A Solid Foundation
Geotechnical Services
New micropile and tieback to support the grade beam
The proximity to downtown Portland, great panoramic views of the city, and unique geography, make houses built along the Southwest Portland Hills very desirable. However, due to the existing topography, construction tends to be very challenging. Structural elements such as foundations are often designed to withstand gravity loads, geotechnical loads, and seismic loads. In the case of this project, the constant rain had eroded and saturated the native soils causing the shallow foundation components to settle and shift a few inches. The movement of the footings eventually damaged the structural beams and bracing supporting the house and deck.
GEOMechanics was retained by the property owner to provide temporary and permanent foundation recommendations. Due to the many different forces acting in different directions, GEOMechanics decided that a grade beam with micropiles and tiebacks would be the best foundation alternative. Micropiles and tiebacks are structural components used by geotechnical engineers in order to transfer/distribute surface loads into deeper soil stratums. Both micopiles and tiebacks, obtain geotechnical capacity/resistance through skin friction between soil or rock and the pile grout. For this specific project, both micropiles and tiebacks were drilled several feet into the native weathered bedrock in order to provide the necessary support.
Damaged timber beam
Additionally, slope stability analysis performed by our firm, determined that the factor of safety for the static condition was below industry standard. For these reasons, the tiebacks were designed to provide lateral support and the micropiles were designed to support axial loads, provide out of plane lateral capacity, and provide shear resistance in order to increase the slope stability Factor of Safety.
The entire project consisted of replacing the existing shallow foundation with a new grade beam supported by micropiles and tiebacks, the removal and replacement of the damaged timber beam with a new steel beam, and the repair of the bracing.
Construction Services
During Construction, GEOMechanics provided Special Inspection Services in order to verify design assumptions and compliance with project requirements. Some of the services included:
- Special Inspections during excavation. It is critical that the new foundations components are founded on good native soils in order to reduce the risk of settlement and erosion.
- Special Inspection services during drilling procedures to ensure that the foundation components will achieve the required geotechnical capacity.
- A Static load test was performed in compliance with the Oregon Structural Specialty Code. This ensures that the new foundation components have achieved the required capacity and meet Industry Standards.
Thanks to the experience and great coordination between all the parties involved, this challenging project was completed successfully.
Load transferring procedures
Author: Juan Escoto, Senior Staff Engineer, (971) 901-2067
