WO1996011309A1

WO1996011309A1 - Method of stabilizing earth for building earthen walls and structures

Info

Publication number: WO1996011309A1
Application number: PCT/US1995/013534
Authority: WO
Inventors: David C. Easton
Original assignee: Easton David C
Priority date: 1994-10-07
Filing date: 1995-10-05
Publication date: 1996-04-18
Also published as: ZA958455B; AU4005595A; IL115527A0

Abstract

A method for building earthen walls and structures by preparing a composition of well chosen earth, clay and soil aggregate, preferably mixing the earth with a small amount of binding or stabilizing agent, feeding the mixture to a conveying apparatus such as gunite equipment, applying a high pneumatic pressure to the mixture held in a gunite chamber, hydrating the mixture, and propelling the hydrated mixture against a sturdy form or backboard at a force sufficient to create an impact to bind the soil aggregate together in the structure.

Description

METHOD OF STABILIZING EARTH FOR BUILDING EARTHEN WALLS AND STRUCTURES

Field of the Invention

This invention provides a novel method of building stabilized earthen walls for housing and other structures which is more efficient, less costly, and results in a wall of equal or superior strength, longevity and aesthetics as achieved in traditional methods.

The present invention is based upon the discovery that direct pneumatic force, such as that provided by gunite equipment, rather than manual or pneumatically-assisted tamping, can successfully be used to impact soil and create an earthen wall so strong that it exceeds the seismic standards of the most severe earthquake regions in the United States, such as California Zones 3 and 4. The utilization of direct pneumatic pressure to impact the soil dispenses with much of the labor previously required to manually tamp the soil and enables structures of stabilized earth to be built more efficiently and on a production scale.

Walls of stabilized earth can be monolithic, load bearing structures which are earthquake safe, fire proof, termite proof, and water resistant. They can serve as structural walls, including building and retaining walls, or in boundary fences, fireplaces, fountains and ponds. The walls can be used as primary structural material or in combination with conventional materials, such as wood, metal and plaster. The buildings formed by these walls are energy efficient, quiet, and long-enduring. The strength and durability of stabilized earth walls are related to the selection and use of appropriate soils, precise control of the component ratios, thorough mixing, and the use of proper forming and reinforcing methods. By using apparatus of the gunite industry to regulate the mixing and moisture content of the soil/cement materials, the present invention assures consistent, high-quality results.

The walls built according to this invention will normally have a stucco-like texture on the exterior and a smooth finish on the interior (resulting from the use of a smooth form) which can be finished with traditional materials such as plaster, paint, or tile, if desired. Since these walls are inherently waterproof, they do not require painting or other exterior finishes. These walls can be built in a variety of natural earth tones, yielding surfaces of striking color and texture, which require much less maintenance, and are far more durable than conventional walls.

Background of the Invention Rammed earth building is an ancient technique wherein moist earth is repeatedly pounded into structural walls, layer by layer in an enclosed form, using heavy tampers on hand-held poles. It is a slow, tedious and arduous process. The process forces the soil particles into tight molecular and mechanical configurations, strengthening the wall by recreating or reconstituting the rock-like characteristics once possessed by the bound components of the soil aggregate. After the form is filled, the formwork is stripped away, leaving a wall having properties similar to sedimentary rock.

Modern techniques have expedited the process with pneumatically powered foundry rammers or backfill tampers to compact the earth. Binding agents such as portland cement are added to the earth to stabilize it. Regardless of the sophistication of the equipment and technique, however, rammed earth construction continues to be labor- intensive and time-consuming.

Traditional rammed earth techniques have generally and preferably been applied to build structural earth walls that are thick and dense and therefore provide a thermal mass for regulating the temperature within the building. Thermal mass enables the walls to absorb and store heat from the sun during the day for re-radiation at night during the winter, and to absorb heat from interior spaces during the day and release the heat in evening during the summer, decreasing the energy costs of heating and cooling the interior of the building. Unlike walls built from conventional materials, earthen walls naturally possess sufficient thermal mass to allow the wall to continue to absorb the sun's heat during the day, without becoming heat-saturated or merely reflecting heat as would a conventional insulated wall. Thick earthen walls, when shaded from the direct rays of the sun, will stay cool throughout the day, and spaces enclosed by these walls will be in thermal equilibrium with the mass. Protected from direct exposure to the sun, the walls will absorb heat slowly enough that the interior spaces will not increase in temperature until well after the sun has set. Properly designed buildings incorporating stabilized earth walls can maintain indoor air temperatures between 60-80°F during summer months without air conditioning in climates where outside air temperatures exceed 90°F.

