Roto Mixer

PhosphoCrete™ Parking Lot Restoration

Because of the pavement distress, numerous previous geotechnical investigations were completed. MEG reviewed the previous reports and conducted a new investigation that included site observations, geotechnical exploration, laboratory program, and engineering evaluation and analyses in order to determine the proximate cause for pavement distress.

By Jason Alligood, PE and Larry Madrid, PE, D.GE, F.ASCE

Madrid Engineering Group, Inc. (MEG) recently completed a project at a local “big box” store where pavement distress has been a15-year on-going and recurring problem.  The solution? A unique, proprietary chemical stabilization method called PhosphoCrete™.

The project site is located in Lakeland, Florida and was built on previously mined land.  The property was mined for phosphate circa 1930’s based on information previously researched by the original pre-construction geotechnical consulting company.  In typical fashion for mined land during this era the excavation was filled in with the waste clay from the post phosphate removal process.  These clays are pumped back into the excavation in slurry form with moisture contents on the order of several hundred percent.  Typically the waste clay filled excavations are then backfilled with cast overburden spoils.  Many times the sites are left in this condition leaving behind clay soils that are unable to support even their own weight let alone any kind of structure or even vehicular load.  This makes development of such land costly and undesirable.

After the subject property was no longer used for mining, fill was placed over the mine waste to heights on the order of 15 to 20 feet.  The site remained dormant until the 1980’s when borrow material was excavated from the site to use for embankment construction of the Polk Parkway and other nearby earthworks projects.  Reportedly, some portions of the site were excavated to elevations of 5 to 10 feet above what was believed to be the original, pre-mining ground surface.

Because of the pavement distress, numerous previous geotechnical investigations were completed.  MEG reviewed the previous reports and conducted a new investigation that included site observations, geotechnical exploration, laboratory program, and engineering evaluation and analyses in order to determine the proximate cause for pavement distress.  Previous investigations completed by other consultants had concluded that the high moisture content in the waste clay soils were causing strength loss in the pavement section or that potential swell of the clay soils were causing distress in the pavement.  Early on in MEG’s investigation one key observation was made that indicated a more primary condition was likely occurring: we noticed that the distress was occurring primarily around select landscape islands containing oak trees, and also that the extent of distress was generally confined to the tree drip lines.  It was also noticed that the oak trees in the areas of distress were generally larger diameter and more mature than the oak trees in islands where no distress was evident.  Finally, the majority of the distress was at tree islands at a lower elevation of the parking lot where water levels would be expected to be closer to the surface.  MEG suspected that the primary problem was water uptake by the oak tree root systems causing volume change in the clay soils.

SPT borings completed within the areas of concerned confirmed that waste phosphatic clay was present at depths ranging from 2 to 6 feet below ground surface and extended to depths on the order of 8 to 10 feet. To add to the poor soil conditions the surficial sandy soils were loose and organic muck was directly underlying the waste clay soil in most of the borings.  The deeper clay and organic soils would certainly be a difficult condition to contend with during construction.  The waste clay soils tested at depths of 2 to 6 feet bgs had moisture contents between 65 to 74 percent.  With respective plastic limits of 58 to 64 it appeared that the soils were much drier than they would have been originally or even just a few years prior to the investigation.  Tests completed by previous consultants indicated that the deeper clays had moisture contents as high as greater 100 percent.  Although the decrease in moisture content of the more shallow clayey soils could be caused by changes in design drainage patterns it was also a likely indication that the trees were indeed pulling moisture from the clay.

With the conclusion that water uptake from the oak trees was the primary cause for pavement distress MEG recommended that the areas of distress be reconstructed by removing the waste clay soil to a minimum depth of 6 feet bgs and replaced with clean fill limerock or No. 57 stone.  A more financially inexpensive alternative to this was also presented.  This included the use of a proprietary method called PhosphoCrete™ developed by MEG for chemically stabilizing soft, waste phosphatic clay.  Through a chemical process PhosphoCrete™ can permanently alter those clays to create a stabilized platform which provides stronger, more stable conditions on which to construct.

By using this product minimal spoil would be created as the in-situ soils would be modified on-site and then used as backfill.  Only minimal import would be required and was associated mostly with free draining material required in the island areas for the alternate tree replacements.

