Settlement Calculation on High-Rise Buildings

Nations across the world including China have entered the stage of modern urbanization, and there have been common problems such as urban population explosion, housing scarcity, land shortage, environmental pollution, traffic congestion and so on. Even though there are more or less arguments on high-rise and super high-rise buildings, the surging trend of the construction cannot yet be discouraged. This trend not only inaugurates a new epoch of construction industry, but also will necessarily brings human into overground and underground space. The evolution of super high-rise buildings remarkably promotes the research, development. and application of engineering technology of foundation & subgrade. Compared with underground foundation engineering, calculation, design, model experiment and field measurement for superstructure of super high-rise buildings are much easier. All of the above processes can be conducted by computer to completely meet the requirements of engineering. While as research and computerization of foundation, subsoil and geotechnical engineering progress relatively much slowly. Theory of settlement calculation of deep foundation is more complicated, and many questions are still left unsolved today, and might remain unsolvable in short term. It indeed requires arduous efforts of generations to get results.

In 1867, Winkler put forward the most simple linear-elastic ideal model, which assumed that pressure of each point on the medium surface of subgrade is in direct ratio with the vertical displacement of this point, and is completely irrelevant with pressure of other points on the interface between subgrade and foundation. Although this assumption has some disadvantages, it is still applied now. Another ideal model describes subgrade medium by elastic half-space continuum, i.e. the elastic half-space foundation model, which takes into account the continuity feature of subgrade, and integral equation calculation is needed accordingly for the analyzing process of the interaction between subgrade and foundation. So this model has more complex calculation process, and the result of internal force of foundation is larger than real. Afterwards, Filonelko-Borodich (1940), Hetenyi (1946), etc, put forward a kind of intermediate double-parameter subsoil models, which was a intervenient one between Winkler foundation model and elastic half-space foundation model. One of such models introduces the mutual mechanics action between springs to make the result more sensible by breaking the limit that the displacement of subgrade medium surface is within the area of load, based on Winkler subsoil model. Cross isotropic model is a linear-elastic model, which considers the stratum feature of subgrade medium. Recently, application of the layered model is more popular in the field of subgrade-structure interaction analysis, the result is between Winkler subsoil model and elastic half-space subsoil model, and is more reasonable. For the complexity of properties of soft subgrade, new models are still to be researched to consider the factors of non-linear feature, elastoplastic feature, and time-dependent feature of subgrade.

The distribution of additional stress in subsoil, which is the basis of the study and computation of settlement, is vital to the research on deep foundation settlement of super high-rise buildings. The related mechanics includes: ➀ geostatic stress, ➁ additional stress caused by the load of buildings, ➂ contact pressure and contact problems of foundation base, ➃ the planar and spatial distribution of subsoil stress, ➄ the distribution of heterogeneous and anisotropic subsoil stress. Also included is the interactions among superstructure, subsoil and foundation are included, and scrutinized along with foundation type and engineering.

The characteristics of super high-rise buildings are the great height, heavy load, and big settlement. The loads are delivered to the subgrade through foundations, and introduce stress diffusion in the subgrade. For the reason that the subgrade is compressible, deformation, whose main component is vertical deformation, is produced in the subgrade under the action of induced stress, then the settlement or inclination of foundation are induced to super-high buildings. The values of deformation depend on the physical and mechanical properties of subgrade and loads, and is also related to the area, depth of foundation and the shoring structures of the pit. Even foundation settlement of super high-rise buildings has little influence on the safety of structures, but excessive settlement would affect the function and appearance, especially for elevators, pipelines and equipments. If foundation settlement is uneven, the crack, distortion or inclination might be induced, which will affect structure’s safety and even results in collapse when serious. This kind of collapse happens both home and abroad. So the problem of subgrade deformation must be carefully researched and considered, and the settlement must be accurately predicted when constructing super high-rise buildings.

As far as high-rise or super high-rise buildings are concerned, foundations must ensure the stability of buildings and meet the anti-inclining and anti-sliding requirements given the demands of wind-resistance and earthquake-resistance. So it is necessary to have enough buried depth for foundations. The basements need to not only satisfy the demand of buried depth, but also make full use of underground space. Therefore, box and raft foundations, also called compensating foundations, are mostly adopted in the foundation design of super high-rise buildings if the subgrade conditions allow. According to experiences, the basement depth required in complete compensating foundations of high-rise or super high-rise buildings refers to Table 5-1.

For deep foundation of super high-rise building, pile box (raft) foundation is mostly adopt, and pile foundation is usually formed by super-long bored piles with large diameter, only a few super high-rise buildings are of super-long steel pipe pile (for example, for Jinmao Tower in Shanghai, steel pipe pile over 80m long is used. Generally, super-long drilled pile’s diameter d ≥800mm, while pile length l ≥-50m. At present, in the latest Technical Code for Pile Foundations of Building (JGJ94-94) of China, the vertical load capacity and other parameters of under water bored piles are determined by modifying prefabricated piles in Code for Design of Foundation and Subgrade of Building (GBJ7-89) combined with measured data of under water bored piles. In China, more and more super high-rise buildings have been built in soft subgrade area, such as Shanghai, Tianjin, Wuhan, Fuzhou, Shantou, Wenzhou, Wuxi, Nanjing, Zhengzhou, Xi’an and etc, and large number of super-long bored piles are used, and there is undergoing trend. After 100 years’ application and research of bored piles, admittedly there have been much study on short pile, middle-long pile and long pile, and there have also been much data & material accumulated. But for super long pile, there have been reletively less study and test data. The reason is that it is difficult to form a pile and also difficult to reach the ultimate bearing capacity of pile in pile compression test for engineering. So it is very pressing and important to study the performance and settlement of super-long pile. In this chapter, settlement characteristics and engineering application of super-long pile are researched, and some suggestions and opinions about using super-long pile are put forward. Meanwhile, settlement calculation method of pile foundation is presented, which is derived from Code for Design of Foundation and Subgrade of Building (GB50007-2001).

The calculative theory of settlement and analysis of measured settlement for pile-box (raft) foundation of general high-rise buildings and middle high-rise buildings have got rich achievement. Some research results of Chinese experts have reached internationally advanced level, such as Professor Xihong Zhao, Professor Jianguo Dong, Professor Yang Min, Professor Zhu Baili, Professor Hong Yukang and Professor Chen Zhuchang of Tongji University; Academician Huan Xiling, Researcher Liu Jinli and Huang Qiang of China Academy of Architectural Engineering; Academician Zhou Jing of Railway Institute of Science and Technology; Academician Shen Zhujiang of Water Conservancy Institute of Nanjing; Chief Engineer of Civil Design Institute of Shanghai: Huang Shaolou. But the interaction between diaphragm wall and friction pile-box (raft) foundation and the settlement analysis concerning the interaction haven’t been considered in these results. Furthermore, a large number of these super high-rise buildings are lesser then 100m in height. There is little study result for super high-rise buildings higher than 150m. So far, there are only 12 super high-rise buildings exceeding 200m, 3 buildings exceeding 300m, and only one building exceeding 400m (It is Jinmao Tower, which is 395m in fact, not beyond 400m). So it could be said that the action of diaphragm wall is not involved in the analysis of foundation settlement of these super high-rise buildings. In addition, because of various reasons, the measured settlement data and analysis of these super high-rise buildings is scarce. So the settlement data of engineering case is very valuable, not only at present but also in the future. In this chapter, there is no discussion on various calculation methods or calculation cases, and emphasis is laid on the analysis of effect of diaphragm wall on settlement calculation, study is conducted on the settlement calculation principle and method of friction pile-box (raft) foundation, and then study and analysis is conducted on measured settlement data of Senmao Tower and Jinmao Tower of Shanghai, which can get useful result for further settlement study.

In the igneous rock area of China, such as Qingdao, Shenzhen, etc, rock is mostly granite or granites gneiss, and the eluvium usually formed from slightly weathered granite is very thin, it is about 20–40m. The pile foundation engineering is always designed as one pile under one column, and most of the piles are designed to be manual hole digging pile with large diameter. During the design calculation, the side friction of the large diameter pile is usually neglected, only the bearing capacity is taken into account, when the settlement is analyzed. It is difficult to account for the effect of the retaining structure of the deep foundation, such as diaphragm wall, row piles, etc.

Settlement calculation of deep foundation of super high-rise building is a critical topic for construction engineering. Due to the complexity of geology and geological soil (tri-phase object), the difference with theories, calculated settlement results hardly comply with measured data, and there has’t been any satisfactory formula for settlement calculation, especially for deep foundation, such as box-raft foundation, single pile and pile group foundation. In China, the difference between calculated value and measured data is modify by applying a unified empirical coefficient. This method is admittedly to have the minimum variance of all the settlement calculation methods at present. However, there is so tremendous differentce with the geological condition of each region of China and so great differentce with regional soil characteristics, that calculation result is basically unsatisfactory. Is might be more close to the real condition to apply the regional empirical settlement formula.