اثر افزودن سيمان بر ناهمساني مقاومتي ماسه در بارگذاري‌هاي فشاري و كششي (كاهش فشار محوري)

EFFECT OF PORTLAND CEMENT ADDITION ON ANISOTROPIC STRENGTH OF SAND UNDER COMPRESSION AND EXTENSION TRIAXIAL TESTS

جهت تنش‌هاي اصلي در زمين‌هاي مسطح در اثر رسيدن موج زلزله به سطح زمين دوران مي‌كند و بارگذاري روي المان خاك مي‌تواند از حالت فشاري به كششي تغيير كند. در چنين شرايطي با توجه به رفتار ناهمسان اغلب خاك‌هاي ماسه‌‌يي، مشخص نيست آيا سيمان افزوده سبب درصد يكسان افزايش مقاومت و سختي در هر دو امتداد افقي و قايم مي‌شود يا خير؟ در اين پژوهش، اين موضوع با انجام تعدادي آزمايش سه‌محوري تحت مسير تنش‌هاي متفاوت استاتيكي و با دوران 90 درجه‌يي امتداد تنش‌هاي اصلي بر روي خاك ماسه‌‌يي داراي سيمان بررسي و نتايج نشان داده است كه در اغلب موارد، درصد افزايش پارامترهاي مقاومتي ناشي از افزودن سيمان در بارگذاري كششي بيشتر از فشاري است، اما در هر دو مسير بارگذاري حد بهينه‌‌يي براي افزايش مقاومت وجود دارد.

Poorly graded clean sand sediments formed on coastal zones are usually in a loose state, especially at shallow depths and when saturated, show low strength values even under static loading. This is because under cyclic loading, like seismic excitations, excess pore water pressure undergoes a sudden rise and, hence, decreases the strength to reach a condition near initial liquefaction or excessive settlement. One of the most important reasons behind intensifying the cyclic loading effect is indeed stress reversal, or, when after loading in a compressive manner, a reverse loading equal to twice the compressive amplitude will be applied in such a way that the soil element experiences an extension. In the latter case, the major principal stress direction will rotate from vertical to horizontal direction or the ? change of 0 to 90 degrees will occur. Furthermore, under many monotonic loading/unloading conditions, the major principal stress may also change its direction, with respect to the vertical direction and the stress path on the soil element after from, for example, compression to extension. Therefore, it can be simply concluded that knowing the values of strength in monotonic compression and extension loading and a study of soil behavior in these cases, would be useful in design applications. Portland cement can be mixed with sand to improve its mechanical characteristics. Many studies are reported in literature on this topic, but the effect of principal stress rotation has not been investigated yet. Considering the inherent anisotropy of most sands, it is not clear whether the added cement will contribute to an equal increase in strength and stiffness in vertical and horizontal directions or not. In this study, the effects of adding Portland cement to clean sand on strength and stiffness characteristics are investigated through experimental investigation using undrained triaxial compression and extension tests. Different mixtures of Portland cement (1.5, 3 and 5 %) have been added to Firuzkooh sand and the specimens have been cast using the wet tamping method. Test results revealed that while adding Portland cement will increase strength and elastic modulus (stiffness) in both stress paths, in most cases the percent of strength and stiffness increases, due to cement addition in extension loading is higher than compression. In other words, Portland cement addition tends to decrease strength and stiffness anisotropy. This is a noticeable point for attention in practical applications when compared to other soil improvement methods.