Soil cement, a compacted mixture of native soil, portland cement, and water, has a number of uses in today’s construction industry. The more common uses today are for stream bank protection, grade control (drop) structures, and pond liners. Soil cement has been proven in all of these uses to be cost effective, aesthetically pleasing, performance and time tested. Other uses for soil cement are retaining walls, streets, shoulders, airports, parking and storage areas.
As the soil cement is placed and compacted, the cement hydrates and the mix becomes a structural slab-like material. After construction and curing, soil cement is hardly affected by water or the freeze thaw cycle, therefore does not pump under construction traffic or rut during spring thaws, and can bridge over soft sub grade.
Almost any native soil can be used for soil cement, but for optimum production through our continuous mixing pugmills, a 2" minus material is recommended (2" minus or less is usually specified for soil cement projects). One cost savings can be obtained from the utilization of on site material. Well graded sandy and gravelly materials with 30-45% retained on the No. 4 sieve work best for soil cement. Material that is deficient in fines or has a high silt/clay content still works well for soil cement but may require a higher cement content.
Portland Type I or II cements are used the majority of the time for soil cement; other types may be used if they comply with the latest ASTM and AASHTO standards. Water is important in soil cement for two reasons. First, it helps obtain maximum density, and, second, it is necessary for cement hydration.
The three fundamental requirements for strong and durable soil cement are proper moisture content, minimum cement content, and laboratory determined densities. A continuous mixing plant is recommended over the mix-in-place technique when accuracy and homogeneous mix is necessary in order to adhere economically to the specifications.
Gears, Inc. and PEI utilize an Aran portable continuous mixing plant whose production is rated between 100 and 1,000 tons per hour contingent upon the engineering properties of the material being used, speed of excavation, and speed of placement.
The production of the mixing plant is monitored by the use of instruments which display the speeds of various feeds to the mixer. Feed rates for soil, cement, and water are variable and are metered with a computerized system based on the weight and density of the native soil.
Since all of the ingredient feeds are volumetric and the mix density is generally specified in terms of weight, it is necessary to calibrate each feed. The purpose of this calibration is to establish an accurate relationship between speed/gate setting and the resultant throughput in tons per hour.
The soil is loaded in the feed hopper by means of a front-end loader or conveyor. A fixed or variable speed belt feeder draws that material from the hopper at a uniform rate and transfers it into the Aran twin shaft mixer.
The cement is metered from the overhead storage silo by means of a cleated belt feeder and enters the same zone as the native material.
Water is drawn from the lower storage tank and metered by a variable speed positive displacement pump system. It is delivered to the mixing chamber which minimizes the potential for escape of dust during the mixing process.