Performance of Contiguous Bored Pile Wall Retention System: A Case Study-Paper

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The paper presents the design, construction, and monitored performance of a temporary retaining system for an 11.5 m-deep basement excavation in a congested urban site surrounded by high-rise buildings and roads.<br /><br />The selected system comprised 750 mm-diameter contiguous bored piles spaced at 820 mm, supported by one level of inclined ground anchors with an approximate 50-tonne capacity. The site had silty clay and clayey sand over medium-dense to dense silty sand and weathered rock, with groundwater about 5 m below ground level. Removing 3 m of surface soil before the main excavation helped optimize the retention system.<br /><br />The wall was initially analyzed using WALLAP and subsequently checked in PLAXIS 2D using a Mohr–Coulomb soil model and a 20 kPa construction surcharge. Both programs produced broadly similar estimates for bending moments, shear forces, and wall movements. WALLAP predicted a maximum deflection of 32 mm, while PLAXIS predicted 25 mm; the design adopted the more conservative results. The system also achieved a factor of safety above 2 against overall instability in the WALLAP analysis.<br /><br />A monitoring program used 26 points at three levels, with total-station measurements taken daily during early excavation and later weekly. Monitoring continued for eight months, through construction of the basement slabs. The maximum recorded wall movement was approximately 21 mm—close to the PLAXIS prediction and below the WALLAP estimate.<br /><br />The comparison between predictions and field measurements indicates that the design assumptions and soil parameters reasonably represented actual site behavior. The case study highlights the value of using multiple analytical tools and systematic monitoring to validate retaining-wall performance during deep excavation.

Keywords

retaining system

deep basement excavation

contiguous bored piles

ground anchors

urban excavation

WALLAP analysis

PLAXIS 2D

wall deflection

soil behavior

structural monitoring