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TECHNICAL FIELD 
     The present invention relates to an apparatus for improved subterranean foundation drainage of particular use for buildings. More particularly this invention relates to an apparatus for allowing water or other liquids in soil to accumulate in hollow, upwardly extending members, to travel by gravity down these members, to pass into an approximately horizontal drain means and from there to travel by gravity into a storm drain system. 
     BACKGROUND OF THE INVENTION 
     Drainage systems are often installed to prevent water from accumulating in the soil near foundations of buildings. This water, acting under hydrostatic pressure, is forced through small gaps in the foundation wall and enters the building. 
     A common method to prevent this is to place an approximately horizontal drain external to and near the base of a foundation wall to channel water away from the foundation and thus relieve the hydrostatic pressure and consequent leakage. 
     This method reduces the hydrostatic pressure in the soil near the horizontal drain. It does not provide effective protection from water which permeates the soil from above, however, since this water percolates downward from the surface of the ground to the bottom of the foundation before it enters the drainage system. While percolating downward, this surface water saturates the soil and applies hydrostatic pressure on the upper foundation walls before it reaches the drainage system at the bottom of the foundation. 
     Another method for preventing foundation leaks is to coat the outer surface of the foundation with a sealant that prevents water in the saturated soil from forcing its way into the building. These sealants are fragile and are often damaged when installed or degrade after installation. 
     Drainage systems have been devised providing a sheet-like barrier covering the buried outer face of a foundation wall. Typical sheet systems are disclosed in U.S. Pat. Nos. 4,840,515, 3,965,686, 4,810,573, and 4,733,989. 
     Such sheet systems typically have interior voids between an inner sheet and a permeable outer sheet allowing water to leave the soil, pass into the voids, and flow by gravity downward into the horizontal drainage tube. Such systems are difficult and expensive to install, as the entire subterranean foundation wall is typically covered from near the surface of the ground to the horizontal drainage tube. 
     Other drainage systems (such as U.S. Pat. Nos. 4,930,272 and 4,869,032, below) channel water from between the walls of a foundation into a horizontal drain located inside the building and underneath a basement slab. For example, DiCello, U.S. Pat. No. 4,538,386, describes a drainage system comprised of horizontal drain pipes laid both outside the foundation wall and inside (within the foundation boundaries) the foundation walls, The internal drain pipes connect to the interior of a foundation wall and allow water to drain therefrom. 
     Many other devices are known in the art for soil drainage in general. For example, Delattre, U.S. Pat. No. 4,246,305, describes an extruded multichannel porous drainage strip for placing in water filled soil. There are also devices for forming a solid wall of backfilled material around a foundation, thus providing a porous path for downward water percolation. Minor et al., U.S. Pat. No. 5,017,042, describe a biodegradable accordion-like container for holding a layer of such coarse drainage material vertically against a foundation wall while the gap between the foundation and the excavation is backfilled. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a durable and inexpensive drainage system for draining soils near a subterranean foundation. 
     It is another object of this invention to provide a drainage system that is not subject to clogging. 
     It is a further object of this invention to provide a drainage system that is installed relatively inexpensively and easily. 
     It is a further object of this invention to provide a flexible drainage system that tailors drainage capacity and cost to the particular soil and rainfall conditions of the particular foundation and weather. 
     The present invention achieves these and other objects by providing a drainage system capable of easy installation and tailorable to a variety of soils and rainfall conditions. The drainage system combines an approximately horizontal drainage means or tiling in combination with upwardly extending hollow risers that direct water in the soil downwardly into drainage o tubes and away into storm or sanitary drains. The horizontal drainage means is not truly horizontal, since a slight incline must be provided to insure that water flows down the drainage means. The hollow risers are mounted adjacent to the face of the foundation wall and provide a downward channel for water that accumulates on the surface and penetrates the upper layers of soil above the drainage tube. The hollow risers are perforated, typically on the side facing away from the foundation, allowing water in the surrounding soils to enter. These perforations may be covered with a filter medium, typically landscaping cloth, fiberglass or mesh screen, to prevent dirt, gravel and silt from filling up the hollow riser and the drainage tube. 
     The lower ends of the hollow risers may be directly connected to the horizontal drainage tube. A &#34;T&#34; connection may be used to join the drainage tube and the hollow risers. The risers may abut the drainage tube, as well. In another embodiment the lower ends of the risers are spaced a short distance from the drainage tube. In a further embodiment the lower end of the risers abut the drainage tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of the present invention, through the earth surrounding the foundation; 
     FIG. 2 is a side sectional view of the embodiment of the invention shown in FIG. 1 with soil removed; 
     FIG. 3 is a partial elevation of an alternative embodiment of the present invention with soil removed; and 
     FIG. 4 is a cross-section of an embodiment of the hollow riser, viewed through line 4--4 in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, means for draining the soil, such as drainage tube 4, extends approximately horizontally along foundation 2 near footing 11 and is underneath the surface 1 of the earth abutting the foundation 2. The means for draining is actually slightly inclined from horizontal to provide a slight angle to lead the water toward the outlet of the drainage system; however, it is generally horizontal as compared to the hollow riser 3 to be described. Any material forming a channel for downward water flow is acceptable, such as plastic, clay, ceramic, gravel or other media. 
