In Part I of this topic, we discussed the assessment and the identification of the cause of settlement of existing structures. Once the causes have been identified, we can then provide alternatives to prevent on-going settlement, if needed. This post will discuss a few of the engineering solutions that are available to mitigate a settlement problem of an existing structure.
It should be mentioned that we do not always propose mitigation or remediation. For instance, if the settlement of the foundation appears to be due to the placement of compacted engineered fill in the 1960s (during construction) over soft compressible clay, OTO likely may recommend that the client delay large and expensive repairs and mitigation, and instead monitor the rate settlement over the next couple of years. In these instances, the rate of settlement often has decreased to negligible amounts and further significant settlement may be unlikely. At that time, we often recommend that the owner proceed with larger structural and cosmetic repairs.
If settlement concerns appear to be due to improper drainage and the introduction of large amounts of water into the soil mass, OTO will provide recommendations for correcting the drainage problems. We often can provide local contractor names, upon request, to help repair or install new drainage systems. Often times, these repair or maintenance tasks can be performed by the owner or facilities manager.
If the cause of the building settlement is the presence of an unsuitable bearing layer, such as loose, non-engineered fill that may continue to compress, or a thick organic peat layer that may continue to degrade, we will recommend a mitigation alternative such as a deep foundation or a soil improvement technique.
A deep foundation system transfers loads through the unsuitable layer to a firm bearing layer, such as driving pilings through a clay layer to bear on a layer of dense sand or bedrock. Deep foundation alternatives to mitigate the settlement of existing buildings may include helical piles or mini piles. Helical piles consist of a central steel shaft with horizontal bearing plates (8 to 14 inches in diameter) welded to the shaft at spacings on the order of 12 inches, which are augered into the soil. Mini piles are drilled, cast in place, cement grouted shafts. The piles are constructed by drilling and advancing casing (three to ten inches in diameter) to a selected depth or bearing stratum, installing a steel reinforcing bar down the center of the casing, and injecting cement grout into the casing. The grout is pumped into the borehole at high pressure, starting at the bottom of the casing and moving upward in order to displace drilling mud or any remaining soil cuttings from the borehole. As the grout is pumped into the borehole, the casing is pulled up to a selected depth at the top of the “bond zone,” allowing contact between the grout and the surrounding soil. Helical or mini-piles are typically connected to the existing footings using an underpinning bracket.
Soil improvement techniques, which improve the existing loose soil so that it can function as a suitable bearing layer, may include pressure or compaction grouting. In compaction grouting, the soils within the improvement zone are densified and strengthened by a systematic, pressurized injection of controlled low mobility cement grout. The goal of the process is to achieve increased strength of the soil mass.
Many factors must be considered in order to recommend the most appropriate engineered solution for settlement issues. OTO will often discuss existing building and soil conditions and proposed mitigation techniques with specialty geotechnical contractors to evaluate possible alternatives costs. OTO maintains relationships with most of the foundation specialty contractors in New England and often can provide two or three independent contractor contacts to the client so that competitive cost information can be obtained. Once the mitigation alternative and contractor is chosen, OTO can assist during construction by documenting the installation and addressing any concerns that arise.
If you have other questions about building settlement, contact Ashley Sullivan at 413-276-4253 or email@example.com to see how OTO can help!