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Reducing the costs for deep foundations of high-rise buildings by advanced numerical modelling
Abstract
When considering foundations for high-rise buildings in urban areas a mayor task is the reduction of settlements and differential settlements of new structures and adjacent buildings to ensure their safety and serviceability. In many cases the soil conditions can lead to deep foundations in order to transfer the high loads of the building into deep soil strata with higher bearing capacities. Compared to traditional piled foundations where building loads are assumed to be transferred to the soil only by piles, the combined piled raft foundation (CPRF) is a rather new approach. A CPRF is consisting of the three bearing elements piles, raft and subsoil. The load share between piles and raft is taken into consideration and the piles can be used up to a load level equal or greater than the bearing capacity of a comparable single pile. This design concept can lead to a considerable cost reduction for foundations of more than 50 % compared to the traditional piled foundation.
... For the case of piled raft system in clay, Shukla et al. [26] stated that the building load inducing stress in the clay preconsolidated pressure is carried by piles in creep state of loading and the remaining load is carried by the raft. Katzenbach et al. [14] indicated that many piled raft systems are designed using the settlement-reducing piled method. Davids [5] showed that the loads of the superstructures are shared between the raft and the piles, with piles carrying about 50-80% of the total load. ...
... Enormous contributions to the development of the piled raft foundation concept have been carried out in Germany during the 1980s and 1990s of the last century. Many piled raft foundations have been constructed in the Frankfurt clay using settlement-reducing piled foundation for heavy high-rise buildings [23,14]. There are also applications in cohesionless soil, such as the Berlin sand [6]. ...
Raft foundations are used when isolated footings cover more than 70% of the building area under a super structure. A combination between the use of piles and raft foundation is known as piled raft foundation. Piled raft foundations proved to be more economical in case of high rise buildings on clay and can provide safe bearing capacity and serviceability requirements. The use of strategically located piles improves the load capacity of raft and reduces the differential settlement. This research sheds some light on the philosophy of using piles as settlement reducers for raft foundations and also the behavior of piled raft embedded into sand. Small scale model tests are performed. The effects of pile length and alignment on the attained ultimate load are experimentally investigated. From the accomplished studies, it has been concluded that as the length of piles and number of piles decrease, the load carried by raft increases. Additionally, it is found that the best and the optimal settlement ratio (S/B%) to design the piled raft as settlement reducer is 0.7%.
Finally the percentage of load carried by raft at S/B = 0.7% is found to be 39%.
... Durum 3 çok sıkı durum için yapılan model testlerde 4,5D ve 3D dizlimdeki E28 ve E27 nolu modeller aynı ötelenme rijitliği sergilerken, 1,5D lik aralığındaki E26 nolu model 5mm düşey yerdeğiştirmeden itibaren diğer modellerden rijitlik kazanımı yönünde ayrışmıştır. [15,39,40,41] göre karşılaştırma grafiği olarak Şekil 14'te sunulmuş bu saha çalışmaları grafiklerde üst limit ve alt limit olarak yer almaktadır. Buna göre saha çalışmalarının neredeyse tamamı yük oturma grafiğinin değersel olarak güvenli kalan bölgelerinden alınan rakamlarla dizayn edilmiş, diğer bir ifadeyle yük oturma grafiğinin göçme veya mobilizasyon başlangıcı bölgelerinden uzak kalan, kazıklı radye temelin güvenli bir biçimde oturma aldığı bölgelerde dizayn ve uygulaması gerçekleşmiştir. ...
Bu çalışmada, %50 ve %85 olmak üzere, iki farklı rölatif sıkılığa sahip kum zeminde 40 adet model test deneyi yapılmıştır. Model temel sistemleri; uygulanan yükün, sadece radyeye, sadece kazıklara ve kazıklı radye temel sistemine taşıtıldığı durumlar dikkate alınarak oluşturulmuştur. Kazıklı radye temeller radyenin zemine gömülü olduğu ve zemine gömülü olmadığı iki farklı durum için irdelenmiştir. Eksenel statik artımsal yükler altında analiz edilen modellerde; optimum kazık aralığının radyenin gömülü olmadığı durumda daha küçük, gömülü olduğu durumda ise daha büyük S/D oranına sahip olduğu belirlenmiştir. Optimum kazık aralığı, orta sıkı zeminde, 1.5D ile 4D, çok sıkı zeminde 4.5D ile 6D arasında bulunmuştur.
