SYNOPSIS: Prior to the introduction of longwall mining to the Greta Seam in New South Wales, Australia, a comprehensive rock mechanics testing programme was carried out to assess the suitability of the longwall method for the particular seam, strata and geological conditions present. The information obtained was used to provide design guidelines for equipment supply and mine layout. The investigations commenced with the collection of basic data including the mechanical properties of the strata and the virgin stress levels. Various modelling techniques (finite element analysis, displacement, discontinuity analysis and physical modelling) were used to provide information on roadway stability, strata caving characteristics, gate-pillar dimensions and longwall support density requirements. RESUME: Avant d'introduire la technique "longwall" au gisement de Greta dans l'etat de Nouvelle Galles du Sud (Australie) on a effectue un important programme d'etudes de mecanique des roches afin de savoir si cette technique y convient, compte tenu de sa geologie et de sa stratification. De plus, les donnees obtenues ont fourni des elements pour le plan de la mine et pour le choix des equipements. On a commence par rassembler des donnees de base telles que les proprietes mecaniques des couches et le niveau des contraintes avant travaux. On s'est servi de plusieurs techniques de traitement (analyse des elements finis, analyse de la discontinuite du deplacement, modèles physiques) pour obtenir des indications sur la stabilite des voies de passage, sur les caracteristiques d'effondrement ou d'eboulement des couches, sur les dimensions entre piliers et sur les piliers de soutènement necessaires au "longwall". ZUSAMMENFASSUNG: Vor Einfuehrung des Strebbaues in das Greta Flöz in Neu-Suedwales, Australien, wurde ein umfassendes Untersuchungsprogramm ausgefuehrt, um die Eignung der Strebbaumethode fuer dieses Flöz, seine Hangend- und Liegendschichten und die geologischen Verhaltnisse zu erfassen. Diese Untersuchung sollte ebenfalls Richtlinien fuer die Gerateauswahl und die Ausrichtung der Grube liefern. Die Untersuchungen begannen mit der Sammlung von grundlegenden Daten bezueglich der gesteinsmechanischen Kennwerte und der urspuenglichen Spannungszustande. Verschiedene gesteinsmechanische Untersuchungsverfahren (Finite Elemente Analyse, Störungs- und Kluftanalysen und Modellversuche) wurden dann angewandt, um Kenntnisse ueber Streckensicherheit, Bruchbauwerte, Strecken-Pfeiler Dimensionen und die Ausbaudichte fuer die Strebbaue zu gewinnen. INTRODUCTION Ellalong Colliery is a new mine being developed to extract Greta Seam coal at depths of cover from 350 to 650m in the South Maitland Coalfield of New South Wales. This Coalfield is located 150 km north of Sydney. The mine is being developed as an extension of Pelton Colliery, which has been mining the shallower Greta Seam coal (up to 350m) since, 1916. Difficulties experienced mining the deeper sections of Pelton indicated that mining by the bord (room) and pillar-system or its derivatives would not be viable at the depths present in Ellalong. Preliminary feasability studies pointed towards the longwall method of extraction. The longwall method had not previously been used in the Greta Seam. This, coupled with the fact that all collieries which had attempted to mine the Greta Seam at depths greater than 350m had closed for a combination of-economic and strata control reasons led to the commencement of an extensive rock mechanics study of mining the Greta Seam at depth in Ellalong. Each of these aims has been achieved to some extent. The mine has been developed in readiness for longwall extraction and a set of longwall equipment has been designed and is on order at the time of writing. When this equipment, becomes operational it is intended to monitor its performance in order to assess the reliability of the predictions. The investigations have been carried out as a co-operative effort by workers from the Australian Coal Industry Research Laboratories (A.C.I.R.L.), Commonwealth Scientific and Industrial Research Organization: Division of Applied Geomechanics (C.S.I.R.O.), the Geology Department of Newcastle University and the Coal Division of Peko-Wallsend Ltd. COLLECTION OF BASIC DATA Since little rock mechanics data was available on the Greta Seam and its adjacent strata the investigations began with the collection of basic data, as discussed below. Sedimentology The sedimentology of the Ellalong area was deduced by examining all available information from surface mapping, adjacent mines, borecores and from underground mapping as mining proceeded. Figure 1 shows typical sections of the Greta Seam and adjacent strata. The floor of the Seam is generally a thin mudstone unit. The Seam varies from 3 to 4.5m thick and may contain up to four distinct bands. The immediate roof is generally a coarse to pebbly sandstone, overlain by various bedded sandstone, conglomerate and laminite strata up to the 400mm thick Pelton Seam The Pelton Seam is from 5 to 15m above the Greta Seam and is overlain by several hundred metres of sandstones and siltstones. In some areas palaeochannels protrude into the Greta Seam from the roof strata. The palaeodrainage system is interpreted as flowing from northwest to southeast in the area of the Ellalong Lease (Rawlings, 1982).

