Unlined Tunnel Research Articles

Plane strain analysis in modelling three-dimensional tunnel excavations

This technical paper aims to clarify the relations between the results predicted by plane strain modelling and the solutions given by 3-d analysis, for a circular tunnel driven in a homogeneous, isotropic, elasto-viscoplastic medium. It presents the results of a nonlinear axisymmetric finite element simulation of tunnelling in a rock mass, which investigates the stress and displacements behind an advancing tunnel face. All the results obtained from the 3-d modelling have been compared with the related 2-d model of similar mesh and material properties. The computational results disprove the widespread assumption that 2-D numerical analysis overpredicts tunnel convergence; such analysis can lead to a significant underestimation error, when a 2-d modelling of a tunnel excavation is used, especially in the prediction of ultimate unlined tunnel convergence. The authors suggest a simple approach for approximating the difference between the 2-d predictions and the more accurate 3-d predictions, although its function and parameters need to be investigated further; corrections of this sort can be obtained by understanding the mechanism of stress redistribution and the influence of the excavation sequence in three dimensions

Tunnel Plug Design at Tyee Lake

High head tunnel plugs must be designed to prevent plug failure as well as hydrofracturing of the surrounding rock. If the water pressure behind the plug exceeds the minimum rock stress, a fracture can develop in the rock which will propagate outward toward the hillside after which this fracture will become a leakage pathway. General criteria for positioning a plug to avoid rock splitting are discussed. The overall problem of sufficient cover for unlined tunnels in rock is thoroughly reviewed via case histories and various design approaches are described. Several reported plug failures are also discussed. Finally, experience with the Tyee Lake Project plug is presented. Operating under a water head of 1,338 ft (580 psi), this plug is 12 ft wide by 10 ft high by 60 ft long and has an 84-in. diameter penstock pipe passing through it. Mini-hydrofracturing tests were used to select the final plug location. Upon filling the tunnel, a water leakage rate of 31 gpm along the rock/concrete contact occurred. This was reduced to 11 gpm by re-grouting. Piezometer measurements taken along the plug perimeter showed a sharp drop in pore pressure over the first 10 ft of plug.

Operation of the underground diversion of the Borisoglebsk hydroelectric station

1. The results of observations and inspections of unlined tunnels prove that exfoliation of rock and falls of individual blocks should be regarded as an ordinary event accompanying tunnel operation. Rejection of the construction of bottom traps (wells, pockets) is impermissible. 2. Only extensive support of the roof and walls over the entire surface can completely protect tunnels from exfoliation of rocks. Under operating conditions this will require shutting down the hydrostation for a long time and considerable material expenditures. 3. Under established conditions it is more rational to periodically, for example, every 5 years, empty the tunnels for a short time for clearing the accumulations of rock material. 4. The installation of intercepting racks helps to reduce the amount of rock being carried by the flow to the units. 5. On the whole the operation of the tunnels of the Borisoglebsk hydrostation and their condition after clearing are considered completely satisfactory.

Underground design at Andekaleka Hydroelectric Development

Andekaleka Hydroelectric Development, Republic of Malagasy (Madagascar), comprises an 8 m high intake dam, a 4 km long unlined power tunnel, a surge shaft, a concrete-lined penstock, four steel-lined branch penstocks, an underground powerhouse, and a 0.5 km long tailrace tunnel. The scheme develops a 235 m fall in the Vohitra River. Two 29 MW Francis turbines have been installed, with space left for two more identical units.This paper describes the geotechnical aspects of the underground design for the scheme. The predominant rock type is good quality granitic gneiss, which required minimal underground support and lining. Where the power tunnel 'daylights' and crosses the Sahantsiva River, steel and concrete lining and drainage tunnels have been provided.The length and design of the steel and concrete lining and the system of underground drainage are described along with the design and support for the powerhouse cavern and the support of the powerhouse crane beams on rock shoulders. Key words: tunnels, penstocks, underground powerhouse, underground drainage, underground support, tunnel lining, Madagascar.

Rock Engineering: La Grande Complex, Quebec

Rock engineering has played an important role in the design and construction management of the La Grande Complex, a 13,75O MW hydroelectric development in the James Bay area of northern Quebec. Confidence in the strength and erosion resisting properties of the precambrian granitic and metamorphic rocks of the region has led to some innovative design solutions as well as a policy of flexible response to rock support requirements. Experience gained with unlined tunnels and channels for hydraulic purposes is discussed. The importance of closely controlled drilling and blasting procedures in some areas of the work such as the crane support ledges at LG2 and a diversion conduit excavation at LG4 is also noted. The success of the flexible, generally optimistic approach to rock excavation and support activities in the Complex is due in large measure to the good communication links existing among design, cost control and construction elements within the management group, the Sociėtė d'ėnergie de la Baie James.