Although the benefits of stabilized earth construction are significant, the inherent difficulties in the rammed earth method have inhibited its widespread use in recent times.

Summary of the Invention

The objects of this invention are to stabilize earth compositions by directly pneumatically impacting them into earthen walls and structures which are durable, energy-efficient and structurally superior. These walls can be erected more quickly, more efficiently and with less expenditures of time and labor than traditional rammed earth methods.

Generally, the invention is carried out by preparing a composition of well chosen clay-containing earth, mixing the earth with a small amount of binding and stabilizing agent such as portland cement, feeding the mixture to a conveying apparatus such as gunite equipment, applying a carefully regulated pneumatic pressure of at least 65 psi under normal conditions to the mixture held in the gunite gun chamber, hydrating the mixture to approximately 18% by weight, and directing the hydrated mixture, at high pressure and at an approximate 45 degree downward angle, from a distance of 2 to 3 feet, against a sufficiently sturdy form or backboard for the walls or structure to be formed. Upon further passes,

the soil composition of the partially formed wall is repeatedly and directly impacted by the projected soil composition. This process creates a build-up of the soil composition against and away from the form, forming the walls to the height and depth determined by the gunite operator. As the wall builds, the earth particles comprising the wall, as well as the particles as they are being shot, are compacted into a tight configuration. The resulting wall approaches the strength and physical properties of sedimentary rock.

The term gunite is used in this disclosure to refer genericaliy to the specialized equipment which was designed and has heretofore been used to apply a thin cementitious coating to an object. In the recommended and general practice of gunite application, compressed air is pumped into gunite equipment where it pneumatically powers the equipment and also passes into a gun chamber in which a very fine and uniform 1:4-1:6 mixture of portland cement and clean sand is also being fed. The air in the chamber is maintained at a substantially constant pressure sufficient to convey the cement mixture onto the object to be coated, generally around 45 psi. The compressed air and pneumatically powered apparatus conveys the mixture from the gun chamber into an attached conduit, usually a hose or a flexible pipe, at the end of which is a water source, and beyond that a nozzle for directing the hydrated cement mixture to the object to be coated. The conventional gunite cement mixture is hydrated to about 4% moisture, resulting in a relatively wet mixture, as it passes through the hose, after which it emerges from the nozzle under moderate pressure sufficient to convey it to the object to be coated and to lightly compact it. The hydrated material is then directed horizontally toward an object or surface to be coated. The material, upon curing, hardens into a high strength cementitious layer which is resistant to weathering, heat, abrasion, and many forms of chemical attack. A common application of gunite is in the building of in- ground pools.

The method of the present invention primarily utilizes earth, a readily available, low-cost material to build the walls. Suitable earth for the practice of this invention may be found at the site itself, eliminating the cost of hauling earth to the site. Use of soil from the site also enhances the aesthetic integration of the walls with the building site. If the building site soil is not suitable, the earth can be obtained from places such as rock quarries, where earth suitable for use in this invention is considered waste which burdens the quarry owner with removal and disposal costs. A binder such as portland cement or a pozzolanic material may be

added to the earth to further stabilize it. As opposed to poured concrete walls, portland cement need only be added in small proportions in order to stabilize the composition. The alternate use of a pozzolanic material, such as fly ash, reduces dependence on portland cement with its

attendant costs and hazards.

Structural wails built in accordance with this invention are preferably between 8 to 24 inches thick. An eight-inch thick wall built according to this invention provides thermal mass and can be structural.

The earthen walls of this invention are built using single-sided vertical forms rather than the two-sided vertical formwork needed to build concrete and conventional rammed earth walls. Besides requiring less wood overall, the single-sided formwork of the present invention can be removed from the earthen wall and later integrated into the building as roof sheathing or the like, or reused as formwork for another building project. This decreases the cost of the project and conserves natural resources.

