AN ALTERNATIVE APPROACH FOR ESTIMATING THE VERTICAL CAPACITY OF L-SHAPED SEGMENTAL UNDERPINNING SYSTEMS IN URBAN BASEMENT CONSTRUCTION-Paper

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The paper presents an alternative method for estimating the vertical capacity of L-shaped reinforced-concrete underpinning systems used to create basements beneath existing buildings. These underpins support building loads while retaining soil and groundwater; their bases experience eccentric vertical loading because the wall stem is offset from the base centre.<br /><br />Conventional design commonly uses Meyerhof’s effective-width method, derived from bearing-capacity theory. The authors note that its predictions are sensitive to uncertain soil parameters and can be highly conservative. They instead apply Adekunte’s work-hardening plasticity model, which represents the interaction between vertical load and overturning moment. Calibrated using field tests on footings with different aspect ratios and embedment depths, the model accounts for both factors, unlike several earlier plasticity models.<br /><br />The method is demonstrated through two basement projects in England. For the Kensington project, the authors estimate central vertical capacity using cone penetration test data, then use the plasticity model to assess capacity under eccentric loading. The calculated allowable overturning moment exceeds the service moment, indicating adequate capacity. Predictions are compared with traditional bearing-capacity methods and other plasticity models; the traditional approaches are most conservative, while the proposed model is least conservative.<br /><br />The paper also describes staged, hit-and-miss underpinning construction and temporary support measures. Reported ground movement during underpinning was negligible, and movement of neighbouring buildings during bulk excavation was limited to a maximum of 3 mm. The authors conclude that the proposed model offers a more consistent, potentially less conservative design approach, while emphasizing its consideration of footing geometry and embedment.

Keywords

underpinning

eccentric loading

vertical capacity

bearing capacity

work-hardening plasticity

Meyerhof method

Adekunte model

reinforced concrete

basement excavation

cone penetration test