Performance of Footings in Rock Based on Serviceability-Paper

Paper
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The paper examines the axial performance of rock-socketed piles and their role in pile groups and piled-raft foundations. Its central argument is that footing design in rock is usually governed by <strong>serviceability—especially settlement and load–displacement behaviour—not ultimate bearing capacity alone</strong>.

Shaft and base resistance mobilise at very different rates. Shaft resistance generally develops at small displacements, while substantial base resistance may require settlement of 10% or more of the pile diameter. Consequently, simply adding factored ultimate shaft and base capacities can give misleading designs. For long sockets, service loads are commonly carried mainly by shaft resistance; base resistance is more important for short sockets and ultimate loading.

Shaft performance depends on more than intact rock strength. Important factors include rock-mass stiffness, pile diameter, rock type, interface roughness, socket cleanliness, concrete–rock contact and construction quality. Roughness and rock-mass modulus strongly affect resistance and stiffness. Smear, drilling-fluid residue, softened rock and poor concrete integrity can substantially reduce performance. Base debris may greatly increase settlement, even when ultimate base capacity is little changed.

The paper recommends displacement-based analysis using suitable elastic or nonlinear methods, supported by a sound geological model and adequate investigation. Triple-tube core drilling, representative strength testing and in-situ pressuremeter or other modulus testing can improve estimates of rock properties. More complete investigation and analysis can reduce excessive conservatism, construction costs and delays.

Reliable design also requires careful construction: appropriate socket support, roughening and cleaning, controlled drilling-fluid management, correctly placed reinforcement and sound tremie concreting where needed. Significant delays between final cleaning and concreting should be avoided. Engineers should observe installation, while pile load and integrity testing can verify performance; testing methods must be selected and interpreted appropriately.

For footing systems, interactions between piles, rafts and surrounding ground must be assessed, often using numerical models with representative rock-mass properties. The paper concludes that integrating investigation, serviceability-based design, construction oversight and verification can produce safer, more economical foundations with better-predicted performance.
Keywords
axial performance
rock-socketed piles
serviceability
settlement
load-displacement behaviour
shaft resistance
base resistance
rock-mass stiffness
displacement-based design
pile groups