Injecting Under Pressure
The construction of a new multistory building near the east side of the Morrison Street Bridge in Portland, OR, adjacent to our client’s property, required the installation of deep foundations due to large amounts of uncontrolled fill soils and soft soils likely due to a pond that once existed at the site. Auger cast piles, a deep foundation system that uses a hollow auger to drill into the ground and fill the hole with grout, were initially used at the adjacent site. Unfortunately, due to the soft and loose soil conditions, and shallow ground water, installation of these auger cast piles caused a concerning amount of settlement to our client’s building.
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As a means to investigate the settlement issue, a subsurface investigation program was carried out by our firm to obtain more information. A total of four test borings were advanced to depths ranging from 35 feet to 65 feet. Onsite soils consisted of coarse grain fills with limited amounts of soft silt, followed by dense gravels. Micropile underpinning and ground improvement by compaction grouting were both considered to re-support the structure. Compaction grouting was ultimately selected as it was the most economical and least disruptive solution for the project. Compaction grouting is a ground improvement technique in which a low slump grout mixture in injected in the soils at high pressures. The grout material is pumped at specific rates and volumes in order to create grout columns that displace (i.e. compact) the soils around it.
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A total of 135 grout columns equaling a volume of approximately 650 cubic yards were injected below the basement of the existing building. After completion of the grouting program, four additional test borings were performed to verify the effectiveness of the program. Comparison between preconstruction and postconstruction Standard Penetration testing (SPT) showed that the average SPT N-value of the grouted stratum increased from 5 blows per foot (bpf) to 10 bpf.
Advantages and disadvantages to compaction grouting
Advantages:
The equipment used to perform the compaction grouting is compact and easy to handle allowing the contractor to work in the basement of the existing building with minimal disturbance.The material cost for compaction grouting is typically lower than other types of conventional underpinning since the grout mixture is largely aggregate with a low cement content.Installation required only a 3-inch diameter be drilled into the existing slab on grade at each location. This minimized damage and prevented complete replacement of the slab.
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Disadvantages:
Compaction grouting requires that the material be pumped at high pressures. If caution is not taken, uplift of the floor slab can occur. For this project, the lightly loaded slab on grade experienced unwanted lift at a few locations
As with any underpinning process, diligent monitoring of the structure is required to reduce the risk of unwanted movement.
Author: Juan Escoto, Senior Staff Engineer
