Mechanical Properties of Gravel Deposits Evaluated by Nonconventional Methods Kuang-Tsung Chang 1 ; Yu-Min Kang 2 ; Louis Ge, M.ASCE 3 ; and Min-Chieh Cheng 4 Abstract: Gravel deposits are materials that contain large particles. The mechanical behavior of such materials is conventionally examined using large-scale tests such as large-scale triaxial and in situ shear tests. However, the large-scale tests may not fully represent the gravel deposits in the field or cannot obtain the deformation parameters. This study estimated the elastic and shear-strength properties of gravel deposits using nonconventional methods. Field investigations at various locations were performed to obtain the material factors, including the gravel content, the diameter in the grain size distribution curve that corresponded to 50% finer (D 50 ), the unconfined compressive strength of the gravel, and the penetration strength of the matrix. Seismic-wave testing was conducted to obtain the elastic parameters at each location. Around each location, the critical slope profiles with greatest gradients or heights were back-calculated to obtain c-phi curves, of which the envelope represents the shear strength of the gravel deposit. The mechanical properties of the gravel deposits were correlated with the four material factors and P-wave velocity using regression analysis. The Young’ s modulus was correlated with the P-wave velocity, the unconfined compressive strength of the gravel, and the penetration strength of the matrix. The cohesion of a gravel deposit was correlated with the penetration strength of the matrix. The Poisson’ s ratio and the friction angle were correlated with the four material factors. The results were successfully tested using data on two gravel deposits at other locations. For the gravel deposits in central Taiwan, the unconfined compressive strength of the gravel and the penetration strength of the matrix significantly affect the elastic properties, and the penetration strength of the matrix controls the shear-strength properties. Although the proposed regression models serve as a preliminary evaluation of the elastic and shear-strength properties of the gravel deposits in central Taiwan, these nonconventional concepts are applicable to gravel deposits elsewhere. DOI: 10.1061/(ASCE)MT.1943-5533.0001287. © 2015 American Society of Civil Engineers. Author keywords: Gravel deposit; Elastic modulus; Shear strength; Seismic wave. Introduction Gravel deposits in central Taiwan have high gravel contents, and the matrix has low cementation. The term gravel used in this study includes gravel, cobble, and boulders, representing particles coarser than 4.75 mm, and the term matrix represents those particles finer than 4.75 mm. Gravel deposits are great sources for construction aggregate. The excavation and construction in gravel deposits need to evaluate their mechanical properties. Standard penetration test in the gravel deposits of Taiwan commonly obtains N values larger than 100. Engineers realize that the very high N values, which are caused by unpenetrable hard gravels and cobbles, do not neces- sarily reflect the shear strength of gravel deposits. Without in situ tests, the gravel deposits are typically assumed as nil cohesion and having friction angles between 32° and 35° in the engineering prac- tice of Taiwan. In most cases, these assumption underestimates the shear strength of a gravel deposit. The mechanical properties of gravel deposits are necessary for quarry design, which ensures stability of excavated slopes. Overconservative estimations of the shear strength of gravel deposits result in design of gentler exca- vated slopes and lead to lower profit in quarries. Gravel deposits are a composite material, consisting of hard gravels and cobbles with soils filled in between. Vallejo (2001) studied the mixtures of glass beads of two different sizes. He noted that if the percentage by weight of the large beads was greater than 70%, the shear strength of the mixtures was controlled by the large beads. If the percentage by weight of the large beads was less than 40%, the shear strength of the mixtures was controlled by the small beads. Vallejo and Lobo-Guerrero (2005) noted that for meaningful test results, the ratio of specimen size to the maximum particle size must be greater than 6. For large particles, the main methods to examine the mechanical properties of gravel deposits are large- scale triaxial tests and in situ direct shear tests. The gravels or cobbles in gravel deposits hamper conventional mechanical tests. Chu et al. (2010) conducted large-scale triaxial tests using speci- mens with diameters of 30.5 cm and heights of 61 cm. Despite the large specimens to accommodate large gravels, the largest particles (5.1 cm) remained much smaller than the largest particles observed in the field. Moreover, the specimens were remolded and might not reflect real gravel deposits in the field. In situ large-scale tests, of which specimens can accommodate large particles in the field, have been used to evaluate strength parameters. The specimens may be remolded (Xu et al. 2011) or undisturbed (Chang et al. 1996; Xu et al. 2007; Coli et al. 2011). These tests show that the composite materials exhibit low cohesion and high friction angles, which may result partly from the low stress levels in the tests. Furthermore, the in situ tests are expensive and time-consuming, and cannot evaluate the deformation behavior 1 Associate Professor, Dept. of Soil and Water Conservation, National Chung Hsing Univ., Taichung 402, Taiwan (corresponding author). E-mail: changkt@nchu.edu.tw 2 Professor, Dept. of Civil Engineering, Feng Chia Univ., Taichung 407, Taiwan. E-mail: yumkang@mail.fcu.edu.tw 3 Associate Professor, Dept. of Civil Engineering, National Taiwan Univ., Taipei 106, Taiwan. E-mail: louisge@ntu.edu.tw 4 Research Assistant, Dept. of Soil and Water Conservation, National Chung Hsing Univ., Taichung 402, Taiwan. E-mail: book31242@gmail .com Note. This manuscript was submitted on June 17, 2014; approved on January 26, 2015; published online on March 26, 2015. Discussion period open until August 26, 2015; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561/04015032(11)/$25.00. © ASCE 04015032-1 J. Mater. Civ. Eng. J. Mater. Civ. Eng. Downloaded from ascelibrary.org by National Taiwan University on 03/28/15. Copyright ASCE. For personal use only; all rights reserved.