Ali Khoja Member Name
Senior Project Geotechnical Engineer
Challenges in urban land development include a high shoring cost for deep excavations. Now, more than 40 years since its inception, the standard of practice in shoring design is still quite conservative, particularly for excavations into competent glacial soils. Moreover, the benefits to evaluating site dimensions (i.e., 3D effects analysis) – an issue that has been investigated in academia but not in practice – is commonly ignored in shoring design. Not accounting for these site-specific details can inflate the cost of shoring, especially for excavations deeper than 50 feet, since standard plane-strain design can often overpredict shoring deformation.
The shoring challenges of deep excavations
Large and deep excavations can extend to around 90 feet deep for high-rise structures in cities. A rule of thumb is that beyond 20 feet deep, the typical method for shoring includes soldier piles with multiple rows of tieback anchors which is a conservative design for glacial soils.
Beyond 50 feet, conventional design becomes very costly, and requires heavier pile sections and longer tiebacks which often introduce extra easements. Since easements require permission from adjoining property owners, they can become problematic. This is especially the case if the proposed high-rise building might obstruct that landowner’s view. It is not uncommon for neighbors to go out of their way to stop developments in certain circumstances.
With that in mind, it is in developers’ best interests to avoid easements where possible. Achieving that goal requires advanced design and often requires city approval. This process varies by location, but some requirements can include:
- Numerical modeling (FLAC3D, PLAXIS3D)
- Ability to demonstrate adequate performance
- Peer reviews
All these items can potentially add significant costs and time to a project.
At what point does a 3D model become cost effective?
Challenges like these make advanced design with 3D effects worth considering. A general guideline is when the length-to-depth ratio for the shoring area is greater than or equal to four, the assumption of plane-strain is generally acceptable. Once the length-to-depth ratios become less than two, (i.e. excavations that are closer to box-shaped in nature), 3D effects become valuable considerations.
The difference between 3D and plane-strain deformation predictions can be significant. Conventional designs often ignore the third dimension, which can promote partial self-support. 3D models offer a more in-depth analysis of the performance of shoring, resulting in more efficient design.
A use case of 3D shoring design for glacial soils
In this example, the shoring designer estimated $200K (USD) were saved by using 3D effects.
Golder worked with a client in Seattle on a 75-foot-deep excavation project for a proposed 41-story building with several levels of underground parking. The site dimensions resulted in a length / depth ratio of 1.6, indicating that the shoring design would benefit from a 3D effects analysis of the site layout and soil conditions (very dense to hard glacially overridden soils).
Rather than relying on conventional design techniques, Golder innovated an advanced design solution by performing a 3D numerical analysis to refine the shoring design – most likely the first time it has been applied to geotechnical practice in Seattle.
The model simulated the excavation sequence and explicitly modeled the soil and shoring design components including tiebacks, soldier piles, lagging, etc. Calibration of the model and soil properties were carried out using a separate model of a completed and well-instrumented excavation which was located only a few blocks from the project site.
The results of the analysis showed lower earth pressures than typical design values used in Seattle, particularly near the corner of the site where the estimated pressures were about 30 percent less than those typically used. The extensive monitoring program implemented during the shoring construction demonstrated the accuracy of the 3D model and set the stage for an economical shoring construction.
In addition to the positive economic impact for the client, the project won the award for “Unique or Innovative Applications” at the American Council of Engineering Companies Engineering Excellence Awards.
Incorporating 3D effects into a design
It has been demonstrated that there are circumstances where 3D effects in shoring design are beneficial and can result in substantial savings for projects requiring deep excavations. It’s important to note that quality inputs are critical to success – a lack of data may mean it’s not viable. To gain the best outcome, develop a 3D numerical model that:
- Contains shoring components that can be modeled individually
- Allows staged construction to be simulated as benchmarks
- Makes use of proper soil characterization
- Can be calibrated against a case history (where possible)
With this data on hand, a model incorporating the 3D effects will enable designers to design more efficient shoring solutions for the project, delivering a much more cost-effective and efficient construction.