Shaking up tunnel design3 September 2003
During Waterpower XIII, the construction and maintenance of tunnels was the subject of a number of technical papers, as Carrieann Davies reports
LOCATED on the shores of Lake Erie and only a few miles from the wonder that is Niagara Falls, New York's city of Buffalo seemed an ideal place to hold a conference on hydro power. This was a view no doubt shared by many of the delegates who attended the recent Waterpower XIII conference in the city.
Held from 29-31 July 2003, Waterpower XIII aimed to offer delegates the opportunity to examine new ideas, technology and approaches to enhance the role and contribution of hydro power. After attending a number of the technical sessions, I felt the conference seemed to do just that.
One area I found particularly informative were the technical presentations which focused on the issues of constructing and maintaining tunnels. An interesting paper came from Matthew Gass, of Hetch Hetchy Water and Power, the municipal power and water supplier for San Francisco, US.
In his paper, entitled Friction reduction of unlined power tunnels for increased power and energy production, Gass shared first hand experience of a power tunnel project where the frictional losses of the tunnel were decreased using a hammer mounted on a small excavator to remove rock projections from the tunnel flow stream.
He explained how the bulk of the Hetch Hetchy's power comes from two separate water and power systems. One of the power plants in the system is supplied by the Cherry and Eleanor reservoirs, which jointly provide the water for the downstream Holm power house. The two reservoirs have an annual runoff of approximately 403Mm3. The power tunnel for Holm power house starts at the base of the dam at Cherry reservoir and extends 8969m to the start of the penstock. Gass explained that approximately 90% of the tunnel is unlined, horseshoe shaped, and was constructed using conventional drill and blast methods. A 2074m long penstock transports the water from the end of the power tunnel to the power house. This water then feeds two six jet pelton type turbine generators with an upgraded maximum combined capacity of 170MW.
Originally, the maximum total output of the plant was to be 135MW with a flow of 22.9m3/sec. Gass explained how economic studies completed in the early 1950s had water conveyance and plant components sized for the relatively low capacity factor of the plant and maximum expected flow rates. Since the completion of the project in 1959, many factors affecting the capacity and loading of the plant have changed. During the original study for the sizing of the power tunnel, the value of additional energy was only US$3/MWh and additional power valued at US$17/kW per year. The final design of the unlined tunnel sections resulted in a 12 inches diameter horseshoe shaped cross-section with a minimum cross section of 12.3m2.
The design and construction was relatively straightforward for the time and geology of the region, said Gass. Both ends of the tunnel are concrete lined due to the lack of ground cover for the expected pressures. Before the downstream lined section of the tunnel the design included a rock and sand trap, 158.5m long with the tunnel enlarged to a 6m diameter horseshoe shape in the trap section. At the current higher flow rate of 28.3m3 it is estimated that the unlined sections of the tunnel produce 1.6m of head loss per 304.8m.
In 2000, Gass reported, the Cherry Power tunnel was dewatered to remove and clean the full rock and sand trap in the tunnel. The work was done using special two-yard diesel powered muck loaders Wagner Model ST-2D Diesel Scooptrams. The rock trap section of the tunnel was wide enough for this style vehicle to pass but not to turn around. As many as four tunnel muckers would work at one time, dumping their spoils as a group.
To increase their production, governors on the primary drive engines were adjusted to allow higher vehicle speed on the trip into and out of the tunnel. The tunnel outage was planned for a minimum of 45 days. This relatively high rate of material removal presented an opportunity for additional work to be done in the tunnel to reduce frictional losses.
For the project, the contractor was directed to perform some trial work in the tunnel upstream of the rock trap. A hydro ram attachment was added to the arm of a compact excavator. The excavator used an IHI-30 JX Compact Excavator with a 33.3 Hp Izuzu engine was a relatively small unit that would fit through the smaller lined sections of tunnel. The machine operators were directed to break any rock protrusions that visually extended from the tunnel walls into the tunnel flow stream.
The goal of this work was to get data on production rates of rock removal and estimates on improved friction factors from smoothing the tunnel wetted surfaces and increased tunnel flow cross sections. From this operating experience it was determined that the sharper the rock protrusion away from the base tunnel wall and floor, the easier it became to break off the material.
These rock protrusions also had the greatest negative affect on the overall tunnel friction factor. Gass said it was also is important to note that true surface contours of the tunnel walls have a much greater effect on the frictional losses then a small resultant increase in tunnel cross-sectional area due to material removal.
It was calculated that if the existing tunnel could be increased in size from 12.3m2 to 13.3m2 a head loss reduction of 0.5 to 0.6m/ 304.8m would be seen at the 28.3m3 plant flow. The author explained that it was estimated that an additional 290MWh/yr would be produced at the downstream power plant for every 0.3m of head reduction at full load.
The power plant was tested for maximum output before and after the cleaning of the rock trap and additional tunnel work. With a three-foot lower forebay level, and no modifications made to the turbines or generators, the maximum combined plant output increased by 1.82MW. It was estimated that the rock trap cleaning resulted in a 0.9m reduction in losses and the extra tunnel work created an additional 4.3m head loss reduction at full load.
