India has a very long coastline in proportion to its land mass, with approximately 7,000 kilometers of coastline, the 18th longest in the world. As a result, the country is particularly vulnerable to the effects of coastal erosion, with frequent storm events causing significant land loss and disrupting state highways and private property. Hence, the concept of geotextile erosion control comes into existence.
A rock seawall, which consists of large boulders placed over a non-woven geotextile, is one of the most common types of coastal protection. These can be built quickly after an event and are usually made of readily available and locally sourced rock.
The tough geotextile remains out of sight and out of mind for most people driving past a rock seawall, bravely weathering the storms. It is, however, critical to the success of a rock wall system. Surprisingly, this geotextile application is one of the earliest known uses of a textile in a civil engineering application.
WHAT IS THE FUNCTION OF A ROCK SEAWALL?
Large rocks placed directly on an erodible coastline will not protect against the sea’s aggressive action. The rock breaks up some of the wave energy while also keeping the geotextile firmly in place. The geotextile, on the other hand, is responsible for holding the soil in place and preventing erosion and scour.
To accomplish this, three critical properties must be obtained from the geotextile in order for it to perform its intended functions: water flow rate, soil retention, and puncture resistance.
1. PERMITTIVITY OR WATER FLOW RATE
The geotextile behind the seawall must allow water to flow freely while preventing hydrostatic pressure from building up. A geotextile works in steady-state one-way flow conditions in most filtration applications, such as a subsoil drain; however, the surging of the waves creates a two-way flow situation in a seawall. The ebb tide is a critical condition to monitor because when the water level drops and the soil on the bank become saturated, the groundwater must be able to flow freely back to the sea.
2. RETENTION OF SOIL
Aside from water flow, it must also hold back fine soil particles without clogging for long-term geotextile erosion control. If the soil is allowed to wash out, the system will fail completely. This soil retention function could be considered the primary function of the textile in the sea wall, as well as the primary purpose of the entire system.
3. RESISTANCE TO PUNCTURE
To prevent the waves’ constant and powerful action from moving the rocks, the designer will usually try to use the largest size rock readily available. Furthermore, the rock will typically have broken faces rather than rounded edges to aid in stability on the sloping sea wall. The large size and sharp edges pose a significant risk of puncturing or ripping the geotextile during application. If the textile is torn during construction, it will be unable to perform its primary function of soil retention. In some cases, this can cause the sea wall to begin slowly failing shortly after construction.
In order to protect the geotextile and enable it to perform for the life of the structure, we must select a geotextile grade that is robust enough to withstand the installation of the specific rock size intended for the site without puncturing. There are several standards and guides which can be used to assist with this selection, and the elongation, puncture resistance and tensile strength of the fabric will be important parameters. The method of rock placement is also important; for example, dropping large rocks from a high digger bucket or tipping large rocks from a dump truck down the seawall will rip almost any grade of textile. Careful placement and lower drop heights are essential for system integrity.
If using a geotextile grade to match the intended large rock size is not practical, a layer of smaller rock can be placed over the textile as a cushioning layer to protect it during installation.
Let us now look at a variation on this theme. Although the composite seawall system of geotextile filter layer and rock is very effective, the rock can be unsightly and harsh in areas where the public has easy access to the seashore. The rock in this application can be replaced with geotextile sand containers. These are large sandbags made of a special UV and vandal-resistant fabric in a light tan color to blend in with the sand. They serve the same purpose as the filter layer, but with a number of additional advantages. They cause very little installation damage, can be used in areas where there is no rock, are filled with sand from the existing environment, and are much more aesthetically pleasing and soft to walk on.
So, the next time you see a rock wall protecting our valuable coastline, take a closer look and recognize the role of Singhal Globals’ geosynthetics “geotextile erosion control” in environmental protection.