As growing human populations settle more and more in low-lying coastal areas and near waterways, flood protection systems are becoming increasingly important engineered structures. Threats from sea-level rise as well as extreme weather events both appear to be here to stay, and provide greater stresses on these structures. In order to provide strong, stable levee systems that also preclude leakage of (possibly contaminated) water, geosynthetic materials are becoming a standard technique for constructing levees. Geosynthetics are polymer plastics utilized in geoconstruction projects, and include a wide variety of types, materials, and functions, as will be described later. As we examine geosynthetic usage in levees, we focus our attention primarily on two functions: to provide reinforcement for structural stability against flooding events and to prevent leakage through the levee, thus not only leading to a more durable structure, but also preventing the potential spread of water contamination. Let us turn to a famous case to demonstrate the first function.
The LPV 111 levee in New Orleans, Louisiana, was first built in 1955 and subsequently raised in the 1970s to a height of 17 ft (Kelsey 2020). Following Hurricane Katrina in 2005, LPV 111 suffered major scour damage and required repair to protect against future storm surges. Therefore, in 2010-11, engineers worked to repair a 5.2 mile stretch of the levee, raising it to a new height of 27 ft. To accomplish this feat in a shortened timeline, 450,000 square yards of polyvinyl alcohol (PVA) geogrids were utilized to distribute load over deep soil mixed piles (see Figure 1). After evaluating several other alternatives for carrying load--T-walls were too expensive, lightweight fill would have been too risky in the event of another flood, staged construction with wick drains would have taken too long to cause consolidation, stability berms would encroach on surrounding wetlands, and stone columns would not be able to be built in the extremely soft soils on site--deep soil mixing was finally selected for the project. Deep soil mixing essentially consists of drilling down a specified depth to firmer soil and mixing in cement to create an unreinforced pile. In this case, depth of piles varied substantially, but 50 ft is representative of average conditions. At the time of construction, this project represented the largest deep soil mixing project in the world, totaling in at 1.7 million cubic yards of material mixing! The geogrids themselves were HUESKER Fortrac 400MP models, manufactured from creep-resistant yarns and typically used for railway support, slope stabilization, and other similar applications. They can withstand a variety of pH conditions anywhere between 2 and 13, a key criterion for the high pH soil environment at the site, and show no more than 5% maximum elongation over a 100 year design life. These characteristics, along with the decreased number of piles that would be needed, made the HUESKER model a good choice for providing geosynthetic reinforcement to the levee. As a result of geosynthetic usage, fewer piles were required to be mixed, offering savings in cost, time, and factor of safety.
Though reinforcement is certainly a popular function for geogrids, controlling internal leakage and drainage offers another area for geosynthetics to prove their worth. Numerical, centrifuge-based, and other models have been developed to assess leakage levels for varying conditions and geosynthetics (Saran & Viswanadham 2018). The primary phenomenon of interest is the buildup of pore pressures within the levee: adequate drainage materials, such as coarse-grained soils or geosynthetics, are necessary to prevent pressure buildup, which leads to reduced effective stresses needed to keep the levee structurally sound. Compared to sand drainage layers, geotextiles and geocomposites can offer lower costs, better quality control, and easier installation. Unlike sand, which causes the phreatic surface to be confined within the levee itself due to a high permeability region, the synthetic materials cause the surface to gradually migrate towards the levee toe, which can be attributed to the blocking of geotextile pores by fine soil particles once seepage begins (see Figure 2). Study results indicate that, as expected, proper drainage reduces the risk of catastrophic failure, and that sands, while effectively reducing pore pressure, do not avert collapse for as long as geotextiles do. Another option to combine the benefits of the two materials is the sandwiched structure, or a geotextile placed within a sand layer. Though further testing is needed in this domain, sand layer thickness may be reduced by around 25% in this setup relative to the no-geotextile scenario, thus providing cost savings to counter the effect of an added geotextile.
