كليدواژه :
پي لبه دار , لبه تحتاني , ظرفيت باربري , نشست , مدل سازي فيزيكي
چكيده فارسي :
به دليل گسترش روزافزون نوع جديدي از روش هاي بهسازي پي هاي سطحي در ژئوتكنيك، بررسي عملكرد استفاده از صفحات قائم تحتاني در زير پي هاي سطحي، به عنوان پي هاي لبه دار ضروري است. پي هاي لبه دار مي توانند به عنوان جايگزين مناسب براي پي هاي سطحي، نيمه عميق و عميق در كاربري هاي پي هاي مخازن نفت و گاز، توربين هاي بادي و سكوهاي حفاري استفاده شوند. رفتار پي هاي سطحي مربعي واقع بر ماسه محصور شده توسط صفحات قائم، با استفاده از مدل سازي فيزيكي كوچك مقياس مطالعه شد. تاثير پارامترهايي شامل نسبت طول لبه به عرض پي، اندازه پي، مقاومت برشي ماسه و زبري سطوح پي، بر رفتار پي هاي لبه دار از نظر ظرفيت باربري، نشست و ضريب واكنش بستر، تحت بارهاي فشاري مورد ارزيابي قرار گرفته و با عملكرد پي هاي سطحي و نيمه عميق مقايسه شدند. نتايج نشان داد كه ظرفيت باربري و نشست پي هاي لبه دار نزديك به مقادير پي هاي نيمه عميق، در عمق و عرض يكسان است. نتايج حاصله نشان داد كه وجود لبه، ظرفيت باربري و ضريب واكنش خاك را افزايش و نشست پي سطحي را كاهش مي دهد. افزايش در ظرفيت باربري و كاهش در نشست پي هاي سطحي، با افزايش طول لبه و كاهش مقاومت برشي ماسه، افزايش مي يابد. نسبت ظرفيت باربري پي هاي لبه دار به پي هاي سطحي، در محدوده 2/4 تا 5/1 برابر مشاهده شده و مقدار نشست پي لبه دار نسبت به پي سطحي تا حدود 91% كاهش يافت. ضريب واكنش بستر، در محدوده بين 1/5 تا 4/23 برابر بهبود يافت. بر پايه تحليل نتايج، نمودارها و روابطي جهت تخمين مقادير ظرفيت باربري، نشست و مدول عكس العمل بستر پي هاي لبه دار ارائه شده است.
چكيده لاتين :
As the new type of improvement methods of shallow foundations in the geotechnical engineering are improving, it seems necessary to study the performance of vertical plates beneath the periphery of shallow foundations known as skirted foundations. Skirted foundations are steel or concrete foundations, which have a top raft and a relatively thin plate constructed beneath the periphery. The skirts penetrate the soil beneath foundation which encompass and confine the soil very firmly. The skirts and confined soil behave as a unit to transmit loads to the soil at the level of skirt tip. The benefit of skirted foundations compared with deep foundations such as piers and piles lies in their ease and short time of installations. The skirted foundations can be used as an appropriate alternative for shallow foundations, pier and deep foundations in applications such as oil and gas storage tanks, wind turbines, oil drilling platforms, harbor, and offshore and jacket structures. The behavior of square shallow foundations resting on confined sand by vertical plates as skirt, was studied using small scale physical modeling in the laboratory. The effects of parameters including ratio of skirt length to foundation width, foundation size, the shear strength of sand, and roughness of skirt and foundation surfaces on skirted foundations behavior in terms of increasing bearing capacity, the settlement reduction, and improvement of subgrade reaction modulus were assessed under compression loading, and results were compared with the performance of shallow and pier foundations. Modeling test analyses revealed the overall improvement of square skirted foundations performance compared to shallow and pier foundations. Results of this study showed that the values of bearing capacity and settlement of skirted foundations are almost close to those of pier foundations of the same width and depth. Also, it was observed that the existence of skirt, increases the bearing capacity and modulus of subgrade reaction, together with decrease of settlement of shallow foundation. The enhancement of the bearing capacity, as well as the reduction in the settlement of shallow foundations increases with increasing skirt depth and decreasing the shear strength of sand. So that, bearing capacity ratio (BCR) of skirted foundations to surface foundations was observed to be in the range of about 2.4 to 5.1 times for the different values of L/B. Furthermore, the value of skirted foundation settlement decreased up to 91% of that a surface foundation in the case of having skirt depth/foundation width of 2.0. The modulus of subgrade reaction improved in the range of about 1.5 to 4.23 times due to skirt existence. From the accomplished laboratory tests, it was found out that skirted foundations resting on loose sand, are more beneficial than in the case of resting on medium and dense sand. In cases where structures are very sensitive to settlement values, the skirted foundations can be used to gain a much lower settlement, at the same allowable bearing capacity. In this paper, based on the analysis of the obtained results, charts and equations are presented to estimate bearing capacity and settlement and subgrade reaction modulus of skirted shallow foundations in terms of those of surface foundation and sand relative density, skirt depth to foundation width ratio and its roughness.
