An apparatus for drilling annulus ring hole, comprising: a down-the-hole (DTH) percussion hammer assembly, the DTH percussion hammer assembly comprising one or more DTH percussion hammers, an annulus ring structural housing having an outer diameter (OD) and an inner diameter (ID) for accommodating the DTH percussion hammers within space between the OD and the ID of the annulus ring structural housing; one or more drill pipes being connected serially with each other forming a string of drill pipes, the string of serially connected drill pipes being connected to the DTH percussion hammer assembly; a top pipe with one or more exhaust openings being connected to the string of serially connected drill pipes; an air or fluid distributor being connected to the top pipe; and a rotary head providing rotational turning motion for the apparatus, and the rotary head being connected to the air distributor.

FIELD OF THE INVENTION 
     The present invention relates generally to civil building construction. More specifically, the present invention relates to techniques and equipments used in building structural foundation piling. 
     BACKGROUND 
     Conventional ground drilling equipments or techniques normally drill holes with a single or outer diameter (OD) target. Depending on the ground condition, a casing sometimes is inserted in the ground to prevent collapse of soil when the ground condition is loose. In the case where the ground strata are stable, no casing is necessary. In either case, all the materials inside the hole will needed to be excavated away in the process of drilling. 
     One design of drilling equipment has a single down-the-hole (DTH) percussion hammer. With this design, the hole drilled has a maximum possible size of about 1500 mm in diameter. Such single DTH hammer drilling will result in complete excavation of all materials inside the hole drilled as well. 
     Another design of drilling equipment has a number of DTH percussion hammers arranged over a circular disc and housed inside a circular housing. Because all the materials of the circular hole drilled will be fully excavated, this design is limited only to drilling solid circular holes, not annulus ring shaped holes. 
     It is also common to use a rotary drilling machine to drill holes with auger in relatively soft ground or core barrel in harder ground condition. This kind of drilling method will also entail the excavation of all materials inside the hole leaving only a solid circular hole. 
     There is also another drilling technique that utilizes a casing oscillator to first drive a casing down into the ground by oscillation and pushing. In order to continue driving the casing down into the ground, it is necessary to excavate the materials inside the cased hole during the drive by tools such as hammer grabs and chisels. As such, this drilling technique cannot be used to form annulus ring hole. The penetration speed is also very low. 
     SUMMARY 
     It is an objective of the presently claimed invention to provide an apparatus for drilling an annulus ring hole in the ground. That is, by using a drilling apparatus with a number of down-the-hole (DTH) percussion hammers driven by compressed air or pressurized fluid, such as water, arranged and allocated in an annulus housing, to drill an annulus ring shaped hole in the ground with different outer diameters (OD) and inner diameters (ID), which can be designed to match certain requirements of a drilling or a building structural foundation piling project. The OD of the annulus ring hole typically can range from 200 mm to 5000 mm (or larger). To meet the various OD and ID configurations requirement, the number of DTH hammers and their distribution positions over the drilling area are configurable. 
     The presently claimed invention provides an apparatus for drilling an annulus ring hole without excavating or disturbing the ground material in the central interior area of the annulus ring hole. Only the materials inside the annulus ring area being excavated leaving a column of materials in the central interior area (the circular area within the ID) of the annulus ring hole. The high percussive power of the apparatus allows penetration of hard rock ground material such as granite with reasonable speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are described in more detail hereinafter with reference to the drawings, in which 
         FIG. 1  shows the side view of one embodiment of the presently claimed annulus ring hole drilling apparatus; 
         FIG. 2  shows the bottom view of the DTH percussion hammer assembly one embodiment of the presently claimed annulus ring hole drilling apparatus; 
         FIG. 3  shows the cross sectional view of one embodiment of the presently claimed annulus ring hole drilling apparatus; 
         FIG. 4  shows the cross sectional view of the DTH percussion hammer assembly of one embodiment of the presently claimed annulus ring hole drilling apparatus; 
         FIG. 5  shows the top pipe with exhaust openings of one embodiment of the presently claimed annulus ring hole drilling apparatus; 
         FIG. 6  shows the drill bit of one embodiment of the presently claimed annulus ring hole drilling apparatus; and 
         FIG. 7  shows various exemplary drill bit cutting face profiles and various exemplary drill bit arrangements of more than one circumferential layers of drill bits. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, apparatus for annulus ring hole drilling and the like are set forth as preferred examples. It will be apparent to those skilled in the art that modifications, including additions and/or substitutions may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation. Although the embodiments described in the following pertain to using compressed air as the force delivery medium, pressurized fluid, such as water, can be used in place of compressed air without substantial modification to the presently claimed invention. 
