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RELATED APPLICATION 
     This application is a Divisional of allowed U.S. application Ser. No. 09/418,681, filed on Oct. 14, 1999 now U.S. Pat. No. 6,648,552. 
    
    
     CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has certain rights in this invention pursuant to Contract No. DE-AC07-94ID13223, DE-AC07-99ID13727, and Contract No. DE-AC07-05ID14517 between the United States Department of Energy and Battella Energy Alliance, LLC. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a sensor system for monitoring the structural integrity of an underground waste containment barrier, and leakage therefrom of waste products or byproducts, and for improved characterization of zones of interest. 
     2. Background Art 
     It is often necessary to form a containment barrier around a hazardous waste site to stop or prevent the migration of contaminants into the nearby soil and water tables. The containment barrier must prevent the migration of contaminants both horizontally and vertically away from the waste site. Therefore, a properly constructed containment barrier may be compared to a huge bathtub, with the hazardous waste contained within four side walls and a generally horizontal floor. 
     A typical, currently-used method of containment is to physically remove the hazardous waste and haul it to a permitted storage facility. However, such method is costly, impractical, and dangerous. Digging up sites with buried drums, radioactive dusts, or other airborne wastes may actually release the contaminants, spreading them into the atmosphere and through the soil. 
     In response to this problem, a number of suggestions have been made for placing containment barriers around hazardous waste sites, without removing the waste. One approach for doing this is disclosed in International Publication Nos. WO 94/19547 and WO 93/00483 by Halliburton Nus Environmental Corp. The Halliburton system uses a row of high pressure jets to shoot a slurry into soil surrounding a hazardous waste site, somewhat liquefying the surrounding soil. The slurry cuts a path through the soil as it intermixes with the liquified soil. Gravity and/or mechanical means pull the row of high pressure jets through the mix of liquified soil and slurry, after which the liquified soil and slurry harden into a protective barrier. 
     The above-described system has a number of shortcomings, including the possibility of further spreading contaminants by the use of hydraulic jets, the difficulty of maintaining balance between the amount of slurry needed for cutting and the amount of slurry needed for hardening the soil, the difficulty of providing a barrier of consistent strength since it would depend in part upon the soil composition encountered and the amount of slurry deposited, and, finally, the lack of testing of excavated soil to know whether soil surrounding the waste site has become contaminated. 
     Another suggested approach for installing a containment barrier around a hazardous waste site is disclosed in patent application Ser. No. 08/925,101, filed Sep. 8, 1997, now U.S. Pat. No. 6,016,714 issued Jan. 25, 2000. In this approach, a multi-layer containment barrier is put in place under a hazardous waste site without disturbing any buried waste, in a simple and efficient fashion. The disclosure in the above-noted co-pending patent application is incorporated herein by reference. 
     In any approach to holding hazardous waste, it would be desirable to monitor the site in terms of both the structural integrity of any containment barrier put in place about the waste material, and leakage of contaminants away from the site. Additionally, it would be desirable to monitor material being excavated from around a waste site in preparation for emplacement of a containment barrier for the site, to determine the extent of contamination of surrounding soils and thus the possible need to extend the containment barrier to a location completely surrounding all contaminated materials and soils. Finally, it would be desirable to efficiently and inexpensively install a long-term monitoring system soon after or simultaneously with the installation of the containment barrier. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a sensor system for sensing a variety of physical parameters of a buried waste containment site. 
     It is also an object of the invention to provide such a sensor system especially suitable for use in connection with a containment barrier disposed under and around a buried waste site. 
     It is a further object of the invention to provide such a sensor system for monitoring the structural integrity of such a containment barrier. 
     It is also an object of the invention to provide such a sensor system for sensing leakage of contaminants from a buried waste containment site. 
     It is still another object of the invention to provide such a sensor system, in accordance with one aspect thereof, for monitoring soil and material excavated from a buried waste containment site. 
     It is an additional object of the invention to provide such a sensor system, in accordance with another aspect thereof, for sensing physical parameters of soil being excavated, during the excavation process. 
     It is a further object of the invention to provide such a sensor system which may be readily installed at a buried waste containment site simultaneously with the installation of a containment barrier. 
