Underpinning

This includes mass concrete, pier and beam and pile and beam underpinning as well as mini-piling

Introduction

When planning for these operations the designer should take account of the following principal hazards which could be encountered.

Possible principal hazards

• structural instability with its attendant risks to workers and occupants of the building
• confined spaces and the risks of lack of ventilation and exposure to fumes, noise and vibration
• excavations and risks of collapse if insufficiently supported together with possibility of water inflow etc.
• hazards of buried and overhead services such as gas, electricity and water and the hazards of damage to them whilst drilling, or driving, or by vibration, settlement or impact
• hazards associated with restricted access for heavy plant and materials
• manual handling hazards associated with the handling of casings, reinforcement, trench sheets and concrete often in confined spaces
• health hazards of contact with contaminated arisings, groundwater, adhesives, cement etc. including the inhalation of fumes and dust

Considerations

The designer should consider access, working space, where services are located and the proximity of other workers and the general public. Consideration of the possibility of contamination, hazardous materials, the extent of manual handling and the stability of structures is also important.

Possible design options to avoid hazards

• is it possible to strengthen or repair the structure instead of underpinning?
• can the ground be stabilised?
• consider structural survey, site investigation or desk study to understand problem
• consider shallow foundations to reduce possibility of entrapment
• consider mini-piling to reduce excavation of contaminated material
• Mini-piles and needling may cause less disturbance
• consider that excavation may de-stabilise the structure
• avoid starter bars
• define sequence of support to avoid instability
• specify monitoring of structure or ground during the work after defining allowable movement