In a typical box culvert, at the interface between rockfill layer and sub-grade, a layer of geotextile filter is usually added to perform separation function. With the addition of geotextile filter layer, it avoids the intermixing of widely different soil granulations so as to reduce long-term settlement.
Moreover, it also prevents interpenetration of rockfill into sub-grade so that the deformed configuration of rockfill may impair its intended function of load spreading.
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This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.
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