Although unreinforced stabilized earth walls can attain a compressive strength of 800 psi and greater, building code regulations in earthquake-prone sections of the United States, for example, in California, require steel reinforcement in walls. In the present invention, unlike conventional rammed earth, an engineered system of steel reinforcement can be readily incorporated into the walls. When using conventional rammed earth techniques, workers must tamp the soil compositions between the two sides of the formwork, commonly 18-24 inches apart. In order to build the initial bottom portions of the wall, the workers must stand inside the forms while tamping. The presence of a network of steel reinforcing bars, as well as utility boxes and conduits, severely handicap the worker's ability to construct the wall. Further, as the workers progress higher in their ramming process, these reinforcing bars and conduits would directly interfere with their tamping and prevent the soil from being impacted as tightly as it should be. The present invention provides a solution to these problems because it is carried out using one-sided forms, allowing full horizontal access to the wall space. Direct pneumatic impaction of the soil composition, unlike manual tamping, is not substantially impeded by the reinforcing bars and conduits.

While the invention will be described in connection with a preferred embodiment, it is to be understood that the invention is not specifically limited to that embodiment but is intended to also encompass all alternatives, modifications and equivalents as may be included with the spirit and scope of the invention as defined in the claims.

Detailed Description of the Invention

An earthen wall built in accordance with this invention is preferably erected on a reinforced concrete perimeter foundation. The reinforcement consists of horizontal steel bars run in a continuous line embedded in the perimeter of the foundation with vertical reinforcing steel dowels, extending vertically out of the foundation line, set at regular intervals. Suitable foundation techniques to support the walls built in accordance with the invention are, e.g., a typical spread footing and stem wall, a concrete slab with thickened edge, a grade beam poured directly into a trench, or a gunite foundation.

Once the foundation is laid, the formwork for the earthen walls is put in place. The formwork is built by constructing a frame of vertical and horizontal wooden studs and facing the frame with 3/4 inch plywood. To erect the formwork, the inside building line is chalked on top of the foundation grade beam, already in place. Stud plates are set into the top of the grade beam so that the plywood of the formwork, when placed against the outside of a plate, will meet the chalk line from inside its perimeter. Beginning at one corner of the building, the first sheet of plywood is set on edge with its long dimension on the grade beam in line with the comer and up against a stud plate. The first vertical stud is attached to the plywood. If the walls are shot from outside the building, the formwork assembly should be screwed together from the inside for disassembly once the walls are built.

A second sheet of plywood is set on edge on the grade beam so that it butts against the first sheet and creates the first corner. A second vertical stud is set at this corner joint and screwed through into the second sheet of plywood. The corner can be reinforced by screwing the two vertical studs together.

With the first corner established, one vertical stud is set every 16 inches along the plywood, with a screw through the studs into the plywood. To ensure a straight and plumb wall, the corner should be stiffened with a brace and every third or fourth vertical braced with kickers staked to the ground.

The edge of the third sheet of plywood is butted against the end of the second sheet, creating a plywood seam. A vertical stud is placed over the seam and the sheet is braced. Placement of the plywood continues in this manner until the second corner is reached. The plywood is cut if necessary to align with the inside building line.

The procedure of the first corner is repeated. Two vertical studs are screwed together for stiffening, then the plywood and braces are set until the entire perimeter is completed with one course of plywood.

When the first course is completed, one row of studs is set around the inside perimeter at a height of about three feet for alignment and stiffening. The vertical studs are then checked for plumbness and the braces adjusted as necessary.

The second course of plywood is then stacked on the first, making sure that the sheets fit together as tightly as possible. The seams are staggered from the first course.

When the second plywood course of the perimeter is completed, another row of horizontal studs is set, this time at about the six-foot level.

The dimension of the top and final course of plywood is adjusted to the predetermined height of the wall. If necessary, the top of the

forms can extend above the top of the finished wall and a chamfer strip can be nailed to the forms to indicate where the wall ends.

A final horizontal stud is set at the top of the forms and braced back. The inside building line should be straight and true.

When the entire perimeter wall formwork is set to full height, the next step is to place volume displacement boxes, "VDBs" so as to form openings for the doors and windows.

