ارزيابي ظرفيت باربري و نشست پي‌هاي سطحي واقع بر ماسه با قابليت روانگرايي با استفاده از مدل ‌سازي فيزيكي

Evaluating Bearing capacity and settlement of shallow footings rested on liquefiable sand using physical modelling

روانگرايي يكي از خسارت بارترين عوامل خرابي ناشي از زلزله است. بالا رفتن فشار آب و كاهش تنش موثر ناشي از تحريك موجب كاهش سختي و ظرفيت باربري پي‌ها مي‌‌شود. حتي اگر خرابي‌هاي ناشي از كمبود ظرفيت باربري اتفاق نيفتد، به‌ هر حال امكان ايجاد نشست وجود دارد كه مي‌تواند سازه را از حالت سرويس‌دهي خارج نمايد. در كاربردهاي مهندسي، نشست‌هاي ناشي از روانگرايي با استفاده از كرنش‌هاي حجمي ناشي از بازتحكيم ماسه روانگرا تخمين زده مي‌شود، حال آنكه مطالعات اخير نشان‌دهنده اهميت سهم كرنش‌هاي برشي ناشي از وجود سازه در كنار كرنش‌هاي حجمي است. مطالعه حاضر اثر توليد فشار آب بر ظرفيت باربري و نشست‌هاي برشي پي‌هاي سطحي را با استفاده از مدل‌سازي فيزيكي مورد بررسي قرار داده است. نتايج حاكي از كاهش ظرفيت باربري ناشي از توليد فشار آب بوده اما حتي در حالت روانگرايي كامل ظرفيت باربري قابل توجهي وجود دارد كه مي‌تواند در مسائل مهندسي مورد توجه قرار گيرد. فشار اضافي آب زير پي همواره مقداري كمتر از فشار اطراف پي بوده و در هيچ‌يك از آزمايش‌ها، روانگرايي كامل دقيقاً در زير پي مشاهده نشده است. همچنين اثر ضريب اطمينان طراحي بر نشست‌ها مورد بررسي قرار گرفته است.

Bearing capacity failure and seismically induced settlement of buildings with shallow foundations rested on liquefied soils have resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in freefield. They commonly disregard residual strength of liquefied soil in their design procedures. Previous studies of this problem have identified important factors involving shaking intensity, the liquefied soil’s relative density and thickness and the building weight and width. Newly studies have also showed that shear deformation combined with localized volumetric strains during partially drained loading are dominant mechanisms. Bearing capacity degradation due to high excess pore pressure development also has been reported in previous studies. Two series of physical modelling experiments involving shallow footing rested on liquefied sand have been performed to identify the mechanisms involved in liquefactioninduced building settlements and bearing capacity degradation. Experiments have performed on Babolsar sand with moderate relative density in a box with two plexiglass sides to observe sand deformations. Earthquake waves can cause pore pressure build up in saturated sands but complete liquefaction always do not occur. Anyhow excess pore pressure generation can cause bearing capacity degradation and excessive settlements. Various pore pressure ratios have been generated by static seepage through box base to assess bearing capacity degradation and excessive settlements before and in complete liquefaction conditions. First series of experiments are consisted of 8 tests related to bearing capacity measurements of square and spread foundation in pore pressure development conditions. Results show bearing capacity reduction due to excess pore pressure development, but there is remarkable strength even in complete liquefaction that is related to postliquefaction strength of liquefied sand. Square foundations are more affected than spread footings by excess pore pressure ratios. Foundation’s loading behaviour and Shape factors are not affected by excess pore pressure ratios. Pore pressure decreased with loading increscent under the middle of foundation because of particle rearrangement and always was below the induced excess pore pressure. Complete liquefaction has never observed under footing. Safety factor selection is a challenging step in shallow foundation designs for engineers because of its economical view. Recent studies show the important role of shear deformations in shallow foundations as discussed before. In second test series of this experimental study, foundations firstly loaded to some safety factors and then its settlements due to pore pressure build up has measured, loading then increased to complete bearing capacity failure. This series are consisted of 12 tests for two types of foundations and various excess pore pressures, only shear deformations are assessed in this series because there is no volumetric deformation as excess pore pressure is constant during the tests. Results show increase of foundation settlements with safety reduction progressively, settlements for safety factors below 2 are negligible.