The supporting capacity of soft, compressible ground may be increased and settlement reduced through use of compression reinforcement in the direction parallel to the applied stress or tensile reinforcement in planes normal to the direction of applied stress. Commonly used compression reinforcement elements include mix-inplace piles and walls. Strips and membranes are used for tensile reinforcement, with the latter sometimes used to form a moisture barrier as well.
a. Mix-in-place piles and walls.
Several procedures are available, most of them patented or proprietary, which enable construction of soil-cement or soil-lime in-situ. A special hollow rod with rotating vanes is augered into the ground to the desired depth. Simultaneously, the stabilizing admixture is introduced. The result is a pile of up to 2 feet in diameter. Cement, in amounts of 5 to 10 percent of the dry soil weight, is best for use in sandy soils. Compressive strengths in excess of 200 kips per square foot can be obtained in these materials. Lime is effective in both expansive plastic clays and in saturated soft clay. Compressive strengths of about 20 to 40 kips per square foot are to be expected in these materials. If overlapping piles are formed, a mix-in-place wall results.
b. Vibroreplacement stone columns.
A vibroflot is used to make a cylindrical, vertical hole under its own weight by jetting to the desired depth. Then, 1/2- to 1- cubic yard coarse granular backfill, usually gravel or crushed rock 3/4 to 1 inch is dumped in, and the vibroflot is used to compact the gravel vertically and radially into the surrounding soft soil. The process of backfilling and compaction by vibration is continued until the densified stone column reaches the surface.
c. Strips and membranes.
(1) Low-cost, durable waterproof membranes, such as polyethylene, polypropolylene asphalt, and polyester fabric asphalt, have had application as moisture barriers. At the same time, these materials have sufficient tensile strength that when used in envelope construction, such as surrounding a well compacted, fine-grained soil, the composite structure has a greater resistance to applied loads than conventional construction with granular materials. The
reason is that any deformation of the enveloped soil layer causes tension in the membrance, which in turn produces additional confinement on the soil and thus increases its resistance to further deformation.
(2) In the case of a granular soil where moisture infiltration is not likely to be detrimental to strength, horizontally bedded thin, flat metal or plastic strips can act as tensile reinforcing elements. Reinforced earth has been used mainly for earth retaining structures; however, the feasibility of using reinforced earth slabs to improve the bearing capacity of granular soil has been demonstrated.
(3) Model tests have shown that the ultimate bearing capacity can be increased by a factor of 2 to 4 for the same soil unreinforced. For these tests, the spacing between reinforcing layers was 0.3 times the footing width. Aggregate strip width was 42 percent of the length of strip footing.
d. Thermal methods.
Thermal methods of foundation soil stabilization, freezing or heating, are complex and their costs are high.
(1) Artificial ground freezing.
Frozen soil is far stronger and less pervious than unfrozen ground. Hence, artificial ground freezing has had application for temporary underpinning and excavation stabilization. More recent applications have been made to back-freezing soil around pile foundations in permafrost and maintenance of frozen soil under heated buildings on permafrost. Design involves two classes of problems; namely, the structural properties of the frozen ground to include the strength and the stress-strain-time behavior, and thermal considerations to include heat flow, transfer of water to ice, and design of the refrigeration system.
Heating fine-grained soils to moderate temperatures, e.g., 1000C+, can cause drying and accompanying strength increase if subsequent rewetting is prevented. Heating to higher temperatures can result in significant permanent property improvements, including decreases in water sensitivity, swelling, and compressibility; and increases in strength. Burning of liquid or gas fuels in boreholes or injection of hot air into 6- to 9-inch-diameter boreholes can produce 4- to 7-foot-diameter strengthened zones, after continuous treatment for about 10 days. Dry or partly saturated weak clayey soils and loess are well suited for this type of treatment, which is presently regarded as experimental.