Cambridge Geotechnical Centrifuge Operations

The Cambridge Geotechnical Centrifuge routinely tests models made of up to 0·2 m3 of soil, at accelerations up to 125g. Self-weight effects in models and in corresponding prototypes are similar within a few per cent. Seepage and diffusion of pore water into the plane section of an unlined tunnel serve to illustrate scaling relationships. The stress/strain behaviour of soil alters with change of effective pressure p′and specific volume v. A theory of plasticity explains cohsolidation and yielding in shear at states above critical, but failures below critical state involve rupture or fracture. The range of values of a new equivalent liquidity LI5 = LI+0·5 log (p′/5)associated with yielding is 1·9<LI5<1, with rupture is l<LI5<0·5, and with fracture is 0·5<LI5. One example of centrifuge modelling is for tunnels in soft ground near critical states (LI5 = 1), in which plastic collapse is observed closely to match upper and lower bound calculations of theory of plasticity. In contrast, another example of centrifuge modelling is of flow-sliding in ground further from critical states. In two famous cases—Mississippi River Bank flowslides and Champlain Sea quick clay flowslides—model tests suggest that at the moment of failure it is the sudden occurrence of fissures that destabilizes ground when its permeability greatly increases. La Centrifugeuse Géotechnique de Cambridge effective à des essais routine de modèles de sol, pouvant atteindre un volume de 0·2 m3 et des accélérations 125 g. Les effets du poids intrinsèque, dans le cas des modèles et des prototypes correspondants, ont été identiques à quelques pour cent près. L'infiltration et la diffusion de l'eau interstitielle dans la section plane d'un tunnel non enduit servent à illustrer les relations d'échelle. Le comportement contrainte/déformation du sol varie en fonction du changement de la pression effective p′ et du volume spécifique V. Une théorie de plasticité explique la consolidation et le fluage cisaillement à des états au-dessus du niveau critique mais, en dessous de ce niveau, il y a rupture ou fracture. Comme gamme de valeurs d'une nouvelle liquidité équivalente LI5 = LI+0·5 log (p′/S) associée au fluage, nous avons l·9<LI5<l; dans le cas de la rupture, nous avons 1<LI5<0·5 et dans le cas de fracture, 0·5 LI5. Un des exemples de modélisation en centrifugeuse concerne des tunnels en sol mou s'approchant des états critiques (LI5= 1), pour lesquels on peut observer que l'affrais-sement plastique correspond très sensiblement aux états limites inférieurs et supérieurs de la théorie de plasticité. Par contraste, un autre exemple de modélisation en centrifugeuse que l'on donne concerne les glissements par liquéfaction dans des terrains dont l'état n'est pas aussi proche de l'état critique. Dans deux cas bien connus, à savior les glissements par liquéfaction des Berges de la Rivière Mississippi et ceux de l'argile sensible de la Mer Champlain, les essais de modèles conduisent à penser qu'au moment de la rupture, c'est l'apparition brusque de fissures qui provoque la déstabilisation du terrain au moment où sa perméabilité s'accroît considérablement.

Head-Loss Measurements on Hydroelectric Conduits

Prototype head-loss measurements on tunnels and other water conduits are reported. The measurements were done on water-power conduits of the Hydro-Electric Commission of Tasmania, Australia. Friction factors are given for an unlined tunnel, a mechanically bored partly lined tunnel, and a fully lined tunnel. Steel and woodstave friction factors are included together with factors for a concrete-lined trapezoidal canal. The effect of slime growth on head losses is discussed.

Mapping of joints in Pre-cambrian granitic rock masses

The joints on the wall of a 665 m long unlined access tunnel to a cavern system were mapped along a horizontal scan line. Several outcrops close to the tunnel were mapped in the same manner.

Optimum shapes for unlined tunnels and cavities : Richards, R; Bjorkman, G S Engng Geol, V12, N2, July 1978, P171–179

Optimum shapes for unlined tunnels and cavities : Richards, R; Bjorkman, G S Engng Geol, V12, N2, July 1978, P171–179

Optimum shapes for unlined tunnels and cavities

The geometry of unlined tunnels and cavities which do not alter the mean stress (or first invariant) of an original biaxial stress field or isotropic stress field with a superimposed linear gradient is described. For the biaxial field either with or without internal pressure on the tunnel wall, such hole shapes (called harmonic holes) produce the absolute minimum stress concentration for any possible hole shape. Also, for an isotropic stress field with a superimposed linear gradient the hoop stresses at the tunnel wall are significantly less for the harmonic shape than for a circular hole of the same size.

