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Geotechnical Issues are Important for Additions, Renovations, and Remodels

With the current economy, greenfield developments are less prevalent. While additions, renovations, and remodels are generally smaller in scale, geotechnical issues are still important. In several respects, foundation and other geotechnical problems can be more difficult to deal with when undertaking construction in a restricted or confined area.

A typical condition would be where a sloping site was graded by cutting and filling to provide level ground for development. Often the better portion of the site, with a limited thickness of fill, was selected for the original building location. When it comes time for the addition, the available area can consist of a substantial thickness of poor fill placed in an uncontrolled manner. Since the initially unused part of the site was intended only as a landscaped area, fill could have been placed over topsoil that is unsuitable for foundation support. The early involvement of a geotechnical engineer can assist in identifying such problems.

Building an addition immediately adjacent to an existing structure can also be difficult. Fill placed against the outside of foundation walls may not be adequately compacted. Where there are deeper basements or machine rooms, a substantial thickness of uncontrolled fill could exist. This may lead to a requirement for underpinning, either temporary or permanent, a relatively expensive undertaking. One aspect often ignored when considering underpinning is that as the depth increases the lateral load on the underpinning from the supported soil and other loads behind also increases. Tie-backs or soil nails for lateral support may be required for the existing foundation and underpinning.

Where the building is sitting on marginal soils, perhaps soft glacial lake sediments, the ability of the existing footings to handle the additional loading from the new footings is called into question. For an older building, erected when there was less control over construction, the footings may even be supported on fill of unknown origin, further complicating the assessment of additional settlement. Where the ground was preloaded/ surcharged for the original building, a similar process may not be possible for the addition, as the new surcharge can cause additional settlement of the existing adjacent footings. Several options can be considered, including tying in new to old footings to reduce differential settlements, a more lightly loaded transition zone between new and old buildings, or offset footings with cantilevered grade beams. A geotechnical engineer can advise on an economical approach.

An interesting situation arises where a new high-rise building with foundation loads mandating a deep piled foundation is constructed right next to an older low- to medium-rise building on shallow spread footings. The installation of a more economical pile type may not be feasible because of the effect of vibrations on the existing building and its occupants during driving. The geotechnical engineer can monitor vibrations and advise on locations and depths of pre-drilling to allow the use of driven piles.

Pavement rehabilitation is another procedure that adds or retains property value. A parking lot in poor repair is not a good introduction to customers and consumers. Our experience with many recent bituminous concrete pavement projects indicates that inadequate design and construction practices are more often the cause of distress than a pavement coming to the end of its proper design life. Poor construction, often combined with limited construction monitoring and testing, typically shows up as an insufficient thickness of bituminous concrete and/or granular base/subbase or a soft subgrade condition left untreated when the pavement section is placed. In the rush to close out a project, the only check on the pavement may be to confirm it is black with white and yellow lines.