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GEOTECHNICAL CONSTRUCTION DSM Excavation Reinforcement A municipality required an excavation support wall for the installation of a new sewage pump station. This wall would not only have to allow work to be performed in a 40-foot deep excavation, but also effectively cut-off groundwater flow into the excavation and prevent any soil movement on adjacent properties. Other methods were considered including sheetpiles, soldier piles and lagging, and conventional slurry diaphragm walls.
Geo-Con's Deep Soil Mixing (DSM) method was chosen over these because it allows vibration free installation, there are no open trenches, and it has the ability to place an impermeable soilcrete "lagging" before excavation. Besides the obvious benefits from a safety standpoint, DSM was also more appealing to the local residents because of the eliminated vibrations and shorter construction schedule. DSM also relieved concerns over the effect on adjacent residents from a lowered water table or soil movements.
The site was located in the heart of a residential neighborhood. Requirements called for the installation to proceed without interruption of service. To accomplish this, the piping to the existing pump station was rerouted around the boundary of the new pump station.
To allow for efficient use of space on the restricted site, the batch plant was reconfigured to fit between structures of the existing pump station. The DSM rig was assembled in the middle of the footprint for the excavation containment wall. Once assembled, the rig was able to install the wall around itself due to the ability of the augers to rotate 270 degrees about the leads. This was particularly important as the wall alignment was within a few feet of the adjacent structures.
The project called for a structural wall with a maximum permeability of 5x10-6 cm/sec. and internal bracing at two levels. A design mix was developed and implemented in the field to achieve these goals. This mix consisted of a fluid cement-bentonite slurry which was injected through the augers and mixed in-situ. The resulting soilcrete (a mixture of soil and slurry) obtained an approximate UCS of 100 psi and served as lagging between the structural beams.
In order to install the beams for structural reinforcing, a template was used. After the augers completed the mixing process of a stroke, the template was positioned to guide the beam. Once the beam was plumbed, it was lowered into the fluid soilcrete, virtually under its own weight.
Samples were taken of the fluid soilcrete directly from the wall and placed in molds that modeled in-place conditions. Permeability tests on these samples showed an in-place permeability of 1x10-6 cm/sec.
To create a "bathtub" so that groundwater would be kept out of the excavation, the DSM wall keyed into a natural clay layer. This clay layer was approximately ten feet below the limits of the excavation. By preventing an influx in groundwater, excavation was able to proceed in the dry. Only a two-inch electric pump was needed to remove water trapped within the "bathtub."
The DSM wall installation was completed in two weeks. This was particularly important on the project since limited access prevented more than one major task from being performed at a time. Immediately after installation, the DSM rig was dismantled and removed. The General Contractor then began the excavation sequence.
As excavation proceeded to each bracing level, the Contractor was able to simply scrape the soilcrete with the excavator to expose the beams. Once exposed, the walers for the internal bracing were attached directly onto the structural beams.
Controls were built into the DSM system to continuously monitor verticality about both axes. This ensured that strict tolerances were maintained during the DSM wall installation. As a result, the Contractor was able to use the DSM wall as the back form to pour the cast-in-place permanent wall.
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