fig1 illustrates one embodiment of an isolation dam located in a passageway 1 formed in a deep mine of surrounding salt rock 2 or other incompentent or soluble rock and which is filled with unsaturated brine or high pressure water . the fragmented or broken salt rock 9 is removed around the area where a first dam 5 is to be installed . the dam 5 is made of an impervious material such as poured concrete . the high pressure side of the dam 5 in contact with the passageway full of brine adjacent surface of the passageway are coated with an elastomer liner 3 . the function of the liner is to prevent penetration of brine to the rock adjacent the dam 5 . a second dam 6 is constructed in a similar manner to the first dam 5 and spaced downstream therefrom . the area between the two dams is equipped in a sump 10 which is connected to a sump withdrawal passageway 11 having a valve 15 . after the two dams have been constructed and are in place , pressurized diesel fuel or similar hydrophobic material 8 is injected by means of an injector 7 into the area between the two dams . microcracks in the host rock 9 surrounding the open area between dams 5 and 6 are partly closed by pressure as well as filled with the hydrophobic liquid , thereby resisting the entry of high pressure brine through the cracks . sampling mechanism including sampling valves 13 are connected through lines 14 to the high pressure water or unsaturated brine area 1 . access passages 12 are located within the dams 5 and 6 for maintenance or inspection purposes and pass generally centrally and horizontally therethrough . these access channels 12 are equipped with ports 4 . once the construction thus far described is in place , the high pressure water or brine in are 1 can be monitored through the sampling mechanism 13 from time to time . if any brine enters into the area where the pressurized diesel fuel 8 is located , its higher specific gravity will cause it to fall to the bottom of the area into the sump 10 . the sump valve 15 is turned on and the brine with drawn until diesel fuel appears . thereafter , diesel fuel can again be injected through area 7 to maintain the correct pressure . fig2 - 4 illustrate an alternative embodiment of a high pressure isolation structure , indicated generally at 20 , in accordance with the present invention . the high pressure isolation structure 20 , which may alternatively be termed an expansion joint or bulkhead , provides an alternative method of accelerated rock repressurization and is illustrated as being positioned within an underground passageway 22 formed in rock , such as saltrock or other incompetent or insoluble rock 24 . while the passageway 22 , as well as the aforedescribed passageway 1 , is illustrated as having a horizontal orientation , it will be understood that the passage or drift may have an inclined orientation and provide passage to a substantially larger underground cavity or chamber ( not shown ) which has previously been mined out or which is in the process of being mined . alternatively , the passageway or drift 22 may comprise one of a plurality of access passages to an underground chamber as employed in the storage and isolation or high - level nuclear waste . the expansion joint or bulkhead 20 includes a pair of generally similar shaped dam members 28a and 28b which are formed of a relatively high - strength solid material , such as concrete or other similar material , resistant to the elements , within an underground passage formed in saltrock or other incompetent of soluble rock formation and which enables pouring and hardening in situ . the dam members 28a and 28b are separated by an expansion joint , indicated generally at 30 , which in the embodiment illustrated in fig2 has a generally horizontal orientation and includes a pair of steel liner plates 30a and 30b . as illustrated in fig4 the liner plates 30a and 30b extend substantially the full horizontal width of the upper and lower dam members 28a and 28b , respectively . prior to forming the high pressure isolation dam or bulkhead 20 , the rock surrounding the passageway 22 is excavated or fragmented to form an expanded recess . in the illustrated embodiment , the recess has a generally octagonal transverse configuration , as illustrated in fig4 with the uppermost and lowermost surfaces being defined by mutually intersecting inclined surfaces 32a and 32b , and 34a and 34b , respectively . in this manner , the isolation dam or bulkhead is fixed at a selected position along the length of the passageway . the high pressure or wet side of the bulkhead 20 and the adjacent peripheral surfaces of the passageway 22 are coated with an elastomer liner 36 similar to the aforedescribed elastomer liner 3 , which serves to prevent penetration of brine into the rock adjacent the dam or bulkhead . referring to fig4 and 7 , the liner plates 30a and 30b extend substantially the full lateral width of the associated dam members 28a and 28b and have their opposite marginal edges affixed to a high pressure reinforced elastomer gasket 40 which extends along the full longitudinal lengths of the liner plates and also along their forward and rearward transverse marginal edges . the elastomer gasket 40 is of generally u - shape and has one edge portion clamped between the liner plate 30a and a flange plate 42 as by bolts 44 . the opposite free edge portion of gasket 40 is secured between the marginal edge of liner plate 30b and a flange plate 46 by similar fastener bolts 44 . the elastomer gasket 40 is affixed to the upper and lower liner plates 30a and 30b about their full peripheral edges so as to cooperate with the plates in establishing a substantially closed expansion chamber . a plurality of spacers , such as indicated at 50 , are preferably affixed to a selected one of the opposed surfaces of the plates 30a and 30b so as to maintain at least minimal spaced relation therebetween to facilitate introduction of a pressurized fluid medium into the expansion chamber as will be described herein below . in forming the high pressure isolation structure or bulkhead 20 , suitable forms ( not shown ) are initially assembled within the passageway 22 to facilitate forming of the lower dam member 28b to a predetermined height , such as approximately one - half the vertical height of the bulkhead . the liner plates 30a and 30b and associated elastomer gasket 40 are then affixed to the upper surface of the lower dam member . thereafter , the upper dam member 38 is formed in a conventional manner such that its upper peripheral surface engages the opposed inner surfaces of the previously created expansion joint chamber within the rock 24 . the upper and lower dam members 28a and 28b are