Ezekiel's Water Project Tunnel Feasibility Estimation

This estimate of the cost and construction method for the Ezekiel's Water Project tunnel was prepared by a leading senior tunnel design engineer. This estimate agrees with two other estimates, that the tunnel can be constructed within five years at a cost of one to one and one half billion US dollars.

EZEKIELíS WATER PROJECT
TUNNELLING DESIGN AND CONSTRUCTION METHODOLOGIES

COMMENTARY

DESIGN INPUT DATA

Length
72km approximately

Diameter
(internal)10m

Lining system
Pre-cast concrete segmental, smoothbore

Strata
Rock of varying hardness

Gradient
1 in 1000

Internal pressure
1 bar at western end and 7.2 bar (max) at eastern end

Inlet structure
Open excavation inside cofferdam
Facility to control inlet flow volume and velocity

Intermediate shafts
Circular, either segmentally lined or diaphragm wall
Headwork structures to house tunnel access equipment
Diesel powered electric generation
Provision for man and vehicle access to tunnel

Outlet structure
Open excavation inside cofferdam
Facility to control outlet flow volume and velocity

DESIGN METHODOLOGY

A comprehensive site/ground investigation (SI) will need to be commissioned along the route of the tunnel. The design and commissioning of such will he focussed on tunnelling, and will be designed, carried out and supervised by a company experienced in tunnelling. The design of the SI will be critical to itís value as the information gleaned from the terrain and rock strata will be relied upon by the tunnelling contractor.

The inlet and outlet structures would be designed as reinforced concrete structures capable of handling the anticipated flows and designed to minimise inlet and outlet head losses. They would probably be constructed within sheet piled cofferdams.

The shafts would be sunk at intervals along the tunnel. Spacing of the shafts could be as close as 500m or as far apart as 5km. There are many factors involved in the decision, too many to cover here. The shafts would either be sunk as PC concrete segmentally lined or as diaphragm walled shafts. The final decision will probably depend upon the local availability of expertise in one or other field. Headwork structures would be reasonably simple in design and construction and would house access and maintenance equipment and a power generator.

The tunnel would most likely be driven by several tunnel boring machines TBMs in an effort to control the duration of the contract. The TBMs would be equipped to built a segmental lining in the tunnel, although consideration might be given, in good rock conditions to casting an insitu concrete lining.

DESIGN FEES

It is anticipated that the design fee would be approximately US$8M and the fee for site supervision of the construction work would be approximately US$3M.

  • Feasibility study of alternatives including data searches
  • Design of site/ground investigation
  • Carrying out of site/ground investigation, analysis and testing
  • Preparation of factual & interpretative geotechnical reports
  • Liaison with all interested parties
  • Preliminary design for approval
  • Detailed design for construction (drawings and documentation)
  • Preparation of Operation & Maintenance Manuals
  • Preparation of tender and contract documentation
  • Advice and assistance in the procurement process
  • Assessment of tender returns from contractors
  • Construction supervision and design support during construction
  • Preparation of as-built drawings and documentation

The contract for the site investigation would ideally be let well in advance of that for the construction contract and would cost approximately US$2M.

CONSTRUCTION CONSIDERATIONS

In deciding the best methods of tunnel excavation and construction consideration needs to be given to:

  • Financing and specific requirements of stakeholders
  • Maximum allowable out-turn cost
  • Allowable construction programme
  • Geology and itís implications on programme and cost
  • Alignment constraints, above and below ground
  • Operational requirements of the finished works
  • Availability of expertise, equipment and labour
  • The form of design/construction contract and the apportionment of risk
  • Geo-political environment

CONSTRUCTION COSTS

We estimate that driving the tunnel using several TBMs and lining it with a pre-cast concrete segmental lining will cost between US$925M and US$1,450M at 2003 prices. This is what it will cost ĎThe Clientí and includes for the Contractors recovery of overheads and profit. In global terms this equates to US$12,800 per metre and US$20,100 per metre of tunnel. This would include for the following.

  • Mobilisation and demobilisation of plant, equipment and labour
  • Excavation and construction of the 10m ID tunnel
  • Excavation and construction of the basic inlet and outlet structures
  • Excavation and construction of the basic intermediate shafts
  • Construction of the headwork structures at each intermediate shaft

So much depends upon the type of TBMs required, the access requirements and the mode of working. It isnít really possible to narrow this down further. No allowance has been made for electrical and mechanical (E&M) works, although this is thought to amount to an additional 5-10% only.

CONSTRUCTION RISKS

Inside the construction industry tunnelling is believed to be the riskiest business. Risks affecting programme and cost can at best be managed, not eradicated. Most of the risks that affect programme and cost are associated with unforeseen ground conditions and their affect on tunnelling progress. Notwithstanding this, health & safety during construction should also be of concern.

Risks associated with unforeseen ground conditions include but arenít limited to:

  • Excessive unforeseen groundwater inflows
  • Excessive unforeseen soft ground inside what was foreseen as a rock drive
  • Excessive unforeseen rock inside what was foreseen as a soft ground drive
  • Unforeseen increases in rock hardness
  • Unforeseen increases in abrasivity
  • Unforeseen increases in instu rock stresses
  • Unforeseen problems removing excavated material from the tunnel

Each of these Ďpossibleí events has the potential to affect TBM mechanical reliability. Progress of the TBM can be slowed; wear on the TBM bearing and cutter-head can be excessive; and construction costs can rise exponentially.

We have made no attempt to either list or quantify the geo-political or economic risks that might be applicable to such a construction project. To do so effectively, would be a time consuming process.

QUALIFICATION

Please bear in mind that the above commentary has been pulled together with the best intentions of providing helpful advice. We have not had access to any factual or interpretative plans, drawings or geotechnical information, nor have we had access to any performance or materials specifications.

We will not underwrite the accuracy of the technical or financial advice. It has been pulled together in a very short time frame with very little knowledge of the proposed tunnel route, geology, geo-political environment, funding and/or risks. Every effort has been made to provide accurate information, in a very short timescale.