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When designing pile foundations, static design equations, pile driving formulae, static loading tests or stress wave analyses can be employed to estimate the axial capacity of single piles. Both laboratory and field tests show that soil exhibits time-dependent behaviour. An important result is that soil gains additional strength and stiffness with time due to time-dependent processes such as ageing. Similarly, results show that the capacity of piles increases, to a certain extent, with time after installation due to time-dependent processes in the soil. However, most design methods do not take this into account. This implies that the design methods used today do not in general make use of the full capacity of piles.
This thesis is based on a number of scientific papers and reports that deal with elements of pile design and time effects in soils in general. With regard to pile design, focus is placed on estimation of the axial pile capacity by static design equations and especially the influence of time on pile capacity. In respect of time effects in soils in general focus is placed on observed time-dependent behaviour of soils and models developed to capture this behaviour. The thesis consists of two parts. Part I deals with pile design, whereas Part II deals with time effects in soils in general. The two parts should be looked upon as steps forward towards a more reliable pile design and towards a better understanding of time effects in soil in general.
An increase in pile capacity with time after installation is denoted "set-up". It is important to quantify set-up because time effects offer potential practical benefits if piles have been, or can be, driven months or even years before any critical loading events can occur - as in carefully staged construction, or when reusing pre-installed aged foundations. Further, design methods that take no account of time will be subject to considerable error unless they consider a tightly specified age range. Time functions (relation between time after installation and capacity), which have been offered for quantifying set-up for piles in clay, are investigated based on a set of static loading tests. In the literature it is suggested that the pile capacity increases with the logarithm to time after installation which is confirmed in this thesis. In continuation of this, it is analysed whether the magnitude of the set-up is related to the properties of the clay surrounding a pile. Statistical analyses show that the rate of set-up is constant and thereby independent of clay properties. The devised time functions are compared to existing time functions. Set-up for piles in clay is in Denmark quantified by employing a so-called regeneration factor in the static design equations. The possibility of introducing a regeneration factor dependent on time elapsed since driving and undrained shear strength is investigated.
Design methods for piles in clay and sand have been a controversial matter within geotechnical engineering for many years due to their empirical nature. Therefore, the design of piles has remained a constant source of attention, especially with regard to the methodology for predicting the capacity. Three very different design methods for piles in both clay and sand are assessed by comparing predicted capacities with measured capacities from established databases of static loading tests. The three calculation procedures in consideration are proposed by the Norwegian Geotechnical Institute, the Imperial College in London and the American Petroleum Institute, respectively. Guidelines are given for the choice of design method to be used in different circumstances (e.g. load specifications, length of pile, pile material). In order to evaluate the design methods for piles in clay, it is necessary to correct for time between pile driving and pile testing. Results of testing the calculation procedures against the available data by employing different time functions are presented.
In respect of time effects in soils in general, the purpose of the study is twofold. Firstly, a concise review of time-related phenomena (creep, stress relaxation, rate effects and accumulated effects such as ageing) observed in connection with laboratory tests on clay and sand is presented. These phenomena are present in both clay and sand. However, they are more pronounced in clay than sand. The review reveals essential characteristic situations for different types of soils, i.e. whether the time-dependent behaviour can be characterised as isotach or non-isotach. Apparently, isotach behaviour is adequate for describing time effects in clays in many situations. In contrast, sand exhibits non-isotach behaviour. Secondly, a concise review which categorizes and describes the basic features of existing models as well as their advantages and limitations is presented. Existing models can be used for modelling isotach behaviour. Thus, existing models and concepts can in principle be used to model time-dependent behaviour of clay whereas this is not the case when considering sand.
|Place of Publication||Aalborg|
|Publisher||Department of Civil Engineering, Aalborg University|
|Number of pages||177|
|Publication status||Published - 2006|
Bibliographical notePDF for print: 303 pp.
- Behaviour of Soils
- Time Effects
- Time-Dependent Behaviour