The Impact of Rigid Inclusions on the Design of Footings and Slabs-Paper

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The paper examines how rigid inclusions (RIs) affect the structural design of slabs-on-grade and footings. RIs improve soil bearing capacity and reduce settlement, but may create stress concentrations beneath foundations. Their effects depend on soil conditions, RI spacing and layout, and the thickness and stiffness of the load transfer platform (LTP), which distributes loads between RIs and surrounding soil.<br /><br />Using PLAXIS 2D and 3D models, the authors evaluated slab stresses and footing bending moments and shear forces under different soil profiles, LTP configurations, and RI layouts. Slabs are more sensitive to RI effects because they are relatively thin and often designed as unreinforced concrete. Thin LTPs can concentrate loads at RI locations, increasing slab tension and the risk of cracking. The authors recommend checking slab stresses when the LTP is under 2 ft (610 mm) over competent near-surface soils, or under 3 ft (910 mm) over soft or loose soils. Thicker LTPs generally distribute loads more uniformly.<br /><br />Footings were less sensitive. In most modeled cases, RI-supported footings experienced bending moments and shear forces equal to or below those for equivalent uniform soil support. One exception was a footing supported by three RIs with an LTP, where shear was 10–20% higher; further checking is advised if shear capacity is limited.<br /><br />To reduce footing demands, the authors recommend placing RIs within one-quarter of the footing width from its center, including a central RI where possible, and using radial rather than grid layouts. Overall, RI effects should be considered jointly by geotechnical and structural designers, particularly where support is nonuniform or the LTP is thin.

Keywords

rigid inclusions

slabs-on-grade

footings

load transfer platform

soil bearing capacity

settlement reduction

stress concentrations

PLAXIS modeling

bending moments

shear forces