

c ¼kn

wherec is the permittivity (s1), knthe cross-plane permeability (n for flow normal to the plane, cm/s), t the fabric thickness at a specified normal pressure (cm)

Permittivity is often used when comparing geotextiles of different thicknesses The testing procedure for measuring permittivity is fundamen-tally the same as for measuring soil permeability Geotextile permeability can be obtained by simply multiplying the measured permittivity by the fabric thickness The geotextile manufacturer typically provides these values

Long-term flow compatibility (anticlogging criteria) may be based on the percent open area (POA) for woven geotextiles, and on porosity for non-woven fabrics (www.fhwa.dot.gov) POA is a comparison of the total open

area to the total area of the geotextile Porosity is the relationship between the volume of voids and the total volume of the geotextile, reported as a

for woven geotextiles or a porosity>30% for nonwoven fabrics When geo-textiles used as filters fail, the most likely reason is clogging.Koerner (2005)

describes a number of scenarios where excessive clogging has been observed from experience One approach is to allow a certain amount of fine sediment

to pass through a more open geotextile with POA>10% for woven fabrics

or porosity>50% for nonwoven fabrics But in order to use these criteria, one must feel confident that neither the loss in retention, nor the passing of material into the drainage system, would promote a significant problem to the application for which it is used

When designing geosynthetic filters, it is important to consider both sur-vivability (ensuring resistance to installation damage) and durability (resis-tance to chemical, biological, and ultraviolet light exposure) AASHTO M288-06 provides specifications for allowable strength and elongation values ASTM 5819 provides a guide to choosing appropriate durability test methods Additional ASTM standards listed at the end of this chapter are available for specific strength and survivability tests

8.1.1.1 Geotextile Filter Applications

The use of geotextile filters have become commonplace for a wide array of applications If properly designed, a geotextile filter may act as the sole fil-tering medium between an appropriate soil and drain or well Where plastic pipe is installed for drainage or a well, a fabric “sock” is often placed around the (sometimes open) end of the pipe, covering the perforations to prevent material from entering the drain For many applications where granular soil filters had traditionally been used, geotextiles have taken their place in new designs and construction Common applications include filtering for gravel drains, filters for zoned earth dams, and filtering within engineered roadway layers Commonly, geotextile filters used for highway applications involves geocomposites to provide filtering and drainage as described inSection 8.2 Geotextile filters also have become an integral part of drainage design behind both rigid and flexible retaining walls As described in Chapter 7, the need to provide long-term drainage so as to prevent buildup of hydro-static pressures behind retaining walls is paramount to their survivability Geotextiles have become the norm for filtering drainage water from the backfill soil to assure that drainage will continue unobstructed In some

cases where the volume of drainage water is very small, the geotextiles may provide for both functions of filtering and drainage For flexible wall sys-tems, such as might be constructed with stone-filled, wire baskets (gabions)

or other free-draining wall systems, geotextiles are now used almost exclu-sively to provide the necessary filtering function When used in this fash-ion, they are actually also functioning as a separator between different materials In another type of application, geotextiles have been used as fil-ters beneath coastal erosion control structures made of placed rock riprap, articulated concrete blocks, or concrete block mattresses These types of erosion control, or armoring, can be used to face the upstream slopes of earth dams subjected to wave action In these applications, the filters may need to be designed to handle flow in both directions, as tides and waves can push water through the protective stone/blocks, building up water pressures in the soil beneath, which must then be dissipated back through the erosion protection without loss of the soil material beneath

In another type of erosion protection application, geotextiles are often secured directly to the ground surface of steep slopes subjected to heavy rainfall and/or prone to surface sloughing

A surface application of geotextiles as filters is for control of “dirty” construction runoff by constructing silt fences or fabric-wrapped, granular material to trap the suspended, fine-grained material and allow relatively clean water to flow away While often not designed specifically for each application, there are readily available products available for construction runoff filtering Where large amounts of runoff are expected, silt fences may be constructed to capture transported sediments The design of silt fences is based on the amount of flow expected and relies on a certain amount of intended clogging of the fabric in order to form a sediment

“trap.” While not really a soil improvement application, it is related to similar flow and filtering functions and criteria Silt fence design is covered by geosynthetic references such as Koerner (2005) and Holtz

et al (1997)

Another filtering application is in the stabilization of dredged materials and other high water content “sludges” by placing these materials into fabric containment “bags.” This allows fluid to drain out of the material, making it easier to transport and/or dispose of Commonly known as Geotubes®, they have also been used extensively for shoreline protection, breakwaters, levees, beach rebuilding, and as a component for reclaimed land These con-finement applications will be discussed inChapter 16.Figure 8.2depicts an example of dewatering of marine spoils with Geotubes

8.1.2 Geotextile Drains

As mentioned above, the use of geotextiles for hydraulic applications is pri-marily for filtering functions, but geotextiles can provide drainage under cer-tain conditions When geotextiles are placed in an application where fluid flow occurs within the plane of the fabric, they can provide a limited amount

of drainage Except for the consideration of flow direction, the criteria for soil retention and long-term flow compatibility described in the previous section on filtration are virtually the same This leaves only the discussion

of adequate flow and in-plane permeability Just as the variable thickness due to compressibility was addressed for measurement of cross-plane perme-ability for the filtering function, it is handled similarly for in-plane drainage For this, the geotextile industry uses a term called transmissivity to describe the flow rate within the fabric Transmissivity is defined as

wherey is the transmissivity (cm2

/s), kpthe in-plane permeability (cm/s), t the thickness (cm)

A recent innovation integrating hydrophilic and hygroscopic yarns into a high-strength woven geotextile incorporates a true wicking component that draws water from the ground and is able to transport it away from critical components of projects located in high moisture environments, or where moisture introduced by rain or snow can be drawn out of the subsoil

Figure 8.2 Dewatering of dredged sediment with Geotubes Courtesy of Infrastructure Alternatives.

(www.tencate.com).Figure 8.3shows a graphic of how these types of geo-textiles are able to draw water out of a soil without pumping, vacuum, or relying on induced pore pressures Figure 8.4 shows a close-up view of the wicking geotextile fabric Case studies have shown very good results

in reducing standing water and frost heaving for difficult roadway applica-tions This type of geotextile has been successful in reducing perpetual frost heave problems in Alaska highways, as well as reducing pumping and flood-ing problems in other environments with high moisture soils

Figure 8.4 Close-up view of wicking geotextile fabric Courtesy of Tencate-Mirafi Figure 8.3 Mirafi ’s H 2 Ri woven geotextile capable of wicking moisture from subgrade soils Courtesy of Tencate-Mirafi.

Just as with filters, geosynthetics have now become commonplace as sub-stitutes for natural soil materials for drainage applications In some cases, thick, nonwoven geotextiles can provide some amount of drainage function While this may be adequate for low-volume flows, the drainage capacity is highly dependent on stresses that will compress the fabrics and reduce flow area

8.2 GEONETS, GEOCOMPOSITES, AND MICRO SIPHON DRAINS

In-plane drainage using geosynthetics is usually designed with either geonets (usually in combination with a geotextile) or with a geocomposite, a class of geosynthetics often designed principally for in-plane drainage These hybrid geosynthetics are made by combining different types of geosynthetic com-ponents, and serve the purpose of providing both filtration and drainage Geocomposites are typically combinations of a drainage (and sometimes bar-rier) material with a geotextile filter to prevent soil migration into the drain-age system Geosynthetics used for draindrain-age include perforated plastic pipes (or “geopipes”), geonets (ribbed materials intended to convey in-plane flow), and corrugated geomembranes (which can provide substantial in-plane flow capacity as well as a hydraulic barrier) Geosynthetic hydraulic barriers are discussed inSection 8.3

Geonets are typically formed by two biplanar sets of relatively thick, par-allel, polymeric (usually polyethylene) ribs bonded in such a way that the two planes of strands intersect at a constant acute angle, forming a diamond-shaped pattern (Figure 8.5) The configuration of the nets form

Figure 8.5 Biaxial geonet.

a network with large porosity that enables relatively large in-plane fluid (and/or gas) flows While they have considerable tensile strength, they are used exclusively for drainage applications Their initial use was almost exclusively for environmental applications, such as hazardous, liquid, waste impoundment, or landfills to collect and drain leachate fluids, and for leak detection Geonets have also been shown to provide effective capillary breaks where moisture intrusion due to capillary rise is a concern They have now become more widely used for drainage behind retaining walls, in slopes, in hydraulic structures (e.g., dams and canals), in large horizontal areas (e.g., golf courses, athletic fields, and plaza decks), and as drainage blan-kets beneath surcharge fills and embankments In order to prevent soil intru-sion into the voids, geonets are generally used in conjunction with geotextiles and/or geomembranes (Figure 8.6) While traditional biaxial geonets were never really intended to support any tensile or shear load, newer triaxial versions of geonets have been designed to provide even greater flow with added load capacity in both compression and shear (Figure 8.7) The triplanar structure provides minimal geotextile intrusion and greater flow capacity through longitudinal channels Their higher rigid-ity, tensile strength, and compressive resistance make them suitable for appli-cation within roadway pavement systems, beneath highways and airfields, and beneath concrete building slabs

Where larger drainage volumes are needed, geocomposites consist of corrugated, “waffle” type, or “dimpled” geomembrane cores with large porosity, attached to a geotextile for filtration and to prevent soil intrusion (Figure 8.8) Geocomposite drains may be configured to act as a central drain

Figure 8.6 Geonet geocomposite drain.