Analysis of arching around shallow tunnels using the boundary element approach

This paper describes a study of the effect of soil arching around shallow tunnels using the Boundary Element Method. Due to the different stiffnesses of the soil and the underground structure, then, under surface loading, the differential displacements generate shearing (arching) forces which increase or decrease the load on the tunnel. Under elastic conditions, the soil parameters (elastic modulus (E) and Poisson’s ratio ( )) both affect these arching forces. The Boundary Element Method is an effective numerical technique to achieve accurate results as it deals directly with the tractions at the tunnel/soil interface. Moreover, it is more efficient than the Finite Element Method in this case because no elements are needed within the soil itself

Effects of root geometry and transpiration on pull-out resistance

Pull-out resistance has been identified as one of the key reinforcement mechanisms for a plant root system to increase slope stability, but the effects of root geometry coupled with plant transpiration on pull-out behaviour are not well understood. This letter presents and interprets a set of centrifuge pull-out tests on some newly developed plant root models that are capable of simulating the effects of transpiration. Three idealised and simplified root geometries were considered, namely tap-, heart- and plate-shaped. All tests were carried out under identical rainfall conditions at high-g, where the stress state of the soil and root dimensions can be modelled more closely to field conditions. The test results revealed that, after a rainfall event, pore water pressure retained by the tap- and heart-shaped roots (which have longer root depths) was much lower than that retained by the plate-shaped root. The presence of soil suction enhanced the pull-out resistance significantly due to increased tendency of constraint dilatancy upon soil–root interface shearing. Among the three root geometries, the tap- and heart-shaped roots were identified to be more favourable in resisting pull-out because they consisted of a vertical taproot component that effectively mobilised soil–root interface friction against pull-out. </jats:p

Performance of coir fiber addition for clay as a sub-grade for pavement design

Clay soil behavior often becomes problematic with building construction, it is about the shrink-swell behavior in clay when influenced by water content. Coconut husk fiber (coir fiber), in general, is an industrial waste which is still infrequent to be re-used. This study aims are to determine the effect of coir fiber addition for mechanical stabilization of clay soil in terms of CBR (California Bearing Ratio) value. Soil samples used for laboratory tests were collected from Ulee Glee area of Pidie Jaya Regency. According to AASHTO classification, the soil category is A-7-5 (25) while for USCS lassification, the soil is OH (Organic High). The percentage of added coir fiber was 0%, 0.2%, 0.4% and 0.6% of the dry weight of the soil with a coir fiber length of 2 and 3 cm. Two treatments of clay-coir fiber mixing method were applied namely direct mixing and mixing by layers. The results of natural soil compaction test obtained that OMC (Optimum Moisture Content) value and dry soil weight (γdmax) was 26.8% and 1.34 gr/cm3 respectively. The highest CBR results were obtained for clay soil with 0.4% coir fiber 3 cm (direct mix) with CBR value = 17.7%. Furthermore, the lowest CBR value is 10% for percentage of 0.2% with coir fiber length 2 cm (by layer). In general, high organic clay soil with coir fiber mixture addition is able to increase the CBR values if compared to CBR of natural soil which is 8.15%. Thus, the use of coir fiber in this study is able to improve soil bearing capacity which is useful for construction material in the site