Foundation Undermined by Excavation After Rain

Operations should stop at once. The tension crack and slope sloughing are evidence that the soil supporting or laterally restraining the foundation may have moved. A tower crane foundation must remain on competent bearing and be protected from undermining; continuing to lift adds dynamic load, wind moment, and slewing forces to a foundation whose support is now uncertain. The plumb bubble is only a local indication of mast verticality over a short reference length. It can remain 'inside the circle' while the pad has settled unevenly, tilted, or lost bearing at one edge, because the bubble does not measure foundation elevation, crack width, or soil displacement. It also cannot detect anchor-bolt movement or structural distortion. The superintendent cannot authorize continued operation. The decision to return to service must come from the competent person responsible for the crane, working with a registered professional engineer and the manufacturer's foundation requirements. Until that decision is documented, the crane should be secured, the base area barricaded, and no personnel allowed near the crack or under the jib. The laborer near the crack should be removed because the excavation slope itself is a separate collapse hazard.

A practical sequence is:

• Survey the foundation and mast: use a level or total station to check pad elevation at multiple corners and mast plumb over its full height. This detects settlement or tilt that the bubble cannot show.
• Inspect the pad and anchor bolts: look for new cracks, spalling, tilt, and anchor-bolt elongation or pull-out. Any movement here changes the load path from mast to soil.
• Verify bearing and drainage: expose or confirm the soil at the pad edge, confirm that native bearing soil is still present, and confirm that water is being directed away. The crack may be a drainage failure symptom.
• Evaluate the excavation: the engineer should check slope stability, shoring, and whether the excavation has removed lateral support or created a failure surface under the pad. A crane foundation near a slope needs a global stability check, not just a concrete check.
• Inspect the crane structure: check mast sections, bolts, and base connection for distortion caused by movement. A foundation problem can preload or distort the mast.
• Compare to the design: the engineer should compare measured conditions with the foundation drawing and manufacturer's requirements. If bearing is lost, the fix may be underpinning, shoring, drainage, or relocation, not merely monitoring.

Each check answers a different failure mode: bearing loss, tilt, structural damage, or water-driven erosion. Passing one does not clear the others.

The stop should be treated as a formal stop-work condition. Document the crack width and location, photographs with scale, slope conditions, rainfall, pump location, survey readings, and the exact time operations ceased. Record who made the stop decision and who was notified. The engineer's evaluation, any monitoring readings, and the manufacturer's correspondence should be attached to the crane inspection record. Communication should go to the superintendent, excavation contractor, crane operator, rigging crew, and concrete crew so no one plans lifts around an unverified crane. A barricade should keep personnel and equipment off the crack zone and out of the excavation influence area; the crane should be left with the trolley and load block secured per the manufacturer's out-of-service procedure and no suspended load. If monitoring is used, define trigger values and who has authority to stop work again. Do not reopen based on a verbal 'looks okay.' Reopening should require the competent person's documented inspection and the engineer's written direction, followed by a pre-shift briefing that explains the restriction and the exclusion zone.