Sentence:  The paving downstream consisted of 18x18x8-inch blocks laid on edge over a brick base which in turn was underlain with a gravel or brick chip filter. Since there was some controversy as to whether an <WSD>underfilter</WSD> was necessary, the first test was made with the  filter omited. 
Meaning of the word marked with <WSD> tags:  A filter that is positioned beneath something.