... Enormous contributions to the development of the piled-raft foundation concept have been done in Germany during the 80's and 90's of the last century. Many piled raft foundations have been constructed in the Frankfurt Clay using settlement-reducing piled foundation for heavy high-rises (Sommer et al., 1985;Katzenbach et al., 2003). There are also applications in noncohesive soil, like the Berlin Sand (El-Mossallamy et al., 2006). ...
... This confirms the observations reported by Katzenbach et al. (1998) and Poulos (2001) from numerical analyses of raft on different numbers of settlement reducing piles. An important relationship between the settlement ratio, SR, and the proportion of load taken by piles (sometimes called relative cap capacity) was introduced from case histories in Germany (Schmitt et al., 2003;El-Mossallamy et al., 2006;Phung, 2010). The results of the present tests at 25 mm settlement level are plotted on this relationship as shown in Fig. 22 used in a preliminary design of raft on settlement reducing piles (El-Mossallamy et al., 2006;Phung, 2010). ...
An experimental program is conducted on model piled rafts in sand soil. The experimental program is aimed to investigate the behavior of raft on settlement reducing piles. The testing program includes tests on models of single pile, unpiled rafts and rafts on 1, 4, 9, or 16 piles. The model piles beneath the rafts are closed ended displacement piles installed by driving. Three lengths of piles are used in the experiments to represent slenderness ratio, L/D, of 20, 30 and 50, respectively. The dimensions of the model rafts are 30 cm × 30 cm with different thickness of 0.5 cm, 1.0 cm or 1.5 cm. The raft-soil stiffness ratios of the model rafts ranging from 0.39 to 10.56 cover flexible to very stiff rafts. The improvement in the ultimate bearing capacity is represented by the load improvement ratio, LIR, and the reductions in average settlement and differential settlement are represented by the settlement ratio, SR, and the differential settlement ratio, DSR, respectively. The effects of the number of settlement reducing piles, raft relative stiffness, and the slenderness ratio of piles on the load improvement ratio, settlement ratio and differential settlement ratio are presented and discussed. The results of the tests show the effectiveness of using piles as settlement reduction measure with the rafts. As the number of settlement reducing piles increases, the load improvement ratio increases and the differential settlement ratio decreases.
... Enormous contributions to the development of the piled-raft foundation concept have been done in Germany during the 80's and 90's of the last century. Many piled raft foundations have been constructed in the Frankfurt Clay using settlement-reducing piled foundation for heavy high-rises (Sommer et al., 1985;Katzenbach et al., 2003). There are also applications in noncohesive soil, like the Berlin Sand (El-Mossallamy et al., 2006). ...
During the last decades, the quick growth of cities all over the world has led to a rapid increase in the number and height of high-rise and super high-rise buildings. High-rises often rest on pile foundations, which are designed using the conventional method, where the piles take the full load from the superstructure. Recently it is increasingly recognised that the use of piles to reduce the foundation settlement and differential settlement can lead to considerable savings. Only a limited number of piles, called settlement-reducers, may improve the ultimate load capacity, the settlement performance, as well as the required thickness of the raft. In this article the result from the Author's experiment study, which strongly supports the concept of settlement-reducers in non-cohesive soil, are reviewed. Applications of FEM in design of piled-raft foundations for high-rises are also discussed.
Due to urbanization, the high-rise building has become a common and growing phenomenon in all over the world. If a raft foundation does not meet the design requirement by itself, then the addition of a pile (piled-raft foundation system) may be possible with the raft. In this paper, the design of piled-raft foundation system has been carried out as the approximate assessment of the required number of piles, asses where piles may be required and refine piling requirements on their locations. The design strategies for the design of piles have been shown, and it has been demonstrated with the effective and efficient foundation can be designed by utilizing the significant capacity of pile. These strategies improve both the ultimate load capacity of the pile and settlement or differential settlement of the raft. In this analysis using of pile as settlement reducer and the condition in which the approach may be successful is derived. The characteristic behavior of the piled-raft foundation system has been considered in the effect of number of piles, nature of loading, raft thickness, and effect of load level on settlement. The behavior of piled raft is determined by complex soil-structure interaction effects, and understanding of these effects is useful for the reliable design of such foundations. The study brings out the effects of number of piles, pile length, raft thickness, and pile configurations; along with the soil-structure interaction by mean of the numerical modeling validation is compared with the measured and computed results.
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