Research studies into the behaviour of tunnels and tunnel linings in soft ground : ATKINSON, JH CAMBRIDGE UNIV., GB ORR, TLL CAMBRIDGE UNIV., GB POTTS, DM CAMBRIDGE UNIV., GB 5F DEPT. ENVIR. TRANSP. ROAD RES. LAB. SUPPL. REP., 176UC, 1975, 34P

The report covers work carried out in the Cambridge University engineering department under a Transport and Road Research Laboratory sponsored research contract to study the behaviour of tunnels and tunnel linings in soft ground. The work consists of model studies coupled with theoretical analyses of the deformations around, collapse of, and lining loads in, shallow tunnels in sands and clays. In the model tests soil displacements were measured by lead shot radiographic techniques and lining stresses and moments were found from strain gauges bonded to the inner and outer surfaces of thin aluminium linings. The theoretical analyses of stresses and deformations were based on finite element and associated fields methods using models for the soils obtained from a critical state approach to soil behaviour. The report discusses the scope and objectives of the research and presents and discusses in some detail the results of model tests on a lined and an unlined tunnel in overconsolidated clay. It also considers methods of estimating the stability of lined and unlined tunnels. (A) /TRRL/

Ouvertures Souterraines Soumises A De Fortes Pressions D’eau Ou D’air

A water pressure higher than 50 kp/cm2 and an air pressure higher than 40 kp/cm2 have been used in the unlined tunnel and closed surge chamber of a hydroelectric power plant in Precambrian rock. The rock conditions demanded are discussed. In particular the resistance to hydraulic fracturing, but also the leakage problem through the rock mass, is mentioned. Some practical advice for the design is put forward.

Transient Stresses Produced around Tunnels by Traveling Waves

The present paper is concerned with the transient states of stresses and deformations around the lined and unlined tunnels during the passage of traveling waves. In the analysis, the tensor code and the finite element method are used and three types of surrounding media are assumed, such as the linear elastic medium, and the elasto-plastic media with yield criteria of von Mises as well as of Drucker. The lining material is assumed linear elastic. The traveling waves discussed are the step-shaped and the bell-shaped.The following results were obtained;(1) The results obtained by the finite element method well coincide with those obtained by the tensor code analysis as well as those by the analytical procedure.(2) The transient maximum stress concentrations are generally lower those under the static loading of the same intensity as the traveling waves.(3) The transient stress concentration due the single half-sinusoidal wave is almost equal to the stress concentration of the steady-state due to the sinusoidal harmonic wave train, so long as the wave length is larger than approximately three times of the radius of circular tunnel. As the wave length decreases, the maximum stress concentration decreases.(4) For the lined tunnel, as the lining becomes stiffer, stresses become higher in the lining whereas lower in the surrounding medium. The rate of increase of the stress concentration factor is slightly less than the ratio of the Young's modulus of the lining to that of the medium.(5) The stiffer the lining is, the smaller the yielded zone becomes. However, the properties of the surrounding media have little effect on the stresses in the lining.(6) A wide yielded zone may be formed in the surrounding medium in the source side of the traveling wave due to the reflected tensile wave from the tunnel.

Mossyrock Project—River Diversion and Care of Fish

The preservation of the anadromous fish in the Cowlitz River was an essential feature of the Mossyrock Project. The final solution to this problem included the construction of a salmon and a trout hatchery downstream of the project. However, during construction of the dam and prior to completion of the hatcheries it was necessary to maintain the natural fish runs to spawning areas upstream of the damsite. This was accomplished by designing two gothic shaped unlined diversion tunnels such that they satisfied the requirements essential to the passage of fish as well as satisfied requirements for diversion of the Cowlitz River. The unlined tunnels were constructed with no difficulty and successfully passed three large floods. The fish runs were maintained during construction and are now being maintained with the aid of the two hatcheries downstream.

Closure to “Hydraulic Design of Unlined Rock Tunnels”

Closure to “Hydraulic Design of Unlined Rock Tunnels”

Closure to “Hydraulic Properties of Small Unlined Rock Tunnels”

Closure to “Hydraulic Properties of Small Unlined Rock Tunnels”

Gravitational stress field around a tunnel in soft ground: Reply

The finite element method has been used for determining the stress distribution and the displacements due to gravity around an unlined tunnel driven through a semi-infinite medium, characterized by three idealized material behaviors reflecting approximately a short term behavior of natural undisturbed insensitive and sensitive clays. The knowledge of stress and displacement fields around an unlined tunnel can be used for evaluating the need for supports according to the acceptability of expected deformations.

Discussion of “Hydraulic Design of Unlined Rock Tunnels”

Discussion of “Hydraulic Design of Unlined Rock Tunnels”

Discussion of “Hydraulic Properties of Small Unlined Rock Tunnels”

Discussion of “Hydraulic Properties of Small Unlined Rock Tunnels”

Discussion of “Hydraulic Design of Unlined Rock Tunnels”

Discussion of “Hydraulic Design of Unlined Rock Tunnels”