Complex sentence: The arch was lined with two (lengthways to a thickness of 20 inches) or three courses of brickwork depending on the strength of the surrounding ground; volcanic matter was used in the mortar, to obtain a quick setting and strong cement.
Split into simpler sentences: The arch was lined with two courses of brickwork, lengthways to a thickness of 20 inches. Volcanic matter was used in the mortar, to obtain a quick setting and strong cement.