Pile and Pile Group Capacity: Some Findings from Centrifuge Tests

The London Geotechnical Centrifuge Centre at City University London is a major experimental research facility for the Geotechnical Engineering Research Group. Some findings from two recent doctoral research projects are presented. The projects were aimed at improving our understanding of piled foundations. Pile group behaviour was studied with particular emphasis in determining the efficiency and load capacity of non standard group arrangements. The project was related directly to a development in London UK in which high capacity foundations were constructed using a Perimeter group arrangement of minipiles since that was all that could be constructed given onerous site constraints. The centrifuge research gave new insights into how pile groups carry large loads and demonstrated that Perimeter group arrangements make much more efficient use of piles than Grid groups. The second project was again industry driven and demonstrated that modifying the profile of a pile shaft can give significant increase in capacity with relatively little additional pile material

Behavior and efficiency of perimeter pile groups

Groups of piles are commonly used as high capacity foundations. It is recognised that the load distribution among piles in a group will vary and it is thought that the inner piles are likely to make a relatively small contribution to the total load carried. The essence of the research undertaken is to establish the relative effectiveness of pile groups with either no inner piles (perimeter group) or a single central pile (target group) when compared to the more commonly used grid group arrangement. Pile groups in which the central piles were omitted were used for the Cannon Place redevelopment in london and provided the impetus for the research project. The main research technique used is geotechnical centrifuge modelling. Samples of overconsolidated kaolin clay were prepared and tested on the centrifuge at City University london. This provided a firm clay into which pile groups could be installed in a wide variety of arrangements. Three or four different pile groups were located in each centrifuge model and loaded to failure using a strain-rate controlled load actuator. The individual model piles were made of 5 mm diameter aluminium rod placed in holes pre-drilled in the consolidated kaolin prior to the centrifuge test. All piles extended to a depth 250 mm in the clay giving an lId ratio of 50. The ranges of pile groups tested are linear, circular and square perimeter, circular and square target and square grid. Single pile tests provided the reference pile capacities used to normalise the data from the 23 centrifuge models tested. The experimental work was complemented by a parametric numerical modelling study using the finite element programme Abaqus. This gave insight into the pile-soil interaction and permitted a more meticulous analysis of the soil stresses and displacements. In addition, the numerical modelling enabled extension to the original variables tested as part of the centrifuge experiments and the soil shear strength and lId ratio were varied. The pile groups failed in one of two ways: either as individual piles with the piles settling into the ground with no noticeable settlement of the soil surrounding a pile, or as a block with the soil contained within the outer ring of piles settling by the same or xxiv similar amount as the piles. The change from block failure to individual pile failure often occurred at a pile centre-to-centre spacing of about two pile diameters though variables such as number of piles, the presence of a target pile and the strength of the soil all had an effect. The efficiency of a pile group is defined as the load capacity of a pile group expressed as a ratio of the number of the piles in the group multiplied by the load capacity of a single isolated pile. It was demonstrated that a grid group arrangement was the least efficient of the groups tested, whereas a perimeter group arrangement could achieve higher efficiencies of greater than unity and the inclusion of a target pile could further enhance the group efficiency. It has been shown that a target group comprised of 17 piles (16 piles plus one central pile) has a significantly higher efficiency than a 5x5 grid group comprised of 25 piles, such that the capacity at lower settlements is the same for both groups

Numerical modelling of perimeter pile groups in clay

The load distribution among piles in a group varies such that the inner piles often carry a smaller share of the total load compared to the outer piles, which is a result of increased soil–pile interaction. The main objective of this paper is to establish the relative effectiveness of pile groups with no inner piles (perimeter group), when compared to the more common grid configuration. The numerical investigation utilized the finite element programme ABAQUS and considered a range of variables that affect pile group behaviour including number of piles, pile spacing, length/diameter ratio, and soil strength. It was demonstrated that a complete grid group is less efficient than a perimeter group, where efficiency is defined as the load capacity of the whole group expressed as a ratio of the number of piles in the group multiplied by the load capacity of a single isolated pile. Efficiencies close to unity were observed for some perimeter groups. Perimeter groups also showed that a “block” type group failure could occur, where piles were placed at a spacing of less than 2.0 pile diameters,d, centre-to-centre. This often, but not always, led to a reduction in the efficiency of the pile group

‘Ethnic group’, the state and the politics of representation

The assertion, even if only by implication, that ‘ethnic group’ categories represent ‘real’ tangible entities, indeed identities, is commonplace not only in the realms of political and policy discourse but also amongst contemporary social scientists. This paper, following Brubaker (2002), questions this position in a number of key respects: of these three issues will dominate the discussion that follows. First, there is an interrogation of the proposition that those to whom the categories/labels refer constitute sociologically meaningful ‘groups’ as distinct from (mere) human collectivities. Secondly, there is the question of how these categories emerge, i.e. exactly what series of events, negotiations and contestations lie behind their construction and social acceptance. Thirdly, and as a corollary to the latter point, we explore the process of reification that leads to these categories being seen to represent ‘real things in the world’ (ibid.)

