Geosynthetics 2015

February 15-18, Portland, Oregon

Some Innovative Alternatives for Protection from Erosion Forces in Civil Engineering Applications J. N. Paulson, P.E., Milliken Infrastructure Solutions, LLC, USA ABSTRACT Erosion control has been a part of earthwork construction and site development since the Romans built roads and aqueducts. Control of storm water in order to maintain a structure’s integrity is fundamental to long term successful life of that structure. The following lists just some of the erosion control solutions commonly used in civil engineering for erosion control. They have been separated into standard or established systems, and some of the new systems being used in the marketplace. This is by no means an inclusive collection as there are many variations on those presented, as well as new systems always being introduced. There are a variety of conventional solutions for hard armor erosion control that have traditionally been used to provide protection for the underlying soils. Riprap, formed and poured concrete for slope or drainage structure paving, and concrete filled fabric forming mats have been utilized to protect underlying soils from migration. Concrete and fabric formed mats can provide a hard, durable surface, but both require access to the site for ready-mix trucks or concrete pumping equipment for more remote areas. Riprap is useful in slowing water velocity down, but is not water resistant, requires maintenance, and there are construction challenges when placing large rock sizes in remote locations. There are a number of new products and systems that are being used for erosion protection, including continuous soil (or concrete) bags, a filled artificial turf system to name a few. Another relatively new material is the Geosynthetic Cementitious Composite Mat (GCCM). This material provides a three dimensional fabric containing concrete powder, backed by a PVC film to aid hydration and provide water resistance. It is a unique means of delivering a hard concrete layer in areas where the ability to get or install concrete is a challenge. Product and system features and beneficial uses of these new erosion protection systems will be explored, along with applicability assessments.

1. ESTABLISHED EROSION CONTROL SOLUTIONS

1.1 Riprap - Also known as rip rap, rip-rap, shot rock, rock armor or rubble, is rock or other material used to armor

shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour, water or ice erosion. Riprap is produced from a variety of rock types, commonly granite or limestone, and occasionally concrete rubble from building and paving demolition It can be used on any waterway or water containment where there is potential for water erosion. Riprap provides soil protection and energy dissipation (slowing the water velocity) in some installations. Rock size and layer thickness are considerations in each application. Many projects are not designed but rely on past history.

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Figure 1 – Riprap for Lake Shoreline protection

Riprap is designed by class or size, based on water flow velocities, depths, and slope angles of areas to be protected. Class 2 riprap is larger than class 1. For riprap to be cost effective transportation distances must be short and access to the installation location must be good. Wire contained riprap, or rock filled gabions, are another hard armor solution for high flow conditions. In many installations a geotextile filter is installed below the rock to prevent soil fines migration from the covered slope (HEC 15, 2005). 1.2 Concrete – Formed or slip-formed concrete and shotcrete have been permanent solutions to erosion control, providing a permanent concrete layer protecting the underlying subgrade from water’s erosive forces. Ditch lining and slope paving continue today as commonly used means of protection by highway departments across the country. Concrete’s main advantage is cost, and ease of use. Challenges to using concrete include site remoteness, and ability to pour in standing water. In high freeze/thaw areas higher strength and more durable concrete is required to be used.

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Figure 2 – Concrete Slope paving Slope paving concrete is usually a minimum of 4 inches, probably more having to do with constructability than designed thickness. As HEC 15 states, “Despite the non-erodible nature of rigid linings, they are susceptible to failure from foundation instability. The major cause of failure is undermining that can occur in a number of ways. Inadequate erosion protection at the outfall, at the channel edges, and on bends can initiate undermining by allowing water to carry away the foundation material and leaving the channel to break apart.” 1.3 Sandbags – Sand filled fabric bags have been used for temporary protection for over a hundred years. Simply, a bag is filled with locally available soil, the bag tied off, and this soil unit is hand placed and stacked in protection positions. Advantages of sandbags are their portability, and the ability to use any soil available. The temporary nature of the fabric used to make these bags is subject to degradation under extensive UV light which makes these a short term protection erosion control solution. Design, if designed, is primarily based on weight of each bag. Many times a means of interlocking the bags is used. In some cases the wall serves as a retention structure, and is designed accordingly.

