A Side-by-Side Comparison of Shaft Excavation locally in Qatar

Abstract

The city of Doha has grown and expanded rapidly in the last 20 years, but this growth has strained the city's infrastructure, including its aging sewerage system and treatment facilities. The Public Works Authority (Ashghal) has therefore launched several strategic schemes which involve shallow and deep sewer tunnels and a new sewage treatment works (STW) to serve Doha's growing population in the years to come. One such schemes is the C853/1 Wakrah-Wukair Drainage Network Branches (WWDNB), which includes more than 15 km of bored pipes between 400mm and 2.4m inner pipeline diameter, with more than 60 shafts ranging in depths from 10m to 47m. The geology in the Wukair area is made up of several distinct strata, typically ranging in material strength between 10 and 48MPa, and it is the range of strengths that decides the method of shaft excavation. This paper will provide a production comparison between two types of excavation methods adopted for the same depth shafts and geological conditions. In the first method, the Contractor adopted the more traditional method using an excavator with appropriate attachments to break the ground, where this progressed in a cyclical manner until the shaft base was reached. The second method incorporated a piling rig to drill a specific pattern of holes within the shaft envelope, which enabled a faster excavation cycle and allowed for time and cost savings as well as improved safety. This paper will assess both methods, why both were utilised, and the lessons learnt.