in the description to follow and as previously mentioned , the terms &# 34 ; rock &# 34 ;, &# 34 ; soil &# 34 ;, and &# 34 ; earth &# 34 ; are periodically interchanged , it being recognized that soil or earth as commonly understood is essentially disintegrated or finely ground rock . the processing of unconventional foundation materials such as those associated with landfills , dredged materials and mine tailings should also be considered within the purview of the invention . in addition , it is also recognized that a selected contaminated soil which may be present on or near a construction site may be processed with the method and apparatus of the invention in order to treat and remediate the soil and at the same time convert the soil to an improved condition for use in a construction project at the site . as described earlier , when a weak soil ( particularly a clay soil ) is heated to temperatures above 200 ° c ., significant improvements in the engineering properties of the soil begin to occur . interstitial water is driven off in an irreversible reaction which begins to reduce the plasticity and the shrink - swell characteristics of the soil . at temperatures approaching 500 ° c ., the soil becomes non - plastic which greatly enhances its utility as a foundation material . the soil begins to take on brick - like qualities at temperatures above 900 ° c . soil melting begins at temperatures exceeding 1 , 100 ° c ., with the resulting rock - like material routinely exhibiting compressive strengths above 15 , 000 psi , which is several times greater than unreinforced concrete . because of wide variations in soil constituents , temperatures at which improvements in engineering properties take place may vary considerably . therefore , the above temperature limits should be considered as representative . candidate soils intended to be processed by the method of the invention may be readily tested in a laboratory in order to determine how their stabilization characteristics and engineering properties change when exposed to a range of temperatures from drying to melting and vitrification . the gradual and steady improvement in engineering properties resulting from this ability to heat soil to any selected temperature up to its melting stage thus provides construction operations with a wide range of soil improvement options with virtually any soil material . this flexibility permits soil specifications to be customized to meet prescribed strength requirements . for example , each one - foot soil layer of a foundation design can be specified and made to conform to specific engineering properties , and therefore in order to achieve these properties can be heated to a specific temperature . this capability thus results in an economical foundation thermal stabilization plan by permitting the soil strength to be less in the lower depths of a foundation . furthermore , the time required to convert local soils into lower strength construction materials will be reduced considerably , greatly increasing the throughput of the soil improvement process . thus the construction project could be completed more expeditiously . as another example of how the invention may be applied , it is noted that a military operation on a battlefield or in a remote location may not have access to conventional equipment and construction materials required for timely construction operations , particularly for vehicle trafficability and mobility over cross - country terrain . the capability to rapidly convert virtually any local soil into a satisfactory construction material by the invention method thus may provide an important determinant in the success of a military operation . in another example , construction operations in areas devastated by natural or man - made disasters , such as earthquakes , where routine construction materials cannot be readily transported to a construction site , may now take beneficial advantage of the teachings of the invention . based upon the recognized physical characteristics of soil as described and upon what is believed to be a novel use of plasma arc torches , the disclosure that follows describes a specific embodiment as exemplary of the invention . as next explained , the embodiment described is based on use of a rotary kiln heated with a plasma arc torch schematically illustrated as being supplied by appropriate utility sources 17 , i . e . plasma gas , water and power , and equipped with a conventional utility control 15 which enable the heat and thus the internal kiln temperature to be adjusted . the soil to be processed is fed into one adjustably elevated end of the kiln and discharged out of the opposite lower end as an improved construction material with properties dependent on the temperature to which the soil was heated while in the kiln . the tilt of the kiln is adjustable to control the rate of flow of material though the kiln and thus the residence time as somewhat schematically illustrated in fig1 . under an ideal situation , the internal kiln temperature is maintained at a constant relatively high temperature . thus , the residence time of the soil in the kiln becomes the principal variable factor in achieving the desired improved soil engineering properties . making reference to the illustrative embodiment of the invention , fig1 illustrates in a schematic elevation view , a mobile , rotating kiln 12 and associated apparatus for supporting and driving kiln 12 