You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to apparatus and methods for installing underground piping such as water and sewer mains, and in particular to apparatus and methods for installing such piping in narrow trenches. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are three methods commonly used for installing buried utility piping such as water and sewer mains. The “sale trench” method involves excavating a trench with its sides sloped at sufficiently shallow angles such that the potential risk of cave-ins is effectively eliminated. While providing optimal safety to workers, the safe trench method entails excavation of very large volumes of soil and the placing and compacting of corresponding large amounts of backfill. Because of the safe trench&#39;s sloped sides, this method disturbs and disrupts the use of a comparatively large surface land area. 
         [0003]    The “trench shield” method reduces the necessary amount of excavation by using a heavy shield or enclosure, to protect workers in the trench. The shield is reinforced to resist forces and pressures that would be exerted against the shield in the event of a cave-in, making it safely feasible to use a trench that is narrower than a safe trench, and without needing the trench sides to be backsloped (or at least not as shallowly as a safe trench). The shield is moved along the length of the trench as new sections of pipe are added to die pipeline, to protect the work area surrounding each newly-added pipe section. The trench shield method thus provides safety to workers while reducing excavation and backfill requirements, but it has significant drawbacks nonetheless. Although, the trench can be narrower and less sloped than in the safe trench method, it still needs to be quite wide in order to accommodate the shield. Moving the shield, each time a new pipe section is added, entails a considerable amount of time, effort, and expense. These factors are exacerbated by the fact that the shield is necessarily quite heavy, especially if it is made long enough (as is preferable) to protect along the full length of a typical pipe section (which is commonly six meters long). 
         [0004]    The third conventional method of installing underground piping is by inserting the pipe into a pre-bored hole. This method is very expensive, and its practical feasibility in a given situation will depend on a variety of variable factors (such as soil properties). 
         [0005]    For the foregoing reasons, there is a need for pipe installation apparatus and methods that are practically and economically feasible in a broad range of field conditions, while requiring less excavation than conventional trenching methods and ensuring optimal worker safety. The present invention is directed to this need. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In general terms, the present invention encompasses a method and apparatus for installing piping (especially jointed piping) in a narrow and substantially straight-walled trench, without need for workers to enter the trench. A first piping trench section is excavated to a desired length, using conventional equipment such as a track-mounted backhoe (also referred to as a trackhoe), with a bucket width typically in the range of 36 inches. Preferably, the bucket has a “spoon” attachment which farms a narrower secondary channel (or “sub-trench”) centered in the trench, for receiving piping. An equipment set-up area (or “working zone”), typically having a length of about 10 meters, is excavated at one end of the first trench section, for receiving the pipe installation apparatus of the present invention. The working zone is excavated in accordance with “safe trench” methods, to ensure the safety of workers operating the apparatus. Sand bedding is deposited into the trench (or, in the preferred embodiment, into the sub-trench), by workers and/or equipment at ground level. This process can be facilitated by having bedding sand deposited in small piles along the projected route of the trackhoe, prior to trench excavation. Upon completion, of excavation of a given section of trench, the hoe operator can scoop up some of the piled sand and deposit it in the trench (or sub-trench). 
         [0007]    A first section of pipe is fed into the pipe installation apparatus, which is actuated so as to push the pipe section into the piping trench. The leading end of the pipe section is supported on a pipe sled which it pushes over the sand bedding as the pipe is pushed into the trench. The leading edge of the pipe sled has an upward curve or is otherwise configured to prevent the pipe from digging into the sand bedding, and at the same time serves to level and at least partially compact the sand bedding. When the apparatus has pushed the first pipe section into the trench, a second pipe section is fed into the apparatus and coupled to the first pipe section, and the apparatus then pushes the joined pipe sections further into the trench. Additional pipe sections are added until a pipe string has been laid along the full length of the first trench section. 
         [0008]    A second trench section may then be excavated, along with an associated second working zone. The pipe installation apparatus is moved to the second working zone and is actuated to install a pipe string in the second trench section until it meets the pipe string previously laid in the first pipe section, and the two pipe strings are coupled to each other. The procedure is repeated as necessary to complete the full pipeline required for the project. 
         [0009]    The method of the invention also provides for the installation of telescoping temporary spacers at locations along the finished pipeline where valves, tees, or other fittings need to be installed. Provision may he made for the safe installation of these fixtures during the trench excavation, by enlarging the trench to “safe trench” standards in the intended vicinity of fitting. The locations where temporary spacers need to be installed in the pipeline may be determined during pipeline installation operations using conventional measuring or surveying techniques. This may be facilitated by use of a known device such as a metering wheel or meter tally, mounted to the pipe installation apparatus, for measuring the length of pipe that has passed through the apparatus, thus enabling workers to make accurate determinations of where spacers should be installed. After a given string of piping and associated fittings has been positioned, a compressive force is applied to the string to firmly seat all joints between the various components. Most conveniently, this compressive force may he applied using the bucket of the trackhoe. The trench and all working zones may then be backfilled and compacted as required. 
