Gabion stability

Abstract

South Korea is reclaiming land by constructing dams and polders. The large tidal differences along the Korean coast make building these dams a challenging job. One of the solutions in South Korea to cope with the high flow velocities in closure projects is to apply sack gabions. These are steel nets with rocks inside them that weigh up to 3 tons. It is not clear how stable these sack gabions are exactly. The objective of this report is to make a preliminary study on the stability of sack gabions. In 2006, the Saemangeum estuary in South Korea was closed with a dam. During the closure sack gabions were used in the bed protection, sill construction and dam heads. In a field trip to the Saemangeum project useful data was collected on the stability of gabions. Experimental data of RRI on model tests on the stability of gabions was also obtained. In addition model tests were done in Delft. All data are compared to come up with an advice for calculating the stability of gabions. To calculate the depth averaged critical velocity for 3t - 5t rock with 3t sack gabions mixtures, formula [15] proves to be useful. ucM = 2.513*x+5.4 [15 Where: ucM = critical velocity of a mixture of 50% rocks of 3.0 to 5.0 t and 50% 3.0 gabions x = the proportion of gabions in the mixture 0.2 < x < 0.5 For the calculation of the local critical velocity for a bed of sack gabions, it is advised to use Izbash' formula with a gabion stability factor (Gamma in formula [10]) while calculating the nominal diameter of a sack gabion as in formula [1] (a mass based approach): Delta Dn = (Beta*Uc2)/(Gamma*2*g) [10] Dn = (M/Rs)^1/3 [1] Where Gamma = 1.26 for sack gabions (while for loose rocks Gamma = 1) Also a qualitative analysis of the Delft model tests is made that leads to several considerations for the design of gabion bed protections: When applying gabions one has to take into account the difference in behavior between gabions and loose rocks. Another behavior that needs to be investigated further is the effect of applied pressure on a gabion bed. There is still much unknown about the stability of gabions and the report mentions several things that should be further investigated. Also it is advised to use sluices in the Netherlands or in South Korea as flumes for extensive prototype tests on the stability of gabions.