Experimental research into the internal stability of a base layer of recycled asphalt granulate mixed with sand under wave loading.

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4. Experimental research into the internal stability of a base layer of recycled asphalt granulate mixed with sand under wave loading.

The Netherlands has 600 kilometers of asphalt revetments on sandy dikes to resist severe wave attack. More recently, base layers have been used consisting of granular material, that gives additional protection. As the phreatic line in the dike can be high, the internal stability of these layers needs attention, as part of a failure mechanism that leads to disintegration of the granular material due to wave action and as a result unallowable S-shaped deformation. The apparent cohesion of a base layer consisting of asphalt granulate mixed with sand that was studied here, can disappear due to cyclic loading (wave action), as fatigue of the bituminous material occurs. A field research was done to determine the layering, the composition and density of this base layer. The material was sampled for further laboratory research in which the upward flow gradients based on tests in a large wave flume in the material just beneath the asphalt layer were applied. These upward gradients, being larger than 1, can cause local liquefaction if the cohesion in the granulate material is insufficient. In the laboratory research permeability was determined for three different mixtures of asphalt granulate and sand (6%, 14%, 21% of sand) that cover the variability in the field, under variable hydraulic conditions, including cyclic loading. 

In the laboratory tests an upward water flow was induced in the asphalt granulate-sand mixture, and increased step by step. Across the sample the hydraulic head was measured with small standpipes at 5 locations. The sample containing volume has a diameter of 300 mm and the sample height was 300 mm as was found in the field. Sample preparation commenced with installation of the soils in 50 mm lifts which were pluviated under water by constant tamping. Upon placement of the sample, hydraulic heads were applied across the sample and the head differences, flow rates, and permeant water temperature were monitored periodically. As local inhomogeneities are expected to arise, two arrays of standpipes across the sample height were used and camara images were taken. It is concluded that the internal stability of the samples is insufficient, the apparent cohesion vanishes due to cyclic loading and the permeability increases drastically. This implies that the base layer material disintegrates and on the dike it will form an unwanted S-shape profile that finally causes failure of the asphalt layer.

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