Work in Progress…
Here you see a photo from a few months earlier along with the current condition of the site. The contractor has been attacking the ground with vigor and the result has been a somewhat unanticipated seepage of water coming out of the embankment of earth seen in the background of the photo on the right. The water seems to have been pent up on one of the layers of earth about 6 feet below the level of Muskogee Avenue on the north edge of the site. The water in this case is referred to in engineering and construction terms as “perched water” and has turned the site in to a muddy mess.
Site Photos
The FSB Team received a call from the contractor requesting that we come to the site to confer on how to address the situation. The parties assembled at the site for this ad-hock discussion are the geotechnical engineer, the earthwork subcontractor, the general contractor, Gary Adair/FSB civil engineer, Tom Bush/FSB structural engineer and Lowe Runkle/FSB project architect and construction administrator.
The main concern revolves around addressing the potential effect of the seeping water on the required compaction of the excavated area for the building and the following operation of placing about 2 feet of select fill over the compacted sub-grade which needs to remain dry for proper compaction.
Why does this matter? First we need a solid base (layer of ground) over the entire area of the building footprint to create a structural platform for the 2 feet of select fill (the amount of fill recommended by the geotechnical engineer). The footings and slabs will be placed on the 2 foot layer of select fill which will provide a common layer of soil with the same characteristics for all the foundations to bear upon. The other issue is to address the approach to subsurface drainage of any continued seepage once the foundations and below grade walls are in place. The discussion leads to an alternative location for the perimeter drainage system.
The parties agree upon the following series of actions:
- The contractor will need to effectively address the rerouting of the seeping water away from the area of the building footprint either through the early placement of the subsurface drainage system or by other means of dewatering the area.
- A structural base layer will be created by pushing rock not greater than 3” diameter into the sub-grade creating a surge layer. This will probably entail an average of 6-9” of rock being rolled into the now soft muddy ground to create the structural platform for the placement of structural fill on top.
- The perimeter drainage system will be lowered about 2 feet from the bottom of the footing to the bottom of the structural fill layer.
- It is critical that the dewatering effort allow the sub-grade to dry and to keep the structural fill material dry.
These effective and timely solutions came about from getting several minds and perspectives on the situation together in an ad-hock brainstorming session at the site.
Other discoveries during excavation include a variety of uncharted piping below grade, buried concrete slab and an area of contaminated soil. The contaminated soil amounted to about 2 dump truck loads of earth in an area where automotive maintenance probably occurred in the previous car dealership that was on the site. Not uncommon and easily dealt with.
While these unknown conditions create sufficient levels of anxiety, the unfolding challenges of the construction phase provide tremendous adrenalin filled rushes of excitement.
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Design Matters 