PhosphoCrete is based on a deep soil stabilization technique called lime columns, used in Scandanavian countries to strengthen in-situ soft marine clays.  The Lime Column technique involves mixing clays with quicklime, or CaO.  Hydration occurs immediately, stiffening clays, then a pozzolanic reaction takes place over time by reaction to silica that is naturally occurring in the clays.  Lime column methodology was successfully tested on waste phosphatic clays around 1990 – 1994 by Bromwell & Carrier, Inc. on a funded research effort (Florida Institute of Phosphate Research). PhosphoCrete is a further refinement of this research and is a shallow soil mixing modification to the lime column method, and recognizes that quicklime is not the only pozzolan available and is a relatively expensive commodity.  A proprietary chemical mixture is utilized in the PhosphoCrete method to strengthen the clays.

Review of the original recommendations by the design/construction team including the store owner ultimately resulted in a revised construction plan that included excavating to shallower depths of 4 to 6 feet utilize and using PhosphoCrete™ to treat the in-situ clayey soils.

In general, the project included a process of excavation, treatment, and replacement of existing clayey soils within areas of the parking lot selected by the civil engineer where distress was observed.  The existing soil was a mixture of sand, clayey sand and waste phosphatic clay which were blended with PhosphoCrete™ Base mixture prior to placement.  The existing soil was excavated and placed in a designated on-site mixing area.  The blending process included intimate mixing of the existing clayey soils with sandy soils with PhosphoCrete™ Base mix.

The blended PhosphoCrete™ was then transported by front end loader back to the excavation, compacted, and tested for strength.  Compaction generally consisted of using a backhoe bucket for the placement of the first one to two lifts, then utilizing a small Bobcat and/or front end loader to place the upper 1 to 2 lifts.  Loose lifts were typically 12-inches thick.  A high strength geotextile (Mirafi HP370 15-foot wide rolls, with a MARV ultimate wide width tensile stress of 3600 lbs/ft) was used as a separator blanket and was installed at the base of the excavation,quipment and operator were at risk of breaking through the surficial clays and dropping through the soft materials below.  The blanket helped to spread out the load and reduce the contact pressure.  This improved the ability to compact and place the PhosphoCrete when very soft clay soils created difficulty in construction of the first lift.  In the worst case condition, one additional 1-foot thick layer of PhosphoCrete™ treated material was placed first, followed by the geotextile, to further stiffen the base of the excavation.  Occasionally, crushed concrete base material stockpiled on-site was used at the bottom of excavations where very soft soil conditions existed to help bridge over the clays to construct the first lift and place the geotextile.  Each 12-inch lift was tested at several locations within each sub-area using a hand cone penetrometer and pocket penetrometer to record strength readings of the individual lifts to compare with natural in-situ conditions.

MEG provided a full-time, on-site technician to monitor the excavation, mixing of existing soils with PhosphoCrete™, placement of treated soils, and to record the relative density readings. Readings were taken on each lift and for each excavation area.   Strength readings were also completed on untreated waste phosphatic clay excavated from the subareas to compare pre-stabilization conditions to post-stabilization conditions.  In general, there was an increase in both penetration test values as compared to the base readings performed within each excavation area.  In areas where very soft to soft soil conditions existed the pocket penetrometer results typically encountered an increase in values from in-situ readings of 0.25 to 0.75 tons per square foot (TSF) to between 2 and greater than 4.75 TSF (maximum value of probe).

Results and Conclusions

Based on the observations and testing completed by the field technician, and follow up post-testing completed during and after construction, the specialized earthwork was completed in general conformance with the specifications provided for the job and expectations of Madrid Engineering Group.  The testing indicated an immediate strength gain due to hydration which facilitated in placement of the soils in a well compacted, uniform manner.  Based on laboratory bench scale testing, it is further anticipated that the material will continue to strengthen over time by pozzolanic reaction in the presence of moisture from groundwater and localized infiltration through the tree islands.  Therefore, the strength increases measured immediately after placement of PhosphoCrete™ lifts of 2.6 to 20 times initial strength are the minimum strength gains.  Laboratory testing of Phosphocrete™ indicates typical strength gains of 10 to 50 times the initial strength of the soil after 28 day curing/reaction periods.

In addition, one regular observation made during the earthwork process included visible tree root systems within the clayey and waste phosphatic clay layers that were excavated, which is consistent with our original observations and conclusions for a proximate cause of the pavement distress as described in detail in our original geotechnical report.