         [0010]    The present invention, also provides for a novel articulated packer apparatus especially adapted for compacting backfill in narrow trenches, such as in accordance with the method of the invention. The compaction apparatus may be independently self-propelled, or it may have a hydraulic drive system served by hydraulic fluid delivered by flexible hydraulic lines from the pipe installation apparatus. In preferred embodiments, the packer is remotely controlled so that it does not require an onboard operation, thereby further enhancing worker safety. 
         [0011]    Accordingly, in a first aspect the present invention is an apparatus, for installing piping in a narrow trench. 
         [0012]    In a second aspect, the invention is a packer for compacting backfill in a narrow trench. 
         [0013]    In a third aspect, the invention is a method for installing piping in a narrow trench. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which: 
           [0015]      FIG. 1  is a perspective view of the pipe installation apparatus in accordance with a first embodiment of the invention. 
           [0016]      FIG. 2  is a plan view of the apparatus- of  FIG. 1 , showing the outriggers in a stowed position. 
           [0017]      FIG. 3  is a plan view as in  FIG. 2 , but with the outriggers in a deployed position. 
           [0018]      FIG. 4  is an elevational cross-section showing a pipe being led through the pipe drive mechanism of the apparatus of  FIG. 1 . 
           [0019]      FIG. 5A  is an oblique partial section showing a pipe being fed through the pipe drive mechanism shown in  FIG. 4 , and illustrating the spring-actuated biasing means of the mechanism. 
           [0020]      FIG. 5B  is an oblique partial section as in  FIG. 5A  illustrating the actuation of the biasing means when a pipe coupling passes through the pipe drive mechanism. 
           [0021]      FIG. 6  is a plan view of the apparatus of  FIG. 1  positioned in a working zone and pushing a partially assembled pipe string into a trench. 
           [0022]      FIG. 7A  is a cross-section through a trench incorporating a secondary channel, shown with an optional laser support structure spanning the trench. 
           [0023]      FIG. 7B  is a cross-section through a working zone, incorporating a secondary excavation for housing the pipe installation apparatus of the present invention. 
           [0024]      FIG. 8A  is a side elevation of the apparatus in operation as in  FIG. 6 . 
           [0025]      FIG. 8B  is a side elevation of the leading end of a pipe string positioned in a pipe sled as shown in  FIG. 6 . 
           [0026]      FIG. 9  is a cross-section through a piping trench during backfilling operations using a remote-controlled articulated packer in accordance with the invention. 
           [0027]      FIG. 10  is a side elevation of the packer shown in  FIG. 9 . 
           [0028]      FIG. 11  is an elevational cross-section of a pipe drive mechanism in accordance with a second embodiment of the invention. 
           [0029]      FIG. 12  is a side elevation of the pipe drive mechanism of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    Referring to  FIGS. 1 ,  2 , and  3 , the pipe installation apparatus of the invention (generally designated by reference character  10 ) has a base structure  20  adapted to rest on a generally level surface, with a transverse bulkhead  30  mounted to base structure  20  at a medial point along the length of base structure  20 . In the Figures and in this specification, bulkhead  30  is representatively shown and referred to as a solid, plate-like structure, but that particular configuration is not essential to the invention. Bulkhead  30  could be of any suitably rigid construction, including an open framework. Accordingly, references herein to bulkhead  30  are to be understood in a non-restrictive sense, and to include transverse frames of other constructions. 
         [0031]    Bulkhead  30  has a front side  30 F and a rear side  30 R. The configuration and construction of base structure  20  may take any form suitable for the functions described herein. In the illustrated preferred embodiment, base structure  20  is of generally rectangular outline (as viewed in plan), with a front end  20 F and a rear end  20 R. Base structure  20  has a pair of spaced side rails  21  extending between, a front frame  22 F having a front frame opening  23 F, arid a rear frame  22 R having a rear frame opening  23 R. Openings  23 A and  23 B are sized to suit the pipe to be laid using apparatus  10 . Base structure  20  has a longitudinal axis extending between front end  20 F and rear end  20 R, approximately midway between side rails  21 . 
         [0032]    Front frame  22 F and rear frame  22 R each incorporate support legs  24 , which optionally may include adjustment means (not shown) for adapting to uneven supporting surfaces. The adjustment means could comprise manually-operated screw-type or ratchet-type jacks, hydraulic cylinders, or any other suitable mechanism, various, types of which are well known in the art. The overall size and proportions of base structure  20  will depend on selected operational design parameters. In preferred embodiments, however, base structure  20  will be configured such that it can be readily transported in the box of a half ton truck. 
