Civil Engineering May 2021 | Vol 29 No 4

14 May 2021 Civil Engineering were also similar in configuration to those used in available research papers and have previously been implemented along various coastal defence projects in South Africa. DESIGN ASSUMPTIONS The design was based on the following assumptions: Q Q A structure design life of five years. Q Q A low level of damage has been consid- ered acceptable during extreme storm events. Q Q Routine inspection and maintenance of the geocontainer revetment is as- sumed and will be required. Q Q It was considered unlikely that the new geocontainer revetment structure will result in changes to the coastline due to the similar alignment and location of the new revetment compared with the existing gabion revetment. The geocontainer revetment alignment had to be “smooth” and similar to the existing revetment with no protruding portions and/or sharp corners that would impact the present coastline configuration and/or create focussing of wave energy. DESIGN APPROACH There are no engineering codes and manuals for geocontainer revetment design. General design guidelines from international and local technical research and case-study papers were therefore adopted for the structure configuration. Ten international research and case study papers, ranging from 2002 to 2021 were reviewed, as well as five South African research and case study papers. The infor- mation obtained from previous research and case studies included: Q Q Minimum performance requirements Q Q External modes of failure Q Q Internal modes of failure Q Q Critical forces on geocontainer structures Q Q Internal force interaction on a geocon- tainer structure Q Q Geocontainer material properties Q Q Stability formulae. Stability calculations Structure stability was determined based on an international research paper titled Grosmasstabliche Untersuchungen zur hy- draulischen Stabilitat (Large-scale studies on hydraulic stability) (Oumeraci et al., 2003). The published formulae were used to verify the stability of the slope elements (wave facing geocontainers) and the crest elements (topmost geocontainers). Formulae for the slope elements (Oumeraci et al., 2003): N S = H S ( ρ E ∕ ρ W –1) D = 2.75 √ ζ 0 Formulae for the crest elements (Oumeraci et al., 2003): N s = H s ( ρ e ∕ ρ w –1) D < 0.79 + 0.09 R c H s For these formulae, two N s (stability) values were used for both the slope and crest elements, as prescribed by the research paper (Oumeraci et al., 2003). The characteristic diameter (D) was also known; thus, the allowable significant wave height (H s ) could be calculated against which the structure would be stable. The calculated wave height was compared to the design wave height. Acceptable elongation percentages of a geocontainer bag Elongation of an individual geocontainer is caused by the internal movement of in- dividual sand particles while wave uprush and down rush forces act on the geocon- tainer structure, as well as when a single geocontainer is handled multiple times from filling to final placement. The sand fill ratio is considered an important factor contributing to the hydraulic stability of a geocontainer structure (Dassanayake and Oumeraci, 2012). Dassanayake and Oumeraci (2012) also stated that the sand fill ratio affects the deformability of a geocontainer, the internal movement of sand, and the interface friction. Too much elongation would lead to excessive internal sand movement that could lead to structural failure. Deformation is a requirement for a geocontainer structure, up to a certain point, but over a certain limit the deformation would lead to struc- tural failure, as stated previously. Based on shared experience from eThekwini Municipality and their paper titled Coastal defences on the KwaZulu- Natal coast of South Africa: a review with particular reference to geotextiles which was published in the October 2012 issue of the Journal of the South African Institution of Civil Engineering (Volume 54, No. 2), an optimal fill ratio of 80% was prescribed and taken as the requirement for the City of Cape Town. The reasoning behind this fill ratio is stated in the below extract from the journal article: “The bags were originally filled to 80% of their capacity (based on the German construction technique (Oumeraci et al., 2003)), ensuring that the sand is sufficiently com- pacted by flooding with water. If the bags are filled any more it becomes difficult to stitch them closed, jeopardising the quality of the stitch. Overfilling the bags also causes rounding. Since some of the bag’s stability is determined by its mass and friction, it was hypothesised that the more rounded it is the less contact each bag would have with the surrounding bags, lowering the stability. Not filling the bags to capacity allows them to be levelled for the next bag layer, as well as providing a large contact area.” Considering the above, it was deemed necessary to investigate the sand fill ratio of a geocontainer for certain elongation percentages in more detail, as different Table 1 Geocontainer stability parameters Coefficient Description N s limit Max stability value before movement H s Significant wave height in front of structure (m) p e Effective density of saturated geocontainer p w Density of sea water D Characteristic diameter ζ Surf similarity parameter R c Crest freeboard