Another interesting paper came from Richard Humphries, James Daly and Wayne Warburton of Golder Associates. In their paper Seismic design of tunnels and rock slopes at hydroelectric projects, they explained that although guidelines have been developed for the design and analysis of dams, no equivalent guidelines are available for tunnels and rock slopes. The authors claim that, as the layout of each hydroelectric project is unique, it is necessary for the designers and owners of these projects to develop performance requirements for every underground excavation and slope. Failure and deformation are important factors that need to be evaluated when selecting the design earthquake for tunnels and rock slopes at these projects.
The authors presented a discussion of the current methods of analysis and made recommendations for the application of these methods and for establishing design criteria for the analysis and design of tunnels and rock slopes.
For projects where the seismicity is low, or even moderate, the static case usually governs the design of tunnels and slopes, and there is little need for close examination of the earthquake case, say the authors. For hydroelectric projects in areas of high seismicity, it is not usually cost-effective to use a single design earthquake for all structures. The consequences of failure of each structure vary from catastrophic if the dam fails to minor if there is cracking in a tunnel lining. They claim an evaluation of the consequences of damage should be coupled with the selection of the design earthquake/return period for each structure.
The paper explains that methods usually used for geotechnical seismic stability analyses for underground works and rock slopes are pseudo-static analysis and dynamic deformation analysis. The pseudo-static method a limit equilibrium method resulting in a calculated factor of safety of a potential failure mass is the most common method used for the seismic analysis of rock slopes and tunnels. In pseudo-static analysis, earthquake loading is assumed to act as constant horizontal and/or vertical forces, which are proportional to the weight of the rock mass. Dynamic deformation analysis builds on pseudo-static analysis by calculating cumulative deformations during an earthquake.
Tunnels are usually considered to be the most earthquake resistant structures at a hydro project and there have been few instances of significant earthquake damage to power tunnels or tailrace tunnels, claim the authors. However, there are many issues to consider in the seismic design for water conveyance tunnels. Power tunnels and tailrace tunnels are often long, linear features, which traverse several rock types and conditions, and may cross a number of major geologic structures.
There have been few cases of damage to water tunnels where the tunnel lining is in immediate contact with the rock. Several tunnels have however recorded damage where there is a gap between the lining and the rock. At these locations, the free-standing lining often cracks or is damaged when it is shaken or when it hits the tunnel rock walls during shaking.
Other points to consider, said the authors, include the increased potential for rockfalls due to seismic shaking in unlined tunnels than in concrete lined tunnels, and that localised damage may be expected where a tunnel is crossed by a fault that ruptures during an earthquake. A number of designs have been used to attempt to accommodate fault displacement, including the use of a flexible lining system, flexible pipe joints, and enlarged tunnel sections.
The authors point out however that the most frequent cause of seismic damage is from earthquake induced landslides at tunnel portals and at shallow tunnels.
In a paper entitled Analysis of water leakages at Lower Kihansi hydro power plant system in Tanzania, East Africa, Leonard B Kassana and Bjørn Nilsen presented research work on water leakage problems, together with brief descriptions of the regional geological setting, geomorphology of the study area and engineering geological conditions of the headrace tunnel system at the Lower Kihansi hydro site.
Located on the Kihansi river, a tributary to Kilombero river, the system includes a 25m high concrete gravity dam, which impounds a small reservoir with a total storage of 1.6Mm3 , of which 1Mm3 can be used for daily regulation. The dam is equipped with sediment flushing gates and an intake structure with trash-racks and gates. The intake connects to the headrace tunnel via a circular unlined 500m vertical headrace shaft.
The hydro-geological conditions of Lower Kihansi are extremely anisotropic with high leakages through sub-vertical east-west orientated discontinuities and minor leakages through North-South orientated discontinuities.
The authors explained that during monitoring it was discovered that there are two main factors influencing the rock engineering conditions of the Lower Kihansi headrace tunnel/shaft system area tropical weathering and faulting/jointing. These factors significantly affect the rock-mass permeability.
Weathering Grades IV and V of rock are always more highly permeable than grades I, II & III explain the authors. At Lower Kihansi, weathering activity can be found as deep as 500m below the ground level. This is mainly facilitated by the existence of sub-vertical joints in the area in which water percolates through. The fault rifting in the area has caused considerable fracturing of the various rocks. Fracture frequency is more significant at shallow levels near the surface than at deep levels of the rock mass, said the authors.
The paper recognises that a review on major historical cases around the world has shown that the most common causes of major leakages/failures during filling or subsequent operations in hydro power headrace tunnels/shafts are due to, but not limited to, inadequate rock cover, erosion of fissure fillings, deformable rock mass, or a specific geologic feature with high hydraulic conductivity. To some extent, uncontrolled filling process may be detrimental to the tunnel leakage and stability.
The papers mentioned above were just a few of those that dealt with the design of tunnels and dams. The issues mentioned illustrate that design is at the forefront of many an engineers mind and new developments that increase the safety and stability of these structures are likely to be the result.