     According to the present invention, a plurality of hollow risers 3 extend upwardly from tube 4. In this embodiment these risers are made of PVC drain pipe, preferably about 4&#34; in diameter, although sizes ranging from about 2&#34; to 8&#34; in diameter are effective for a typical house foundation. Referring to FIG. 2, riser 3 extends upwardly from tube 4 creating an angle φ of 0 to 20 degrees with the foundation wall 2. Preferably angle φ is between about 3 and 10 degrees and most preferably about 5 degrees. This angle defines a void 8 between the riser and the foundation. 
     At the top of each riser 3, means for covering the upper end to prevent the entry of soil, such as cap 9, may be disposed. In this embodiment the cap is made of landscaping cloth. Numerous other methods of covering the end of the riser to prevent dirt from entering are acceptable, such as a pipe cap, a &#34;T&#34; joint or fiberglass. The advantage to landscaping cloth and fiberglass is that these materials pass water relatively easily, yet resist the passage of dirt. Each riser is perforated with holes 7 passing through from the outside to the inside surface of the riser. In a preferred configuration, the holes are approximately 9/16&#34; in diameter, spaced approximately 4&#34; apart and located in two longitudinal rows. In this embodiment, illustrated in FIG. 4, the holes 7 are located in two rows on the earth-facing side 17 of the riser, and the riser has a smooth foundation-facing side 16. This foundation-facing surface provides a smooth imperforate channel for water to flow down the riser. Other spacings or diameters may be used depending on the requirements of the system. For example, closer spacing could be used to increase water flow into the risers. 
     A filtering means such as screen 10 (shown only partially) covers the holes in the riser. This filtering means should be such that it allows water to pass from the soil to the interior of the riser, yet inhibits the passage of the soil itself. Again, landscaping cloth is a preferred material. At the bottom of the riser is a gap 6 between riser 3 and tube 4. This gap is typically about 4&#34; to 12&#34; in height, most typically about 9&#34;. Gap 6 is filled with backfill material 5, such as pea gravel, which allows water to pass through relatively easily, yet inhibits the passage of dirt or other particulate matter. 
     When riser 3 and tube 4 are covered with backfill, the void 8 defined by the angle φ between riser 3 and wall 2 is loosely filled with backfill material. This void provides an additional channel to o move water away from the wall 2 toward riser 3 and into drainage means 4. 
     As shown in FIG. 1, risers 3 are spaced apart from each other. Typical spacing varies from 4&#39; to 12 feet depending upon the soil type and water conditions, with closer spacings used in problem drainage areas such as especially wet soils, clay-based soils, or grades that tend to direct surface water toward a foundation rather than away from it. Risers smaller than 4&#34; in diameter may require a closer spacing to provide enough drainage. An 8&#39; spacing with risers 4&#34; in diameter is effective for most soil and water conditions. 
     In use, water entrained in the soil is forced through a filter screen 10 and through holes 7 into the hollow riser 3 by hydrostatic pressure. Referring to FIG. 2, riser 3 is at an angle φ, and since the earth-facing side 17 of riser 3 is perforated and the foundation-facing side 16 is not, water entering the holes 7 flows inside the riser to the foundation-facing side 16 and then down a longitudinal channel formed by the non-perforated side 16 to the bottom of the riser. It then travels through the filter of backfill 5 in gap 6 and enters the drainage tube 4. It travels down the drainage tube and typically enters a storm or sanitary drain. 
     FIG. 3 shows a further embodiment of the invention. Cap 18 of hollow riser 3 is oblong, shaped as a &#34;T&#34;. An oblong shape prevents the riser from rolling down the wall when placed in position. In this embodiment, cap 18 is also provided with a landscaping cloth cover for the open ends of the &#34;T&#34;s to prevent dirt from entering the riser. Other methods are available to cover the ends of a &#34;T&#34;, such as pipe caps or fiberglass, for example. The advantage of landscaping cloth, fiberglass or similar materials, is their porosity to water and relatively impermeability to dirt. 
     A further feature of this embodiment is the &#34;T&#34; shaped lower end 4 of the hollow riser 3. The drainage tube 15 intersects the &#34;T&#34; shaped lower end 14 of the riser, thus permitting water to travel into the drainage tube directly without passing through an earth filtering gap 6 as embodied in FIG. 2. This &#34;T&#34; shape is provided in this embodiment by attaching a &#34;T&#34; joint to the lower end of riser 3. 
     Another embodiment of riser 3 has a &#34;T&#34; shaped lower end that is proximate to, but does not intersect, the drainage tube.

Summary:
This invention relates to the draining of foundations by using an elongate subterranean drainage structure located approximately horizontally and parallel to the foundation in combination with a plurality of elongate upwardly extending hollow drain structures extending from the structure toward the surface of the earth. Hydrostatic pressure of water in the soil forces water through holes in the upwardly extending drain structures. The water then passes rapidly to the bottom of the upwardly extending drain structures by the force of gravity and thereupon into the horizontal drain structure wherein it is carried away from the foundation.