Special features in tendering, award and contract - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 352

settlement. The interruption scenarios envisaged in the contract should also regulate the
method of payment for shorter and longer stoppages. The quality criteria should also be
specified for the tunnel lining, and in order to enable comparability of tenders, the calcu-
lation model, the waterproofing class to be achieved, the tolerances of the lining and the
required fire protection for the tunnel lining should all be specified.
The tender documents should include special requirements for the tunnelling concept with
regard to settlement, calculation basis for the stability of the face, technical requirements
for the shield construction and additional measures undertaken from the shield. Control
and regulation programmes including quality assurance system and loading actions on the
segment lining should also be specified.
9.3.4 Costs as a decision criterion
In the consideration of costs (cost security), the decision phases and the parties involved
also have to be taken into account. The decision phases are structured into design phase,
tendering phase and construction phase.
Design phase or tender design. Three cases should demonstrate in a simplified manner
the influence that a consultant can exert on tendering:
Case 1. The consultant roughly estimates the cost of a shield drive without producing a
full design basis and comes to the conclusion that the cost is higher, resulting in a recom-
mendation to the client that there should be no provision for a shield tunnelling in the
tender documents and no approval of an alternative proposal for mechanised tunnelling.
Case 2. A shield tunnel drive is indeed fundamentally permitted as an alternative proposal,
but the design and geological information required for the production of a tender are not
included in the tender documents. The alternative proposal that is received in this case is
either incomplete, too expensive or also even too cheap, if the price has been manipulated
rather than estimated (for example by omitting additional measures).
Case 3. A shield tunnel drive is planned parallel to shotcrete tunnelling and the tenders are
open to free competition. This procedure is only known to have been fully implemented
in Switzerland.
The consultant has a decisive influence on the acceptance and implementation of shield
tunnelling.
Tendering phase. Mechanised tunnelling, which is tendered on the basis of a design
intended for shotcrete tunnelling, is normally more expensive (about 10 to 15 %). This
means that mechanised tunnelling often has no chance of winning the contract. When,
however, shield tunnelling is tendered in fair competition with a design that considers
shield tunnelling, it has good chances of competing with the shotcrete method.
As a conclusion, it should be stated that the comparison of construction methods should be
subject to competition and not the opinion of the consultant.
But the question of costs is not concluded at this stage. The phases of construction and
final payment also have to be considered.