A business and technical challenge
Every structural engineer’s nightmare is to receive a call when a vibrating or rotating piece of equipment is started up and are told that the foundation is vibrating excessively. It is important to ensure that supports for these types of equipment are designed based on best practices, especially when they are placed in unfavourable soils.
Unfavourable soils are those that have low bearing capacity, especially when they are made of several metres of clay.
Best practices are those recognized for designing foundations to accommodate vibrating or rotating machinery. This type of equipment and its foundations raise many challenges for structural engineers who need to really understand the following:
- the loads the equipment transmits and where they apply on the foundation.
- the operating frequencies associated with the loads.
- the dynamic characteristics of the soil.
- the acceptable limits for the foundations when exposed to vibration.
- the type of construction and materials used.
- the surrounding conditions.
Structural engineers must also be on site to quickly meet challenges that can arise during construction.
With so few solutions available to engineers and the soil not being suitable to support the weight of foundations and equipment, piles remain the sensible choice. There are several types of piles: H-piles, tubular, precast concrete, widened base and caisson piles. There are two criteria for pile selection: it must be technically suitable, and it must reflect the experience the selected specialized contractor has with the type of piles.
A concrete example
As part of a turnkey mandate carried out by BBA for an electric utility, the challenge was to build four synchronized condensers weighing 350,000 kg each, including a 136,000 kg rotating component. This heavy equipment generates large torque and harmonic dynamic loading during operation, ramp-up and ramp-down.
Because the condenser mechanism can only experience very little vibration while operating, the foundations had to be designed with enough rigidity to reduce any induced vibration and ensure smooth operation. However, they could not be too rigid because their resonance frequency coincided with the operating frequencies. The balance between flexibility and rigidity was key to designing these foundations.
Moreover, BBA had to address the requirement that ensured the natural frequency of the foundation system was at least 20% higher or lower than the condenser operating frequency to avoid potential resonance issues.
Unfavourable soil conditions: 18 metres of clay!
During the initial study, geotechnical borehole data revealed that all four sites had a layer of soft clay up to 18 metres thick above till and weak rock. This is a unique engineering challenge due to the softness or the lack of rigidity of the clay layer. However, the combination of large foundations and the soil provided the system with a lot of damping. Although the process equipment was identical for all four locations, the foundation design itself could not be a “one-design-fits-all” solution because of varying soil conditions. The caisson design was customized for each location to reflect the various soil parameters at each specific site.
The combination of unfavourable soil conditions at each location and the strict requirements for smooth equipment operation called for a custom solution. The main challenge was to design a foundation system that had lower natural frequencies than the operating frequency, without compromising its rigidity. The soil-structure interaction also had to be considered since the geotechnical aspect of the site represented the greatest risk to the design.
An adapted and beneficial solution
During the detailed design phase, BBA chose to use a concrete block foundation system on bearing piles. We had two reasons for doing so: these piles provided a natural frequency range outside the target range and the selected specialized contractor had already had installation experience with them in the past. The primary natural frequencies of the designed structural systems were all below the prescribed operating frequency. Over-tuned concrete block foundations require a more rigid foundation, increasing constructability costs and difficulty. Under-tuning occurs when the frequency of the foundation or supporting structure is designed to have a primary frequency below that of the vibrating equipment.