     It is also an object of the invention to provide such a sensor system in which sensors may be installed and removed after the buried waste containment site is in place. 
     The above and other objects of the invention are realized in a specific illustrative embodiment of a sensor system for a buried waste containment site having a bottom wall barrier and/or sidewall barriers, for containing hazardous waste. The sensor system includes one or more sensor devices disposed in one or more of the barriers for detecting a physical parameter either of the barrier itself or of the physical condition of the surrounding soils and buried waste, and for producing a signal representing the physical parameter detected. Also included is a signal processing device for receiving signals produced by the sensor device and for developing information identifying the physical parameter detected, either for sounding an alarm, displaying a graphic representation of the physical parameter detected on a viewing screen and/or a hard copy printout, etc. 
     In accordance with one aspect of the invention, the sensor device disposed in one or more of the barriers comprises a strain or crack transducer for detecting strain or cracking and thus possible leakage locations in the barrier in which the transducer is disposed. One embodiment of such a transducer includes a grid of detecting elements disposed in the barriers to detect strains wherever they might occur. 
     In accordance with another aspect of the invention, one or more access tubes are disposed in or below the barriers with at least one end of the tubes extending from the barriers to allow access thereinto. Sensor devices are then disposed in the access tube or tubes and coupled to the signal processing device through the one end of the tubes. The access tubes provide protection for the sensor device without inhibiting operation thereof. Also, use of access tubes allows for selective removal and deployment of a variety of sensors. 
     In accordance with still another aspect of the invention, the sensor device is adapted to detect radiation that may be leaking or may have already leaked through the barriers, and/or the presence of RCRA metals. Also, a sensor device may be provided to detect volatile organic compounds using fiber optic spectroscopy deployed in the access tubes. 
     In another embodiment of the invention, conveyor apparatus is provided for removing and carrying away excavated earthen material. Disposed above the conveyor apparatus and above any material being carried by the conveyor apparatus is one or more sensor devices for detecting various conditions and components of the material being carried. The sensor device is coupled to a processing device for developing information identifying the condition or components detected by the sensor device, just as with the sensor device disposed in the containment barriers described above. 
     In another aspect of the invention, sensor detectable tracers could be used to verify barrier integrity. Specifically, tracers could be placed within the barrier with sensors outside the barrier to determine whether the tracers have migrated through a breach in the barrier, or stayed in place. 
     In a further aspect of the invention, sensors or sensor arrays are installed in or about a barrier simultaneously with the installation of the barrier. For example the sensors or sensor arrays could be disposed between layers of a multi-layer barrier as the barrier is being installed in a trench dug for that purpose. 
     As indicated earlier, one approach to installing a containment barrier around a waste site involves the use of high pressure jets shooting a slurry into soil surrounding the waste site. This is also known as grouting, and typically involves a grouting beam or arm which carries the jets and which is moved along a locus to both remove soil and produce the containment barrier with a mixture of slurry and soil. In accordance with an aspect of the present invention, a sensor or sensors are disposed on the grouting arm to detect physical properties of the soil through which the arm moves, to thus determine whether contaminants have leaked from the waste site into the surrounding soil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the invention will become apparent from a consideration of the subsequent detailed description presented in connection with the accompanying drawings in which: 
         FIG. 1  is a perspective view of a plot of ground contaminated by hazardous waste; 
         FIG. 2  is a perspective view of the plot of ground with the hazardous waste contained by a protective ground barrier; 
         FIG. 3  is a side, schematic view of sensor apparatus positioned above a conveyor carrying excavated material, in accordance with the present invention; 
         FIG. 4  is a perspective view of a grid sensor system deployed in a containment barrier, in accordance with the present invention; 
         FIG. 5  is a side, schematic view of a fiber optic strain/crack sensor system deployed in a containment barrier, in accordance with the present invention; 
         FIG. 6  is a schematic view of a gamma spectroscopy sensor system suitable for use in the present invention; 
         FIG. 7  is a side view of a barrier placement machine suitable for constructing a multilayer underground barrier and for simultaneously deploying sensor devices in the barrier; and 
         FIG. 8  is a side, cross-sectional view, enlarged, of the multi-layer underground barrier of FIG.  7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a typical waste site  11  is shown containing drums  13  filled with hazardous waste, both on the surface  15  and buried under the ground  17 . Contaminants  19 , leaking from the drums  13 , threaten to migrate into a water table  12 , unless some type of containment barrier can be provided. 