ABSTRACT The paper presents a stability investigation of shaft intersections and pit bottom roadways in the vicinity of upcast and downcast shafts at a colliery in the U.K. Ascheme of instrumentation was aimed at monitoring the strata displacement surrounding the pit bottom roadways and shaft intersections with a view to ascertaining the origin of the strata movement and effectiveness of the control measures. The borehole instrumentation results together with a theoretical analysis suggested that the cause of instability could be attributed to the drivage of a roadway within the shaft pillar. A semi-empirical technique was used to calculate the amount of support resistance required to reinforce the pillar to take additional abutment load. A rock bolting system was designed to provide a required increase in thrust capacity of the roadway support. The results indicated that roadway closure was effectively controlled after installation of bolts indicating efficacy of the control measures adopted. INTRODUCTION Recent events in the international energy market have culminated in placing more emphasis on the exploitation of indigeneous energy resources and revitalising coal industry, in the U.K. In order to increase the potential recovery of coal the existing shafts must be utilised to their maximum capacity. Most shafts are constructed to have a life span of at least 50 years, and in some cases in excess of 100 years. These modifications may be enlargement of shaft intersections to accommodate loading facilities, drivage of vertical bunker for storing additional output from the new seam or drivage of new roadways. The most critical part of such modification of shaft structure is design and location of these intersections specially in view of their size, geometry, geological environment and interaction. Interaction of new construction is of vital importance and may o cause major instability of shaft intersections and pit bottom roadways. The paper describes the pit bottom instability caused by the interaction of a roadway drivage in a vicinity of access shafts at a colliery in South Yorkshire in the U.K. The application of rock bolt reinforcement system, as a successful remedial measure for restoration of stability is discussed. SITE DESCRIPTION The coal mine involved is situated in South Yorkshire coalfield, in the United Kingdom, some 36 miles north of Nottingham. The present mining operations are concentrated to Haigmoor Seam, producing about 0.6 million tonnes per annum from two mechanised working districts. The access to Haigmoor coal seam was obtained by sinking two shafts in 1914. Figure 1 shows the pit bottom layout. The downcast shaft, was originally sunk up to a depth of 660 m which was filled up to 652 m level in 1976 and the winding inset was maintained at 623 m level below the surface. Shaft intersection was enlarged to house a skip-bunker, automatic skip loading pockets and control instrumentation. The upcast shaft had a winding inset at about 595 m below surface and was equipped with a pair of double deck cages and was mainly used for men and material winding.