as it is mounted on a trailer 21 , supported on roadway 19 , as required for a complete thermal plasma soil improvement process according to the invention . the specific dimensions assigned to apparatus will vary since the size of kiln 12 will be directly dependent upon the power level of plasma arc torch 14 used in the process . typically a 1 megawatt plasma arc torch 14 will require a kiln 12 with an inside diameter of approximately five feet , while a 500 kilowatt torch 14 will require a minimum inside diameter of about three feet . the length of kiln 12 ( typically 20 feet long for a 1 megawatt torch ) will also be dictated by the range of residence times considered appropriate for the specific soil improvement application . rotating kiln 12 has the ability to be positioned at an angle of a few degrees up to an angle of approximately 20 degrees from horizontal through the actuation of jacks 16 by a suitable control 25 . this range of angles permits the soil residence time in the kiln to be regulated considerably . kiln 12 preferably has a capability to rotate 1 - 5 revolutions per minute on rollers 18 . the exhaust gas g from kiln 12 is passed through the untreated soil 20 in the feeder hopper 22 to preheat the soil . a clod breaker 26 is located at the base of the feeder hopper 22 to break up large lumps of soil before entering kiln 12 and to control the feed rate of the soil to the kiln 12 . a similar clod breaker 28 is located in a discharge hopper 30 to break up fused clods of treated soil . a side discharge system 32 is designed to empty the improved soil into a carrier alongside the mobile system , or to allow it to fall directly on the ground surface . vitrified soil exhibiting high strength characteristics may require additional processing through a rock crusher in order to break it into proper size pieces for its planned use in the construction project . fig2 is an enlarged cross section of the plasma heated soil improvement kiln 12 . kiln 12 comprises an open cylinder containing a series of metal fins 36 which help to agitate soil 20 and carry soil particles 20p close to the top of the kiln diameter before the soil tumbles through the hot plasma - heated gas stream . kiln 12 is lined by a suitable refractory material . kiln 12 rotates in the direction indicated by arrow a by means of driven rollers 18 engaging peripheral rims 46 . the soil particles 20p drop through the hot gas space 50 . the speed of rollers 18 and thus the speed of kiln 12 , is controlled by a suitable kiln speed control 27 ( fig1 ). fig3 illustrates in an alternative embodiment a conveyor belt loading system 60 which can be used to transport the untreated soil 20 to the kiln feeder hopper 22 . soil 20 is placed in the loading hopper 72 and fed to a conveyor 74 of the conveyor belt system 60 by a front end loader ( not shown ). hopper 72 may also be equipped with a clod breaker 76 . though not shown , it is recognized that in a variation of the illustrated fig3 embodiment , loading hopper 72 is replaced with a feeding system which picks up soil directly from windrows , or alternatively , from a rotary plow breaking up in - place , undisturbed materials , e . g . along the bed of a roadway under construction . in this variation , a continuous , moving apparatus consisting of the fig1 mobile soil conversion kiln and the fig3 conveyor belt system capable of slowly progressing along the roadway is achieved . fig4 schematically illustrates the courses that could make up a typical flexible pavement 80 . in this typical construction , a compacted subgrade 82 is first formed on the natural subgrade 84 . the compacted subgrade 82 in turn receives various courses 86 of an improved subgrade material which in turn receives subbase courses 88 . base course 90 is placed on the uppermost of subbase courses 88 and the surface course of pavement 92 is placed on top of base course 90 . prime , tack and seal coatings are not shown for purposes of simplifying the drawing . what should be recognized is that each of the soil materials 82 , 86 , 88 and 90 illustrated in fig4 could beneficially use soils with different engineering properties . this can now be achieved by processing a quantity of soil from a single source according to the method of the invention , processing portions of the soil to achieve relatively high but different temperatures whereby to cause each portion to acquire its own set of engineering properties suited to its particular application . for example , the soil making up the lower level of the improved subgrade 86 can be processed by the invention process to reach a comparatively low temperature to improve its properties only to a minimum extent , such as reduced plasticity , prior to being placed and compacted . each of the courses making up the upper layer material in layers 86 as well as the subbase course layers 88 may also be individually processed to reach ever increasing temperatures and thus causing each upper layer of material to have better engineering properties than those of the other courses . additionally , the soil normally making up the natural subgrade can be processed according to the method of the invention to further enhance the construction . the embodiment described above and illustrated in the accompanying drawings is a particular example and is not to be construed as a limitation of the scope and principles of the invention as more fully set forth in the appended claims .