     One such containment barrier  21  is shown in  FIG. 2  to include side barriers or walls  23  and a floor or horizontal barrier  29 . The side barriers  23  may be made using conventional methods and interconnected to the horizontal barrier  29 . Additionally, the waste site  11  could be completely encapsulated by forming an upper barrier cover (not shown) and interconnecting it with the side barriers  23  and front and rear barriers  25  and  27  (front barriers  25  are shown in phantom line in FIG.  2 ). The afore-cited co-pending patent application describes how containment barriers of the type described may be constructed using apparatus such as that to next be briefly described. 
       FIG. 3  is a side, schematic view of one embodiment of excavated soil sensor and assay equipment, in accordance with the present invention.  FIG. 3  shows a conveyor  710  on which excavated soil  700  (from a waste containment site) is being carried for ultimate deposit. Disposed above the conveyor  991  for detecting various physical parameters and contaminants of the soil  700  are a gamma ray spectrometer  704 , an X-ray fluorescence detector  708 , and a hood  712  for collecting vapors rising from the soil  700  and passing the vapors to an analyzer  716 . Disposed under (or could be over) the upper section of the conveyor  991  is a scintillating fiber bundle  720  coupled to an optical-to-electrical convertor  728 . The gamma ray spectrometer  704 , X-ray fluorescence detector  708 , analyzer  716  and optical-to-electrical converter  728  are all coupled to a monitor  732  for processing signals received from the various components shown for displaying information represented by the received signals or for taking other action. 
     The gamma ray spectrometer  704  is provided for making measurements of the energies of particles emitted by different radioactive sources in the soil  700  to thereby distinguish among the sources and identify them. The gamma ray spectrometer  704  supplies signals to the monitor  732  identifying the different sources of radioactivity, and the monitor processes these signals to provide a display, hard copy printout, or other indication to a user of what sources of radioactivity are present in the soil  700 . Gamma ray spectrometers are well known in the art. 
     The X-ray fluorescence detector  708  is provided for detecting the presence of RCRA metals in the soil  700 . The detector  708  supplies signals to the monitor  732  indicating whether or not RCRA metals have been detected, and the monitor then develops a suitable display, printout, etc. This type of detection is well known. 
     The hood  712  collects whatever vapors may be emitted by the soil  700 , but in particular volatile organic compounds, and these are supplied to the analyzer  716 . The analyzer  716  could include a variety of devices for detecting the presence of volatile organic compounds including an acousto-optic tunable filter (AOTF) infrared spectrometer or a Fourier-transform infrared spectrometer. Either of these devices is suitable for detecting the presence of volatile organic compounds and both are well known in the prior art. If volatile organic compounds are detected by the analyzer  716 , the analyzer supplies signals to the monitor  732  identifying the volatile organic compounds and this information may then be displayed, provided on a hard copy printout, etc. 
     The scintillating fiber bundle  720  is provided to detect the presence of radiation emanating from the soil  700  being conveyed on the conveyor  991 . The fiber bundle  720 , in the presence of different types of radiation, emits light of a characteristic frequency, and this light is then supplied to the optical-to-electrical converter  728 . There, the light is converted to electrical signals for supply to the monitor  732 , for producing a display or other indication of the nature of the radiation detected. 
     Scintillating fiber bundles illustratively may be made of polystyrene fibers, doped with fluorescent compounds that scintillate in response to various kinds of ionizing radiation. This radiation-induced scintillation comprises the light supplied to the optical-to-electrical converter  728  for conversion to electrical signals. Scintillating fiber bundles are commercially available. 
     The monitor  732  might, advantageously, be a conventional computer-based data acquisition and display system, such as a Dell PC with Pentium processor. 