         [0033]    Bulkhead  30  has a pipe opening  32  generally aligned with front frame opening  23 F and rear frame opening  23 R. Mounted in association with bulkhead  30  is a pipe drive mechanism, for engaging a pipe section  70  passing through pipe opening  32  and advancing it toward the front end  20 F of base structure  20  and through front frame opening  23 F. The pipe drive mechanism may take any of several different forms. In the embodiment shown in  FIGS. 1-4 , the pipe drive mechanism includes a plurality of drive wheels  34  spaced radially around pipe opening  32  in association with either the front side or the rear side of bulkhead  30 . In the preferred embodiment (and as best seen in  FIG. 4 ), each drive wheel  34  has its awn hydraulic drive motor  36 . Drive wheels  34  preferably will be rubber-tired, to facilitate effective tractive engagement with pipe  70  without causing damage to the outer surfaces of pipe  70 . 
         [0034]    The pipe drive mechanism shown in  FIGS. 1-4  has a total of four drive wheels, each with its own hydraulic drive motor  36 . However, other pipe drive configurations are readily conceivable. To provide non-limiting examples of alternative configurations, the pipe drive mechanism could have three drive wheels, rather than four as shown. Another alternative embodiment could have four pipe-engaging wheels as shown, but with only two of the wheels being driven (preferably radially opposing each other) and with the other two wheels acting as idlers winch help guide the pipe  70  through pipe opening  32 . Other embodiments could use only a single drive wheel. A further embodiment (shown in  FIGS. 11 and 12 , and described in detail later in this specification) would have six drive wheels mounted in pairs, with each pair of wheels driven by a single hydraulic motor by means of a pair of drive chains. 
         [0035]    Simple embodiments of the pipe drive mechanism may have a fixed configuration for handling pipe of a specific diameter. In preferred embodiments, however, the pipe drive mechanism incorporates wheel adjustment means for adapting to different pipe sizes. In the embodiment shown in  FIGS. 1-4 ,  5 A, and  5 B, the wheel adjustment means is provided by mounting each motor  36  on slide arm  38  which slides within a sleeve  40  which in turn is pivotably mounted to a bracket  41  connected to bulkhead  30 . The radial position of slide arm  38  within sleeve  40  may be controlled by means of set screws or bolts  44  as illustrated, or by any other suitable and conventional means. The wheel adjustment means could of course be provided in various other forms using well-known technology. For example, a hydraulic or pneumatic cylinder could be provided for adjusting the radial position of each drive wheel  34  to accommodate different pipe sizes. 
         [0036]    As illustrated in  FIGS. 5A and 5B , the pipe drive mechanism preferably includes biasing means for biasing drive wheels  34  against a pipe  70  passing through pipe opening  32  so as to optimize the grip or traction between drive wheels  34  and pipe  70 . In the Illustrated embodiment, the biasing means for each drive wheel  34  is provided in the form of a compression spring  42  disposed between sleeve  40  and bulkhead  30 , radially outboard of the associated bracket  41 . When there is no pipe passing through pipe opening  32 , spring  42  biases wheel  34  toward (or even against) front face  30 F of bulkhead  30 , with the clear space between opposing drive wheels  34  being somewhat less than the diameter of the pipe to be installed. Therefore, when a pipe section  70  is then passed through pipe opening  32  in a direction toward front frame opening  23 F, it will be tractively engaged by drive wheels  34  (which are being rotated by their respective hydraulic motors  36 ). 
         [0037]    Springs  42  thus promote and maintain effective traction between drive wheels  34  and pipe  70 . At the same time, they provide resiliency to accommodate imperfections in pipe  70  (for example, out-of-roundness), and to accommodate passage of pipe couplings  72  at connections between pipe sections where, as is common, the outer diameter of the coupling  72  is greater than that of pipe  70 . As shown in  FIG. 5B , the passage of a coupling  72  through pipe opening  32  is accommodated by additional compression of spring  42 , which remains effective to keep drive wheels  34  in tractive engagement with pipe  70  (and coupling  72 ). 
         [0038]    Persons skilled in the art of the invention will readily appreciate that other effective biasing means may be devised in accordance with known principles and technologies, without departing from the essential concepts of the present invention. 
         [0039]      FIGS. 11 and 12  illustrate an alternative embodiment of the pipe drive mechanism having three pairs of drive wheels, with each pair of wheels being driven by a single hydraulic motor. A pair of spaced-apart upper wheels  34 U are rotatably mounted in coplanar relation to a suitable upper beam structure  190 U positioned above pipe opening  32  and extending between bulkhead  30  and a suitable upper support member  192 U connected to or forming part of base structure  20 . As shown in  FIG. 11 , upper wheels  340  are radially oriented relative to pipe opening  32 . Each upper wheel  34 U has a coaxially-mounted upper wheel sprocket  194 U rotatable with upper wheel  34 U. An upper motor support structure  1950 U is mounted to upper beam  190 U at a point between upper wheels  34 U, and supports an upper hydraulic motor  361 U which tarns an upper drive sprocket  196 U lying in the same plane as upper wheel sprockets  194 U. A continuous upper drive chain  198 U is disposed around upper wheel sprockets  196 U and upper drive sprocket  196 U such that actuation of upper motor  36 U will cause rotation of upper wheels  34 U. 
         [0040]    Upper motor support structure  195 U may be of any suitable construction, and is preferably adapted to include or accommodate motor position adjustment means for adjusting the position of upper motor  36 U relative to upper motor support structure  195 U, to facilitate tensioning of upper drive chain  198 U as may be required. In  FIG. 12 , the adjustment means is conceptually shown as incorporating an arm to which upper motor  36 U is mounted and which is slidable within a sleeve member connected to upper beam structure  190 U. However, persons skilled in the art will appreciate that the motor position adjustment means could take various other forms in accordance with well-known design principles and techniques. 
         [0041]    In simple embodiments, upper beam structure  190 U can be rigidly connected to its end supports (i.e., bulkhead  30  and upper support member  192 U), with its position being set to accommodate a specific size of pipe  70 . In preferred embodiments, though, upper beam  190 U is mounted to its end supports using suitable wheel height adjustment means  199 , thus allowing the radial position of upper wheels  34 U, relative to pipe opening  32 , to be adjusted to suit different sizes of pipe  70 . In  FIG. 12 , wheel height adjustment means  199  is shown as comprising an upstand connected to upper beam  190 U and slidable within a capped tubular sleeve connected to bulkhead  30  (or upper support member  192 U), with a coil spring disposed between the upstand and the cap of the sleeve to bias upper wheels  34 U radially toward a pipe  70  passing through pipe opening  32 . A bolt  44  or pin passes through a hole (or holes) in the sleeve and through a vertically slot (or slots) in the upstand, such that the upstand is retained by and movable within the sleeve (to the extent allowed by the slots). Multiple holes can be provided in the sleeve to facilitate adjustment of wheel height adjustment means  199  to suit different pipe sizes. 
         [0042]    The construction shown and described in connection with wheel height adjustment means  199  is for purposes of example only. Persons skilled in the art will appreciate that wheel height adjustment means  199  could take various other forms in accordance with well-known design principles and techniques. 
         [0043]    Below upper beam structure  190 U and pipe opening  32 , a pair of lower beam structures  190 L extend between bulkhead  30  and a suitable lower support member  194 L connected to or forming part of base structure  20 . A pair of spaced-part lower wheels  34 L are ratably mounted to each lower beam  190 L in substantially the same fashion as described in connection with upper wheels  34 U. Each lower wheel  34 L has a coaxially-mounted lower wheel sprocket  194 L, rotatable with lower wheel  34 L. A lower motor support structure  195 L is mounted to each lower beam  190 U at a point between lower wheels  34 L, and supports a lower hydraulic motor  36 L which turns a lower drive sprocket  196 L lying in the same plane as lower wheel sprockets  194 L. A continuous lower drive chain  198 L is disposed around lower wheel sprockets  196 L and lower drive sprocket  196 L such, that actuation of lower motor  36 L will cause rotation of lower wheels  34 L. 
         [0044]    As best seen in  FIG. 11 , the two pairs of lower wheels  34 L are preferably disposed on either side of pipe opening  32  in a canted radial orientation, such that all upper wheels  34 U and lower wheels  34 L can tractively engage a pipe  70  passing through pipe opening  32 , with all wheels&#39; planes of rotation passing through or close to the longitudinal axis of pipe  70 , thus optimizing tractive efficiency. In alternative embodiments, however, the planes of the two pairs of lower wheels  34 L could both be vertical. 
         [0045]    Although three sets of wheels are used in the embodiment shown In  FIGS. 11 and 12 , it would of course be feasible to use more than three sets. However, the use of three sets of wheels is particularly preferred since that configuration helps to ensure that all wheels will have substantially uniform contact with, pipe  70 . Maximum tractive effectiveness with respect to pipe  70  is achieved by driving all wheels  34 U and  34 L, but this is not essential in one variant, only lower wheels  34 L are driven, with upper wheels  340  being idlers; in another variant, only upper wheels  34 U are driven, with lower wheels  34 L being idlers. 
