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Vibroflotation

Case Study: International Mineral and Chemical Corporation Phosphate Plant

Sand Compaction by Vibroflotation (D’Appolonia et al., 1953)

Background

Vibroflotation was chosen to improve the loose sand deposits at the site of the International Mineral and Chemical Corporation Phosphate Plant, located outside of Bartow, Florida. This technique was selected because of its simplicity and its economic superiority. The entire plant construction was projected to cost roughly 12 million dollars, and included an improvement area of 156,000 square feet. With compaction needed to an average depth of 12 feet, the contractor was able to use vibroflotation on 1400 square feet per day, and cost about $0.10 per square foot of vibroflotation improvement.

Site Characteristics

The site was composed of uniform vegetation, with no swamps present within the boundaries of the project. Figure 19 displays a map of boring locations and proposed building footprints. This figure also displays the areas which were improved with vibroflotation. The site is composed of a surface layer of very loose, clean sand varying between 10 and 15 feet in thickness. SPT blow counts ranged from 2 at the surface to 10 at the base of the sand layer. A sandy clay layer with phosphate granules is located below the surficial layer, and is locally called a “matrix.” This layer has an average depth of 15 feet and is fairly dense with a high bearing strength. Beneath the matrix, alternating layers of sand, gravel, and clay compose a mix of stiff and soft strata. The presence of coarse gravel in this lower layer gave rise to the possibility that the SPT values could be artificially high.

 Figure 16: Phosphate plant foundation layout (D’Appolonia et al., 1953)

Figure 19: Phosphate plant foundation layout (D’Appolonia et al., 1953)

Statement of Problem

It was determined that settlement could occur in the loose sand layer and the soft clay layers, located at depths greater than 30 feet. As stated in the site characteristics section, the uppermost stratum consists of very loose sand. It was determined that the average relative density of the sand in this layer was about 33%. The site will be subject to vibrations due to train movement and pumps located within the plant. Both of these sources almost definitely ensure settlement. The compressible clay layers, however, were only considered to be susceptible to a minor amount of compression due to the small foundation stresses and the depth of the compressible layers.

Solution and Design

Several foundation systems were examined to determine the best course of action in minimizing the amount of settlement experienced by the structures. Piles extending 50 to 60 feet to the gravelly clay layer were considered, however it was determined that thepresence of the soft strata at those depths would force the piles to utilize skin friction over tip resistance. The frictional forces exerted on the soft strata would induce consolidation, and result in unacceptable amounts of settlement at the surface.

Short point-bearing piles driven to refusal in the sandy matrix layer were also considered. However, it was determined that the stress exerted on the sandy matrix would also adversely affect the clay layers below, resulting in unacceptable settlement at the surface.

It was determined that spread and mat foundations on compacted sand would result in the minimum settlement. Vibroflotation was selected to adequately compact the sand at depths up to 15 feet. Based on the experience of the engineers, the relative density goal was set at 70% or a dry unit weight of at least 103 pounds per cubic foot. Three different spacing patterns were examined, including the line, square, and triangular pattern. Different spacing distances were also examined for each pattern. It was found that spacing and pattern combinations of 8 feet for the square and triangular patterns, and 7.5 feet for the line pattern all resulted in the same relative density. Ultimately, a triangular pattern with a 7.5 foot spacing was used. Compaction was conducted in one foot lifts, and an average of 2.25 cubic yards of backfill material was added for each probe.

Results

Benchmarks were placed around the site to record the amount of settlement experienced before, during, and after construction. Based on data acquired from the tests and boreholes, in conjunction with the design building loads, the total settlement was estimated to be 3.96 inches. However, at the time of completion of the report (131 days after the conclusion of construction), it was found that building had only settled 0.42 inches compared to the projected 1.0 inch.

Conclusions

It is clear that vibroflotation was an excellent solution for this scenario of very loose sands. It greatly improved the site and yielded less settlement than originally projected, while also offering a better alternative to a deep foundation system. However, it should be noted that this case study is from 1953; many contractors use updated technology and projects today may require higher standards of relative density, typically around 80%.

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