     The sensing and assaying discussed above is for soil excavated as a result of installing a waste containment barrier, for example in accordance with the method described in the afore-cited co-pending patent application. It is also desirable to monitor the barrier itself for integrity and to determine whether leakage of contaminated material through the barrier is taking place.  FIG. 4  is a perspective view of a grid sensor system for monitoring the integrity of a waste containment barrier  800 . In one embodiment, the grid sensor system includes a first plurality of conductors  804  extending generally in parallel in one direction through the barrier  800 , and a second plurality of conductors  808  extending also generally in parallel in another direction in the barrier to intersect with the first plurality of conductors at an end wall  800   a  and a bottom wall  800   b  (and the other end wall not shown) of the barrier  800 . Both ends of the first plurality of conductors  804  and of the second plurality of conductors  808  are gathered and routed to a signal source and processor  812 . The signal source and processor  812  supplies electrical signals to both sets of conductors  804  and  808 , which have a predetermined characteristic impedance. The electrical signals supplied to one end of the sets of conductors will then be received by the signal source and processor  812  from the other end. Any strain, i.e., change in dimension, which takes place in the material of the barrier  800 , for example, such as the development of cracks or openings, will affect the conductors  804  and  808 . The affect will be generally to elongate the conductors where the strain occurs and this will result in a change in the characteristic impedance of the affected conductors. If a strain, for example, occurs near an intersection of one of the conductors  804  and one of the conductors  808 , then the characteristic impedance of those two conductors could be read by the signal source and processor  812  and that would locate the location of the strain as being near the intersection. The change in characteristic impedance can be measured with electrical time domain reflectometry, a well-known measuring technique. Once the location or locations of strain are detected by the signal source and processor  812  (e.g., spectrum analyzer), it signals a monitor  816  which develops an output identifying the location of the strain. The monitor  816  might advantageously be a computer-based data acquisition system, as with the monitor  732  in FIG.  3 . 
     An alternative embodiment to the conductor grid described above for determining integrity of the barrier  800 , is a grid of fiber optic strands disposed in the barrier  800  in the same manner as are the conductors. Assume that the conductors  804  and  808  are simply replaced with fiber optic strands (as shown in a side view in  FIG. 5 ) and that the signal source and processor  812  provides light of a certain intensity and wavelength to one end of strands  804  and  808  and then that the signal source and processor receives from the corresponding opposite ends the light that has been transmitted through the strands. If a change in wavelength and/or intensity of the light in any of the strands is detected by the signal source and processor  812 , such change indicates that strain or cracking has occurred in the barrier  800  at a location near the affected strands. Thus, detecting a change in the wavelength and/or intensity of light in two or more intersecting strands would indicate that the strain or cracking has occurred near that intersection and this information could be supplied by the signal source and processor  812  to the monitor  816  for display or other disposition. For processing the received light, the signal source and processor  812  might illustratively be a commercially available optic time domain reflectometer, or optical spectrum analyzer, interfaced to a personal computer. 
     The spacing between conductors  804  and  808  or between fiber optic strands  804  or  808  could illustratively be about one foot. This would enable identification of the location of strain or cracks in the barrier  100  to resolution of about six inches. 
     Although a grid of either conductors or fiber optic strands were shown and described for  FIG. 4 , it is also possible to detect the location of a strain or crack occurring in a barrier by an array of wires or fiber optic strands extending parallel to one another and just in one direction. In particular, a strain or crack which affects a single wire can be located using electrical time domain reflectometry in which a wavelength shift in a signal applied to the wire indicates a strain or cracking in the barrier, as analyzed by a spectrum analyzer. Electrical time domain reflectometry is a well-known operation. Similarly, the location of a crack or strain affecting a fiber optic strand could be determined by measuring a back-reflected signal (reflected from the crack or strain in the fiber) of an optical pulse sent down the fiber, using optical time domain reflectometry. 
       FIG. 5  shows another embodiment of a fiber optic strain/crack sensor system embedded in a containment barrier made of grout. 