Given the advantages of implementing geosynthetics into levee design, there still exists a choice of where to place the geosynthetic for maximum benefit. One important division can be observed between what are known as geosynthetic-reinforced soil (GRS) and laminar drain reinforcement (LDR) levees (Kurakami & Nihei 2019). The former consists of a geogrid connected to a concrete panel on the back side of the levee in order to resist overflow. However, once the phreatic surface reaches the back slope, instability results and failure can be sudden. The LDR levee combines the advantage of the GRS in resisting overflow with infiltration protection through the use of a sand or gravel drainage layer through which the geogrid runs (see Figure 3). Drainage prevents excessive pore pressure buildup, giving the levee greater resilience to high hydraulic heads (see Figure 4). Other LDR benefits include air ventilation through the drainage layer, prevention of suction of the levee body due to the presence of a geo-fabric, and stronger connection of the concrete panel to the levee. These benefits must be weighed against the cost of implementing a drainage system, the additional geosynthetics needed, constraints in constructability, and so forth.
|Basic Armored||GRS||LDR (20 cm)||LDR (10 cm)|
|Time to Failure (min)||87||102||150*||150*|
|Time to Failure without Scour Protection (min)||112||57|
|Max Concrete Panel Gap (cm)||3||16||19||19|
|Time Between Gap Occurrence and Panel Outflow (min)||1-2||20||3|
This section gives an overview of types of geosynthetics. Geosynthetics (GS) are synthetic polymeric or natural materials in the form of strips, sheets, or structures. They have the advantage of being easy to transport, thus making the construction efficient and eco-friendly. Geosynthetics perform 5 essential functions in construction: separation, reinforcement, filtration, drainage, and containment. Different types of geosynthetics have different merits when applied in construction. Table 2 provides an overview of the primary function of geosynthetics (Dashore, 2016).
|Type of Geosynthetic ||Separation||Reinforcement||Filtration||Drainage||Containment|
|Geotextile||✓ ||✓ ||✓ ||✓ |
|Geosynthetic Clay Liner||✓ |
|Geocomposite||✓ ||✓ ||✓ ||✓ ||✓ |
Geotextiles can be used for separation, filtration, drainage, reinforcement (Wu & Yao, 2020). Geotextiles can separate two kinds of materials, avoiding mixing and losing integrity to the structures. Geotextiles can be used for separation between subgrade and stone base in unpaved and paved roads and airfields, between subgrade in railroads, between sidewalk slabs, etc. Separation is illustrated in the following figure. The separation function of geotextiles can effectively prevent pumping effects created by dynamics.
Geogrids are polymeric materials formed by extrusion, weaving, or welding to form open aperture products of varying strength, strain, and load-carrying capability for applications of soil reinforcement. Based on which direction the stretching is done during manufacturing, geogrids are classified as either uniaxial geogrids or biaxial geogrids (S K, 2016). The following figure is an illustration of typical Uniaxial and Biaxial geogrids
Geogrids are commonly used to reinforce sub-bases or subsoils below roads, as well as retaining walls or other structures. The usage of geogrids in retaining wall construction is in the area of soil backfills. Holding the soil together will help in stable retaining wall construction.
Reinforcing with geogrid makes the whole structure behave as a single mass, thus increasing structural integrity. This helps in confining backfill as well as helps in distributing the loads. The geogrids solve the problems with soft backfill or sloping ground (Mandavkar & Weldu, 2019).
Geomembranes are flexible and watertight polymeric membranes manufactured with a wide range of polymers such as plastic, elastomers, and blends of the polymers. They are thin membranes, and the thickness is often 0.5 mm or more. The material has effective permeability or hydraulic conductivity on the order of 10-10~10-13cm/s, and such low permeability appears a solution to seepage and leakage problems, provides a function of containment.
The design and construction of levees and embankments have numerous factors to be considered, from overall purpose and functionality to location, proximity considerations, and fill material type. These factors may vary from project to project, but an overall sense of steps and procedures should be followed based on past successes. Generally, levees and embankments are comprised of pervious and non-pervious, or coarse-grained and fine-grained, soils, each having a specific purpose. Where pervious materials are mostly easier to move and construct with, fine-grained soils, normally clays in this regard, provide the benefit of having a very low hydraulic conductivity.
When it comes to the design elements of a levee or an embankment, there are several factors that need to be taken into consideration, namely: the foundation, it’s adequacy, which is directly related to soil type; the fill material used to construct the bulk of the levee, whether it be compacted or hydraulically placed fills; and the composition of the impermeable core. All of these factors are directly affected by the overall purpose, and how the embankment is intended to function. The main components of a levee are the waterside slope, which is the side that is in direction contact with the retained body of water; the landside slope, which is the side opposite of the retained body of water, and is usually dry; and the embankment crest, which is the plateau-like feature of the levee, and is often designed with a functional width. The elevation of the levee above the ground surface should take into account an allowance for varying retained water heights. These components, and how they fit together to configure a levee, can be seen in Figure _, below.
