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Restricted Access Site Investigation City of London

Background

The site comprised two adjoining buildings with up to 2 basements levels in the centre of the city of London. It was proposed to replace the buildings with new office developments. A structural and ground investigation was required within the basement areas.

The structural investigation comprised 6 No pits and 30 No horizontal and vertical cores to determine parameters of the existing basements, including wall and floor thickness, retained soil type, and distance to adjacent structures.

The ground investigation required 3 No boreholes to determine the soil profile to a maximum depth of 40m below the basement floor.

Challenges

• Areas of limited headroom and restricted working space
• Stair access only to the basement levels
• Existing tenants resulting in noise constraints
• Poor ventilation
• Maintaining free and clear access to certain areas of the site (fire escapes etc)
  

Solution

This involved the of use portable electric coring and breaking equipment for all structural investigation works, a breakdown compressed air powered shell and auger rig for the ground investigation, and night time working for the operations that created significant noise.

The use of portable 110V electric equipment meant no fumes were created and noise production was reduced. All the structural investigation works could be undertaken during the day.

By using a breakdown rig, the disassembled rig could be carried manually down the stairs, with the air driven motor set on a sled and winched down the stairs. An air compressor was located within a ventilated car parking area on the ground floor, and hoses were run down the fire escape to the lower levels. The borehole rig was erected on the upper basement level, and two aligned vertical core holes were opened through both floors. The borehole could then be cased through to the lower level. Props were placed directly under the struts of the rig in the lower basement floor levels to ensure the stability of the rig. The boreholes all achieved their full depths. All the borehole operations were undertaken at night.