ABSTRACT The features of weak rock in Chinese coal mines is the subject of this paper. An optimum entry layout and support system are proposed. The importance of putting the roadways in the destressed zone and the necessity of using closed supports are emphasized. There are many urgent problems of construction in weak rock which remain to be solved for mining, tunneling, dam construction and other civil engineering applications. These problems are even more serious in those coal mines where the underground geo- 0glcal and operating conditions are complicated, some of which seriously threaten the construction of new mines and the routine operation of the existing mines. The following is a brief account of three aspects. THE FEATURES OF WEAK ROCK Weak rock possesses many features different from that of, hard and medium hard rocks, the most prominant ones eing: low strength, high porosity, strong water absorption, strong weathering Characteristics, high clay constituents, such as montmorillonite, illite, etc., and a certain degree of Swelling characteristics. The principle features of weak rock are:Low uniaxial compressive strength. he mostly commonly met rocks in Chinese Coal mines are argillaceous shale. mudstone, sandy shale, etc., and the uniaxial compressive strength of most strata is below 200 kg/cm, with the lowest figure of the average uniaxial compressive strength being only 33.8 kg/cm (Table 1), which is 5–20 times less than that of medium hard and hard rock. (Table in full paper) The strength of some rocks may reach 500–600 kg/cm. However, depth and slacking and swelling upon wetting make them very low in strength. As soon as the stress reaches the limit of strength plastic deformation occurs. High natural moisture content and strong water absorptivity have a great influence on rock strength. Rock, in general, has a given amount of moisture content and will be softened upon absorbing water. However, the natural moisture content and the degree of softening upon wetting vary with different rocks. Therefore, the natural moisture content and the water absorbability of the rock have a great influence on the rock strength. Even the hardest granite exhibits the character of softening upon absorbing water, and the influence on weak rock is quite obvious. For example, the uniaxial compressive strength of granite is reduced by 13% after water immersion, while that of sandstone in the Datong mine area is reduced by 32–35%, and that of argillaceous shale in the Qiantun Mine, Shengyang Mines Administration is reduced by 80% when the moisture content is increased from 2% to 12%, as shown in Fig. 1. (Figure in full paper) The porosity of the weak rock is generally higher than that of other rocks, that is one of the reasons why it has higher moisture content, strong water absorbability and hydration. The natural moisture content and porosity of the weak rock is far higher than that of the hard rock (Table 2). Therefore, in weak rock water drainage and water-sealing measures are very important to reduce the moisture content and to halt

INTRODUCTION Yubari New Coal Mine has been suffering roadway maintenance problems since the beginning of the mining operation started in 1970. Main roadways driven in thick layers of Horokabetsu shale are situated at a depth of about some 1000m from the surface and at 30m below the working coal seam having mean thickness of 3m. Behavior of the roadways are characterized by the continuing creep-like progress of the road- Way deformation with considerable amount of closure rate which depends on several factors and a great deal of manpower have been obliged constantly to be put into the repair workings. In one case, whole length of the roadway experienced seven times of repairment during 40 months. It is apparent that the present supporting design of the 125xl5mm three piece steel arches installed at O.5m intervals in conjunction with wooden lagging is not adequate to effectively maintain the road- Way of 5.5x3.6m section. To overcome these miserable roadway conditions, both field measurements and laboratory studies have been conducted and effort has been made to clarify the mechanism of the excessive roadway deformation. long with these investigations, effective counter-measures and/or supports have been Searched and tested in the field. FIELD AND LABORATORY OBSERVATIONS Some aspects of the roadway closure A typical example of the vertical closure of the roadway opening is presented in Fig. 1. The measurement started. one week after the completion of the roadway. Mining activities in the vicinity of the measuring section had been absent during the measurement. It is noted that the convergence rate gradually decreased in a few weeks and thereafter kept the constant rate over a period of 200 days. Measurements on the rock movements surrounding the opening, as shown in the exemplified result in Fig. 2,reveals that the floor uplift mainly contributes to the vertical closure, because roof lowering is considerably small. Small amounts of tilt which have been monitored on the rock mass below the sidewall might indicate that the floor uplift is induced neither by the shear failure occurring presumably at the edges of the opening nor by the radial movement around it but by the swelling of the relaxed zone under the floor(Kinoshita et.al.1979). Vertical closure is generally recognized to be superior in magnitude to side closure. Conditions created in rock mass around an opening With an aid of the borehole jack of Goodman type (Goodman et.al. 1968),the relaxed zone developed around an opening was measured at several sections. It is noticeable that this zone extends to a fairly amount of depths below the floor as depicted in Fig,3. Measurement using multi-strain meters revealed also that the rock mass in this relaxed zone showed higher rate of expansion than those in other deeper zone. In addition, observations by borehole viewer direclty confirmed that the density of fissures is remarkable in the relaxed zone. (Figure in full paper) Fig. 4 is a typical pattern of water contents of rock mass around the roadway; In sidewalls(and in floor)they are rather low value, whereas in floor higher values.

