Costa Rica Circunvalacion Norte Project: Lessons Learned from Foundation Drilling-Paper

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The Circunvalación Norte project completed 416 drilled shafts for Costa Rica’s urban perimeter highway in 15 months, achieving an average drilling rate of 32 m per day. Shafts ranged from 4 to 40 m long and were built mainly in silty soils and volcanic lahars. A full-scale static load test supported design capacities of 4,000–6,000 kN; the test shaft’s extrapolated ultimate resistance was 12,056 kN.<br /><br />Construction experience highlighted several practical lessons. Working platforms require both sound initial design and regular maintenance: a muddy, saturated platform delayed concrete delivery, contributing to a cold joint that led to replacement shafts and tripled the cost of that foundation. Spare critical equipment should also be available; a broken Kelly bar and three-week replacement delay halted production and caused substantial lost revenue. In hard lava, predrilling with a down-the-hole hammer reduced rock-socket drilling time from about 10 hours to two.<br /><br />At locations affected by underground water flow, concrete segregation sometimes prevented the tremie pipe from reaching the excavation bottom. Introducing a cement-grout seal before concreting, and ensuring the first concrete charge was fully loaded, prevented further occurrences.<br /><br />All shafts received crosshole sonic logging (CSL) tests. About 3% were rejected for concrete anomalies, most commonly from inadequate cleaning at the shaft tip. Tomography helped assess anomalies and verify repairs. Grouting—most effectively through the CSL tubes—successfully remediated six rejected shafts; others required replacement shafts.<br /><br />Overall, the project demonstrated the value of site-specific testing, careful platform upkeep, clean excavation tips, contingency equipment, and construction procedures tailored to rock and groundwater conditions.

Keywords

Circunvalación Norte

drilled shafts

Costa Rica

static load test

crosshole sonic logging

silty soils

volcanic lahar

rock socket drilling

tremie concreting

grouting remediation