Special features in tendering, award and contract - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 338

- Verification of serviceability.
- Cost-effectiveness including considerations of cost relevance.
- Checking compliance with the intended use.
- Quality requirements for the quality-relevant work categories.
- Control plan structure-ground with the geotechnical monitoring concept.
- Working instructions for the contractor: the contractor returns an acknowledgement
that the aims have been met with the correct implementation of the quality require-
ments. These working instructions should include at least:
- A definition of the construction works,
- Organisation,
- Technical basis,
- Working procedures,
- Testing and checking plan,
- Traceability,
- Accident prevention,
- Environmental protection.
Assignment of risks in the contract. The risks that are inherent in any construction pro-
ject should be borne by one or other of the contract partners alone. The often vaunted
performance specification tender often leads to a transfer of all risks to the contractor.
The Swiss standard SIA 198 “Underground Structures” [218] includes in Appendix 5 a
possible assignment of risks in tunnelling, mostly resulting from ground conditions.
For TBM tunnelling, this results in the following recommendation:
Scope of risk of client
- Presence of gas.
- Collapse due to geological conditions.
- Larger convergences than were assumed in the contract, leading to:
- Jamming of the tunnelling machine,
- Already laid invert segment being laid deeper,
- Enlargement of already bored tunnel sections,
- Rebuilding of the machine to a larger diameter,
- Alteration of the formwork for a smaller diameter.
- Rock quality encountered significantly outside the limits given in the tender documents:
- Cuttability significantly less favourable,
- Bracing of the machine proves impossible or has to be packed due to collapses,
- The invert cannot provide the necessary bearing pressure and the TBM cannot be
supported at the correct level,
- Rock is so fractured that it corresponds to soil.
Scope of risk of contractor
- Change of rock properties inside the agreed limits.
- Tunnelling through variably hard rock layers.
- Stoppages in the tunnelling system due to stickiness of the excavated ground.
- Qualification of the employees.