INTRODUCTION The Japanese Islands are situated at the north-east margin of Pacific Ocean, and thrusted by the Pacific and Philippine Plates. About 40% of total area of them is covered by Tertiary and Quarternary sedimentary rocks, most of which are classified into weak rock. Particularly, the north-east district of Honshu Island is widely covered by so-called "Green Tuff", which is one of the weak and incompetent miocene tuff colored in green or dark blue. In this Green Tuff formation, a peculiar and complicated lead-zinc-copper ore "Kuroko" deposits are burried, which are one of the most important ore deposit in Japan and mined presently in several mines. In Japan, most of coal bearing formations belong to Tertiary system. Although coalification process has been accelerated by geothermal effect and severe tectonic activities to be bituminous coal, the coal seam and coal measure rocks are frequently weak and fractured. Furthermore, frequent volcanic activities and severe tectonic movements have caused many foldings and faults in geological formation in Japan. All of these geological situations have made rock formations fractured and altered, which cause many difficulties related to severe rock pressure in mining, in turn. In coal mine, about 75% of the total output is mined from the depth of over 500m from pit mouth. Although mean depth of faces is not greater than those in Ruhr coal field, the combined effect of severe tectonic stress, high gas content of coal and weak rock formations causes various difficulties in mining, which need various technical developments to control severe rock pressure and to prevent hazards of rock and gas outburst. In Kuroko mine, although the mean depth of stops is about 300m, Cenonic clayey shale as country rock is usually weaker than Kuroko ore which is classified into the weak rock in itself. Therefore, the maintainance of cross cut and haulage level roadway has been encountered with many difficulties concerning with design of support, cost and frequency of repair work in roadways. (Table in full paper) In this paper, the author reviews some features of mechanical properties of rock formation and some technical developments to control severe rock pressure in the incompetent rock formation in mines ROCK STRESS AND ROCK STRENGTH IN MINING AREA State of rock stress in mining area The initial state of rock stress is often measured for the purpose of disigning the underground hydraulic power plant, preventing the rock burst hazard or disigning the mine pillars as well as stopes. Most of Rock stress measurements are carried out by means of stress relief method, particularly door stopper type Strain cell and borehole deformation gage. (Table in full paper) Most of the maximum horizontal stress determined by means of stress relief method trend approximately E-W, and this trend approximately coincides with the trend of tectonic stress determined by geodesic triangulation survey (Oka, et al. 977). Some examples of the initial state of rock stress measured in mining field, are shown in Table 1.