It is important to design and construct the VDBs so that they can be easily disassembled and removed once the wall has been built. They must be stiff enough on their sides to resist the lateral force of the shot earth, and strong enough across the top to support the weight of wet material while it sets up. VDBs are preferably constructed of a double thickness of plywood, the sides supported by 2x4 vertical struts and splayed for removal from the wall. Once the VDB is assembled, it is set in place against the formwork and screwed into the strut from the inside of the building.

Although steel reinforcing is not required to obtain a thermal mass wall, code regulations in some earthquake-prone regions require it. The procedure to install the reinforcement is as follows. After the formwork and the VDBs have been installed, a grid of #3 rebar or prefabricated welded wire panels (4" x 4" x W2.9) is tied to the outside of the formwork and attached to the reinforcing steel dowels extending out of the foundation. The reinforcing grid should be centered in the thickness of the wall to decrease the possibility of voids in the shot earth against the formwork. The rebar should be doubled around all door and window openings, and three continuous horizontal #4 rebar is run at the top of

the wall as bond beam steel.

Anchor bolts for sill plates at the top of the wall or for ledgers that support intermediate floors are put in place before the earth is shot. For ledgers, the anchor bolts can be simply attached through holes drilled in the formwork at the correct location. For sill plates on top of the wall, the bolts are tied off to the bond beam steel.

With all the formwork up, determining precisely the location of doors, windows, roof line, and intermediate floors, the plumbers and electricians install the various pipes, chases, conduits, and boxes. Plumbing lines are wrapped and either butted against the inside face of the formwork or extended through holes drilled in the forms. Electrical boxes are secured to the formwork with either screws or tie wire, and joined together with conduit.

Screed wires are stretched from corner to corner as a point of reference to indicate the outside building line. The wires are run horizontally and spaced about 30 inches apart. They are attached to vertical 4x4 wood posts accurately set at each of the building comers. The inside corner of the 4x4 post should align with the outside corner of the building. The base of the post is anchored at the ground either with stakes or with steel dowels drilled into the grade beam and into the

bottom of the posts. The posts should be braced away from the wall with studs and stakes. A brace at the top is added to secure the posts to tops of the wall forms.

At this point, the earth is prepared. A suitable earth composition is one which contains clean and well-graded aggregate, preferably with the maximum size of the particles not exceeding 3/8 inch in diameter, fines comprising no more than 30% and preferably between 10-20%, and the clay content not exceeding 20% of the aggregate. Since too much clay content in the soil mixture can result in unacceptable cracking and separating upon drying of the material, a sample should be subjected to two tests by a geotechnical laboratory to determine soil suitability. First, a sieve analysis, in which a measured amount of soil is shaken through a series of screens of decreasing size, should be performed. The "fines" or smallest particles, silt and clay, those which pass a 200 mesh screen, should not substantially exceed 30% of the soil. If the total fines substantially exceed 30% of the sample, it should be subjected to an Atterburg Limits Test, which measures the plasticity of the soil sample, an indication of the proportion of clay content in the fines. Acceptably low plasticity measurements will indicate a decreased possibility of shrinkage cracking. If earth found at the building site does not meet these standards, it can be altered by adding the elements which are missing. Alternatively, earth can be obtained from other sites, such as rock quarries where inexpensive but suitable "quarry fines," "quarry waste," "reject sand," or "dirty sand" are often available.

While the clay naturally occurring in some earths will often provide enough cohesion upon impaction to support the walls made according to this invention, an additional binding agent in a ratio of about 1:15 in the soil composition can be added to further stabilize the earth. Preferred binders are pozzolanic materials such as fly ash, portland cement Type I or II complying with A.S.T.M. standard specification C 150-67, or additional clay. In the case of portland cement, the mixing ratio can also be expressed as 1.5 sacks of cement per cubic yard of earth. The binder is mixed by machine into the dry earth for uniform distribution.

Samples of the earth/binder mixture should be compacted into 3" x 6" cylindrical molds, allowed to cure, and tested by a certified laboratory in accordance with A.S.T.M. Standard 39. Minimum 28 day compressive strength should be at least 600 psi. Typical compressive strenths in walls built according to this invention are in the range of 1000 to 1500 psi.