     Referring to  FIG. 1 . The side view of an annulus ring hole drill string  100  is shown in  FIG. 1 . In accordance with various embodiments, the annulus ring hole drill string includes a DTH percussion hammer assembly  101 , one or more drill pipe  102 , a top pipe with exhaust openings  103 , an air distributor  104 , a rotary head connection interface  105  for connecting a rotary head  106 . The rotary head  106  provides rotational turning speed and output torque for the entire drill string  100 . Rotary head  106  is equipped with intake swivel  107  where compressed air is supplied from an external source and delivered through an internal channel down to the rotary head connection interface  105 . Other configurations of drill pipe, top pipe, and air distributor can be adopted. The number and total length of drill pipes  102  adopted depend on the required drilling depth of the annulus ring hole to be drilled. In some cases of shallow drilling, no drill pipe is necessary. 
     Compressed air supplied from the external source to the intake swivel  107  is delivered through the internal channel in the rotary head  106  and down to the rotary head connection interface  105  and through the internal channel in the rotary head connection interface  105  to the air distributor  104  below it. The air distributor  104  diverts the airflow into a number of separate air paths matching the number of DTH percussion hammers employed in the DTH percussion hammer assembly  101 . In this exemplary embodiment, five DTH percussion hammers are employed. In this case, the air distributor  104  ends with five air passages; each is further connected to an internal air-delivery pipe in the top pipe with exhaust openings  103 , to an internal air-delivery pipe in the drill pipe  102 , then to the respective DTH percussion hammer. 
     Still referring to  FIG. 1 . During drilling, while the DTH percussion hammers are impacting the ground, the entire drill string, including the air distributor  104 , the drill pipes  102 , and the front annulus ring drilling percussion hammer assembly  101 , is rotating, driven from the top by the rotary head. The rotary head provides the necessary turning torque for overcoming the drilling friction. The rotation axis is the centre of the annulus ring. Therefore, the percussion and impacting actions provided by each drill bit combined with the continuous turning of the annulus ring drilling percussion hammers result in the complete smashing of the ground materials within the targeted annulus ring drilling area. 
     Referring to  FIGS. 1 and 5 . As shown in  FIG. 5 , internal air-delivery pipes  501  are equipped from top end to bottom end of the top pipe with exhaust openings  103 , where it is further connected to the drill pipes  102  below. Compressed air is delivered through the internal air-delivery pipes  501  in the top pipe with exhaust openings  103 , the internal air-delivery pipes in the drill pipes  102 , then reaches the top connection interface of the DTH percussion hammer assembly  101 . At the top connection interface, the compressed air delivered from the internal air-delivery pipes in the drill pipes  102  is supplied to a receiving port at the top of back head of the DTH percussion hammer assembly  101 . 
     Referring to  FIGS. 1 and 4 . The back head is an annulus housing with a prescribed OD and ID. In the back head, the compressed air is directed through an air channel and presses the top adaptor of the DTH percussion hammer, driving the reciprocal hammering action of the piston  402 . The piston  402  strikes on the drill bit  401  below transferring the hammering force, and in turn the drill bit impacts the ground. After each strike, the compressed air passes through the piston and is released out of the exhaust outlet located at the bottom of the drill bit. The exhausted compressed air simultaneously flushes away broken debris or rock particles, conveying them along both the inner and outer surfaces of the annulus ring structural housing  400 . The broken debris and rock particles that are flushed along the outer surface of the annulus ring structural housing  400  travel upwards along the drill pipes  102  and escape out on to the ground surface. Whereas those that are flushed along the inner surface of the annulus ring structural housing  400  travel upwards along the drill pipes  102 , reach the top pipe with exhaust openings  103 , and escape through the exhaust openings out on to the ground surface. 
     Tie bolts are used to tie together the DTH percussion hammer assembly  101 's front section, main body, and the back head with nuts and lock nuts. When servicing the individual DTH hammer or changing the drill bits, the tie bolts can be loosened or removed. Other known methods of tying together the main elements of the annulus ring hole drilling percussion hammer assembly  101  should be apparent to practitioners skilled in the art. 
