1. Field of the Invention
The present invention relates to methods for mixing soil with a chemical hardener in situ to form soil-cement columns, walls, piles, grids and monolithic overlapping columns. More particularly, the present invention is directed to improved methods for introducing various and consolidating slurries with soil to form deep soil-cement columns, walls, piles, grids, and monolithic blocks of columns.
2. Relevant Technology
For a number of years, multi-shaft auger machines have been used to construct soil-cement columns in the ground without having to excavate and remove the soil. These columns are sometimes referred to as "soilcrete" columns, because the soil is mixed with a cement slurry in situ. Upon hardening, the soil-cement columns possess some characteristics of lower strength concrete columns, but they are constructed without the expense and time-consuming process of removing and replacing the soil with concrete.
Soil-cement columns have been arranged in a variety of patterns depending on the desired application. Some soil-cement columns are used to improve the load bearing capacity of soft soils, such as sandy or soft clay soils. The columns are formed in the ground to help support surface construction.
In other cases, the soil-cement columns have been overlapped to form boundary walls, excavation support walls, low to medium capacity soil-mixed caissons, and for the in situ fixation of contaminated soil or toxic waste.
Because the soil is mixed in situ and because the soil-cement wall is formed in a single process, the construction period is shorter than for other construction methods. The costs of forming soil-cement columns are less than traditional methods requiring excavation of the soil, construction of forms, and the pouring of concrete into the forms in order to form the concrete columns or walls. In addition, because the soil is not actually removed from the bore hole in the process of forming the soil-cement structures, there is comparatively less material brought to the surface which must be disposed of during the course of construction.
Historically, a modified earth digging auger machine is used in the formation of in situ soil-cement columns. Such an auger machine is depicted in FIG. 1. The final boring and mixing operations are performed by multi-shaft drive units in order to make the process more efficient. A shaft typically has attached thereto soil mixing paddles 18 and auger blades 20 which horizontally and vertically consolidate the soil with a chemical hardener to produce columns having a homogeneous mixture of soil and chemical hardener.
As auger blades 20 located at the lower end of each shaft 10 of a multi-shaft drive unit penetrate the soil, the soil is fractured and a chemical hardener slurry is injected into the soil through the ends of the hollow-stemmed augers which are attached to the shaft or through opening 11 in the shaft. The augers penetrate, fracture and lift the soil to mixing paddles which further consolidate the slurry in the soil.
To produce soil-cement columns, a multi-shaft auger machine bores into the ground and simultaneously mixes the soil with a slurry of chemical hardener pumped from the surface through the auger shaft to the end of the auger. The resulting soil and slurry mixture is often referred to as a bore hole. The use of the term "bore hole" does not necessarily mean that soil is removed to create a hole. Although some soil is deposited on the surface due in part to expansion of the soil as it is fragmented and consolidated with the slurry, the majority of the soil remains below the surface as it is mixed.
Multiple columns are prepared while the soil-cement mixture or soil-chemical mixture is still soft. By overlapping the columns at the end of the series of columns, a continuous wall may be formed. This is accomplished by drilling through a portion of the end column of the previous series of columns before that column has hardened. As the auger drills through the soil, the chemical hardener slurry is injected into the fractured soil. This slurry acts as a lubricant and begins the chemical reaction which results in the hardening of the soil-cement columns. Because the auger apparatus must be capable of drilling downward into a bore hole and injecting the chemical hardener slurry, and then must be able to drill back up before the soil-cement column hardens, there is a temporal limit to the depth of a bore hole.
Not only must the auger be removed from the bore hole before hardening of the soil-cement column but the auger must be capable of redrilling through a portion of the last column to form overlapping columns or walls. If the soil is hard or the bore hole is deep, valuable time may be lost in drilling the bore hole and the soil-cement column may harden before the next bore hole is formed.
Traditionally, the mixture of the chemical hardener slurry and the soil is such that it may set up shortly after the slurry is injected. While use of such a method and mixture creates little trouble in many circumstances, certain conditions exist which can result in significant problems due to the untimely hardening of the soil-cement mixture. These conditions include construction of soil-cement structures at greater depths, construction of soil-cement structures in hard soil, and equipment breakdown. Under these conditions, the soil-cement mixture may harden before the soil-cement column is complete which can have a number of detrimental effects including inferior quality soil-cement structures, abandonment of a partially completed soil-cement column, damage to the auger apparatus, or even loss of the auger apparatus due to the hardening of the soil-cement structure while the auger apparatus is still in the bore hole.