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Figure 3a and b – Individual Sandbag and a sandbag wall

2. NEW PRODUCTS AND SYSTEMS

Recently, a series of new materials and systems have been developed and are being used in increasing frequency. Some of these new solutions will be presented below. There are numerous variations on these types of systems, but the groupings attempt to classify these. 2.1 Continuous Large Geotextile bags and Geotextile Tubes - 20 years ago the USACOE developed the use of large bags for erosion protection and flood control These Geobags and Geotextile tubes are just a larger version of the sandbag, but now made with more durable and stronger fabric, and the need for machinery to fill and place these units. Typically a one cubic yard bag is fabricated of high strength woven geotextiles, then supported by a frame, then filled, tied off and placed. The size and mass of the bag makes this system resistant to moving water forces.

Figure 4a and b Geotextile bags and geotextile tubes A subcategory of geotextile bags are what may be called top filled, continuous cell geotextile bags, or a series of bags sewn together to form an interconnected row of soil filled cells. Commercially available systems such as Geotextile tubes, and containers are available. Once again, the mass of the soil filled bag and its interconnectedness make this a system that has been used for shoreline or streambank protection.

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The size and continuity of these systems results in their use as retention structures, retaining standing or flowing stormwater as well as providing protection. When used as such conventional gravity retaining wall design principles apply. Shoreline protection applications in high wave action conditions may result in additional design details to prevent undermining.

Figure 5 – Top filled continuous Geotextile bags or cells 2.2 Articulated Concrete Block Systems (ACB) – In the 1970’s the use of machine made concrete blocks to be placed and used as erosion protection were introduced. Sometimes these systems are hand placed one at a time. Other times they are cabled together to form mats that are crane hoisted and placed. These concrete armor mats are useful where large areas are in need of protection, and the uniformity of these units is useful. There are numerous styles of block shapes, and cabling techniques, but the central approach is to fabricate and cover areas where flowing water is anticipated. Channel linings, dam overtpping and overflow spillways are common applications for ACB systems. Design of these blocks takes into account the mass of each individual block and in high flow applications the limiting shear stress is that which causes the block to lose contact with the underlying soil. Numerous design methods are available (NCMA 2006).

Figure 6 - hard armor cabled ACB System 2.3 Fabric Formed Concrete – Fabrics have been used as a form for concrete in many erosion control applications. Two layers of fabric are joined into an envelope and injected with a very fluid fine-aggregate

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grout to produce a durable concrete lining (Sprague and Koutsourais, 1992). Once hardened, these mats provide a protective covering and erosion protection in many erosion prone areas and in hard to reach areas. Several forms of this type of protection system include a uniform section mat and a “filterpoint” mat with permeable points allowing seepage thru the grouted product.

Figure 7 - Fabric Formed concrete (Photo courtesy of Hydrotex) 2.4 Cellular confinement Systems (Geocells) are small height vertical cellular mats that are stretched out over subgrade, then the cells are infilled with soil or, in some cases, concrete. Originally developed and tested by the US Army Corps of Engineers, these systems were originally developed for subgrade stabilization, but have been used for erosion protection applications on shorelines and slopes. When soil filled, a geotextile is sometimes used as a filter and separator layer underlying the filled cells. Their advantage is the installation flexibility, and the strength of confined soil within these cells. Installation and infiling on slopes can sometimes be a challenge.

Figure 8 - Geocell installation on a slope (photo courtesy of Presto products)

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2.5 Geosynthetic Cementitious Composite Mat (GCCM) Concrete Clothtm

, a GCCM, was originally developed in the UK. This material is a machine made three dimensional fiber matrix than contains a specially formulated cement. The product comes in rolls, then can be placed, positioned, anchored, and once hydrated, becomes a high strength protective layer of concrete. It takes the shape of whatever it is placed upon. The type of product is a machine made GCCM that comes in rolls and thicknesses of up to 13 millimeters. GCCM materials have been used as a protection layer for ditch lining, channel lining and slope protection and soil erosion protection around the world. These systems are considered permanent, and may have the fabric surface or fibers exposed to the elements. There are also field applied systems utilizing composite concepts as well, just entering the marketplace. Design details of importance include anchor trenches to prevent undermining, and proper overlapping in the direction of flow. Durability must be considered in Northern climates subject to winter and designed against freeze/thaw exposure in Northern climates.