ABSTRACT The criterion for discriminating the swelling pressure is the ratio of the the gravity.field to the compressive strength of the rock which is determined emperically. The influences on the compressive strength of clay shale and rock pressure of such factors as the water vibration, dynamic loading caused by Winning and repairs to supports are described, based on practical data. The princeples of support, and preventive measures for weak swelling strata are described. The key is the control of floor heave. INTRODUCTION Various types of weak swelling strata occur in many mine areas in China such as Huainan, Shulan, Shenbei and Changguang. These strata differ greatly in their origin, geological age of formation and depth. Also their compressive strengths are quite different. Excavations in these strata become badly deformed and deteriorated by the violent swelling rock pressure, so that frequent repairs are needed, which have a great influence on ongoing production and mine construction. Therefore, it is of vital importance to conduct research on rock pressure and support in roadways built in weak swelling strata. The deterioration of the roadways caused by the swelling pressure is mainly due to inadequate selection of entry layout and support system by the engineers who did not possess enough data relevant to engineering geology and rock type before excavation and did not realize the nature of the rock pressure in the swelling strata and its harmful effects. Heavy deterioration of the excavation in inevitable under such conditions. In the future, before an excavation is made it is necessary to investigate in detail the engineering geology and make the necessary tests of rock properties. First of all, during design, the entry should not be located in weak swelling strata and in areas of stress concentration. If the roadway has to be driven in a weak stratum, an effective support system should be introduced and Borne strict construction measures should be taken, which might improve the roadway support conditions to a great extent. CONCEPTS OF WEAK SWELLING ROCK STRATA AND SWELLING ROCK PRESSURE Up to now, there is no unified knowledge nor a precise definition of weak swelling rock strata and swelling rock pressure. The BO called weak strata usually refer to those rocks with loose structure, well developed fissures and cracks and low strength. Most of these rocks expand in volume to varying degrees upon wetting, so they are called swelling strata. However, under some mining conditions in the coal mines, even where there is no water, these rocks or clayey rocks with higher strength of up to 600 kg/cm would exert violent swelling pressures (e.g. the rock becomes rhological, expands upon being fractured or cracked and causes floor heave). Based on an analysis of a large quantity of data of the displacements of the soft rock around the excavation, the movement of the surrounding rock is a complex process which involves many factors, including the displacement caused by the expansion of the rock upon wetting,

ABSTRACT INTRODUCTION The bulk of workable coal deposits in India occur in the Lower Gondwanas of Permian age in Raniganj Formations (Upper Coal Measures}, the Barakar Formations (Lower Coal Measures) and the Karharbari Formations. Of the present underground coal production of 76 million tonnos, conventional bord and pillar mining accounts for some 98%, using largely non-mechanised methods. Depths of exploitation vary from 20 to 650 metres, with bulk of the production coming from working depths of 150 to 250 metres. Problems of control of the immediate roof in bord and pillar mining has not assumed so far a serious dimension. This could be ascribed to relatively shallow working depths and competent sandstones as roof rock. Some 15 per cent of working coal mines have had to contend with ground control problems with immediate roof of friable coal, weakly laminated shale and aberrant geological features such as channel sandstones or discontinuities. Supporting in mines, which had here to fore been largely through timber supports, is gradually being supplemented by rock bolting, rope truss and other steel supports. Since the first application of rock bolts in 1955, incremental and evolutionary developments in supporting systems have been made and studies initiated on evaluation of bolting systems (Ghose, A.K.et al 1964). An innovative and economical support system, the rope truss, was developed in 1974 and has emerged as one of the most successful advances in coal mine roof control in India (Raju et al, 1974). Basically the system consists of drilling two holes in the roof over the ribs at an inclination of 45° and to a depth of 1.8m. Two ends of a discarded haulage rope, of a nominal diameter of 22 mm, are grouted in the holes and the rope tightened against the roof by timber lagging. With supplementary vertical grouted bolts, the rope truss system has proved its efficacy in trials carried out in 50 coal mines in different Indian coal fields. Laboratory model studies have also confirmed an additional advantage of this reinforcement system, namely= the post-failure loading behaviour which prevents sudden failures, an observation which has also been corroborated by field experience (Raju and Ghose,198O). Currently, the usage of mechanical and grouted bolts in Indian coal mines is around 100,000, while some 75,000 rope trusses (rope stitching) are being installed every year. In view of the wide diversity of rock mass characteristics in different coal basins in India, based on a study of some 50 case records of rock bolting and/or roof truss application, an attempt has been made to evolve a rock mass classification scheme and guidelines for support design in bord and pillar mining. The paper presents the scheme of classification and evaluates the validity of application. COAL MEASURE ROCKS- VARIABILITY OF PROPERTIES The Permian Coal Measures in India, which represent continental fluviatile deposits, exhibit a wide variability in physical and mechanical characteristics. The roof sandstones are of moderate strength and are usually competent rocks. They exhibit, however, an extremely wide range of compressive strength from 50 to 150 MN/m, depending upon grain size and presence of thin bedded shale and/or silt partings.

