تعيين پارامترهاي اندركنشي خاك-مهاري براي مدل سازي عددي براساس داده هاي ميداني

[Determination of Soil-Anchor Interaction Parameters for Numerical Modeling Based on Field Data]

مهاري ها در سازه هاي ژيوتكنيكي مانند گودبرداري ها نقش ويژه اي دارند. بطور كلي مقطع مهاري در خاك به پنج ناحيه عنصر مسلح كننده، دوغاب، مخلوط دوغاب و خاك اطراف، ناحيه برشي و محيط خاكي تقسيم بندي مي شود. هدف اصلي اين پژوهش تعيين پارامترهاي اندركنشي خاك-مهاري براي مدل سازي عددي گودبرداري به روش مهارسازي با نرم افزار FLAC2D است. اصولا تعيين قطر ناحيه تزريق چالش اساسي براي برآورد نيروي مهاري است. قطر ناحيه تزريق شده طبق روش پيشنهادي اين پژوهش براساس فشار تزريق، حجم دوغاب، پوكي و ضخامت ناحيه برشي تعيين مي شود. اين قطر حدود 40 درصد نسبت به قطر حاصل از حفاري افزايش نشان مي دهد. اين در حالي است كه در محيط سنگي قطر ناحيه تزريق برابر قطر حفاري است. ساير پارامترها با معادل سازي روابط محيط سنگي براي محيط خاكي تعيين شده اند. براي اطمينان از صحت روابط و روش ارايه شده در اين پژوهش اقدام به شبيه سازي آزمون كشش در نرم افزار FLAC شده و با استفاده از نتايج كشش مهار در يك پروژه گودبرداري راست آزمايي صورت گرفته است. نتايج اين پژوهش نشان مي دهد كه ظرفيت باربري نهايي مهاري محاسبه شده از مدل عددي اختلاف قابل قبولي با رابطه پيشنهادي پژوهشگران دارد. همچنين اختلاف تغييرمكان حاصل از شبيه سازي عددي و نتايج كشش مهار در اجرا نيز ناچيز است. بنابراين مي توان از اين شيوه در مدل سازي هاي عددي گودبرداري با سيستم مهاري در محيط هاي خاكي با دقت بالا بهره جست.

The anchored walls consist of nongravity cantilevered walls with one or more levels of ground anchors. Nongravity cantilevered walls employ either discrete (e.g., soldier beam) or continuous (e.g., sheet-pile) vertical elements that are either driven or drilled to depths below the finished excavation grade. For nongravity cantilevered walls, support is provided through the shear and bending stiffness of the vertical wall elements and passive resistance from the soil below the finished excavation level. Anchored wall support relies on these components as well as lateral resistance provided by the ground anchors to resist horizontal pressures (e.g., earth, water, seismic, etc) acting on the wall. A prestressed grouted ground anchor is a structural element installed in soil or rock that is utilized to transmit an applied tensile load into the ground. The basic components of a grouted ground anchor include the anchorage, free stressing (unbonded) length, and bond length. It is of note that the anchor bond length should be located behind the critical failure surface. The first use of ground anchors in the US was for temporary support of excavation systems. The use of permanent ground anchors for public sector projects in the US did not become prevalent until the late 1970s and today, represent a common technique for earth retention and slope stabilization. Anchors play a vital role in geotechnical structures such as excavations. The anchor section in soil is generally divided into five areas including reinforcement element, grout, grout and surrounding soil mixture, shear zone and soil media. Most of the common geotechnical software such as PLAXIS utilizes limited parameters in order to model the complex behavior of anchors; contrarily, modeling the soil-anchor interaction by means of FLAC yields conspicuous accuracy due to considering over ten various parameters. The main objective of the present research is to determine the soil-anchor interaction parameters for numerical modeling of anchored walls using FLAC2D software. Fundamentally, the main challenging issue in estimation of the anchor force is to determine the injection area diameter. According to the proposed method, the diameter of the injection area is determined based on the injection pressure, grout volume, porosity and shear zone thickness. It is demonstrated that the diameter of the injection area in soil medium is approximately 40% greater than the drilling diameter; nonetheless, that of the injection area in rock media is equal to the drilling diameter. The other parameters are determined by equalization of rock media formulas for soil media. In order to ensure the validity of the proposed methodology, the pull-out test was numerically simulated in FLAC2D, besides the numerical results have been then verified with anchor tension data in a field excavation project. The results indicate that the ultimate load of the anchor calculated from the numerical model is in fair agreement with equations postulated by many of former researchers. Correspondingly, there is a negligible difference between the displacement value obtained in the numerical simulation and the field pull-out test results. Henceforth, this method can be utilized in numerical modeling of anchored walls in soil media with high precision.