توسعه يك مدل براي نمايش رفتار مكانيكي خاك‌هاي ماسه‌اي سيماني‌شده

Developement of a Constitutive Model for the Mechanical Behavior of Cemented Sands

چكيده- در اين پژوهش، يك مدل براي نمايش رفتار مكانيكي خاك ماسه‌اي سيماني‌شده ارايه شده است. اين مدل بر اساس جدا كردن رفتار خاك سيماني‌شده به دو بخش بافت غيرسيماني و باندهاي سيماني و تركيب رفتار مكانيكي دو بخش با استفاده از معادلات سازگاري تغيير شكل‌ها و انرژي، طراحي شده است. براي بخش غيرسيماني از يك مدل برپايه‌ي پلاستيسيته عمومي استفاده شده كه با يك مدل رفتاري براي باندهاي سيماني تركيب شده است. تركيب اين دو بخش، نتايجي دلخواه براي مدل سازي رفتار خاك سيماني‌شده در شرايط زهكشي‌شده و زهكشي‌نشده فراهم كرده است. براي كاليبراسيون اين مدل، نتايج مدل سازي به صورت منحني‌هاي تنش محوري اضافي-كرنش برشي، كرنش حجمي-كرنش برشي و تنش محوري اضافي-تنش موثر ميانگين با آزمايش‌هاي سه‌محوري انجام شده بر خاك ماسه اي سيماني‌شده با عامل سيمانتاسيون گچي، مقايسه شده و كاركرد مناسب آن در بازه‌ي گسترده‌اي از فشارهاي همه جانبه و درصد سيمان تاييد شده است .

Cemented sandy soils can be found in different parts of nature. Slopes and natural cuts are observed to be stable for long periods of time. Indeed the stability is related to the cementation and bonding between soil grains which induces an equivalent cohesion for coarse grained soil. Several experimental and theoretical studies have been performed to investigate the mechanical behavior of this category of geomaterials. In present research, a constitutive model is proposed for simulating the mechanical behavior of cemented sandy soils. The model is based on separating the mechanical behavior of cemented soil to two different parts; firstly the uncemented soil matrix and secondly the cemented bonds. The generalized plasticity model developed by Manzanal et al. (2011) is used for predicting the mechanical behavior of the uncemented soil matrix. The model is based on critical state concepts and is able to simulate the behavior of sandy soil in a wide range of confining pressures. It contains sixteen parameters which can be determined using ordinary geotechnical tests for the base soil. Also the elastic-plastic damage bond model proposed by Haeri and Hamidi (2009) is used for cemented bonds. The model has two additional parameters and is able to predict the brittle behavior of cemented bonds besides their degradation with increase in shear strain. Peak shear resistance of cemented bonds increases with confining pressure, however, the axial strain associated to the peak shear strength decreases with enhancement of confining stress. Also cement content is considered in this bond model which is its advantage in comparison with similar ones. Both components have been combined together based on deformation consistency and energy equilibrium equations. Deviatoric stress-shear strain curves besides volumetric strain-shear strain ones have been compared with the results of consolidated drained triaxial tests on a gypsum cemented sand to verify the proposed model. Also deviatoric stress-axial strain besides deviatoric stress-mean effective stress curves of model are compared with results of tests in consolidated undrained state. Results of verification indicate good performance of developed model in a wide range of cement contents and confining pressures. The proposed model has two distinct advantages. At first it considers the effect of cement content as a model parameter and shows variation of the results with this parameter. Secondly, it simulates the soil behavior in a wide range of confining pressures which enables using it in the boundary value problem in geotechnical engineering. However, it should be noted that the model predicts the mechanical behavior of cemented sand in cement contents less than floating limit. Increasing the cement content from the floating threshold changes its role from effective bonding between soil grains to a filler of voids. In this condition, the model can not predict the behavior of cemented soil due to the limitations in the elastic-plastic damage bond model applied in present constitutive model.