New CPT methods for evaluation of the axial capacity of driven piles

High costs associated with offshore foundation installations have provided strong impetus to the offshore energy sector in the search for more reliable design methods. This paper provides a summary of an Industry sponsored project that led to the development of new CPT-based design methods for the evaluation of the axial capacity of driven piles. Particular attention was given to the need for the new methods to be applicable to large diameter offshore piles given that many existing methods are derived by calibration with capacities measured in static pile load tests on smaller diameter onshore piles. The basic mechanisms supporting the general format of the expressions proposed for shaft friction and end bearing in sands and clays are described. It is shown how the new expressions, which are calibrated against a database of the most reliable load tests reported in the literature, lead to better predictions of capacity compared to other methods and can also satisfactorily predict the capacity of piles driven into deposits comprising interbedded layers of sand, silt and clay. Recommendations for the prediction of pile displacements at working loads using CPT data are also presented.New CPT methods for evaluation of the axial capacity of driven pilespublishedVersio

Pile groups under axial loading: an appraisal of simplified non-linear prediction models

The settlement behaviour of vertically loaded pile groups has been the subject of an extensive body of research over the past two decades. In particular, this work has identified the over-conservatism associated with predictions of pile interaction derived from elastic theory and the corresponding amplification of group settlement relative to single pile values. Researchers have since redoubled efforts to refine settlement predictions for pile groups towards more economical design, largely through more rigorous treatment of soil stiffness non-linearity. Although foundation design engineers are increasingly employing three-dimensional continuum analyses to quantify pile interaction on a site-specific basis, simplified design approaches remain an integral part of preliminary foundation design. The purpose of this paper is to undertake a critical examination of these methods with a view to increasing their potential for take-up by foundation engineering practitioners. A database of simplified models has been collated for the prediction of non-linear pile interaction that exists within vertically loaded pile groups. These models are categorised as either analytical or empirical. The development, limitations and range of applicability of these models are explored in detail in the context of some published case histories.peer-reviewe

Friction fatigue on displacement piles in sand

Experiments with instrumented displacement piles have shown that the ultimate shaft friction that can develop in a given sand horizon decreases as the pile tip penetrates to deeper levels. This phenomenon, which is now commonly referred to as friction fatigue, is investigated here using centrifuge model piles equipped with lateral stress sensors, and by drawing on other experimental data from the laboratory and the field. It is shown that the primary mechanism controlling friction fatigue is the cyclic history imparted during pile installation to soil elements at the pile-sand interface. For a given installation method the stationary lateral stress acting at any given level on a displacement pile can be described as a relatively unique function of the cone penetration test end resistance and the number of cycles imposed during installation. The strong influence of cycling, which is also seen in cyclic constant normal stiffness interface shear tests, is attributed to contraction of a narrow shear zone at the shaftsoil interface that is surrounded by soil with a relatively high lateral stiffness.</p

Centrifuge modelling of the pushover failure of an electricity transmission tower

Centrifuge model tests were conducted to examine foundation failure mechanisms during rapid horizontal pushover of an electricity transmission line support tower, simulating a broken transmission line response or wind gust loading. A model transmission tower supported on four pad foundations in clay and backfilled with sand was loaded horizontally and the loads at each foundation were measured during fast and slow pushover. The tests examined the influence of tensile resistance mobilized at the underside of the footings, which is difficult to reliably incorporate within design practice due to a lack of accepted quantitative design methods. The measured performance of the tower footings was compared with results from a series of tests where a single footing is subjected to purely vertical loading in compression and tension and was found to be in good agreement. The tower response was back-analysed as a simple push–pull model and the calculated uplift capacity of the footing backfill provided a close match to the observed response of the tower footings subjected to slow pushover. During fast pushover, the additional capacity mobilized due to tensile resistance (suction) created by the reverse bearing capacity beneath the base of the footings subjected to uplift was quantified using a suction capacity factor

Uplift performance of transmission tower foundations embedded in clay

The contribution to the uplift stiffness and capacity provided by the clay beneath the base of shallow footings typical in configuration to those employed to support high voltage electricity transmission towers is examined. Pore pressures developed at the base of appropriately scaled footings founded on clay were measured over a wide range of uplift rates in a geotechnical centrifuge. These measurements, coupled with data from tests on identical footings founded on sand, are used to provide insights into the influence of uplift rate on the failure mechanism and footing capacity. Data from a series of undrained triaxial extension tests, conducted over a range of strain rates, are presented and these data combined with finite element back-analyses of the centrifuge uplift tests are used to provide designers with a means of assessing the capacity and load–displacement response of footings on clay subjected to high rates of uplift in service

The influence of effective area ratio on shaft friction of displacement piles in sand

As a cone penetration test (CPT) induces similar strain paths to closed ended pile installation, its results are directly applicable to the evaluation of radial stress on closed ended piles. For CPT data to be used to evaluate the shaft capacity of open-ended piles, an understanding of the relative magnitude of radial stress on open and closed-ended piles is necessary. This relative magnitude is explored using cavity expansion theory to simulate the stress field as soil flows around a pile tip. The end condition of the pile affects the flow field. These analyses allow the difference in shaft friction on open and closed-ended piles in sand to be linked to the pile area ratio and plugging behaviour, with friction angle having a small influence. A function of the effective area ratio, which combines the area ratio of the pile and the incremental filling ratio, is proposed for estimating the difference in radial stress along open-ended piles as compared to closed-ended piles.</p

Rate effects on the vertical uplift capacity of footings founded in clay

This paper describes results from a series of uplift tests performed in a drum centrifuge on shallow square footings founded on clay and sand, with an overlying sand backfill. The test series investigated footings of various widths loaded at displacement rates extending over five orders of magnitude. Additional tests were performed to examine the influence on uplift capacity of the direction of load inclination. The uplift tests performed on footings founded on sand enabled estimations to be made of the suction that developed at the base of the footings founded on clay. It is shown that the suction mobilised at very fast rates may be assessed using existing undrained bearing capacity approaches and a (rate corrected) operational undrained strength similar to that measured in triaxial extension. The uplift rates required to reach fully undrained conditions are almost two orders of magnitude higher than those for footings loaded in compression, and the upward footing displacements required to generate maximum resistance can be up to 10% of the footing width. These findings have important implications for shallow foundations such as those used to support high-voltage electricity transmission line support structure

Pile groups under axial loading: an appraisal of simplified non-linear prediction models

The settlement behaviour of vertically loaded pile groups has been the subject of an extensive body of research over the past two decades. In particular, this work has identified the over-conservatism associated with predictions of pile interaction derived from elastic theory and the corresponding amplification of group settlement relative to single pile values. Researchers have since redoubled efforts to refine settlement predictions for pile groups towards more economical design, largely through more rigorous treatment of soil stiffness non-linearity. Although foundation design engineers are increasingly employing three-dimensional continuum analyses to quantify pile interaction on a site-specific basis, simplified design approaches remain an integral part of preliminary foundation design. The purpose of this paper is to undertake a critical examination of these methods with a view to increasing their potential for take-up by foundation engineering practitioners. A database of simplified models has been collated for the prediction of non-linear pile interaction that exists within vertically loaded pile groups. These models are categorised as either analytical or empirical. The development, limitations and range of applicability of these models are explored in detail in the context of some published case histories