As-Built Scanning of Excavation Shoring for Build Back Analysis & Coordination

October 26, 2017

Abstract

Emergency consultations with Designers at the time of Work happening on a construction site are costly and cause delays. This white paper describes a method of mitigating a subset of said emergency consultations; specifically to do with earthworks shoring coordination on future build back. Said method involves the use of 3D scan data of existing excavation and shoring Works, subsequent processing and of said scans, and finally analysis of the as-built conditions versus the future build back.

Introduction

Earthwork is not an exact science. Significant monies are spent year over year across the globe towards making earthworks more accurate and efficient. While this effort is not fruitless, the method described herein is not for increasing the initial accuracy of the work at the potential expense of time and efficiency. The method described herein is to instead proactively measure and account for the inherent inaccuracy of the completed temporary earthworks; thus, minimizing install time while maximizing efficiency and mitigating emergency design rework due to inaccuracies discovered during build back.

We begin with a completed temporary earthworks shoring system whose primary component is an approximately 20m wide, by 20m deep, by 150m long excavation. The shoring system is based on a secant pile[1] arrangement. Due to the scale of this excavation, there is significant wale and strut based bracing spanning along and between the shoring piles. It is important to note that a 20m deep pile may be upwards of 300mm off from center-line at the bottom.

Add to the above with a 2m thick foundation slab spanning the length and breadth of said 20m wide by 150m long excavation. This foundation slab is designed to be poured up to and against the temporary earthworks shoring piles; with all the foundation slabs reinforcement steel to be between 40mm and 60mm away from the edge of slab.

Thus, knowing exactly where the bottom of the secant pile faces are around the perimeter of the excavation is necessary to ensure proper fabrication of the foundation slab reinforcement steel. If this coordination is not done, significant time and money would have to be spent to rectify the now field discovered conflicts.

SOLUTION

Even before a point cloud laser scan is taken of the completed temporary earthworks, it is useful to generate a 3D model of the as-designed shoring system. This will serve the dual purpose of allowing an initial assessment of the as-designed shoring system vs the future build, as well as having the as-designed shoring system ready in 3D to compare against the point cloud.

Fig.1 As-designed shoring system, note the ideal placement of the piles

As well, already having a 3D model of the as-designed shoring system makes it significantly quicker to apply field driven design changes to the final as-designed model.

Fig.2 As-designed shoring system once fully completed, note the significant changes in the wale and strut bracing

Thus, once the point cloud laser scan is complete and brought in for comparison with the as-designed 3D model, points of build back concern are readily identifiable.

Fig.3 Scan data compared to the fully completed as-designed model

As well, should the need arise, it is straightforward to compare the point cloud laser scan to the original as-designed model.

Fig.4 Scan data compared to the original as-designed model

The next step is to adjust the as-designed shoring model to match the point cloud laser scan data and turn it into a highly accurate and incredibly useful 3D As-Built Shoring Model

Fig.5 Scan data used to adjust as-designed model into an As-Built Model

Once the 3D As-Built Shoring Model has been developed, it is an elementary task to compare this 3D As-Built Shoring Model against the 3D model of the build back.

Fig.6 Annotated plan view comparison of the 3D As-Built Shoring Model against the 3D build back model

Fig.7 Enlargement of Fig.6

Conclusion

Thus, the 3D point cloud laser scan has been used to update the completed temporary earthworks shoring system 3D As-Built Model. This in turn has been fully coordinated against the build back 3D model, maintaining minimum install time with maximum efficiency and mitigated emergency design rework thanks to proactive review and adjustment of the build back well in advance.