1. Field of the Invention
This invention relates to new forms of granular and haphazardly arranged aggregate employed in septic drainage fields, underground burial storm-water storage, and drainage systems. In particular, such aggregate must provide interstices an void passageways therethrough for the percolation of drainage water and for the evaporation of moisture in air or gases ventilated through the aggregate, while providing load-carrying stability as backfill, stabilizing the surface grade and carrying pedestrian and vehicle traffic within predetermined load limits.
Specifically, the present invention is a specially-shaped block for use as synthetic aggregate, the shape providing a certain combination of void and compressive strength when the blocks are used to fill a volume.
2. Prior Art
There are a number of applications in which there is a need to fill an underground volume with material that will provide some compressive strength able to support loading from above, and some void for providing a passageway for air or fluid, and also, in some cases, some thermal insulation isolating what is below or within the volume from the thermal environment above ground. In these applications, the material must almost always be transported to the underground volume to be filled, and then distributed in some manner throughout the volume.
In the past, stone aggregate has been used. Stone aggregate, however, is sometimes not uniform in its size, shape or composition, so in using it there is uncertainty in meeting the requirements of an application. In addition, stone aggregate has to be mined (as gravel), affecting some ecosystems. Moreover, stone aggregate is heavy and so more costly to transport and to distribute throughout the volume of an application. Finally, even when using washed, crushed stone as aggregate, since all stone has silt, there will still be a membrane formed from the silt of the aggregate in the bottom of any volume filled with the aggregate. Silt build-up is unpredictable and, in case of a drain-field application, greatly reduces the lifetime and efficiency of the drain system.
There is also in the prior art the use of expanded polystyrene tubes as aggregate. Houck (U.S. Pat. Nos. 5,015,123 and 5,051,028) teaches using various shapes such as plastic balls, styrofoam peanuts, tubes, and chunks of polypropylene, PVC or other light material. Houck therefore teaches using for aggregate foam in shapes that either fit closely together, as in the case of a cube, or that cannot be fit closely together, i.e. will always have in any arrangement a substantial percentage void.
The use of odd shapes of one or another kind of material for use as aggregate is also taught by Minvielle et al. (U.S. Pat. No. 4,411,555). However, the shapes taught by Minvielle et al. rely on internal passageways for allowing the movement of air or fluid.
In all of the prior art, void is provided in the volume to be filled either by choosing irregular shapes to comprise the aggregate or choosing hollow, nonclosed shapes providing internal passageways. In many applications there is a need for substantial void in the settled aggregate, often over 40%. In addition, there are some applications where the aggregate must provide greater compressive strength than is usual. A hollowed open shape can be made of different sizes to change the percentage void, or can be made more or less hollow, but there will always be a significant void in any assembly of these shapes because of the ever-present internal passageways. Moreover, these hollow open shapes are not capable of providing the same compressive strength as solid shapes made of the same material.
In the case of irregular solid shapes, the percentage void is fixed by the shape. Making the same shape larger or smaller will not significantly change the percentage void. Thus, the compressive strength offered by these shapes is also fixed, because compressive strength depends in some way inversely on the void provided by the shapes comprising the aggregate.
Thus, in transporting any of these shapes to an application, because there is no way to arrange the shapes of the prior art to eliminate void, the void must also be transported. Although the void weighs nothing, it occupies volume, and transporting it has a cost.
What is needed as aggregate are shapes that fit together for transporting to an application and that, when distributed throughout a volume, will almost never fit together, but will instead provide a desired void. What is also needed is a shape that can be tailored to provide different combinations of void and compressive strength, depending on the application.