         [0046]    Persons of ordinary skill in die art will appreciate that other variants of the drive mechanism of  FIGS. 11 and 12  may be readily devised without departing from the principles of the present invention. To provide one non-limiting example, pulleys and drive belts could be used instead of sprockets and drive chains. 
         [0047]    The operation of the pipe drive mechanism to advance pipe toward and through front frame opening  23 F will necessarily result in an opposite reactive force acting against base structure  20 . Accordingly, anchorage means must be provided to resist this reactive force in order to prevent rearward displacement of the apparatus  10  (i.e., to transfer the reactive force to the ground in the vicinity of apparatus  10 ). It may be possible in some operative circumstances, when the magnitude of the reactive force is small, for the anchorage means to be effectively provided by frictional or mechanical resistance between base structure  20  and the surface upon which it rests. In preferred embodiments, however, and as shown in  FIGS. 1 ,  2 ,  3 , and  6 , the anchorage means is provided in the form of a pair of outriggers  26 , one on either side of base structure  20 . One end of each outrigger  26  is mounted to base structure  20  (preferably, but not necessarily, near front end  20 F thereof) so as to be pivotable about a vertical axis. The other end of each outrigger  26  has an anchorage member  27  (such as a steel plate or blade) adapted to penetrate into and to be retained within a soil mass. Each outrigger  26  has a hydraulic, cylinder  28  extending; from a point near anchorage member  27  to a selected connection point on base structure  20 . Actuation of hydraulic cylinder  28  is thus effective to move outrigger  26  in a generally horizontal plane between a stowed position (as shown in  FIG. 2 ) and a deployed position (as shown in  FIGS. 3 and 6 ). Effective result: have been achieved using hydraulic cylinders  28  having a 2-inch bore and an 8-inch stroke, with a working pressure of 3,000 pounds per square inch. However, hydraulic cylinders with other characteristics may be suitable or appropriate depending on site conditions arid desired operational, criteria. 
         [0048]    It will be appreciated that the anchorage means described above and illustrated in the Figures represents an exemplary embodiment, and other effective anchorage means may be devised without departing from the principles of the present invention. 
         [0049]    In simpler embodiments of the invention, pressure hydraulic fluid for actuating the hydraulic wheel motors and hydraulic cylinders of the anchorage means could be provided from a source external to apparatus  10 . In preferred embodiments however, apparatus  10  is a self-contained unit, and therefore includes a power control system, conceptually indicated in  FIGS. 1 ,  2 ,  3 , and  6  as comprising a power module  50  and a control module  60 . In the preferred embodiment, power module  50  incorporates a gas or diesel engine (with various accessories including a fuel tank), a hydraulic pump which is driven by the gas or diesel engine, and a hydraulic fluid reservoir. To provide one non-limiting example, beneficial results have been achieved using a 20-horsepower gas engine driving a Vickers™ Model 45D50A1A122R hydraulic pump with 1-inch lines. Control module  60  incorporates hydraulic system accessories such as manifolds, valves, and valve actuators for controlling flow of hydraulic fluid between the fluid reservoir and hydraulic motors  36  associated with drive wheels  34 , via hydraulic hoses  37 . In the preferred embodiment, power module  50  and control module  60  are mounted to base structure  20  in association with auxiliary rails  21  extending between rear frame  22 R and bulkhead  30 , but other mounting arrangements are possible without departing from the essential concept of the invention. 
         [0050]    Persons skilled in the field of the invention will be sufficiently familiar with the principles of power systems and hydraulic drive and control systems so as to be readily able to devise one or more embodiments of a power module  50  and a control module  60  suitable for use with the present invention, without need to set out detailed hydraulic schematics or component particulars for purposes of this patent specification. 
         [0051]      FIGS. 6 ,  7 A,  7 B, A, and  8 B illustrate how the apparatus  10  of the invention may be deployed in the field for purposes of installing underground piping. As shown in  FIG. 7A , a piping trench  80  is excavated along a desired path, using suitable equipment such as a conventional trackhoe. As may be seen from  FIG. 6  and in particular from  FIG. 7A , trench  80  may be comparatively narrow, with vertical or near-vertical sidewalls  80 W if the soil is sufficiently cohesive. As indicated by reference characters  81 , it may in some cases be desirable to backslope the upper regions of sidewalls  80 W. If soil characteristics are such that sidewalls  80 W require some amount of backsloping, the backslope angle can generally be significantly sleeper than would be warranted when installing pipe using safe trench methods. 
