|
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.
EZEKIEL’S 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 it’s 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 it’s 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 ‘The Client’ 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 isn’t 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 aren’t 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 ‘possible’ 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.
Next: Desalination Estimate
|