Site preparation, foundations, rail/base conditions and ground bearing requirements
Site preparation
Before erecting a self-erecting tower crane, the site must be prepared: ground must be level, compacted, and capable of supporting the crane's maximum load. For static cranes, a concrete foundation designed by an engineer is required. For trailer-mounted cranes, outriggers must be fully extended on stable ground, often with timber or steel spreader pads to distribute load. Rail-mounted cranes require properly laid tracks with correct gauge and ballast.
Ground bearing
Ground bearing pressure is the load per unit area exerted by the crane's outriggers or base. It must not exceed the ground's bearing capacity. Factors include soil type, moisture content, and underground services. If unsure, a geotechnical assessment is needed. Never set up over voids, drains, or uncompacted fill.
Traps in questions
A common trap is assuming any flat ground is suitable; it must also be strong enough. Another trap is forgetting to check for underground services. Also, outriggers must be fully extended and pads used; partial extension reduces stability. For rail cranes, track condition and alignment are critical.
Worked example / rule of thumb
Rule: Use spreader pads to reduce ground pressure; the pad area should be large enough to keep pressure below the ground's capacity. Example: If outrigger load is 10 t and ground capacity is 100 kPa, required pad area = (10 t × 9.81 kN/t) / 100 kPa ≈ 0.98 m², so a 1 m × 1 m pad is minimum.