Displacement and stress nephograms of the existing tunnel (Z = 0·50D): (a) displacement nephogram at λ = 0·5; (b) displacement nephogram at λ = 0·3; (c) displacement nephogram at λ = 0·1; (d) stress nephogram at λ = 0·5; (e) stress nephogram at λ = 0·3; (f) stress nephogram at λ = 0·1