TECHNICAL NOTES On the Effects of Subgrade Erosion on the Contact Pressure Distribution under Rigid Surface Structures M. Menaa 1 ; M. A. Meguid 2 ; and G. Assaf 3 Abstract: The performance of rigid surface structures such as concrete pavements and slabs-on-grade supported by a deteriorated subgrade and experiencing local contact loss is investigated experimentally and numerically in this study. A laboratory setup has been designed to facilitate the simulation of subsurface erosion and measure the changes in contact pressure at selected locations under a slab-on-grade supported on granular material. The presence of erosion voids under a slab-on-grade can lead to rapid increase in the contact pressure in the immediate vicinity of the void in addition to an increase in tensile stresses at the outermost fibers of the slab. This preliminary study suggests that efforts to detect and arrest the growth of erosion voids under slabs-on-grade should be made before the voids reach the size where significant loss of support develops and the tensile strength of the slab material is exceeded. DOI: 10.1061/ASCEGT.1943-5606.0000097 CE Database subject headings: Slabs; Soil erosion; Deterioration; Voids; Subgrades; Underground structures. Introduction Several geotechnical engineering structures e.g., pavements, slabs-on-grade, and footings transfer pressure to the ground through surface contact with the supporting soil. The design of these structures usually assumes that full contact is established throughout their service life. Slabs-on-grade are usually made out of reinforced or unreinforced concrete placed on a sub-base layer over a prepared subgrade. Failure of these rigid slabs is usually attributed to two main factors. The first involves aspects related to material failure, which includes fatigue of the concrete and other construction defects. The second category is attributed to the loss in reaction support. Once a void space develops under the slab, it starts to increase due to plastic deformation of the sub-base course or subgrade, temperature curling of the slab, and subgrade erosion Huang 1993. Erosion of the subgrade is known to develop for many reasons, some of which are the dissolution of soluble rocks such as karst limestone Newton 1984, dynamic loading caused by construc- tion related activities Tharp 1999, and the presence of nearby leaking pipes Hauser and Howell 2001. For soil layers underlain by heavily jointed bedrock, surface water entering in the soil is usually allowed to penetrate into the joints of the underlying bed- rock causing erosion of the overburden soil. Subgrade erosion phenomenon can be very problematic when the unreinforced con- crete slab experiences deformations that induce excessive tensile stresses in the outer fibers of the concrete or when the supporting soil around the void experiences excessive shear stresses. A sche- matic of eroded subgrade under a rigid slab-on-grade is shown in Fig. 1. The bearing capacity of strip footings underlain by subsurface voids has been investigated by several researchers e.g., Baus and Wang 1983. Results showed that, for a given void size, the bearing capacity decreased when voids were introduced immedi- ately under the footing. This was explained by the reduction in the shear strength of the supporting system as the void became closer to the footing. Performance of concrete slabs-on-grade has also been extensively studied in the past two decades with emphasis on modes of deformation Rajani 2002 and failure mechanisms e.g., Mailvaganam et al. 2001. However, little attention has been paid to the effects of erosion voids on the distribution of contact pressure at the soil-structure interface. The objective of this study is to investigate the performance of a rigid slab-on- grade subjected to subsurface soil erosion. An experimental study is conducted to examine the changes in contact pressure during and after the erosion process. Elastoplastic finite-element analy- ses, validated using the experimental results, are then performed to investigate the role of void size and location on the stresses developing in the slab-on-grade and the supporting soil. Experimental Study One of the challenges of the experimental program was to de- velop a suitable technique to simulate subsurface soil erosion un- derneath an existing slab-on-grade and measure the corresponding pressure at the contact surface between that slab and the soil. The physical model used in this study consisted of a rigid steel tank to contain the granular material, a mechanism to artificially simulate 1 Graduate Student, Département de Génie de la Construction, École de Technologie Supérieure, 1100 Notre Dame West, Montreal, Canada PQ H3C 1K3. 2 Assistant Professor, Civil Engineering and Applied Mechanics, McGill Univ., 817 Sherbrooke St. West, Montreal, Canada PQ H3A 2K6 corresponding author. E-mail: mohamed.meguid@mcgill.ca 3 Professor, Département de Génie de la Construction, École de Technologie Supérieure, 1100 Notre Dame West, Montreal, Canada PQ H3C 1K3. E-mail: gabriel.assaf@etsmtl.ca Note. This manuscript was submitted on July 12, 2008; approved on February 2, 2009; published online on February 23, 2009. Discussion period open until March 1, 2010; separate discussions must be submitted for individual papers. This technical note is part of the Journal of Geo- technical and Geoenvironmental Engineering, Vol. 135, No. 10, October 1, 2009. ©ASCE, ISSN 1090-0241/2009/10-1538–1542/$25.00. 1538 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / OCTOBER 2009 Downloaded 17 Feb 2010 to 132.206.38.53. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyright