Cities all over the world are confronted with ever growing problems in keeping alive and functioning. This means that at an ever increasing speed the underground has got to be used where there are no good solutions anymore above ground for many vital elements. Subsurface construction used to be too expensive in many cases but now gives a much wider scope because of new developments in building techniques and because of a growing awareness of what should be put underground to preserve precious space above ground for people to use to its best advantage. Problems to be solved both in hardrock and in soft ground are often very big since the city has got to remain functioning and both construction as well as maintenance should be done as un-obstrusively as possible. Examples from Oslo Norway, and Amsterdam Netherlands will be given. HARDROCK, THE OSLO EXPERIENCE
The City of Oslo, surrounded by hills, is situated at the end of the Oslo fjord(Fig. 1). The built up areas of the central part of Oslo consist mainly of 4-5 storeyed buildings which were erected in the later half of the 19th century. The greater part of these buildings are founded on clay deposits. Newer buildings can be considerably higher, but are then founded on rock. As in most established cities planning and building of new highways, a public transport system, and transportation systems for sewage and storm water drainage confronts the city planners and engineers with many problems. Very often the only solution is subsurface constructions. This chapter will deal with the more important aspects of rock constructions under urban areas with reference to experience gained by various public works departments in the Municipality of Oslo. Especially in two fields much new practical knowledge has been obtained recently, i.e. in the use of full face tunnel boring machines (TBM) and in grouting techniques against ground water leakage.
1.2. Geological conditions.
The geological conditions in Oslo are known to be very complex(Fig. 2). The rocks originate from three different geological periods. The oldest are the pre-Cambrian gneisses (1800 million years old). We find these mainly in the eastern parts of town, but they also form the foundation for Akershus Castle which lies in the centre of the town. The cillnbro-silurian sedimentary rocks arc about 570 to 350 million years old. These rocks consist mainly of different kinds of shale and limestone. These appear in the western and central part of town. The youngest rocks are the permian rocks which are about 250 million years. During the permian age the area around Oslo underwent great changes including large volcanic activities. The rocks from this period consist of java, basalt, syenite and porphyrite. These igneous rocks are found in the northern part of the town and also in the form of sills and dykes penetrating the sedimentary rocks and even the pre-cambrian bed-rock. There are large differences in the compressive strength of the different types of rocks. Some examples will show this:
In connection with the geotechnical study of a proposed underground nuclear plant, a 300 m deep test hole was drilled at Ontario Hydro''s Darlington site approximately 65 km east of Toronto. The hydrofracturing method was used to determine the state of stress in the Paleozoic limestones (0-220 m depth) and the Precambrian granitic gneiss (220-300 m). The results of these measurements indicated a state of high horizontal stress along the depth of boring and a consistent orientation of principal stresses in each geologic unit. These results were confirmed by a borehole TV camera survey, and by overcoring tests at shallow depths in the same general area. The measured stresses were used in an assessment of the stability of large reactor caverns for housing the nuclear components of a proposed 4 x 850 MWe underground nuclear power plant in Precambrian hard rock. It was found that under ambient temperature conditions the state of stress alone did not critically affect cavern stability. Next, the occurrence of thermal shock in the cavern under a loss-of-coolant accident condition was also analyzed. To aid in the design analysis of the thermomechanical stability, a series of tests were carried out on the thermal properties of Precambrian rocks at elevated temperatures, including thermal conductivity, thermal diffusivity and coefficient of thermal expansion. The test results on thermal properties are presented in the paper, along with the results of the finite element analysis and other pertinent rock mechanics tests performed.
Underground siting of nuclear power plants has been receiving increased interest in various parts of the world owing to some potentially important benefits over equivalent surface installations. Reactors housed in deep rock provide an additional safety margin in containing the release of radionuclides, in reducing the damage due to earthquake in high seismic areas, in protecting against sabbotage, aircraft crashing, hurricanes, etc., in supplying high-pressure gravity-fed cooling water to the fuel core in emergency situations, and in minimizing the amount of land use and environmental impact (Oberth and Lee, 1979). These and other potential advantages are still being weighed in the United States against the major drawback of higher construction costs. Norway and Sweden, however, built their first underground power facilities some 20 years ago (McHugh, 1964; ENEA Report, 1962). In Canada, Ontario Hydro has recently embarked on an extensive investigation of the technical and economic implications of building a large 4 x 850 MWe underground CANDU (Canadian Deuterium Uranium) power station in Ontario. The location of the proposed power plant will have to satisfy several siting requirements including the proximity to a major load center to cut transmission costs, and the availability of an adjacent large body of cooling water. A third requirement, that of suitable host rock formation to ensure water tightness and cavern stability was the subject of a preliminary geotechnical investigation which included a geological surface study at the Darlington site, the drilling and coring of an NQ borehole (UN-l), and a number of borehole tests such as permeability and stress measurements.
Compressed Air Energy storage (CAES) in underground caverns and Underground Pumped Hydro Storage (UPHS) are systems which can be used for electrical power generation during peak demand periods. Key features for economical use of such systems involve the structural and leakage stabilities of the storage caverns over operational lifetimes of some 30 years. A goal of this paper is to establish a consistent set of stability and design criteria for caverns in hard rock, which are both practical to the designer and applicable to numerical modeling and failure probability assessment techniques. To formulate stability criteria, the phenomenological modes of potential instability are identified for the physical situations and "quantified" in terms of appropriate constitutive laws of material behavior.
Compressed Air Energy storage (CAES) in underground caverns and Underground Pumped Hydro Storage (UPHS) are two methods for storing energy that can be used for generation of electrical power during peak demand period. In the CAES concept, air is compressed and stored in underground caverns during off-peak demand periods, and withdrawn and used with surface turbine systems to generate electrical power during peak demand periods. In the UPHS concept, water is pumped from underground caverns to an upper reservoir during off-peak demand periods. During periods of peak demand, electrical energy is produced when the water in the. surface reservoir is permitted to flow down a penstock shaft, through a water-power turbine system, and into an underground reservoir. For these systems to be economical, the structural and leakage stabilities of the storage caverns must be maintained over an operational lifetime of some 30 years. The objectives of this paper are to examine stability and design criteria for CAES and UPHS caverns in hard rock in terms of performance criteria and acceptability limits, and to present some numerical modeling results which illustrate the mechanical and thermal responses of the caverns during construction and operation.
The underground layout of a compensated CAES system in hardrock consists of a collection of parallel storage caverns, each of which is connected at either end by inclined entry’s to common (air and water) crosscut tunnels. The crosscut (air) tunnels are connected to the surface compression/generation equipment by an air shaft and to a surface water reservoir by a water inlet/outlet shaft. Water from the surface reservoir provides a compensating head for maintenance of constant air pressure (with varying volume) in the storage caverns. This is designated a "wet" system because of the rise and fall of water in the caverns during the withdrawal (generation cycle) and injection (compression cycle) of compressed air. To prevent air ejection through the water shaft, the base of the shaft and any intermediate water storage caverns must be situated at greater depth than that of the air storage caverns. Because of turbine machinery limitations and pressure losses during the generation cycle, a maximum air pressure of about 7 to 7.5 MPa is required, dictating a facility depth of 715 to 765 m.
Komada, H. (Central Research Institute of Electric Power Industry) | Nakagawa, K. (Central Research Institute of Electric Power Industry) | Kitahara, Y. (Central Research Institute of Electric Power Industry) | Hayashi, M. (Central Research Institute of Electric Power Industry)
In unlined underground rock caverns, liquid petroleum and petroleum gas can be stored by making use of natural or artificial ground water pressure. In case of applying the unlined underground storage caverns to the cracky rock mass, it seems to be most important that the behaviour of the ground water through the rock mass surrounding the caverns is studied in advance. This study, therefore, discusses the following subjects concerning unlined underground oil storage caverns.
A cross-hole high-frequency acoustic investigation of a granitic rock mass subjected to sustained heating is reported. Compressional and shear-wave velocity measurements along four different paths between four vertical boreholes made prior to turning on the heater, during 398 days of heating and after the heater was turned off correlated well with the presence of fracture zones, in which the fractures were closed by thermal expansion of the rock upon heating. When the rock mass cooled, the velocity measurements indicated a greater intensity of fracturing than had existed prior to heating. Laboratory compressional and shear-wave velocity measurements have been made on intact rock specimens obtained from the site and subjected to axial stress. When used to interpret the increases in velocities measured in the field upon heating the rock mass, increases in horizontal normal stresses to between 30 and 40 MPa were inferred. Increases of these magnitudes agree with stress measurements made by the other techniques. The ratio of measured compressional to shear-wave velocity appears to provide a sensitive measure of the fraction of crack porosity containing water or gas.
One of the more promising methods developed in the past few years for geotechnical site investigation and the characterization of rock masses is the higher-frequency acoustic wave technique. The high frequencies employed permit detection of discontinuities and the outlining of zones having different physical properties between boreholes or behind surface boundaries in much more detail than the conventional low-frequency seismic methods.
Price, Malone and Knill (1970), McCann, Grainger and McCann (1975) and Auld (1977) describe the use of acoustic measurements between boreholes for geotechnical purposes. Price and colleagues employed the results of their study to determine the optimum rock-bolt pattern to stabilize a rock mass. McCann and colleagues used the between-hole technique to delineate interfaces between homogeneous media, to detect localized, irregular features and to estimate the degree of fracturing in the rock mass. Auld used between-hole acoustic measurements to determine the elastic properties of the rock mass.
Acoustic techniques employed within a borehole have been described by Geyer and Myung (1971), Myung and Baltosser (1972) and by King and colleagues (1975, 1978). The application of acoustic borehole logs in detecting fractures, for rock classification and in determining the in situ elastic properties of rock have been discussed by these workers and by Carroll (1966, 1969) and Coon and Merritt (1970).
In this paper are described the results of a research project involving cross-hole acoustic measurements in a fractured granite rock mass subjected to thermal stresses. The acoustic research project is itself part of a comprehensive rock mechanics and geophysics research programme associated with large-scale heater tests in an abandoned iron-ore mine in centralSweden, as described by Witherspoon and colleagues (1979).
This paper relates Kansas City''s experience in developing underground space. It focuses on the technical, legal and psychological aspects. Kansas City, Missouri, in the heart of the United States, is setting an exciting pace for the development and use of underground space. Their activities are being monitored throughout the world by those who hope to either emulate or modify and improve upon this experience. Kansas City is serving as a proving ground for underground development. Fortunately, Kansas City has a very extensive ledge of limestone located near the surface, extending as far as fifty miles in each direction. This ledge, known as Bethany Falls, outcrops in the river bottoms and is generally accessible from surface roads and highways. The quality is uniform and high in calcium carbonate making it ideal for a number of uses including agricultural lime, cement production, mineral filler, concrete and asphalt aggregate, plus many others. Mining is accomplished in a conventional manner on a room and pillar basis, leaving twenty-five foot square pillars on sixty-five foot centers .. This results in a regular geometric pillar pattern yielding approximately 85% of the underground area as open and usable which virtually doubles the size of the surface area of the property. It is as simple as driving back into the hillside from surface streets to gain access to the underground area, thus not having to change depth to go underground. Overburden averages one hundred forty feet above the mined out area. One of the main attributes of underground m1n1ng in Kansas City is that its initial purpose was to serve the aggregate rock needs of a growing industry of surface development. Great Midwest Corporation, for instance, owns 2,000 acres and has the mining rights below all of it. During the years that this corridor was being developed, they served surface construction with concrete and asphalt aggregate rock. Now that the corridor has grown to become one of the most active industrial districts in the metropolitan area, their surface property is relatively unused and is available for surface development. By mining 85% of the area from under the surface, they have nearly doubled the developable area. Instead of 2,000 surface acres, they have a combined potential of 3,500 acres that can ultimately be used. Obviously, the mineral extraction cannot all take place in an area this extensive during the few years that the area growth is taking place. Great Midwest Corporation mines about a million square feet annually which gives an adequate inventory of underground space to meet market demands for reuse for development purposes. To prepare the subsurface for mining and subsequent reuse, the hillside is excavated to expose the bluff. The mining simply extends directly into the bluff. Because the most unstable portion of the mine is generally the forward rooms closest to the bluff, experience shows that it is advisable to leave a solid rock mass of two hundred to three hundred feet thick to help eliminate structural problems.
The following perspective was presented for discussion at Rockstore ''80. It is intended to address, in very general terms, the following three questions:
1. Upon what experience can we base geological investigations for subsurface nuclear waste isolation facilities?
2. What are the additional requirements to be met, for which our previous experience must be extended?
3. What approaches can be adopted to meet these new requirements?
Geological characterisations provide input to all the subsequent phases of repository development, including performance assessment. Consequently, the quality and accuracy of safety assessments can directly reflect the quality and accuracy of geological characterisations. Before presenting this perspective, it is appropriate to explain the meaning of some terms to be used in this discussion. The word "system" is not used in the traditional geological sense, but in the more universal sense to denote a body of interacting and interdependent parts. The disposal medium is viewed as a part of the system to be employed, not to be treated in isolation from the geological system of which it is only a part. An additional term, characterisation, is used in the recognition that we cannot expect to define all the parts of a system as complex as the geological system within which disposal of waste is to be carried out. The overall objective must be to gain a knowledge of those processes pertinent to containment within the system which is sufficient to rationalise the potential for waste/system interaction.
AN HISTORICAL PERSPECTIVE
A new class of industrial development has appeared within the last 30 to 40 years. This class of engineered structures can be identified on the basis of the consequences of operational failure. For these structures, such failure is apparently of potentially far greater severity, in terms of public health, than is the case for other industrial development. In recognition of this fact, a far greater level of effort and care in siting, design, and construction is warranted. Examples of this class of structures are nuclear power plants and such chemical facilities as LNG plants. Commonly, the design lives of such facilities are approximately 40 years. Regulation of such development has been established at a national level.
The relevant national and other regulatory bodies have sought a much more stringent degree of performance assessment than previously known. Because of the potential impact which such a facility can have in terms of escape of hazardous material in the event of operational failure, a knowledge of the natural environment of the facility is required. The process of safety assessment is thus dependent upon a wide range of scientific disciplines, not simply upon that which governs the industrial process of concern, such as chemical or nuclear engineering. Yet it would be unreasonable to assume that man has an equivalent level of understanding of each technical discipline of relevance. To assume that the levels of knowledge of nuclear physics and of geologic processes were similarly advanced at the onset of nuclear power development, for example, would be absurd.
The problem of energy storage is of central importance for and improved energy economy. Long term heat storage requires very low construction costs. Many underground storage alternatives can meet these requirements. In the big scale, hot water accumulation in rock chambers and in-ground hot water pools are competitive solutions. Direct storage of heat in soil masses is another alternative. The paper describes some new techniques for underground thermal storage.
The consumption of energy is closely related to external conditions like the daily activity pattern of the society and the weather fluctuations. When thermal energy for heating is produced by burning of fuels, then the production can be readily adjusted to the actual consumption level. The fuel itself is a good energy storage. In countries with cold climate, the energy required for heating of buildings is very large. In Sweden, for example, about half of the total energy consumptions used for heating. 90 % of this thermal production is based on oil. Measures for reducting the dependency on oil are discussed in many countries. Where the oil can be substituted with other fuels, like coal or biomasses, there will be no additional need for thermal storage in the system. However, other production methods for thermal energy also utilized or will be used in the near future. Examples of such methods are solar energy, industrial thermal waste and combined power and thermal production. Here the production and consumption patterns are very different on every time scale (daily, weekly, yearly) and thus thermal energy storage is required. Characteristics of thermal storages for heating purposes are low energy density (temperatures usually below 100º C) and low degree of utilization (once on a few times a year). Consequently the installation costs for these storages must be kept very low in order to make them economically competitive. The addition of a thermal storage to a combined power and heat production plant will enable the power production to be adjusted accord.ing to the consumption pattern of electrical energy. It will then not be necessary to waste any heat during peak power generation periods, just because there is not a concurrent thermal load in the system. In pure thermal, production units (central heating stations) a thermal storage has the benefit of reducing the required peak capacity of the heater (as well as of the redundant heater system). Maybe the most important advantage created by thermal, storages is that they permit the utilization of such thermal excess energy from the industry, that otherwise would have been wasted, and of thermal energy from solar collectors. This energy can only be produced during the warm season of the year, but will be consumed during the cold season.