Breathing Current Domains in Globally Coupled Electrochemical Systems: A Comparison with a Semiconductor Model

Spatio-temporal bifurcations and complex dynamics in globally coupled intrinsically bistable electrochemical systems with an S-shaped current-voltage characteristic under galvanostatic control are studied theoretically on a one-dimensional domain. The results are compared with the dynamics and the bifurcation scenarios occurring in a closely related model which describes pattern formation in semiconductors. Under galvanostatic control both systems are unstable with respect to the formation of stationary large amplitude current domains. The current domains as well as the homogeneous steady state exhibit oscillatory instabilities for slow dynamics of the potential drop across the double layer, or across the semiconductor device, respectively. The interplay of the different instabilities leads to complex spatio-temporal behavior. We find breathing current domains and chaotic spatio-temporal dynamics in the electrochemical system. Comparing these findings with the results obtained earlier for the semiconductor system, we outline bifurcation scenarios leading to complex dynamics in globally coupled bistable systems with subcritical spatial bifurcations.Comment: 13 pages, 11 figures, 70 references, RevTex4 accepted by PRE http://pre.aps.or

The London Geotechnical Centrifuge Centre at City University London

The London Geotechnical Centrifuge Centre located at City University, London is one of four currently active centrifuges in the UK. The centrifuge is well used and much of the geotechnical research at City University employs physical modelling but supported by a large element testing facility and numerical modelling capability. The centre was established in 1990 and the facility was extensively upgraded to provide more space for sample preparation and model making in 2004. In 2012 the centre is supporting 4 doctoral research projects in addition to visitors from China, Italy and UK. The focus of research is urban construction processes with an underlying theme of sustainability. Over the last 10 years the group has established a capability for tackling complex modelling problems and is currently investigating the application of smart instrumentation and control at high g

Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV

We report a measurement of the longitudinal double-spin asymmetry A_LL and the differential cross section for inclusive Pi0 production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. The cross section was measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T < 11 GeV/c and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of Pi0's in their parent jets was found to be around 0.7 for electromagnetically triggered events.Comment: 6 pages, 3 figures, submitted to Phys. Rev. D (RC

Measurement of the parity-violating longitudinal single-spin asymmetry for W±W^{\pm} boson production in polarized proton-proton collisions at s=500\sqrt{s} = 500 GeV

We report the first measurement of the parity violating single-spin asymmetries for midrapidity decay positrons and electrons from $W^{+}$ and $W^{-}$ boson production in longitudinally polarized proton-proton collisions at $\sqrt{s}=500$GeV by the STAR experiment at RHIC. The measured asymmetries, $A^{W^+}_{L}=-0.27\pm 0.10\;({\rm stat.})\pm 0.02\;({\rm syst.}) \pm 0.03\;({\rm norm.})$ and $A^{W^-}_{L}=0.14\pm 0.19\;({\rm stat.})\pm 0.02 \;({\rm syst.})\pm 0.01\;({\rm norm.})$, are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized DIS measurements.Comment: 6 pages, 4 figures, submitted to Physics Review Letter

High pTp_{T} non-photonic electron production in pp+pp collisions at s\sqrt{s} = 200 GeV

We present the measurement of non-photonic electron production at high transverse momentum ($p_T >$ 2.5 GeV/$c$) in $p$ + $p$ collisions at $\sqrt{s}$ = 200 GeV using data recorded during 2005 and 2008 by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The measured cross-sections from the two runs are consistent with each other despite a large difference in photonic background levels due to different detector configurations. We compare the measured non-photonic electron cross-sections with previously published RHIC data and pQCD calculations. Using the relative contributions of B and D mesons to non-photonic electrons, we determine the integrated cross sections of electrons ($\frac{e^++e^-}{2}$) at 3 GeV/$c < p_T <~$10 GeV/$c$ from bottom and charm meson decays to be ${d\sigma_{(B\to e)+(B\to D \to e)} \over dy_e}|_{y_e=0}$ = 4.0$\pm0.5$({\rm stat.})$\pm1.1$({\rm syst.}) nb and ${d\sigma_{D\to e} \over dy_e}|_{y_e=0}$ = 6.2$\pm0.7$({\rm stat.})$\pm1.5$({\rm syst.}) nb, respectively.Comment: 17 pages, 17 figure