Before beginning concrete placement for drilled shafts, what is vital to confirm?

Confirming subgrade stability is essential before beginning concrete placement for drilled shafts because the integrity and performance of the drilled shaft heavily depend on the stability of the ground that will support it. Proper subgrade conditions ensure that the concrete will set and harden effectively, maintaining the designed load-bearing capacity of the shaft. If the subgrade is unstable, it can lead to sinks, shifts, or failures in the foundation, which can jeopardize the entire structure.

Other options, while important in their own right, do not carry the same immediate structural implications as subgrade stability. For example, while the absence of underground utilities is crucial to avoid damaging existing infrastructure, it does not directly affect the structural integrity of the drilled shafts during their initial placement. The proper mixture of concrete is critical for achieving the desired strength and durability, but this can be managed and adjusted after confirming the site conditions. Availability of labor is also necessary for the operation to proceed, but it is more of a logistical concern than a direct factor in the engineering safety of the drilled shaft's installation. Therefore, the priority lies with ensuring that subgrade stability is confirmed before any concrete placement begins.