Building collapse can be due to a few factors:-
1. Poor design - not taking into account the necessary design considerations leading to a weak/poor design
2. Poor quality or wrong material used - this can be due to purchase of cheaper material from the "back lane" factories or manufacturers who do not meet the necessary requirements or standards. This could also be due to failure to test the material used for the construction.
3. Poor supervision - the consultants or assigned supervisor not doing his/her job professionally - sometimes allowing construction which is not following the design or drawings.
In the case of a design & build contract - the contractor is liable for whatever he as constructed. This is because the consultants are part of his team and the consultants have a professional responsibility and liability towards their design and supervision to ensure the final product is as per specification, drawings and quality which have been mutually agreed and binded in the contract.
JKR is supposed to be auditing the contractor & the consultant in a D&B contract, since the responsibility for the end product still lies with the contractor's team. However, if JKR fails to do regular / proper auditing ' I believe JKR can also be roped in to be partially responsible due to failure to do the necessary audit.