Sentence:  A contractor is paid for excavating a certain amount of rock when constructing a tunnel; the line drawn in a cross section of the tunnel that defines this limit of excavation is called the <WSD>payline</WSD> (fig. 12). Any material excavated outside the <WSD>payline</WSD> is defined as overbreak, and a contractor is not paid for excavating that material. 
Meaning of the word marked with <WSD> tags:  The planned limit of an excavation, beyond which a contractor is not paid for any excavation work.