       FIG. 6  shows a side schematic view of another embodiment of the present invention in which hollow access tubes  604  are disposed in a containment barrier  600 , with the tubes being placed into the barrier (or below) during emplacement of the barrier. The access tubes  604  are used to deploy, among others, radiation sensors, such as scintillating fiber bundles or thermoluminescent dosimeters, X-ray fluorescence sensors for detecting the presence of RCRA metals, and/or a fiber-optic spectroscopy system to detect volatile organic compounds. The access tubes  904  could be emplaced in the barrier  900  using a variety of known deployment methods. The access tubes  904  may be placed in the bottom wall of the barrier and/or the sidewalls thereof. 
       FIG. 6  shows a specific embodiment of a sensor system carried in the access tube  604  to include scintillating fiber bundles  608  (best seen in the enlarged view  612  of a section of the barrier  600  and tube  604 ). The scintillating fiber bundles  608  were discussed earlier in connection with  FIG. 3 , and operate to emit light of different frequencies depending upon the type of radiation to which the fiber bundles are exposed. Fiber optic strands  616  are carried by the access tube  604  and coupled to the scintillating fiber bundles so that light emitted by the fiber bundles when exposed to radiation is carried by the fiber optic strands to a monitor  620 . The monitor  620  would include an optical-to-electrical convertor for converting the light to electrical signals for processing by a signal processing circuit to develop information identifying the type of radiation detected which information could then be provided to a user. 
     X-ray fluorescence sensors could also be deployed in the access tube  604 , for detecting the migration of RCRA metals through the barrier  600 , in a manner similar to that discussed in connection with FIG.  3 . Conductors would be coupled to the X-ray fluorescence sensors for carrying signals to the monitor  620  for processing and display of information relating to the presence of RCRA metals. 
     Fiber-coupled optical systems based upon Raman and/or fluorescence spectroscopies could also be deployed in access tubes in or around the barrier to detect and identify volatiles permeating through the containment barrier and through perforations  628  in the tube  604 . Such systems operate by transmitting an excitation signal from a laser to a sample volume at the distal end of an optical fiber, or fiber bundle, and then sampling and analyzing the excited gas in the volume with a second fiber. This signal is then returned to a spectrometer and analyzed to determine the type and concentration of volatiles present. The systems can be multiplexed to obtain samples from multiple locations beneath the barrier. Using available microchip laser technology, the laser itself can be fiber-optically coupled and placed in the access tubes. A fiber optic spectroscopy sensor  624  at the distal end of a fiber  626  is shown in the enlarged views  612  and  614  of the access tube  604 . 
     Two other types of sensor systems could utilize the tube  604  of  FIG. 6  including acoustic sensors and radar sensor systems. Acoustic sensors could be used to determine barrier emplacement performance and to gather information about waste pit contents. Typically, arrays of acoustic transmitters would be disposed in tubes extending through the bottom wall containment barrier, for transmitting acoustic signals upwardly through the waste pit contents. Arrays of acoustic receivers are deployed on the surface or just under the surface at the top of the waste pit for receiving transmitted acoustic signals. The acoustic receivers in effect measure the propagation of various seismic waves, such as pressure waves, shear waves, raleigh waves, etc. (through the waste pit contents), such propagation depending upon the elastic properties of the contents. The arrays of transmitters and arrays of receivers are coupled via control cables to signal source and processor equipment and monitors for processing the acoustic signals and displaying information determined from the sensors, in a manner similar to the systems discussed earlier. 
     A radar system could also be used to map barrier performance. With such a system, transmitters could be deployed in the tubes extending in the bottom wall of a barrier containment system to transmit electromagnetic waves upwardly through the waste pit contents to electromagnetic wave receivers deployed on or near the surface of the waste pit. Heterogeneities in the waste pit contents (e.g., different soils, objects, moisture content, etc.) have different electromagnetic properties, transmitting electromagnetic waves through the waste contents and then receiving and mapping the transmitted signals will provide data about the contents and the performance of the waste containment barrier in containing the contents. Of course, the transmitter arrays and receiver arrays would be coupled by cables (or telemetry devices) to signal source and processor equipment and monitors for displaying the data derived from the transmission and reception of electromagnetic waves through the waste pit contents. 
     Although the acoustic sensor system and radar system described above were defined as transmitting signals from the bottom of the waste pit up to the surface thereof, it is obvious that the transmitters could be arranged on one side of the waste pit, with receivers arranged on the opposite side and that the signals could be transmitted effectively horizontally through the waste pit contents. In this case, the transmitters would be deployed in access tubes located on one side of the waste pit, with receivers deployed in access tubes located on the other side of the waste pit. 
