Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena Modeling unconfned compressive strength of fne-grained soils: Application of pocket penetrometer for predicting soil strength Fatemeh Mousavi a, ⁎ , Ehsan Abdi a , Shaaban Ghalandarayeshi b , Deborah S. Page-Dumroese c a Department of Forestry and Forest Economics, Faculty of Natural Resources, University of Tehran, Karaj, Iran b Department of Statistics, Gonbad Kavous University, Golestan Province, Iran c U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Moscow, ID, USA ARTICLE INFO Keywords: Soil type Regression Moisture content UCS PPT ABSTRACT In recent years, the pocket penetrometer (PPT) test has been used to measure soil compaction and has gained attention due to its simplicity, high speed, and low cost. This study aimed to predict soil strength using a regression relationship between unconfned compressive strength (UCS) and PPT. We collected 45 large soil samples from a forest district in a part of the Hyrcanian Forest. In the laboratory, soil samples were classifed according to unifed soil classifcation system. Samples were classifed as either silt or clay with a low or high liquid limit. These four soil groups were, brought to four diferent moisture contents (14%, 25%, 31%, and 36%; n = 180 measurements). Samples were then brought to a constant dry unit weight (1.387 kg/m 3 ). Once the samples were prepared, we measured soil compression strength using a PPT and then determined UCS. We modeled our lab results to determine if PPT could be used as a surrogate for UCS. As soil moisture content increased UCS decreased non-linearly. The behavior of silt and clay with low liquid limit plasticity were similar; likewise, the silt and clay with high liquid limit plasticity produced similar results. According to the values of Eta-squared, soil moisture is more important factor than soil type class in explaining the variance of UCS. In addition, PPT data can accurately predict soil UCS (R 2 = 0.98). Therefore, PPT data can be used to predict UCS of fne-grained soils with good accuracy and very low cost. 1. Introduction Understanding soil mechanical properties is critical for geotechnical analyses and designs to guarantee project success. One of the most important soil properties for construction activities is soil resistance to penetration (a measure of soil strength). There are two principal methods for measuring soil penetrability or penetration resistance: (1) dynamic methods, in which a probe is driven into the soil by a slide hammer or falling weight and (2) static methods, in which a probe with a cone or blunt tip pushed into the soil at a constant velocity (Bradford, 1976). Another important soil physical characteristic is shear strength, which is defned as the magnitude of shear stress that a soil can sustain. Soil shear resistance is a measure of friction and cohesion (i.e. inter- locking particles or cementation). Frictional shear strength is a function of gravitational forces and is common in coarse-grained soils; cohesion shear strength is a function of surfcial forces common in fne-grained soils (Robertson and Chock, 2017). Soil cohesion is not a constant parameter and is a function of the load transferred by the structure (e.g. building, bridge) to the soil and soil moisture content (Tavenas, 1976). Soil cohesion also depends on inter-granular loads and testing method and, when no lateral load is applied to the sample, adhesion is con- sidered to be soil shear strength (Chang and Cho, 2019). Unconfned compression tests are used to determine adhesive soil strength (Güneyli and Rüşen, 2016). Generally, testing for unconfned compression strength (UCS) measures the adhesive soil strength using a strain-controlled method (Cao, 2018). This test determines the com- pression strength of a soil cylinder with no lateral load. It is similar to an unconsolidated undrained triaxial test; both are used to determine cohesion in fne-grained soils. In saturated clayey soil samples, soil strength decreases with increased soil moisture content but, in un- saturated soil, when dry weight is fxed, soil strength decreases slightly with increasing saturation. Unconfned compression test has an ad- vantage over a direct shear test, in which stresses and strains are uni- formly created in the soil sample and fractures occur in the weakest part of the soil. This soil characteristic is important for calculating safe bearing capacity and soil strength (Patel and Patel, 2012). Assessing road strength is another phase of construction or main- tenance operations (Patel and Patel, 2012) and road trafc https://doi.org/10.1016/j.catena.2020.104890 Received 2 March 2020; Received in revised form 28 August 2020; Accepted 29 August 2020 ⁎ Corresponding author. E-mail address: fmusavi@ut.ac.ir (F. Mousavi). Catena 196 (2021) 104890 0341-8162/ © 2020 Elsevier B.V. All rights reserved. T