In the heart of densely populated cities and sites surrounded by existing structures, deep excavation work transforms from a simple construction step into a complex engineering challenge. Every movement in the soil can impact surrounding elements, and every decision made on-site can have unforeseen consequences for neighboring land and buildings. In this article, we present a real-life engineering case in which the pile-driving process generated vibrations that impacted an adjacent property. We detail how the execution method was changed to rotary bored piling as a safer alternative, while highlighting the technical differences between both methods and the lessons learned from this real experience.
The Beginning of the Problem
The project was a multi-story commercial building in a relatively tight site within an urban area. Excavation to a depth of approximately 8 meters was required to reach the foundation level. The loose sandy soil increased the risk of collapse, so secant piles were selected as the retaining system. These consist of a series of piles executed in such a way that part of each pile overlaps with the next, forming a continuous wall that prevents soil or water leakage. Initially, these piles were executed using mechanical driving methods. However, as soon as the pile-driving operations began near the neighboring plot, concerning signs started to emerge:
- Severe vibrations were felt by workers and residents of the adjacent property.
- Hairline cracks appeared in the boundary wall of the neighboring site.
- Slight level changes were recorded at certain points within the adjacent plot.
Analysis of the Problem
A quick technical assessment revealed that the issue was due to:
- The use of mechanical pile driving, which relies on dropping a massive iron hammer onto the pile head, generates very strong shock waves that propagate through the soil and negatively impact nearby structures.
- Absence of a prior vibration study to assess the potential impact.
- Proximity between the pile line and the boundary of the neighboring property.
- Lack of an early soil movement monitoring system, such as inclinometers and settlement markers.
Corrective Action Plan
Mechanical driving operations were immediately halted in the affected zone. The issue was presented to project managers and senior engineering leadership, and a key technical decision was made: to switch from mechanical pile driving to rotary bored piles (Bored Piles) as the new execution method.
Clarification: Rotary bored piling is not a design method but an execution technique. The pile is installed by gradually drilling the soil with an auger or rotary tool, instead of driving it with impact force.
Therefore, secant piles can still be executed using rotary bored methods to form an interlocking retaining wall without relying on high-vibration techniques like mechanical driving.
A Question May Arise:
Doesn’t rotary bored piling also use heavy equipment that causes vibrations?
Yes, but here lies the technical difference:
In mechanical driving, a hammer drops with significant force, creating high-peak acceleration shock waves—this poses a major risk to nearby soil and structures.
But In rotary bored piling, the equipment acts like a slow-turning screw or drill, removing soil gradually. This generates much lower amplitude, longer duration vibrations, without sudden impact waves. It is therefore considered the optimal method in sensitive or densely built environments.
Supporting Measures:
- Installing precise monitoring instruments such as inclinometers and settlement markers to detect any movement in the soil or shifts in nearby walls.
- Promptly repairing minor damage to the neighbor’s boundary wall.
- Communicating with the neighboring landowner to explain the issue and the corrective action plan, while committing to repair or compensation as needed.
Lessons Learned:
- Conducting a comprehensive vibration impact study is critical before execution, especially in sandy soils or adjacent to existing buildings.
- Early consideration of less disruptive execution methods, such as rotary bored piling, can prevent future problems.
- Installing a soil movement monitoring system from the project’s outset ensures continuous assessment of the site.
- Transparent communication with neighboring landowners helps build trust, reduces conflict, and limits potential liabilities.
Technical Conclusion:
In urban environments, choosing the right pile execution method is not a minor detail—it is a strategic engineering decision.
For example, mechanical driving generates intense shock waves that can negatively affect the soil and nearby buildings, while rotary bored piling produces gentler, slower-impact vibrations, making it the preferred choice for sensitive areas. the success of this project was not merely about changing the method; it stemmed from sound engineering judgment, thorough geotechnical analysis, precise real-time monitoring, and honest collaboration with surrounding stakeholders. this case reaffirms that civil engineering is not just about concrete and steel—it’s the art of making the right decision at the right moment, to ensure the safety of people and structures.