         [0052]    In preferred embodiments of the method, a secondary channel  82  is excavated at the base of trench  80 . Secondary channel  82  may be formed using any suitable method. Preferably, secondary channel  82  will be formed concurrently with trench  80 , using a trackhoe with an auxiliary blade or “spoon” permanently or removably attached to, and extending downward from, the cutting edge of the trackhoe bucket. The geometry of the “spoon” will be selected to suit the desired cross-sectional dimensions of secondary channel  82 , which, in turn will depend on the size of pipe to be installed in secondary channel  82 . As desired, a different, size of “spoon” may be used for each pipe size; alternatively, a given size of “spoon” may be used for a range of pipe sizes. 
         [0053]    The depth of trench  80  (and, in the preferred embodiment, secondary channel  82 ) needs to be controlled within reasonably close tolerances in order to ensure that the installed pipeline will be at the intended grade and slope. This is accomplished in accordance with well-known level surveying methods, preferably using a stationary surveyor&#39;s laser  200 . For this purpose, and as may be seen in  FIG. 7A , a laser support structure  210  may be provided at a convenience location, spanning trench  80 , for supporting the laser  200 , which emits a visible beam in a constant horizontal plane. As trench excavation proceeds, a worker carrying a surveyor&#39;s rod of suitable length holds the rod on the bottom of trench  80  in location as directed by the trackhoe operator. The laser beam intercepts the scale on the rod, enabling the trackhoe operator to determine the current depth of trench  80 , and to determine the extent to which additional excavation may be required. 
         [0054]    To prepare for use of the pipe installation apparatus  10  of the present, invention, a working zone  84  is excavated at the end of trench  80 , generally as shown in  FIGS. 6  and  7 B. The length of working zone  84  (as measured parallel to trench  80 ) will preferably be in the range of 10 meters, but in general will be selected to suit various practical factors including the dimensions of apparatus  10  and the desired extent of worker access space around apparatus  10 . Working zone  84  has sidewalls  84 W which are backsloped in accordance with “safe trench” methods as appropriate to suit soil conditions. A machine pit  88 , with sidewalls  88 W, is excavated at the base of working zone  84  to accommodate apparatus  10 , leaving a generally level access area  86  adjacent to apparatus  10  as appropriate. Machine pit  88  is excavated within reasonable tolerances to facilitate effective engagement of anchorage members  27  with sidewalls  88 W. As best seen in  FIG. 7B , machine pit  88  is excavated to art appropriate depth such that once apparatus  10  is positioned therein, front frame opening  23 F, rear frame opening  23 R, and pipe opening  32  of bulkhead  30  will be in general alignment, both horizontally and vertically, with the base of trench  80  (or, in the preferred, embodiment, with secondary channel  82 ). 
         [0055]    After working zone  84  and machine pit  88  have been excavated, apparatus  10  is positioned in machine pit  88  as shown in  FIGS. 6 and 7B . Outriggers  26  are then deployed, by actuation of hydraulic cylinder  28 , such that their anchorage members  27  penetrate and securely engage sidewalls  88 W of machine pit  88 . As shown In  FIGS. 7A and 8B , a layer of sand bedding  110  is deposited in the bottom of trench  80  (or, in the preferred embodiment, secondary channel  82 ). A first pipe section  70 A is fed manually through rear frame opening  23 R and pipe opening  32  so as to engage drive wheels  34 , which in turn advance first pipe section  70  forward through front frame opening  23 F. Leading end  72 A of first, pipe section  70 A is then engaged with a pipe sled  90  as shown in  FIGS. 6 ,  7 A, and  8 B. Pipe sled  90  has a sole plate  92  adapted for sliding over sand bedding  110 , with a contiguous upturned prow member  94  that prevents pipe sled  90  from digging downward into sand bedding  110 . Pipe sled  90  also has a sleeve or bracket  96 , of any suitable configuration, for receiving and retaining leading end  72 A of first pipe section  70 A. 
         [0056]    The apparatus  10  is then activated so as to advance first pipe section  70 A and pipe sled  90  into trench  80 , with pipe sled  90  acting to level and to some extent compact sand bedding  110  as it passes thereover, and with the horizontal reactive force induced by this operation being transferred into sidewalls  88 W of machine pit  88  through outriggers  26  and anchorage members  27 . Pipe sled  90  may be suitably heavy or may have supplemental weighting to enhance its effectiveness for purposes of levelling and compacting sand bedding  110 . 
         [0057]    When the trailing end  74 A first pipe section  70 A approaches rear frame opening  23 R, the forward advance of first pipe section  70  is temporarily stopped so that a second pipe section  70 B can be coupled to trailing end  74 A of first pipe section  70 A. The apparatus  10  is then reactivated so as to advance the pipe string (comprising first and second pipe sections  70 A and  70 B) further into trench  80 . Hits mode of operation is carried on, with additional pipe sections being added as required, until leading end  72 A of first pipe section  70 A has advanced to a desired final position. At that stage, apparatus  10  may be re-positioned in a second working zone  84  a selected distance back along trench  80 . A second pipe string is then advanced into the trench until it meets and is coupled to the trailing edge of the first pipe string. This procedure is repeated as required until the entire pipeline required for the project has been laid in trench  80 . 
         [0058]    The distance between working zones  84  will be selected to suit a variety of factors, including but not limited to the size and weight of pipe being installed and the mechanical capabilities of the particular apparatus  10  being used. As a general rule, the power required to advance a pipe string into trench  80  will be greater for heavier pipe sections, and will increase as the length of the string increases. It has been found that working zone intervals in the range of 50 to 100 meters are typically sufficient for installing 6-inch to 12-inch plastic wafer mains, using an apparatus  10  compact enough to be transported on a half-ton truck. However, larger or smaller working zone intervals may be practical or desirable for particular combinations of variable design factors and project requirements. 
         [0059]    At one or more locations along the length of the pipeline being installed, it will commonly be necessary to install valves, tees, cleanouts, or other fittings. To accommodate such fittings, the method of the invention provides for the installation of collapsible spacers (not shown) in such locations. The spacers may be of any suitable construction. In the preferred embodiment, however, each spacer comprises a first pipe section and a smaller second pipe section which can slide in telescopic fashion within the first pipe section. Preferably; each pipe section has a linearly-arrayed series of pin holes for receiving a retainer pin. The second pipe section is positioned as desired within the first pipe section, with at least one pin hole, of each pipe section being In alignment, whereupon one or more suitable retainer pins can be dropped through the aligned pin hole(s), thus temporarily fixing the length of the spacer (to suit the length of the fitting to be installed in the, spacer location). One end of the spacer will be a “male” end and the other end will be a “female” end, adapted for engagement with typical pipe sections  70  being laid in trench  80  (or secondary channel  82 ). 
         [0060]    The collapsible spacers thus make it possible to install the full length of the pipeline, using the apparatus and method of the present invention, in a continuous fashion without needing to interrupt pipe-laying operations to install valves and tees and the like. After the pipeline has been laid out incorporating all required spacers, workers can enter a secondary “safe” working zone which has been excavated around each spacer to install the required fitting. The spacer is “collapsed” by removing the retainer pin(s) and then telescoping the two spacer sections, thus disengaging the spacer from adjacent pipe sections  70  to Which the spacer had been temporarily connected. The required valve or other fitting is then connected between the adjacent pipe sections  70 . 
         [0061]    After all spacers have been replaced with their corresponding valves, fees, or other fittings, the entire pipeline string is ready to be backfilled. Prior to that step, however, the connections between the various components are preferably made more secure by applying a compressive force to the string, so as to firmly seat all joints. Such a compressive force may be applied using the bucket of a trackhoe. 
         [0062]    After all required pipeline strings have been positioned and connected as desired (and after the pipe installation, apparatus  10  has been removed), all trenches  80 , secondary channels  82 , working zones  84 , and machine pits  88  may be backfilled and compacted as appropriate. In many if not most cases, it will necessary or desirable for the backfill  115  to be compacted to specified densities to prevent excessive settlement as backfill  115  consolidates over time, and methods and equipment for achieving such backfill densities are well known, in the interests of worker safety, however, it is desirable be able to compact, backfill  15  in narrow trenches  80  without the need for workers to descend into them. 
         [0063]    For this reason, compaction of backfill  115  in trenches  80  is preferably carried out using a remote-control led articulated packer  120  as illustrated in.  FIGS. 9 and 10 . In the preferred embodiment, packer  120  has a front section  120 A plus a rear section  120 B of basically construction. Front section  120 A has a roller drum  122 A mounted to a peripheral frame  126 A by means of suitable bearings  124 ; similarly, rear section  120 B has a roller drum  122 B mounted to a peripheral, frame  126 B by bearings  124 . Frames  126 A and  126 B are coupled by a suitable articulation linkage (conceptually indicated by reference character  160 ) whereby front and rear sections  120 A and  120 B may swivel relative to each other about a substantially vertical axis Z. The articulation linkage may incorporate steering means for selectively controlling relative swivelling of front and rear sections  120 A and  120 B. The steering means preferably will include at least one hydraulic steering ram, although other types of steering mechanisms may also be used. Although not essential, linkage  160  preferably will also provide for at least a limited degree of swivelling about a transverse horizontal axis. 