The foundation of a levee often requires a minimum level of preparation before construction can begin. The general process involves clearing and grubbing of the plan area, which is the complete removal of any obstructional objects, including organic matter, at the ground surface; stripping, or the removal of low growing vegetation and the upper 6 to 12 inches or organic topsoil; and the proper disposal of the removed material. An explorational trench is then dug to determine if there are any undesirable features or buried objects located along the alignment of the levee. The dimensions of the trench vary depending on the underlying soil and the overall configuration of the embankment; however, a minimum depth of 6 feet is required, unless the height of the levee is less than 6 feet, in which case the trench depth should match the height. Trenching, in this regard, allows for inspection of seepage channels or other potentially destabilizing site conditions. As a final foundation preparation measure, the surface where the fill material is to be placed should be scarified, or broken up, to a depth of no less than 6 inches. This is to ensure a strong bond between the foundation soil and the placement material, reducing the potential for a plane of weakness at the interface. The foundation soil should be kept dry before scarification, and scarification should occur just before the first lift of the fill material is placed. Any soft or organic material found within the foundation plan area should be removed and replaced with more competent, compacted soil.
Coarse-grained materials have the benefit of providing stability by their higher unit weights; however, the trade-off is that water is able to flow freely through their respective void space. Since this is the case, a high permeability material needs to be balanced with that of a low permeability material, i.e., a fine-grained soil, or a clay-like material in this regard. The low permeability layer, which is often used at the core of an embankment, in tandem with the underlying foundation, creates longer flow paths for the permeating water. With the mixed-use of fill material type, utilizing their respective benefits, potentially destabilizing hazards, such as settlement, global instability, external and internal erosion, and tension cracking, can be mitigated through proper design. In an attempt to design a more efficient structure, and to further aid in mitigating failure, different types of geosynthetics can be used. The most common type of geosynthetic used in levee and embankment design is geogrid. Geogrid is generally designed to extend a factor of the levee height into the levee, and is often laid out in several layers throughout the structure height.
As previously mentioned, the type of fill material, along with the level of compaction effort applied, will ultimately dictate the embankment geometry. Semicompacted fills are usually fine-grained borrow materials that are far more wet of their optimum water content and are used where the design requires much shallower slopes for stability. Uncompacted fills are generally highly organic borrow materials that, like semicopacted fills, are used for shallow slope construction. When the foundation of the levee has adequate strength, and when space requirements are a factor for design, compacted fill construction would then observe the procedures for earth dam building.
Hydraulically placed soils are mostly coarse-grained, pervious materials. Since levees that are constructed of hydraulic fills generally have low densities and large plan areas, they are very susceptible to liquefaction. Hydraulic fills also tend to erode when overtopping occurs, or when a non-pervious layer cover layer is breached. This type of fill is mostly used for stability berms and seepage berms, but not normally for embankments; however, where human life is not threatened by levee failure, or where an embankment includes impermeable core, hydraulic fills can be used.
The impermeable core portion of a levee is often a structure composed of fine-grained, non-pervious soil located at the very center of the levee and anchored into the foundation material. The intention of having an impermeable core, along with the underlying foundation, is to create a longer flow path for the permeating water. Although the clayey soil used has the benefit of low permeability, it is prone to destabilization due to surface cracking and internal erosion. This is another good opportunity to include geosynthetics into the design. In this specific case, a top layer of geogrid has the potential to minimize the extent at which surface cracks could form, as well as a layer of geotextile to act as a filter to prevent internal erosion.
Much of levee design, including the use of geosynthetics, is rightly concentrated on structural strength requirements to prevent catastrophic breaches and flood events; however, there are several environmental consequences that can arise from improper design and construction as well. Levees principally fail due to overflow, infiltration, and erosion; these three failure modes each carry concerns related to contamination of water bodies and human exposure (Dortch et al. 2007). For example, the presence of heavy metals and organic compounds in floodwaters could threaten ecosystem health, and moreover, human health, especially in those water bodies used by humans for drinking water or agricultural production. Bacteria present a biological concern for similar reasons, and turbidity levels could be problematic for drinking water systems. Finally, salinity can present an issue, particularly in coastal and marine environments, when salty or brackish waters are allowed to mix with freshwater, whether surface or underground.