ABSTRACT INTRODUCTION A 9 m thick XIV seam is being worked on by three ascending slices with hydraulic sand stowing at Noonodih-Jitpur colliery in Jharia coalfield in India. The seam is accessed by two fully concrete lined shafts 4.88 m in diameter and 450 m deep. In early 1976 signs of ground movement were noticed in the shaft pillar area. The ground movement was associated with damage to reinforced concrete lining in shaft insets, distortion and damage of rectangular and arched steel supports in roadways, pump-house, underground sub-station and storage bunker, etc. Roof falls also took place in some roadways. The Central Mining Research Station, (CMRS), was approached to investigate into the causes and magnitude of ground movement and to suggest methods to deal with the ground movements. This paper describes the investigations, results obtained, and recommendations. MINING METHOD There are nine seams upto an average depth of about 500 m; namely XII, XIII, XIV, XV, XV-A, XVI, XVI-A, XVII and XVIII, in the area where the mine is located. Of these XVII and XVIII have already been extracted by bord and pillar system with caving/hydraulic sand stowing. The XVI and XVI-A seams were being extracted by longwall/bord and pillar system with caving/hydraulic sand stowing. The XV-A seam is unmined and XV seam burnt due to igneous intrusion. The 9-m thick XIV seam was being worked by three ascending inclined slices with hydraulic sand stowing. The XIII seam is burnt and XII seam is unmined. The section of the strata showing relative positions of the seams and the plan of XIV seam shaft pillar and adjoining area are shown in Figure 1. The parting between XIV and XV seam was about 48 m and that between XIV and XVI seam was 181 m. The XIV seam access shafts have 600 mm thick concrete lining throughout. The J1- shaft is downcast and has a pair of 6 t skips with rigid rail guides for hoisting coal, men, and materials. The J3-shaft has a pair of cages with rigid rail guides. Figure 1. Plan of XIV Seam Working In Noonodih-Jitpur Colliery Showing Stowed Areas (Available in full paper) All extraction operations in XIV seam were suspended in March 1977 except essential development. Three panels (BMT-1, BMT-2, and BMT-3) had been extracted in the seam. In BMT-1 panel first slice of 2.1-2.4 m thickness was extracted with a shearer and the remaining two slices were extracted with solid stowing. The BMT-2 panel was similarly extracted, but in this panel the first slice was extracted completely and the second and third slices partially. In the BMT-3 panel only first slice was extracted partially. The thickness of the second and third slices varied between 2.7 m to 3 m. During the extraction of the third slice about 0.6 m to 0.9 m thickness of coal was left along the roof. The faces of the first slices were supported by 40 t friction props and in the second and third slices the faces were supported by wooden props.

ABSTRACT This paper presents a design parameter description of a high throughput (1.6 million barrels of crude oil per day) offshore export terminal and an advancement of state of the art emphasizing the analytical details, design details and full scale load test results for precast, pre-stressed, pre-tensioned, post-tensioned double tee concrete trestle girders spanning 123.69 feet (37.71m). The girders are subjected to heavy design torsion. The paper develops an illustrative example of shear and torsion reinforcement calculations. The girders are part of the 2,525 ft. (770m) long trestle system and 3,450 ft. (l,052m) long jetty trestle for the Yanbu Offshore Terminal export facilities. The trestle includes:A pipeway supporting three 56 in.Ø crude lines, one 30 in.Ø bunker line, one potable water line, one sewage line, a firewater loop and electrical cable trays. A 12 ft. (3.7m) wide maintenance access roadway. The girders are supported over steel jackets and decks which are installed in water depths ranging from 2 feet (0.6m) at the causeway abutment to 114 feet (34.8m) at the jetty area. The superstructure loads are transferred to the foundation through steel piles. The rubble mound causeway involved placement of approximately 650,000 cubic yards (500,000m) of quarried rock. The terminal elements required 48,400 short tons of steel (43,900mt) fabrication for decks, jackets and pilings. 4,400 cubic yards (3365m) cast-in-place reinforced concrete was used for decks and 1300 cubic yards (1000m) was placed in the pile supported abutment. The trestle system required 13,335 cubic yards (l0,200m) of pre-stressed concrete. The most recent cost estimate for the offshore marine terminal which will accommodate tankers up to the 500,000 DWT class exceeds $150,000,000. INTRODUCTION The need to transport crude oil from the east coast of Saudi Arabia to the west coast has brought about the development of a 746 miles (1,200 km), 48in. (1.22m) pipeline system (Fig. 1). The system, when completed, will transport crude oil from the Ghawar oil field near Abqaiq in eastern Saudi Arabia to Yanbu on the west coast. The development of the Yanbu terminal and loading facility will be the first phase of a new industrial complex at the Red Sea port for the Kingdom of Saudi Arabia. The pipeline and terminals will enable the largest tankers in existence to load crude oil for export and move up the Red Sea to Ain Sukhana where the crude oil can be transported through the Sumed pipeline to the Mediterranean Sea. The tankers can also continue through the Suez Canal, which has been expanded to handle large crude carrier traffic. As a result, the pipeline system will reduce the shipping distance from oil fields in eastern Saudi Arabia to European and North American ports by 2,200 miles (3,540 km). Shipping costs will thereby be reduced. The extensive crude oil transportation project includes:A marine export terminal at Yanbu. An onshore terminal at Yanbu with storage for 11 million barrels (1.7Mm) of crude oil.

This paper reports the result of closure measurements in roadways driven in salt at a depth of 11OOm. The results obtained from a variety of mining geometries are compared and the implications for the design of underground nuclear waste repositories are discussed. Introduction Ever since Project Salt Vault (Bradshaw, 1971) isolation of nuclear waste in mined openings in rocksalt has been rigorously investigated. To eliminate the possibility of radio-nuclide escape to the biosphere, a primary requirement of the design of an underground repository must be the long term stability of the openings. The stability of excavations in rocksalt is mainly a function of the mining layout, the percentage extraction, depth below surface and the time dependent behavior of the material. Additional factors to be considered in the case of nuclear waste storage are the influence of heat and radioactivity on the behavioral properties of the material. Generally speaking evaporate mining practice is based on either regular room and pillar layouts with fully load bearing pillars or the use of relatively narrow, high extraction panels separated by wide barrier panels. Internal panel pillars are small and designed to yield (Serata, 1976). This paper focuses on a study of the closure behaviour of a variety of mining layouts and configurations driven in salt at a potash mine in North Yorkshire, including:Isolated twin roads. Roads in low extraction panels (25–35%) with 40m pillars Roads in high extraction panels (60–70%) with 4 - 6m wide pillars Roads with 60m pillars driven in the shaft pillar. All roadways are rectangular, 6.7m wide by 3.5m high, driven by means of mechanical miners, except the shaft pillar roads which were drilled and blasted to roughly the same shape and dimensions. The depth below surface is 11OOm given an approximate overburden stress of 25MPa. Roof-floor closure measurements have been taken over periods ranging from days to years. PRESENTATION AND DISCUSSION OF RESULTS Closure measurements from the four basic layouts described above have been collected and are presented in tabular form. The variation of closure rate with time has been found (Hebblewhite, 1977) to follow the curve given by an equation of the general form: C= At where c is the closure rate in mm/day, t is the time from date of excavation in days, and A and B, constants. Experience based on visual inspection over several years has shown that for roadways to be stable they must have closure rates which fall within the envelope given by the equation: C = 34.81 t