THE TWO SCALES OF THERMAL STORAGE
Figure 1 shows the energy consumption for heating of buildings in a cold climate (Stockholm) during a year. This curve has variations on (at least) a daily, a weekly and a seasonal basis.
This paper presents the results of a study for the design of a repository capable of storing 400,000 200 litre drums of both low-and medium-level waste in solidified form. It is shown that in a depth of a few hundred meters, tunnels, caverns, and shafts are the favoured geometrical forms for the disposal of this category of waste.
A concept for the overall installations required for the repository including the aboveground operations building, the main shafts and tunnels and the underground plant is described. Consideration is given to the various techniques for the isolation, backfilling and sealing of the cavities together with the handling, safety and radiation protection aspects.
The entire cost of the repository has been estimated to be in the region of 450 - 500 Mio Swiss francs.
In Switzerland, the National cooperative for the Storage of Radioactive Waste (NAGRA) is responsible for the storage and disposal of all types of nuclear wastes. The general strategy for the waste management and the disposal concepts have been presented in the report "Konzept fuer die nukleare Entsorgung in der Schweiz" in 1978. By 1985, NAGRA has to develop proposals for sate and final disposal. The proposed storage scheme consists of 3 types of repositories corresponding to 3 categories of solidified waste according to their origin, activity and toxicity:
The unprecedented time span of concern and consequent need to predict future geological evolution, coupled with the potential consequences of failure, dictate that geological characterizations prepared for nuclear waste repository evaluation be immensely complex. The complexity of such investigations can, however, be used to optimize the knowledge achieved by a synergistic interaction between the individual geological disciplines. Additionally, we view the use of "back analysis" of geological structures as an essential tool of the characterisation process, for example for the assessment of constitutive relationships applicable during long periods of time.
Considering a sedimentary basin as an example of a geologic system to be characterised, the role of the stratigraphy/lithology and structural geology/tectonics disciplines are reviewed. Comments are included on the role of vertical crustal movements in intraplate environments, as a directly measurable and therefore readily appreciable facet of the evolution of geologic systems. The significance of vertical crustal movement upon strain energy changes is noted. The roles of state of stress and geomorphology/palaeoclimate are introduced and used to illustrate the use of interaction between disciplines to optimize the geological characterisation.
Emplacement in a geological system is a favored solution to the alternative proposals for isolation of high level nuclear waste. Selection of a suitable site, repository design and safety assessment depend upon a characterisation of the geological system within which the waste is to be stored. It must be assumed that the quality and relevance of geological characterisations prepared for repository development will therefore be directly reflected in the subsequent performance of the repository. In general, all activities involved in the isolation of waste in the earth''s crust depend upon the geological characterizations prepared for this purpose.
Because of the potential consequences of failure of a waste isolation facility, it is assumed that the investment in geological exploration will be substantial and thus the evaluation of geological conditions will be detailed. This in itself means that such investigations will be complex. Additionally, however, the comprehension of the geological system which must be achieved by such investigations is unprecedented. The lack of precedence arises largely as a result of the vastly increased time scale of concern. Previous investigations for potentially hazardous facilities have been related to engineering lives of generally less than 102 years; the nature of the nuclear waste to be isolated is such that time spans of the order of 106 years are of concern. Thus, the geologist is rather abruptly faced with the change of perspective determined by a time scale which is increased by approximately 104 years. This means that the geologist must consider evolution of the geologic system during the life of the waste repository. Other reasons which influence the level of the characterisation to be achieved, and the difficulties in achieving such, include the need to minimize disturbance of the rock mass, to account for groundwater flow at a regional scale and to account for thermomechanical effects. However, it is the time scale of concern and the consequence of failure which primarily determine the complexity of the geologic characterisation, and the need to optimize investigation.