1. Stabilising the leaning tower of Pisa
2. Overturning Instability
3. Johnston G. Burland J. (1832). "An early example of the use of underexcavation to stabilise the Tower of ST Chad Wybunbury". http://www.unesco.org/archi2000/pdf/burland.pdf.
4. Some Historic Examples of Underexcavation;Johnston G.;Thomas Telford,2004