MICROPILES FOR COMBINED DEEP SLIP AND STRUCTURAL LOADING-Paper

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The paper examines permanent A-frame hollow-bar micropile systems used to remediate deep slope failures at Gatton Clifton Road and Kin Kin Road in Queensland. Micropiles were adopted after severe wet-weather events in 2011–2013, partly because their smaller installation equipment can reduce road closures and disruption compared with large bored-pile systems. Both projects were approved as trials and instrumented for monitoring.<br /><br />The standard design approach, based on the 2005 FHWA micropile manual, determines the shear resistance needed for slope stability and configures raked upslope and downslope piles connected by a concrete capping beam. However, the method treats the beam mainly as a means of fixing pile heads together; it does not fully address external loads on the beam or their transfer into the piles.<br /><br />At Gatton Clifton Road, FHWA-based design was supplemented with PLAXIS 2D modelling to assess deflections, including scenarios involving loss of passive support from erosion or shallow slips. The modelling showed that assumptions about the capping-beam connection affect predicted deflections.<br /><br />At Kin Kin Road, the A-frame also supported an integrated 1.5 m-high retaining wall used to widen the road. Its earth-pressure loads created additional shear, bending and axial forces. The design therefore combined slope-stability analysis with checks for wall loads, overall deflection, and pile capacity under combined axial force and bending. Including the actual transverse spacing of the pile rows in the PLAXIS model substantially reduced predicted ultimate deflection, from 55 mm to 3.5 mm. The piles passed the combined-load checks.<br /><br />The authors conclude that A-frame micropiles can provide an effective solution for deep slips while also supporting structural loads, but such applications require analysis beyond the FHWA procedure. Geotechnical modelling is useful for assessing deformation, though it has limitations in determining structural forces. Combining static structural calculations with geotechnical model results is complex and may lead to conservative designs. Ongoing inclinometer monitoring is expected to inform future design refinements.

Keywords

A-frame micropiles

deep slope failure

Queensland

FHWA micropile design

capping beam

PLAXIS 2D modelling

slope stability

retaining wall loads

pile deflection

instrumented monitoring