     A resistivity system might also be deployed in the tubes for measuring long-term barrier performance. Such a system utilizes very low frequency electromagnetic fields (approaching the direct-current limit) to perform direct current resistivity measurements of the barrier contents. An electromagnetic wave transmitter would be deployed in a tube near the center of the waste pit at the bottom thereof, to transmit 360 degrees outwardly, with receivers being located outside of the waste pit, either underground or on the surface for receiving the transmitted waves. The resistivity measurements would provide an indication of barrier integrity such as imperfections, cracks and breaks. 
     With the arrangement of access tubes described in particular with respect to  FIG. 6 , it is apparent that various sensors could be deployed in the access tubes simultaneously or one type sensor might be deployed for data gathering at one point in time, then removed and another type sensor deployed in the access tubes for acquisition of different data. Since one or both ends of the access tubes would extend through the surface of the ground, sensor arrays could easily be installed and later removed from the access tubes to make way for a different sensor array. 
     Advantageously, the access tubes  604  could be made of any flexible, electrically neutral material, and may be perforated, as shown at  628  in  FIG. 6 , to allow entry of VOC&#39;s for detection purposes. The access tubes  604  could illustratively have an inside diameter of from 0.5 to 6 inches. The spacing of the access tubes, advantageously, is about three feet. 
     Another approach to monitoring barrier integrity involves the use of a tracer system in which tracers are placed at various locations in the barrier. The tracers could be dye, detectable by fluorescence spectroscopy, visual or chemical testing of samples of soil or groundwater, or ferromagnetic material, detectable by magnetic sensors. The sensors would be placed outside the barrier in positions to detect movement of the tracers and thus a possible breach in the integrity of the barrier. 
     Referring now to  FIG. 7 , there is shown an embodiment of a barrier placement machine  220 . The barrier placement machine  220  includes an operator&#39;s cab  97 , a cutting chain and grout injector assembly  333  including cutter teeth  31  and discharge paddles  33 , a grout receiving conveyor  959 , a soil retaining shield traveling pan  953 , a soil retaining shield consolidator  955 , a side trench excavator  91 , soil conveyor  933 , and track mechanism  975  for moving the entire machine  220 . The machine  220  is depicted in  FIG. 7  in schematic form, and may include all other components necessary for its operation, as understood by those of ordinary skill in the relevant field. 
     As the barrier placement machine  220  moves forward, a trench excavator  91  digs a side trench shown in phantom line at  226 . The trench excavator  91  carries the excavated soil  984  up out of the ground and dumps it on the trench excavator conveyor  991 , which carries the soil backwardly along the machine  220 . Grout or other suitable barrier forming material is then placed within the side trench  226  by the soil retaining shield traveling pan  953  and the soil retaining shield consolidator  955 , along with any other necessary grout injecting devices known to those of ordinary skill, to form the side barrier. The trench excavator conveyor  991  dumps the soil  984  behind the barrier placement machine  220 , refilling the side trench  226 . Simultaneously, the cutting chain and grout injector assembly  333  and soil conveyor  933  operate to excavate earthen material  985  from beneath the in-situ portion of earth  216  without removing said in-situ portion, and discharges the soil  985  above ground as shown in  FIG. 7  where it lies conveniently accessible for testing if desired. 
     The machine  220  further includes a barrier-forming means  953 ,  955  and  224  attached to the excavating means  31 ,  33  and  91  for simultaneously forming a side barrier and a generally horizontal, multi-layer barrier  228  (or could be a single-layer) within the generally horizontal trench  222 , said multi-layer barrier  228  having at least a first layer  202  and a second layer  204 . This is further described in the afore-cited co-pending application. 
     Regarding the horizontal, multi-layer barrier  228 , a horizontal barrier forming mechanism  224  is provided for forming at least a portion of the second layer  204  simultaneously with forming at least a portion of the first layer  202 . More specifically, the horizontal barrier forming mechanism  224  includes: a first injector  232  for injecting a first material for forming the first layer  202  in the horizontal trench  222 ; a mechanism for placing an intermediate shield  234  over the material for the first layer  202 ; a second injector  236  for injecting a second material for forming the second layer  204  onto the intermediate shield  234 ; and a frame  238  to which the intermediate shield  234  is attached for removing the intermediate shield  234  from between the first and second material forming the first and second layers  202  and  204 . The intermediate injectors  232  and  236  and, as an extension of the frame  238 , is advanced horizontally between the first and second layers  202  and  204  as they are formed, as the track mechanism  975  advances the machine  220 . 
     The first and second injectors  232  and  236  are contained within first and second chambers  240  and  242 , respectively. The intermediate shield  234  thus operates as a carrying member coupled to the chambers  240  and  242 . The third, middle layer  212  begins a dispensable, pre-formed roll  244  of barrier material that resides in a suitably sized trench  246 . The roll  244  of barrier material includes a first end  248 . Any suitable attaching means known to those of ordinary skill in the art may be used for attaching the first end  248  of the roll  244  of barrier material to the intermediate shield  234 , such that barrier material is withdrawn from the dispensable roll  244  as the machine  220  advances. In such manner the roll of material  244 , which might comprise a high performance material such as polyethylene or any suitable geo-textile membrane material, is pulled between the first and second layers  202  and  204  as the machine  220  advances. In this embodiment, the barrier material of the roll  244  preferably has sufficient strength to be pulled between the first and second layers  202  and  204  without substantial tearing. 
       FIG. 8  depicts another embodiment of a barrier placement approach in which a dispenser  250  comprises a pre-formed roll of barrier material rotatably disposed between horizontal digging elements  31 ,  33  and the chambers  240 ,  242 . The second injector  236  is positioned to inject the second layer  204  on top of an intermediate shield  234   a  such that said shield  34  separates the second layer  204  and the pre-formed layer  212  as said second layer  204  and said pre-formed layer  212  are being respectively injected and dispensed. The intermediate shield  34   a  thereby operates as a retaining plate. 
     Various sensors  846  ( FIG. 7 ) and  850  (FIG.  8 ), of the types described, may be disposed on the barrier material (geo-textile membrane) of the rolls  244  and  250 , respectively, for sensing barrier integrity, radiation, etc. In this manner, any desired sensor can be deployed between the first and second layers  202  and  204  by being incorporated into the membrane barrier material forming the roll  244  or the roll  250 . In other words, the sensors  846  or  850  can be installed at the same time as the barrier  228  is installed. 
     Referring again to  FIG. 8 , sensors  35  may be installed in the cutting teeth  31  to detect characteristics of the soil being removed such as volatile organic compounds (VOCs), heavy metals and radiation, to determine if contamination has leaked from the waste site. The sensors  35  might illustratively be comprised of scintillating fiber optic bundles, x-ray fluorescence sensors, or fiber-coupled optical systems, for transmitting signals to a receiver located, for example, on the surface to indicate the soil characteristics being detected. 
     In a manner similar to sensors  35  on the cutting teeth  31 , sensors could be mounted on a grouting beam or arm, such as those disclosed in the foresighted International Publication Numbers WO 94/19547 &amp; WO 93/00483 by Halliburton Nus Environmental Corp., for detecting soil characteristics of soil through which the grouting beam is moved to form the containment barrier. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements.

Summary:
A sensor system for a buried waste containment site having a bottom wall barrier and/or sidewall barriers, for containing hazardous waste. The sensor system includes one or more sensor devices disposed in one or more of the barriers for detecting a physical parameter either of the barrier itself or of the physical condition of the surrounding soils and buried waste, and for producing a signal representing the physical parameter detected. Also included is a signal processor for receiving signals produced by the sensor device and for developing information identifying the physical parameter detected, either for sounding an alarm, displaying a graphic representation of a physical parameter detected on a viewing screen and/or a hard copy printout. The sensor devices may be deployed in or adjacent the barriers at the same time the barriers are deployed and may be adapted to detect strain or cracking in the barriers, leakage of radiation through the barriers, the presence and leaking through the barriers of volatile organic compounds, or similar physical conditions.