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This study proposes a simple mathematic model for approximating the level of phreatic surface inside the protected zone in mechanical stabilized earth wall with back drain installation though the position of phreatic surface at the drainage interface (ho) which reflects the maximum level of phreatic surface in the protected zone. The proposed model was established based on dataset taken from 180 simulation cases caried out in Plaxis environment. Regression results present a combination of significant effects and major role to maximum water level in the protected zone (ho) of a ratio of length from upstream water to the drainage face to the wall height (L/H), a soil permeabilities coefficient (k) and a transmissivity of the drainage material (Tnet). The proposed model can facilitate design of drainage material to achieve desired level of phreatic surface in the protected zone.

Recently, considerations in partly replacing traditional concrete fine aggregate contained high river sand is concerned by marine sand to become a concern in Vietnam and the whole world. In the study area of Quang Nam (Vietnam), marine sand is selected to add to concrete fine aggregate because it meets engineering requirements for concrete fine aggregate material about fineness modulus (Ms) and salt content. The main research object is variations of concrete compressive strength (CCS) when adding marine sand to fine aggregate mixes. A proposal of concrete ratio mixes for CCS lab determinations was conducted. Two main non-destructive methods for determining CCS comprise ultrasonic pulse velocity (UPV) and rebound hammer (RH). Results highlight the adding of studied marine sand to concrete fine aggregate mix leads compressive strength of tested concrete block gives unchanged significantly during 90 experimental days in comparison to typical mixing ratio. The effect of marine sand percentage for mixed ratios was mentioned to CCS variations. Series of verification and linear regression is conducted between CCS values in design experimental time (t) to indicate the reliability and strong correlation between them in service of practical engineering aspects.