Pile load test acceptance criterion: During a routine pile load test, the platform is loaded up to 1.5 times the design load. After unloading, the net settlement (gross settlement minus rebound) per tonne of test load shall not exceed which value for the pile to be considered safe?

Introduction / Context:
Pile load tests verify capacity and serviceability. A simple acceptance criterion uses the net settlement per unit of applied test load to judge whether the pile exhibits acceptable stiffness and creep characteristics. Limiting net settlement helps control serviceability issues in the supported structure.

Given Data / Assumptions:

• Pile is loaded to 1.5 times the design (working) load.
• Settlement during loading and rebound on unloading are measured.
• Net settlement per tonne of test load is assessed for acceptability.

Concept / Approach:
By subtracting elastic rebound from total settlement, the residual (plastic and time-dependent) component is isolated. A lower allowable net settlement per tonne indicates a stiffer pile–soil system. Traditional acceptance guidelines adopt a relatively stringent limit around 10 mm per tonne of test load for routine acceptance, especially in granular soils or where settlement control is critical.

Step-by-Step Solution:
Conduct incremental loading up to 1.5 × design load.Record total settlement at each stage; then unload to observe rebound.Compute net settlement = total settlement − rebound.Normalize by test load (tonnes) to obtain mm/tonne and check against the limit.

Verification / Alternative check:
Comparing with established practice and many project specifications, the 10 mm per tonne criterion is commonly adopted for routine compression tests; stricter limits may be specified for sensitive structures.

Why Other Options Are Wrong:

• 15–30 mm: These larger limits risk excessive long-term settlement and are generally less conservative for routine acceptance.

Common Pitfalls:
Confusing total settlement with net; ignoring time-dependent settlement during maintained load; not accounting for test load increments when normalizing.

Final Answer:
10 mm