     The top adaptors of the DTH percussion hammers are located and housed in the back head at prescribed position and are screwed together with their corresponding DTH percussion hammer body. The top adaptor is supported and retained in the back head housing by a pair of bit retaining rings. The chuck of each DTH percussion hammer is mounted at its bottom and is of hexagonal outer sectional shape. The hexagonal chucks are located in place and housed in the front section of the DTH percussion hammer assembly  101 . The hexagonal chucks have the advantage of enabling their corresponding hexagonal housing to withhold any torsion load experienced by the individual DTH percussion hammer along its own longitudinal axis during drilling. It is, however, possible to use chucks of other shapes such as circular sectional chucks. 
     The drilling OD of the annulus ring hole is determined by the radius distance from the exterior gauge dimension of the specially sized drill bit to the centre of the annulus ring drilling structural housing  400 . The drilling ID of the annulus ring hole is determined by the radius distance from the interior gauge dimension of the specially sized drill bit to and the centre of the annulus ring structural housing  400 . 
     The drilling OD of the annulus ring hole is designed to be larger than the OD of the annulus ring DTH percussion hammer assembly  101  and the drill pipes  102 . That is, the exterior gauge of the drill bit  401  is protruded away from the OD of the annulus ring structural housing  400 . The drilling ID of the annulus ring hole, on the other hand, is designed to be smaller than the ID of the annulus ring DTH percussion hammer assembly  101  and the drill pipes  102 . That is, the interior gauge of the drill bit  401  is protruded away from the ID of the annulus ring structural housing  400 . The purpose of this arrangement is to reduce the drag resistance on the surface of the entire drill string as it advances deeper and deeper into the ground. 
     Depending on the requirement of the annulus ring hole, various allocation arrangements of the drill bits are possible. If the difference between the annulus ring hole OD and ID is small, one circumferential layer of drill bits is used. In one embodiment, one circumferential layer comprises five drill bits, which is shown in  FIG. 2 . For annulus ring holes with a large OD-ID difference, two to three circumferential layers of drill bits can be used to cover the large annulus ring drilling area as shown in  FIG. 7 . 
     The front annulus ring percussion hammer assembly  101  shown in  FIG. 1  includes five air driven DTH percussion hammers distributed and assembled over an annulus ring structural housing  400 . The structural housing  400  has a prescribed housing OD and a prescribed housing ID. The surfaces created by the housing OD and housing ID function as the flushing surfaces for exhaust air and drilling debris. The space between the housing OD and ID accommodates the DTH percussion hammers of which the number and size are determined by the required annulus ring hole OD and ID. Each DTH percussion hammer is assembled longitudinally inside this housing space in parallel with the drilling axis of the drill string. All DTH percussion hammers are held and supported by the housing along its longitudinal direction. 
     Referring to  FIG. 2 . Each drill bit  202  is fixed in its position by its two adjacent index blocks  201 . The index blocks  201  prevent the drill bits  202  from self-turning along the axis of its corresponding DTH percussion hammer. The index blocks  201  are specifically built to withstand the torque and thrust forces experienced by the drill bits  202  during the drilling process with the annulus ring drilling percussion hammer assembly rotating and each drill bit  202  impacting the ground. The indexing of each drill bit  202  also restricts it from self-turning so as to allow maximum possible ground impacting area on both the exterior protruding gauge and the interior protruding gauge at all time during drilling. 
     Referring to  FIG. 6 . In accordance with various embodiments, specially designed percussion drill bits with tungsten carbide tips  601  are mounted at the bottom of each drill bit. The drill bit has a special peripheral profile to achieve a larger percussion area on both the annulus OD and ID drilling areas. The drill bit cutting face profile is not necessary circular in shape, and can be triangular, rectangular, or any special profiled shape as shown in  FIG. 7 . The drill bit cutting face profile is designed to achieve maximum material smashing are on both the exterior protruding gauge and the interior protruding gauge. 
     In accordance with various embodiments, pressurized fluid instead of compressed air can be used to drive the reciprocal hammering actions of the DTH percussion hammers. In this case, pressurized fluid, instead of compressed air, is supplied through the intake swivel  107  in the rotary head  106 , delivered through fluid-delivery pipes in the top pipe  103  and the drill pipes  102  to each of the DTH percussion hammers driving reciprocal hammering action during drilling. 
     The foregoing description of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art. 
     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalence.