Foundation of High-Rise Buildings by Minimal Threshold Settlement Micropiles-Paper

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The ALTO project in La Défense, Paris, required the foundations of a 150 m tower’s lightly loaded North Exostructure to settle by a minimum amount, limiting differential movement relative to the heavily loaded tower. The target settlements were 12, 13 and 16 mm for three foundation blocks.<br /><br />The initial proposal used micropiles with long free lengths to generate settlement. This raised concerns about residual shaft friction, reduced bond length, buckling, specialized high-strength steel and uncertainty in predicting micropile movement.<br /><br />The adopted solution separated settlement control from load capacity. Each block was founded on micropiles and a laminated elastomeric bearing pad placed between two reinforced-concrete blocks. The micropiles carried the vertical load, while the pad provided a predictable settlement; a mechanical stop limited its maximum compression. This enabled the use of conventional, fully bonded micropiles.<br /><br />Micropile settlements were estimated using the Frank–Zhao method with high- and low-range soil parameters, including values calibrated from nearby load tests. A 2 mm safety margin was applied. Factory compression tests established pad settlements, and micropile control tests were conducted at the long-term service load.<br /><br />Measured micropile settlements exceeded the required minimums and were generally close to the theoretical estimates, with differences of about 1 mm. Pad tests met or exceeded target settlements for blocks A and C. Block B’s pad settled 1.5 mm less than intended; the project team accepted the shortfall without redesign.<br /><br />The composite foundation successfully addressed differential-settlement requirements while reducing design uncertainty. The project demonstrates how combining micropiles with elastomeric bearings can provide controlled settlement where conventional foundation design would usually seek to minimize it.

Keywords

ALTO project

La Défense

Paris tower foundation

micropiles

differential settlement

exostructure

elastomeric bearing pad

Frank-Zhao method

micropile load tests

controlled settlement