Foundation in Simsima Lime Stone for an Underground Reservoir-Paper

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The paper describes the foundation design for a 120 m × 80 m underground drinking-water reservoir in Doha, Qatar, founded on Simsima limestone. The reservoir’s large size and changing water conditions created two critical loads: when full, its weight could be carried safely by the rock, whose allowable bearing pressure was 450 kPa; when empty during a high groundwater event, uplift could cause the structure to float.<br /><br />A further challenge was an estimated 26 mm horizontal movement at the foundation caused by concrete shrinkage, creep, and thermal contraction. Conventional tension piles or rock anchors could be damaged by bending if required to resist this movement. The selected solution was a purpose-designed tension pile that transfers uplift to the pile through high-tensile steel strands while allowing horizontal movement at the pile head. A sleeve creates an annular movement gap, and a compressible board helps isolate the pile from horizontal forces. The strands are flexible, so compression is not transferred to the piles: downward loads pass directly from the raft into the limestone.<br /><br />The bored piles are 600 mm in diameter and rely on shaft friction for tensile resistance. Depending on the design uplift reaction, piles used 11 or 14 strands and had a 6 m shaft, with a 5 m strand bond length. Approximately 1,000 piles were required. Load tests showed very small movements: a pile designed for 1,000 kN displaced 0.59 mm at working load and 1.41 mm at 2,500 kN, with 0.22 mm residual displacement.<br /><br />The design demonstrates how separating load paths—rock bearing for compression and strand-connected piles for tension—can accommodate both uplift and horizontal movement economically in a large underground structure.

Keywords

underground reservoir

Doha Qatar

Simsima limestone

foundation design

uplift resistance

tension piles

high groundwater

horizontal movement

rock bearing

shaft friction