How safe is a vertical cut in rock?
Often, the construction industry requires deep excavation for basements. But how safe is a vertical cut?
Several factors need to be considered. Firstly, the nature of the rock, its level of cementation and internal friction which depends on size, sorting, and shape of grains. More importantly, are the discontinuities in the rock. Most common discontinuities consist of joints. Joints are features produced by tension and can be horizontal or quasi vertical, depending on the nature and history of the rock. All these aspects need to be considered by the geologist.
Third party properties are directly affected by nearby vertical rock cuts. The question arises, how safe is existing third-party property affected by the excavation?
What does the Law say about this matter?
Legal framework
Subsidiary Legislation 623.06 requires a ground investigation with borehole drilling but does not give details of what should be assessed and who is the competent person to make an assessment on trial pits and rock cores.
Around the world, geologists are deemed the competent persons to make such assessments, but Maltese law is silent about this matter, creating a vulnerability which can have dangerous consequences. Moreover, the law refers to ‘expected strength of the ground materials’ and ‘the presence or otherwise of fissures’. These very vague comments in the law raise a number of questions:
1. What is the difference between the ‘expected’ and actual strength of ground materials?
2. Are fissures the only discontinuities in Maltese rock? Certainly not!
The law ignores the presence of joints (which are very common), faults and caverns.
IS IT SAFE TO BUILD ON A FAULT?
What is a fault? A fault is a discontinuity in the rock where one side has moved relative to the other.
Faults are considered by many to be dangerous, but what about Malta? Malta has many geological faults that criss-cross the islands. Most are normal fault where one block has gone down relative to the other block. However, many are inactive.
Most people, including engineers think that a discontinuity in rock is a fault. This is incorrect, since many discontinuities do not have movement associated with them. Faults are identified by special features associated with them. This is where the geologist is needed to identify whether the discontinuity in the rock is a fault or not.
Although many faults in Malta are inactive, some have been reactivated by recent tectonic movement of Africa with respect to Europe. Fault reactivation usually results in a normal fault becoming a strike-slip fault where one side slides past the other side. These faults can pose problems for buildings over them.
Legal framework
There are no laws requiring the identification of faults in Malta prior to a construction. The only reference to discontinuities in rock mentioned in the law is Legal Notice 623.06 which refers to 'fissures', which are not faults. This serious omission in Malta's laws can be fatal in case of weak material associated with faults and reactivated faults.