As part of the planning process, contractors managing expansion projects should identify the design and method used to support the original building. It may very well involve ground improvement.
With more than three decades of ground improvement design and methods available and used in the commercial industry, thousands of buildings now exist. The more successful building owners will eventually need more space and capacity, and this will lead to expansions.
What does this mean? It means the original building could be resting on poor or weak soils that have been strengthened using a ground improvement design. If any of the reinforcing elements weaken or unravel due to over-excavation and/or encroachment—or the matrix soils are adversely affected from dewatering—the existing building’s foundation and floor slab could be jeopardized.
To manage these risks, contractors should carefully consider the methods for addition support so the original building is maintained in a good condition. It’s just not as simple as it sounds.
What is ground improvement?
Ground improvement, by definition, is the improvement of weak or poor soils beneath foundations and floor slabs. Such methods can provide a cost-effective alternative to deep pile/caisson foundations and over-excavation/replacement, such as soil correction and undercutting.
Unlike support using driven or drilled piles/caissons that bypass weak soils, ground improvement is really a spot-specific strengthening of poor, compressible soils. The improvement allows the use of normal spread footings (usually with a high bearing pressure) and standard floor slabs-on-grade.
The poor or marginal soils are still present beneath the original building, but they were strengthened by the introduction of crushed rock and other granular materials, and more recently from rigid inclusions, such as grout/crushed rock blend or concrete, into the weak soil matrix.
Whether the original building site had uncontrolled fill, soft clay, loose sand or silt, or organic soils such as peat, it’s probable that the expansion footprint is underlain by these same poor soils. Often, high groundwater is part of the mix, too.
If the ground improvement elements are disturbed or weakened under the existing building’s foundation, then the improved soils will try to lapse or revert back to poor soil behavior. This of course could lead to unexpected settlement of the footings or interior floor slab.
How to locate existing ground improvement
To manage these unknowns, the design/construction team should conduct a comprehensive review to identify how the original building is supported. This should be done for every addition/expansion project because identifying the location of the reinforcing elements can be challenging.
Many times, the ground improvement design won’t be found in the original construction design documents. Typically, the ground improvement design was and still is provided as a deferred submittal for review by the design/construction team. Often they are provided to the city, too. The reviewed submittal and any revisions are in storage somewhere, but are not part of the original plans.
To complicate matters, due to not realizing the original building support method, the geotechnical consultant may have just recommended over-excavation of poor soil and replacement with new compacted fill. The report may also show the need for dewatering so soil removal and backfill placement can all be done in dry conditions. Either of these recommendations can create the aforementioned soil weakening and settlement risks.
The design/construction team can do electronic research to investigate the project in question. Most projects dating roughly back to the mid-2000s or so will have digital files of all design aspects; go to the architect’s/engineer’s office and general contractor’s office to find these files.
For earlier projects, the design/construction team may need to dust off paper blueprints and folders. Another idea is to start calling ground improvement product manufacturers or engineers to see if they remember or can find the project in their files.
Let’s say that the research paid off and the improvement design is found. Then the project geotechnical engineer and structural engineer of record can determine methods to avoid the “no-dig zone” near these elements. The no-dig zone varies but is meant to keep excavation out of an area that could compromise the pier(s) or the footings they support.
Solutions for existing ground improvement elements
Hopefully the team can discuss the situation with the original ground improvement designer to put together a good strategy that incorporates avoiding risks that the original designer brings forward.
If the over-excavation approach turns out not to be viable because it would disturb the piers or surrounding matrix soils, or even undermine the footings, then the team must turn to other solutions.
For example, the design team can turn to the original ground improvement method to stabilize the addition’s weak soils in a similar manner. If it worked well in the past, it could work with the expansion project.
Other options include low-vibration ductile iron piles, micro-piles, helical piles or drilled foundations. With very close new footings, these techniques are helpful, especially when the team wants deep foundation support and has grade beams, pile caps and structural slabs.
If the team is intent on over-excavation, then shoring could be installed to avoid disturbance, or the adjacent footings could be underpinned with some type of piles. Dewatering is a potential issue with high groundwater and this could lead to weakening of soils between the piers; this possibility has to be carefully considered. In addition, flowable fill could also quickly backfill limited excavation areas.
Conclusion
Identifying the original building support is always essential when designing and constructing an addition. This is true whatever was done in the past.
If ground improvement was used, then take time to identify the technology, consider the risks and employ methods to avoid or manage them. This will set up the expansion project for success.
