TILT MONITORING CASE STUDY:
Mitigating Risk During Excavation
Tilt monitoring of older existing structure needed during construction
SJV Construction was retained by PEEK Properties, Inc., as the supportive excavation engineer for the construction of a new apartment complex in East Orange, New Jersey. The construction site posed some unique excavation challenges:
Plans called for excavation and construction of lot-line walls, mere feet from a 30-year-old existing building.
This building, an original wood-frame structure with two subsequently built additions, had a large, 3/4" crack that had formed along the butt joint between the additions.
One addition was constructed with 8" block with a return; the other was set on slab-on-grade and was unattached to the original masonry. They were both unfiled and did not meet building code requirements. Of concern was that neither the primary nor the additional masonry had the lateral stabilization typically required.
Given these challenges, SJV's engineer-of-record was concerned that additional movement would be likely to this structure during excavation due to the lack of an appropriate foundation and lateral bracing. He recommended tilt monitoring during excavation and foundation construction to provide continuous information about the movement of the structure during the shoring process.
Veva III Tilt Monitor Reveals a Buried Problem
Photo 1: Existing structure prior to excavation. The red circle indicates reference points used for manual measurements. The green circle indicates Veva III tilt monitor placement.
One Veva III Tilt Monitor was placed on the addition approximately five feet above ground level and in close proximity to the crack to be monitored (see photo 1). Continuous tilt monitoring was provided with hourly recording 24 hours a day. The data uploaded to the Inzwa Cloud platform could be viewed by the engineer-of-record at any time and from any device.
While drilling in the rear of the addition to protect the structure, an object was struck approximately six feet below grade. This was unexpected, as soil engineering analysis previously conducted had indicated no structural elements or protrusions within 18 inches of the wall. Believing it to be a boulder, the excavation continued. However, within an hour the tilt monitor indicated rotational movement of approximately 0.04°, triggering an alert and suspension of drilling.
New test pits revealed an existing footing from a previous foundation extending beyond the lot line and into the construction site. Additional footings were also discovered the following day during drilling, triggering an alert by the Veva III and causing additional work stoppage. New measurements indicated that the wall not only rotated an additional 0.35°, its pitch changed by 0.15° as well, which was not visible to the naked eye.
Rotational movement from striking first obstruction (A). More significant rotational movement, as well as some change in pitch caused by a second obstruction (B). After pile placement was changed to accommodate the newly discovered footings, no additional movement of significance occurred (C)
Conclusion: Active Tilt Monitoring Mitigated Project Risk
The contractor mitigated the likelihood of additional movement by relocating the new piles outside the existing footings. Not only were no additional movements of the wall noted after this change - the crack reverted to its original size approximately a week after drilling concluded.
Real-time monitoring with the Veva III gave the engineers on site the data they needed when they needed it to make crucial changes and avoid causing potentially costly damage to the existing structure and delays to the project.
Photo 2: The width of the crack at the manual measurement points before significant rotation occurred.
Photo 3: The width of the crack after rotation had occurred grew approximately 3/8" , a measurement consistent with the Veva III Tilt Monitor's reading of 0.04 °.