Figure 9 – A GCCM is installed as a slope protection layer under a bridge

3. COMPARE AND CONTRAST SYSTEMS

Characterizing these systems may be complicated by the multiplicity of functions and characteristics including:

1. Protection,

2. Longevity,

3. Impermeability or low permeability,

4. Energy dissipation,

5. Mass to resist hydraulic forces

6. Installation

7. Others

The challenge is to clearly understand system functions so the best system (or systems) may be used for a given application. Design procedures are available for some of these systems, with more being developed. As well, new systems are continually being developed.

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Table 1 characterizes these systems by function providing the authors insight into design considerations and primary functions.

System Basic description

Permanent or temporary

Design considerations

Limitations Function

RipRap Rock of varying size placed on a slope or canal

Permanent for 3:1 of flatter. Temporary for slope steeper than 3:1

Flow velocity and depth

Not available Requires maintenance

Energy dissipation and protection

Concrete Formed and poured layer

Permanent Thickness Rigid, prone to undermining

Protection

Sandbags Small soil filled bags

Temporary unless protected

Mass of bag and size of structure

Labor intensive and temporary

Retention and protection

Continuous large fabric bags

Large soil filled fabric bags or containers

Temporary unless protected

Size of filled bag or container has large mass

Fabric outer layer considered temporary

Retention and protection

Geocells Rigid plastic cellular confinement

Permanent Thickness and infill material type

Installation Protection and retention

GCCM Geosynthetic Cementitious composite mat

Permanent Flow velocity and installation details

Relatively light weight layer

Protection

The above are general descriptions of these systems and is not to be considered all inclusive. More details may be obtained in the references and literature.

4. COMBINATIONS OF SYSTEMS

There are applications where combinations of systems may be used. For example a GCCM may be used as a protection layer for some of the temporary systems listed above. The following photo shows a sandbag wall protected after installation with a GCCM. This provides protection of the fabric from damage due to debris and floating objects and to protect against UV exposure.

Figure 10 - GCCM covering a temporary sandbag wall for flood water control

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This approach was applied to the continuous bags or tube systems where damage to the fabric would compromise the integrity of the soil filled system. In situations where a temporary system needs to become permanent this may solve this challenge. Design of the structure follows conventional design methods while the protection is a function of the level of protection desired for that environment.

5. SUMMARY

Erosion protection in civil engineering applications has been and continues to be a challenge. Solutions may be conventional solutions that have been established for decades, or new innovative systems. There are several new erosion control systems that have recently been developed that augment the currently available erosion control alternatives for erosion protection applications. Several of these are combined geotextile fabric and concrete systems, and soil filled containers. ACKNOWLEDGEMENTS The author would like to acknowledge the photos taken from many and numerous websites of these systems. This is not intended to all inclusive of the many new systems, but illustrative of the varying types commonly available. 1.1 REFERENCES BALOGH A. (1994) “Simplifying Slope Paving” Article, Aberdeen group Pennsylvania, USA HEC 15, “Design of Roadside Channels with Flexible Linings” Hydraulic Engineering Circular Number 15, Third Edition Publication No. FHWA-NHI-05-114 September 2005 NCMA National Concrete Masonry Association, Design Manual For Articulating Concrete (ACB) Revetment Systems, ISBN 1-881384-20-9, NCMA Publication Number TR220, 2006. www.ncma.org Sprague, J. and Koutsourais, M. (1992) “Fabric Formed Concrete Revetment Systems”, Geotextiles and Geomembranes, Elsevier science publishers, ltd, England 2.

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