Drainage element

1. A preassembled drainage unit comprising

a self-contained sleeve having a first part-circumferential portion thereof made of netting having a plurality of openings therein for the passage of water and a second part-circumferential portion thereof made of a biodegradable material for stopping solids from passing therethrough; and

a mass of light weight synthetic aggregate disposed within said sleeve.

2. A preassembled drainage unit as set forth in claim 1 wherein said first part-circumferential portion of netting has mesh openings to allow water and solids to pass through and said second part-circumferential portion is made of kraft paper.

3. A preassembled drainage unit as set forth in claim 2 wherein said paper has a weight of from 40 to 60 pounds.

4. A preassembled drainage unit comprising

a sleeve of elongated tubular shape having a first peripheral portion of net material having a plurality of openings therein for the passage of water and having a pair of radially outwardly directed flaps extending along the entire length of said portion, a second peripheral portion of biodegradable material for stopping solids from passing therethrough separate from said first peripheral portion and having a pair of radially outwardly directed flaps extending along the entire length of said second portion, each said flap of biodegradable material being secured to a respective one of said pair of flaps of said net material; and

a mass of light weight synthetic aggregate disposed within said sleeve.

5. A preassembled drainage unit as set forth in claim 4 wherein said biodegradable material is one of a perforated paper and a plastic film.

6. A preassembled drainage unit as set forth in claim 4 wherein said sleeve is self-contained and defines a tubular circular cross-sectional shape.

7. A preassembled drainage unit as set forth in claim 4 further comprising a perforated pipe disposed within said aggregate and extending outwardly of said sleeve at each of two opposite ends of the drainage unit.

8. In combination,

an array of preassembled drainage units, each said unit including a self-contained sleeve of elongated tubular shape having at least one peripheral portion thereof of biodegradable material for stopping solids from passing therethrough, at least one peripheral portion thereof of net material having a plurality of openings therein for the passage of water and a pair of radially outwardly directed flaps and a mass of light weight synthetic aggregate disposed within said sleeve; and

each of said units being disposed in spaced relation to an adjacent unit within a trench and having a respective flap thereof disposed in overlapping contact relation to a respective flap of said adjacent unit.

9. The combination as set forth in claim 8 wherein at least one of said units has a perforated pipe disposed therein and extending outwardly thereof at each of two opposite ends thereof.

10. The combination as set forth in claim 9 wherein said perforated pipe is disposed in asymmetric relation within said one unit.

11. The combination as set forth in claim 10 wherein each peripheral portion of net material of each said unit is downwardly disposed and each said peripheral portion of biodegradable material is upwardly disposed.

12. A preassembled drainage unit comprising

a self-contained sleeve of tubular circular cross-sectional shape having a first part-circumferential portion thereof made of netting characterized in having a plurality of openings therein for the passage of water and solids and a second part-circumferential portion thereof characterized in being made of a biodegradable material for stopping solids from passing therethrough; and

a mass of light weight synthetic aggregate disposed within said sleeve.

13. A preassembled drainage unit as set forth in claim 12 wherein said biodegradable material is made of one of a biodegradable paper and a biodegradable plastic film.