formed so as to create a generally rectangular recess or cavity , indicated at 52 in fig7 of larger size than the outer dimension of the elastomer gasket 40 about the full periphery of the bulkhead . a suitable bitumen or similar material 54 is preferably injected into the recess 52 so as to encompass the outer peripheral surface of the elastomer gasket 40 while facilitating flexing of the gasket upon separation of the liner plates 32a and 32b . to facilitate injection of a pressurized fluid medium , such as a suitable hydraulic fluid , into the expansion chamber defined between the liner plates 30a and 30b , a plurality of injection tubes or pipes 56 have communication with the expansion chamber through associated flow conduits 58 affixed to the upper surface of the liner plate 38 in communicating relation with orifices in the expansion plate . as illustrated in fig2 the flow tubes 56 extend generally longitudinally of the isolation structure or bulkhead 20 and extend outwardly from an end surface 28c where they are connected to an injection manifold 60 adapted for connection to a source of pressurized hydraulic medium . after forming the isolation structure or bulkhead 20 as thus described , and prior to introducing high pressure fluid into the expansion chamber between the liner plates 30a and 30b , a suitable grouting material is introduced between the outer surfaces of the upper and lower dam members 28a and 28b and the adjacent rock cavity by conventional techniques , such as through suitable conduits ( not shown ) formed within the dam members and through which a suitable grouting material may be introduced . the grouting effects substantially full surface contact with the rock surface peripherally of the bulkhead . thereafter , pressurized hydraulic medium is introduced through the injection manifold 60 and distribution lines 56 and 58 into the expansion chamber between the liner plates 30a , b to effect selected expansion of the dam members against the adjacent rock surfaces to obtain a predetermined rock stress and prevent liquid seepage through any microfracture system existent in the rock . for example , it is known that saltrock has properties similar to asphalt and will flow when subjected to predetermined stress so as to effectively close off any seepage channels formed within the initially created microfracture . on the other hand , the pressure applied to the expansion chamber within the expansion joint 20 may be reduced to apply reduced stress to the adjacent rock surfaces when working within an elastic type rock formation . once a desired rock stress is achieved by the bulkhead 20 , the hydraulic medium between the liner plates 30a and 30b can be displaced with grout or any epoxy and suitable hardener . for this purpose , a drainage manifold system similar to the injection manifold system is formed within the dam member 28b below the liner plate 30b to facilitate discharge of pressurized fluid medium from the expansion chamber when displaced by the grout or epoxy . a drainage manifold 64 is interconnected to discharge orifices formed uniformly throughout the liner plate 30b through longitudinally extending drainage tubes 66 and associated flow conduits 68 as illustrated in fig2 . fig5 and 8 illustrate a further embodiment of a high pressure isolation structure , indicated generally at 74 , which may alternatively be termed a dam , expansion joint or bulkhead . the high pressure isolation structure 74 may be employed as an isolation dam within the passageway 22 in place of the aforedescribed isolation structure 20 . the isolation structure or bulkhead 74 differs from the isolation structure 20 in that it provides for both horizontal and vertical expansion within a single isolation dam , and in some installations may provide improved rock integrity and dam performance in isolating a mining chamber or storage isolation cavity below ground from intrusion of liquids through the passageway 22 . the isolation structure or bulkhead 74 has four similarly configured impervious dam members 76a - d which are maintained in assembled relation by associated expansion joints , each of which is indicated generally at 78 , so as to form a bulkhead having four similar quadrants when veiwed from an end thereof or in transverse section as illustrated in fig5 and 6 . the expansion joints 78 are generally similar to the aforedescribed expansion joint 30 and need not be described in greater detail herein . suffice it to say that each of the expansion joints 78 includes a pair of spaced generally planar liner plate 80a and 80b which are maintained in spaced relation by suitable spacers and which have their outer marginal edges interconnected through elastomer gaskets 40 . the corresponding inner marginal edges of each pair of liner plates 80a and 80b are interconnected by similar high pressure reinforced elastomer gaskets 82 affixed to the respective pairs of plates 80a and 80b in similar fashion to the elastomer gaskets 40 . the elastomer gaskets 82 are received within a central longitudinally extending cavity 84 formed centrally of the dam members 76a - d . similarly , the spaced liner plates 80a and 80b of each of the expansion joints 78 are cooperative with injection manifolds and associated drainage manifolds to facilitate introduction of suitable hydraulic medium between the respective pairs of plates for effecting outward expansion of the dam members 76a - d into pressure engagement with the adjacent rock surface to obtain desired stress within the rock surface and closing of the microfracture system therein . it will be understood that a suitable grouting material is introduced between the dam members 76a - d and the adjacent rock surface prior to expansion of the dam members , as was described in conjunction with the high pressure isolation structure 20 . the hydraulic fluid introduced into the expansion joint of fig5 and 8 may also be replaced with a hardenable grouting material to effect a fixed isolation dam of long duration . the high pressure isolation structures or expansion joints 20 and 74 can be used to replace one or both of the isolation dams illustrated at 5 and 6 in fig1 or they could be used separately in a single dam or bulkhead application . in either usage , the high pressure isolation structures 20 and 74 enable relatively rapid forming or installation within an underground passageway and are of long duration . while preferred embodiments of the present invention have been illustrated and described , it will be understood that changes and modifications may be made therein without departing from the invention in its broader aspects . various features of the invention are defined in the following claims .