Christina Storage PhosphoCrete™ Case Study
South Florida Avenue, Lakeland, Florida
In January 2003, the site contractor at the Christina Storage Center alerted the engineer that he was unable to achieve compaction in the driveway areas at the site due to the presence of soft phosphatic clays at/near final grade. The contractor expressed fear that his earthmoving equipment would break through the crust and sink into the underlying eight-foot deep clays. In addition the clays were also perching the recent rainwater near the surface, which prevented other soils from drying out sufficiently to be compacted. At the request of the engineer, Madrid Engineering Group, Inc. immediately initiated in a geotechnical investigation using the on-site backhoe to dig test pits, and to confirm the near-surface soil conditions including the presence or absence of clays.
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The geotechnical investigation revealed moist sand located at the top of the hill whereas the lower test pits |
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revealed saturated clayey sand at the surface, underlain by very soft waste phosphatic clay (slimes) at a depth of 1 to 3 feet. The clay was the consistency of peanut butter with 61% moisture content, and the sand was saturated. Madrid Engineering Group, Inc. recommended chemical stabilization of the top 2.5 feet of materials using the PhosphoCrete™ method, to provide a stabilized platform on which to place the crushed concrete base and the asphalt for the driveways. The thickness of 2.5 feet was based on the relatively lightweight loading, typical roadway design, the strength of existing soils at the site, and anticipated strength gain of materials stabilized by PhosphoCrete™. |
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Initial Site Conditions |
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In addition to recommending the PhosphoCrete™ method, Madrid Engineering also recommended the installation of a permanent 4-inch (minimum diameter) underdrain system into the clays in the northernmost aisle to drain the excess rainwater that had ponded on top of the clays. The drain was installed to a depth of approximately 4 feet and over the weekend, it effectively removed a considerable amount of water from the sand layer and improved the site conditions for mixing PhosphoCrete™.
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Forty tons of PhosphoCrete™ materials were purchased and 6850 square feet were stabilized in three days. The areas of concern were excavated and moved temporarily to the staging area, where they were laid out in one 8-inch lift. The sand and waste clays were pre-mixed using a motorized roto-till machine provided by the contractor. The PhosphoCrete™ materials were added using the bucket of a front-end loader, amounting to approximately 5 percent mix on a dry weight basis. The PhosphoCrete™ was then mixed into the sandy waste clay with just one pass of the roto-till equipment. After allowing the materials to ‘mellow’ for approximately one half hour, the mixture was then moved back to the excavation and backfilled in continuous lifts of a maximum one-foot each. Loose compaction was achieved using only the weight of the wide track dozer for the first two lifts, then additional compaction was made using the rubber-tired front-end loader on the exposed PhosphoCrete™ and for each additional lift. |
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Adding PhosphoCrete™ In Staging Area |
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Roto-Till Type Mixer |
Madrid Engineering provided continuous construction monitoring for the excavation, staging, mixing, and placement of the PhosphoCrete™ materials back into each of four stabilization areas. We measured the depth, length, and width of each excavation, and also monitored the mixing procedure, approving only when thoroughly mixed, and compaction/placement. Madrid Engineering Group also supplied the propriety mixture of chemicals needed for PhosphoCrete.™
Mixing
PhosphoCrete™ In Staging
Area
Backfilling With
PhosphoCrete™
Field strength tests were completed on site after approximately one week in the ground. Ten penetration tests were completed using the pocket penetrometer at various sites within the mixed areas. All sites achieved strengths of over 3.5 tons per square foot, more than 10 times the initial strength of the clay, well above the expectations for 28-day strength.
After allowing the mix to set over the weekend, backfilling with crushed concrete base took place without incident. No cracking or distress was observed in any of the PhosphoCrete ™ mix areas. Anecdotally, it was observed that a fully loaded dump truck with crushed concrete drove over the treated area with no signs of distress to the soil.
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The site was paved and the owner accepted the project. It is anticipated that the PhosphoCrete™ will continue to improve in strength over time, due to the pozzolanic nature of the mixture and the availability of from the underlying clays, for additional curing. |
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Final Pavement Surface |
