Sentence:  The earth was excavated from the sunken cylinder; the <WSD>shield</WSD> was inserted into it, and the tunnelling began, the target being Wapping, on the opposite bank. The <WSD>shield</WSD> was an iron honeycomb containing thirty-six cells within which men dug the wall of mud before them. 
Meaning of the word marked with <WSD> tags:  A framework used to protect workmen in making an adit under ground, and capable of being pushed along as excavation progresses.