Coastwide
Schematic representation of GSC breakwater model at Wave Mechanics Laboratory, NITK26.

USA - Preliminary investigation on stability and hydraulic performance of geotextile sand container breakwaters filled with sand and cement

Breakwaters are essential constructions providing tranquility to ports and harbour structures, when there is a lack of natural protection measures.

Abstract

Traditionally these massive structures are constructed using natural rocks weighing tonnes. In the present scenario, obtaining huge natural rocks are difficult as well as non-eco-friendly. Geotextiles sand containers (GSCs) emerges as a suitable alternative for the rock armour units of breakwaters and various literatures supports its efficacy. The present investigation aims at analysing the performance of GSCs when filled with a calculated amount of cement and sand. The Hydraulic performance and stability analysis of cement and sand filled geotextile breakwater models are carried out in a 1:30 scaled monochromatic wave flume. When GSC breakwaters are filled with sand and cement, up to 43% increased stability is observed with a considerable decrease in wave runup, rundown and reflection, than sand-alone filled units. As a result, cement-sand filled GSC units can be suggested as a possible alternative to sand alone filled units where vandalism has to be countered.

Introduction

Breakwaters are generally constructed to dissipate wave energy, creating a calm condition inside the harbour area for easy loading and unloading of cargo from a berthed ship at a port. Additionally, a calm harbour area is advantageous for the smooth functioning of ports and harbours. Ocean waves often cause serious coastal erosion during monsoon season and can be catastrophic for the coastal ecosystem, livelihood and communities. Global climate change and associated sea-level rise can cause serious wave actions, inundations and coastal flooding in the adjoining coastal areas1. Recurring cyclones and associated storms surges are a matter of recent concern2,3,4. Therefore, it is inevitable to have protection structures that can reduce the adverse impact of damaging waves on coastal areas.

Coastal protection structures may generally be hard or soft. Sand bypassing, dune replenishment, vegetative protection, beach restoration, etc. are considered soft solutions5,6. Submerged and emerged breakwaters, dikes, seawalls, revetments and groins etc., are adopted as hard solutions7,8. Over the years, rubble mound or rock armoured structures were the most commonly adopted breakwater system9. The alarming increase in the cost of natural rock, reduced availability and prohibition of quarrying in many states demand viable alternatives to rock structures10. Over the years, there have been tremendous innovations in artificial armour units. Concrete Cubes, Tetrapods, Accropods, Dolos, Core Loc etc., are some notable armour units9,11,12,13,14,15. Research extended in designing innovative breakwater structures. Some notable contributions in this regard include semi and quarter-circular breakwater16,17, plate breakwater18, tandem breakwater19, floating pipe breakwaters20,21, pile breakwater22,23 etc. Along with these, geosynthetics are also widely used for various coastal engineering applications. Geosynthetics refers to a wide range of natural or artificial polymeric material than can be used for various civil engineering applications24. Geosynthetics include geotextiles, geomembranes, geogrids, etc. (used in civil engineering applications including road construction, waste management, slope protection etc.) with geotextiles being widely used for coastal engineering applications25,26. Coastal engineering applications of geotextiles include revetments27, embankments28, breakwater29, armour units of breakwaters30,31 and other coastal protection structures8,12,32,33,34,35. Geosynthetic sand containers (GSCs) are proved to possess various benefits over conventional rock constructions36. It can supply a wide range of uniformly sized armour units, which is very difficult in the case of rocks37,38. Cost per unit volume can be reduced when the size of containers is large, reducing construction time significantly. Fill ratio affects stability and shape due to interlocking and flexibility. It can also be stacked to steeper slopes when compared to conventional structures. Another attractive feature of the geotextile constructions is the insitu filling capability of the tube or containers with locally available materials, making the construction cost-effective and rapid39. Despite the fact that GSC units produce visual impact, the above-stated advantages make sand-filled geotextile units a viable alternative to the primary and secondary armour units of a conventional rubble mound breakwater. There have been fewer attempts to quantify the performance of geotextile units in a breakwater structure, motivating one to pursue the present study.

Physical experimentations conducted by Elias et al.30 investigates the efficacy of sand encapsulated geotextile containers as the armour units of breakwater structures. The study confirms the utility of geotextile sand containers (GSCs) up to a wave height of 3.96 m (prototype) when used as armour units of breakwaters. Various arrangements, including single layer, double layer, and slope parallel placement have been investigated, with double layer arrangement exhibiting up to 18% higher and slope parallel placement showing up to 11% lower stability than single layer placement. As described in the study, these GSC units are susceptible to damage that can be incidental, biological or vandalism. Incidental damages include boats making a direct hit or anchoring on the structure, fishing involving sharp tools tearing the geotextiles, leading to sand loss40. Driftwood and ice also cause incidental damages29. Rodents and rats nesting around the geotextile containers, damaging it, is grouped as biological damage41. Food waste encourages the colonisation of rats resulting in them tearing the geotextiles. Vandalism or deliberate destruction of GSC structures is a major concern and remains a prime disadvantage. It is reported that exposed GSC structures attract the curiosity of the native people or tourists and ultimately cut or damage it with sharp tools. Vandalism due to knife cut resulted in the failure of Kirra Groyne (Gold Coast, Australia), Submerged Reef at Kovalam, Kerala, St Clair revetment (Dunedin, New Zealand) etc.29,41. Once the containers are cut, the sand leaks out due to wave attack leading to total deflation and failure. As a possible solution for this problem, the present study investigates the feasibility of mortar mixture in filling the GSCs of breakwaters. When the GSC units are filled with mortar, the units harden, forming a rigid structure. This reduces the risk of vandalism as even when the external geotextile cover is damaged, the inner solidified unit remain intact.

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