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Many studies have been focused on soil-related problems so far. These may include consolidation, soil settlement, or swelling for clayey soils. However, before some incidents like liquefaction phenomena had been recognized, sands were not defined as a sort of problematic soil. After some liquefaction and flow failure took place during Niigata and Alaska earthquakes in 1964, comprehensive studies were carried out to explain sands behavior under various situations. One of the most crucial factors that can affect soil behavior is fines content and their types. Many investigations have been conducted on behavioral characteristics of clean and silty sands since liquefaction first observed in these kinds of soils. Most of studies suggest that sand liquefaction potential boosts with an increase in silt content until a threshold value and after that it starts to decline as silt content increases. Most of the studies conducted so far have mainly focused on clean sand or its mixtures containing non-plastic fines with not much attention paid to the soil mechanics of the critical state of clayey sands. The reason for this neglect might have been the misconception that plastic properties in clay prohibit flow behaviors and liquefaction. However, the studies of Northridge 1994, Kokaeli 1999, Chi Chi 1999, and Niigata 2004 earthquakes have indicated that notable settlements occur in soils containing considerable amounts of clay, resulting in great destruction. The researchers have emphasized that more detailed investigations are needed to determine the critical state behavior of clayey sands The steady state line or critical state line is one of the main factors used in the critical state topics. It is used for investigating the variation of liquefaction potential, calculating state parameters and also determining the parameters affecting on the collapse or state boundary surface shapes. Sand skeleton void ratio, equivalent void ratio and the interaction of sand and fines are being the main subjects of sand critical state behavior researches. In this paper the possibility of equivalent steady state line is investigated to order to reach a unique frame work for interpretation the critical state behavior of clayey sands. The inter granular interaction of coarse and fine grains and the fine communication in inter granular contacts are investigated to reach the equivalent void ratio. An appropriate and useful relation is presented to calculate the fine contribution percent in inter granular contacts and so the equivalent void ratio trough performing numerous cyclic triaxial on different combination of sand and clay, evaluating the variation of liquefaction resistance and finally mathematical analysis the results. Increasing the fine content has increased the instability of combinations. Also it has been shown that the different steady state lines can be converted to a one equivalent steady state line using the proposed relation. The latter means that it would be possible to predict the steady state line of every optional mixture of sand and fine trough performing some tests just on clean sand. In order to certificate the proposed relation, it has been verified based on some previous researches.

Keywords: Steady State Line, Cyclic Triaxial, Equevalent void ratio

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