         [0064]    Roller drums  122 A and  122 B are fabricated of steel plate in a fashion similar to rollers of known compaction equipment, with a continuous cylindrical outer plate  123  arid circular side plates  125  enclosing an inner chamber  127  that may be filled with ballasting material (such as water), in the illustrated embodiment, side plate  125  on roller drum  122 A is inset a suitable distance from the edge of outer plate  123  to define a a recess  125 F in which a suitable packer drive/braking mechanism (schematically indicated by reference character  150 ) may be disposed. The packer drive/braking mechanism could take a variety of forms, only a few of which are described or illustrated herein. 
         [0065]    In preferred embodiments, the packer drive mechanism incorporates a reversible hydraulic motor having a “neutral” mode. In the preferred embodiment, the output shaft of the hydraulic motor is fitted with a drive sprocket that engages a drive chain attached to the outer lace of side plate  125  (such as by welding) in a circular configuration concentric with the drum&#39;s axle, thereby causing the drum to rotate in a selected direction. Alternatively, a sprocket, could be concentrically mounted to side plate  125 , and driven by means of a drive chain disposed around the hydraulic motor&#39;s drive sprocket and the sprocket mounted to side plate  125 . 
         [0066]    The packer braking mechanism may work on principles analogous to automotive dram brakes, with one or more brake, shoes (with appropriately curved brake pads) that may be urged radially outward into contact with the inner face of outer plate  123  within recess  125 F so as to retard and stop the rotation of the associated roller drum. 
         [0067]    The sizes of roller drums  122 A and  122 B and their associated frames  126 A and  126 B will be determined to suit the width of trench  80  in which packer  120  is intended to be operated, as well as the roller mass required to achieve the desired level of backfill compaction. Satisfactory results have been achieved using roller drums having diameters of approximately 42 inches. 
         [0068]    In the embodiment shown in  FIG. 10 , front section  120 A of packer  120  has a platform  165  disposed above roller drum  122 A and supported from frame  126 A by suitable structural support members  132 . The purpose of platform  165  is to support auxiliary components (schematically indicated by reference character  170 ) associated with packer drive/braking mechanism  150  and its remote control system. In preferred embodiments, the auxiliary components will include a hydraulic pump operably connected to the hydraulic motor of the packer&#39;s drive system, and a gas motor for driving the hydraulic pump. 
         [0069]    The remote control system for the packer drive/braking mechanism  150  may be either a wireless (e.g., radio-controlled) or hard-wired system, in accordance with, well-known technology. In alternative embodiments, the packer may have a seat (and possibly a cab) for a riding operator, rather than being remotely controlled. 
         [0070]    In preferred embodiments, as shown in  FIG. 10 , packer  120  has a second platform  130  carrying a water tank (schematically indicated by reference character  140 ), which may be used for adding water to backfill in the trench as may be required to achieve desired or required backfill compaction standards. 
         [0071]    Also in preferred embodiments, packer  120  may be equipped with an adjustable “dozer” blade at either or both ends of packer  120  (as schematically indicated by reference characters  180 A and  180 B in  FIG. 10 ). Dozer blades  180 A and  180 B will ideally be adjustable for both blade height and blade angle, by means of suitable hydraulic rams operably connected to a hydraulic pump included in auxiliary components  170 . This pump could be the same pump that serves the hydraulic motor associated with packer drive/braking mechanism  150 , or it could be a dedicated pump serving only the dozer blades. 
         [0072]    It may be seen from the foregoing that the present invention enables she installation of utility in narrow and substantially straight-walled trenches, thus requiring considerably less excavation and backfill than in conventional pipe installation methods, while eliminating or limiting the need for workers to enter the trenches. 
         [0073]    It will be readily appreciated by those skilled in the art that various modifications of the present invention may be devised without departing from the essential concept of the invention, and all such modifications are intended to come within the scope of the present invention. 
         [0074]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following that word are included, but items not specifically mentioned are not excluded. A reference to an element by the Indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element.

Summary:
In a method for installing underground piping, a pipe installation apparatus is temporarily positioned at one end of a pipe trench, in which bedding sand has been placed. The apparatus has a pipe opening through which pipe can pass, and a plurality of rubber-tired pipe wheels arrayed around and biased radially inward toward the pipe opening. At least one wheel is a motor-operated drive wheel. A section of pipe introduced into the pipe opening will be tractively engaged by the drive wheels and pushed through the pipe opening and into the trends The drive motors are disengaged as required for connection of additional pipe sections, or for placing temporary spacers in the pipeline to facilitate subsequent installation of required pipeline fittings. The leading end of the pipeline engages a sled which rides over and levels die bedding sand while preventing the pipe from digging into the sand. The need for workers to enter the pipe trench is thus reduced or eliminated, making it possible to safely install piping in steep-walled trenches.