The earth/binder mixture is then fed into a standard gunite apparatus such as the Lova sold in the United States by Reed Manufacturing or the Hydrostatic Rotary Gun available from Blastcrete Equipment Co. A flow of compressed air is supplied to the apparatus and the air pressure in the gun chamber holding the earth mixture is maintained preferably at the highest pressure the apparatus can attain, preferably no less than 65 psi when the application site or form is at a distance of no more than approximately 100 feet from, and at a height of no more than approximately 10 feet above, the gun. Generally, the pressure should be increased from 65 psi by at least approximately 5 psi per additional 50 feet of distance from the gun to the form. That distance, generally determined by the length of a flexible pipe leading from the gun, is preferably no greater than about 50 feet, as the soil composition must be hydrated and applied at a height of 12 feet or more above the gun. Also, because the soil composition used in this invention generally creates more friction in transit than conventional gunite cement, yet must be applied with greater impact, the distance between the gun and the application site should be minimized. Preferably, the means for conducting the soil composition to its application site should not exceed 150 feet.

The mixture is hydrated with clean water at the nozzle. Moisture content is continuously monitored and maintained at between approximately 10-20% by weight of dry mass. Proper hydration is achieved when the rebound, which is the shot material that does not adhere to and rebounds from the wall, is between approximately 5 and 0% of the propelled mass. Although the water content of a conventional gunite concrete mix is generally less than that of the mixture used in this invention, the absorptive clay in the earth in the latter mixture causes it to appear drier than standard clean sand gunite. The

soil mixture of this invention should be hydrated to appear drier than conventional gunite cement, although a greater amount of water in relation to mix is actually being used. The emerging hydrated mixture is directed against the formwork at a downward angle of approximately 45° and at a distance of about 2 to 3 feet from the form to maximize the force of the impact.

The shooting is begun at one comer of the building and moves laterally along the foundation, building the earth out to the full thickness of the wall (typically 18 to 24 inches) in maximum lifts of 30 inches, completely embedding the reinforcement, if any. The material should emerge from the nozzle in a steady, uninterrupted flow. Using the screed wires as a guide, the excess earth is trimmed to a flat and true building line.

Immediately upon completion of the first pass around the building, the second one can begin and the process repeated until the desired height of the wall is reached. Loose material from waste and rebound should be blown off the top of a course before placing the fresh material. The shot earth wall will have obtained enough strength to support itself, without the aid of the formwork, within 24 hours. The forms are removed carefully from the walls to avoid marring the inside surfaces. The wires which had been holding the electrical boxes and rebar to the formwork, and the screws which had been holding the VDBs in place, are cut.

Any voids in the earthen structure should be filled with a moist earthen mixture handpacked into the void and trowelled off.

The interior smooth surface may be coated with a polymer sealing compound, although this is generally unnecessary.

Conventional window frames, doorframes and roofs are used with the earthen walls of this invention to create comfortable, energy-efficient and aesthetically pleasing living, working or storage spaces.

Claims

What is claimed is:

1. A method of forming a stabilized earth structure comprising pneumatically propelling a hydrated soil composition comprised of a natural binding agent and soil aggregate against a form at a force sufficient to create an impact to bind the soil aggregate together in the structure.

2. A method according to claim 1 wherein the stabilized earth has a compressive strength of greater than approximately 300 psi.

3. A method according to claim 1 whereby the soil composition is pneumatically propelled by means of a gunite apparatus comprising (a) a gun comprised of a chamber to receive compressed air, the natural binding agent and the soil aggregate, (b) means for conducting the compressed air, natural binding agent and soil aggregate from the gun to the form and (c) means for hydrating the soil composition prior to propelling it against the form.

4. A method according to claim 3 wherein pressure in the gun is maintained at greater than approximately 65 psi and when the form is at a distance of no more than approximately 100 feet from, and at a height of no more than approximately 10 feet above, the gun.

5. A method according to claim 1 whereby the soil

composition is hydrated to between approximately 10 and 20% of the weight of the soil aggregate and natural binding agent.

6. A method according to claim 1 whereby the soil aggregate is hydrated to a degree which results in approximately between 5% and 10% rebound of propelled soil composition which does not bind in the structure.

7. A method according to claim 1 wherein the soil aggregate is comprised of particles no larger than approximately 3/8 inch in diameter.

8. A method according to claim 1 wherein the soil aggregate is comprised of no more than approximately 30% by weight of fine particles which pass a 200-mesh screen.

9. A method according to claim 1 wherein the natural binding agent is comprised of a clay material.

10. A method according to claim 9 wherein the clay material comprises no more than approximately 20% of the combined weight of the soil aggregate and natural binding agent.

11. A method according to claim 1 wherein the soil composition is further comprised of an added binding agent.

12. A method according to claim 11 wherein the stabilized earth has a compressive strength of greater than approximately 600 psi.

13. A method according to claim 11 wherein the added binding agent is comprised of portland cement.

14. A method according to claim 11 wherein the added binding agent is comprised of a pozzolanic composition.

15. A method according to claim 11 wherein the added binding agent comprises less than about 10% of the combined weight of the soil aggregate and natural binding agent.

16. A method according to claim 15 wherein the added binding agent comprises between about 6 and 8% of the combined weight of the soil aggregate and natural binding agent.

17. A method according to claim 1 wherein the structure is comprised of a wall having a thickness of about 8 inches or greater.

18. A method according to claim 1 wherein a means for reinforcing the structure is embedded in the stabilized earth.

19. A method according to claim 1 wherein utility conduits or hardware to be located at least partially within the dimensions of the earth structure are positioned prior to formation of the structure.

20. A method of building a stabilized earth structure comprising (1) feeding a soil composition comprised of a natural binding agent and soil aggregate to a means for pneumatically propelling the soil composition comprising (a) a gun means for receiving compressed air and conveying the soil composition, (b) a means for conducting the soil composition and compressed air from the gun means to a form and (c) means for hydrating the soil composition prior to propelling it against the form, (2) hydrating the soil composition and (3) propelling the resulting hydrated soil composition against a form at a force sufficient to create an impact to bind the soil aggregate together in the structure.

21. A method according to claim 20 wherein the stabilized earth has a compressive strength of greater than approximately 300 psi.

22. A method according to claim 20 whereby the pressure in the gun means is maintained at greater than approximately 65 psi when the form is at a distance of no more than approximately 100 feet from, and at a height of no more than approximately 10 feet above, the gun means.

23. A method according to claim 20 whereby the soil composition is hydrated to between approximately 10 and 20% water by weight.

24. A method according to claim 20 whereby the soil composition is hydrated to a degree which results in approximately between 5% and 10% rebound of propelled soil composition which does not bind in the structure.

25. A method according to claim 20 wherein the soil aggregate is comprised of particles no larger than approximately 3/8 inch in diameter.

26. A method according to claim 20 wherein the soil aggregate is comprised of no more than approximately 30% by weight of fine particles which pass a 200-mesh screen.

27. A method according to claim 20 wherein the natural binding agent is comprised of a clay material.

21 A method according to claim 27 wherein the clay material compr .as no more than approximately 20% by weight of the soil composition.

29. A method according to claim 20 wherein the soil composition is further comprised of an added binding agent.

30. A method according to claim 29 wherein the stabilized earth has a compressive strength of greater than approximately 600 psi.

31. A method according to claim 29 wherein the added binding agent is comprised of portland cement.

32. A method according to claim 29 wherein the added binding agent is comprised of a pozzolanic composition.

33. A method according to claim 29 wherein the added binding agent comprises less than about 10% by weight of the soil composition.

34. A method according to claim 33 wherein the added binding agent comprises between about 6 and 8% by weight of the soil composition.

35. A method according to claim 20 wherein the structure is comprised of a wall having a thickness of about 8 inches or greater.

36. A method according to claim 20 wherein a means for reinforcing the structure is embedded in the stabilized earth.

37. A method according to claim 20 wherein utility conduits or hardware to be located at least partially within the dimensions of the earth structure are positioned prior to formation of the structure.

38. A structure formed according to claim 1.

39. A structure formed according to claim 4.

40. A structure formed according to claim 17.

41. A structure formed according to claim 18.

42. A structure formed according to claim 19.

43. A structure formed according to claim 20.

44. A structure formed according to claim 22.

45. A structure formed according to claim 35.

46. A structure formed according to claim 36.

47. A structure formed according to claim 37.