The following are summaries of papers published in other parts of ICE Proceedings during 2008 that readers of Geotechnical Engineering may find of interest. Summaries of all papers in ICE journals are freely available and fully searchable at the ‘journals on-line’ section of the ICE website. See www.ice.org.uk/journals for details.
Gibraltar runoff: a steep challenge for decommissioning
M. Cooper, P. Stubbs, T. Carter, S. Dunn
Proceedings of the Institution of Civil Engineers, Civil Engineering, 161, No.1, February, 35–41, doi: 10.1680/cien.2008.161.1.35
Much of the British government's water supply in Gibraltar over the last century was achieved by fixing corrugated iron sheeting on steep sand slopes flanking the rocky peninsula to catch rainfall. However, introduction of desalination plant has rendered the massive catchments redundant. Over the past 30 years they had also started to become a liability, with large sheets becoming detached in high winds and two major landslides resulting from concentrated wetting of underlying sand. This paper describes the challenging design and construction of a decommissioning scheme for the east-side Ministry of Defence catchments, involving removing 13 ha of sheeting, installing 78 km of stabilising ground anchors, placing a temporary coir matting and seeding with indigenous plants and all on a 250 m high, 34 slope.
Heathrow Terminal 5: tunnelled underground infrastructure
I. Williams
Proceedings of the Institution of Civil Engineers, Civil Engineering, 161, Special Issue 1—Heathrow airport Terminal 5, May, 30–37, doi: 10.1680/cien.2008.161.5.30
Terminal 5 at London's Heathrow airport was one of the largest infrastructure projects in Europe over the last five years. It included approximately 14 km of tunnelling to provide underground infrastructure for rail, road and effluent discharge. In all cases the tunnels were delivered without major incident, thanks to a single integrated team which made all aspects of risk management central to its delivery philosophy. This paper describes the four key projects for which the key aspect of delivery was tunnelling – the airside road, the storm water outfall and the Piccadilly line and Heathrow Express extensions. It discusses the range of aspects that made up the delivery of this infrastructure, ranging from design through to performance of tunnelling machines and the control of surface settlement.
Heathrow Terminal 5: building substructures and pavements
T. Dawson, K. Lingham, R. Yenn, J. Beveridge, R. Moore and M. Prentice
Proceedings of the Institution of Civil Engineers, Civil Engineering, 161, Special Issue 1—Heathrow airport Terminal 5, May, 38-44, doi: 10.1680/cien.2008.161.5.38
This paper describes the design and construction of the vast piled basement structures for the three terminal buildings at London Heathrow's £4·3 billion Terminal 5 project, together with 1 million m2 of associated aircraft pavements. The basements are up to 20m deep and involved the excavation and reuse of 6·5 million m3 of gravel and clay. The aircraft pavement involved a number of innovations including development of a new high-strength concrete, which delivered a thinner construction and resulted in programme and environmental benefits.
Teton dam, USA: uncovering the critical aspect of its failure
B. Muhunthan and S. Pillai
Proceedings of the Institution of Civil Engineers, Civil Engineering, 161, Special Issue 2—Learning from failures, November, 35–40, doi: 10.1680/cien.2008.161.6.35
This paper provides a retrospective analysis of the failure of the 93 m high Teton dam in Idaho, USA, on its first filling in June 1976. Water from the near-full reservoir cascaded downstream, killing 11 people and causing massive economic losses. The failure shattered the confidence of the engineering community at large, enraged the public and politicians, and led to a series of dam safety regulations in the USA and other jurisdictions around the world. The conclusion to be drawn is that, in building major structures, engineers should expect the unexpected.
Learning from collapse of piles in liquefiable soils S. Bhattacharya, A. Blakeborough, Suresh Dash
Proceedings of the Institution of Civil Engineers, Civil Engineering, 161, Special Issue 2—Learning from failures, November, 54–60, doi: 10.1680/cien.2008.161.6.54
Piled structures continue to fail during earthquakes despite being designed with the required safety factor against bending due to lateral loads. This paper suggests that the reason for this may be that when the ground liquefies, its reduction in lateral support allows piles to buckle under their axial load. The mechanism could have led to the collapse of many pile-supported structures in earthquakes worldwide and is likely to cause more damage in the future. The paper concludes that the practical implications could be far-reaching, requiring reassessment of existing structures in liquefiable soils as well as rewriting seismic design codes to take better account of the effects of axial load.
Influence of anchor head on dynamic response of anchorages
A. Ivanovic and R. Neilson
Proceedings of the Institution of Civil Engineers, Engineering and Computational Mechanics, 161, No. 1, March, 27–34, doi: 10.1680/eacm.2008.161.1.27
The load of the anchorage installed in the laboratory was assessed by a non-destructive technique initially using a system based on the application of the impulse load and observation of the frequency response. An anchor head assembly, different from that commonly used, was then deployed as part of tests to assess the load of the anchorages. A dynamic model, developed previously, was used, to replicate the results obtained from the experiments. It was shown that the anchor head model required enhancement in order to obtain the range of frequencies found from the experiments. A set of tests with the more complex anchor head set-up was undertaken in order to obtain the characteristics, in this case stiffnesses, of the anchor assembly interfaces. The sensitivity of different components within the anchor head assembly is discussed showing the influence of the complexity of the head.
Analysis of soil—reinforcement interaction in reinforced soil beds
H. B. Raghavendra
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 1, February, 9–15, doi: 10.1680/grim.2008.161.1.9
A reinforced soil bed is a composite material composed of stiffer materials called ‘reinforcement’ embedded in the soil bed. Reinforcement inclusion in a soil bed results in a significant improvement in the bearing capacity and in reduced settlement. As a result, soil reinforced with strips, fabrics, sheets, grids and cells has become commonplace. Research is being carried out worldwide to study the effect of reinforcing elements in soil beds. The current paper describes an attempt to understand the soil–reinforcement interaction in soil beds that carry footing. The aim of the present study is to understand the changes brought about by the inclusion of reinforcement inside the soil system in terms of altered stresses and displacements at different increments of loading. The investigation uses the finite-element technique for the analysis and consists of a study of: (a) nodal displacement vectors, (b) deformed meshes and (c) failure initiation and progression. Based on the results of the above analysis, an attempt is made to understand clearly how the stiffer reinforcement enhances the load-carrying capacity, restricting the settlement. Results of the analysis are presented by means of illustrations between unreinforced soil beds and reinforced soil beds.
Bearing capacity of square footing on reinforced pond ash
A. K. Bera, A. Ghosh and A. Ghosh
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 1, February, 17–22, doi: 10.1680/grim.2008.161.1.17
This paper presents the ultimate bearing capacity of square footing on pond ash reinforced with a single layer of geotextile reinforcement. The effect of different vital bearing-capacity parameters on the ultimate bearing capacity (qru) of square footing on reinforced pond ash has been studied. Parameters considered include depth of the reinforcement layer below the footing (u) and friction ratio (f= ψ/φd)—that is, the ratio of the pond ash reinforcement interface friction angle (ψ) to the direct shear friction angle of pond ash (φd). The value of u/B (B being the width of footing) corresponding to the maximum ultimate bearing capacity is considered as optimum u/B in the current study. In the present investigation the value of optimum u/B ratio was found to increase with increase in friction ratio (f). With increase in friction ratio (f), bearing capacity ratio (BCRu (qru = qu) where qu is the ultimate bearing capacity of square footing, of the same size as that for qru, on unreinforced pond ash) increases. In the present investigation BCRu increases from 1·1180 to 1·4158 with a corresponding increase in friction ratio (f) of 0·7073 to 0·9230.
Linear analysis and comparison of displacement granular pile anchors
M. R. Madhav, B. Vidyaranya and V. S. Kumar
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 1, February, 31–41, doi: 10.1680/grim.2008.161.1.31
Granular piles can resist only compressive and shear loads owing to their inherent nature. By a simple modification of providing a pedestal/geogrid at the bottom and attaching a cable to the same, they are made to resist pullout/uplift forces. This paper presents an analysis of granular pile anchor (GPA), considering it and the in situ soil to behave linearly and the in situ ground to be semi-infinite. A parametric study presents results in the form of variations of normalised shear stress, displacement influence coefficient and axial uplift force with depth with relative stiffness factor. Two methods for the estimation of deformation moduli of the GPA and the in situ soil are proposed. Based on the estimated values of the moduli, the displacements of GPA were estimated and the results compared with test results of Kumar (2002). The predicted displacements compare well with the measured ones.
Testing coir geotextile drains for soft ground improvement
K. S. Beena and K. K. Babu
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 1, February, 43–49, doi: 10.1680/grim.2008.161.1.43
The results of an experimental investigation to explore the effectiveness of coir geotextile vertical drains for ground improvement are reported. The time settlement behaviour of clayey soil, as influenced by coir geotextile drains, is summarised and the construction and installation details are also discussed. Two types of coir drains, a circular type and a rectangular type, using two varieties of coir geotextiles were tested in three configurations, namely a single drain and triangular and rectangular patterns. It was observed that the time for consolidation was very much reduced due to the installation of coir drains irrespective of the type of coir geotextile used, type of coir drain and the layout of the drains. Furthermore, in addition to ease of construction these drains are eco-friendly and will not pose any environmental problems.
Soil improvement by internally reinforced stone columns
T. Ayadat, A.M. Hanna and A. Hamitouche
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 2, May, 55–63, doi: 10.1680/grim.2008.161.2.55
Granular columns have been used as an effective technique for improving the engineering behaviour of soft clays and loose sand deposits. This paper presents the results of an experimental investigation on sand columns internally reinforced with horizontal wire meshes made of plastic, steel and aluminium materials. Loading tests were performed on prototype reinforced sand columns in a stress-controlled chamber that contained normally consolidated clay. The objective of this investigation was to establish the load-carrying capacity of these columns in light of the presence and intensity of the reinforcement. The performance of the reinforced sand mass was examined, by direct shear testing, in terms of angle of shearing resistance, strains to peak strength, bearing ratio, and brittleness of the system post-peak. A theoretical model is proposed to predict the load-carrying capacity of these columns. The values predicted by the proposed theory compared well with the results of the present experimental investigation as well as those available in the literature. A design procedure is presented for practitioners.
Eccentricity effect on a footing supported by a stone column
M. Sadek and I. Shahrour
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 2, May, 65–70, doi: 10.1680/grim.2008.161.2.65
This paper includes analysis of the response to vertical loading of a single footing supported by a stone column. It mainly concerns the influence of footing eccentricity with regard to the stone column on the behaviour of the footing–stone column system. An eccentricity of e = 20 cm is generally accepted in engineering practice. Analyses are carried out using three-dimensional, elasto-plastic finite-element modelling. Computations for different eccentricities show that the footing eccentricity slightly affects the vertical stiffness and bearing capacity of the footing–stone column system.
Ground improvement by small-diameter timber piles
S. Ghosh, S. Mukherjee, R. B. Sahu, A. Ghosh and N. Som
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 2, May, 89–102, doi: 10.1680/grim.2008.161.2.89
Geotechnical engineers often encounter problems in designing foundations of structures on soft clayey soil. There may be a need for ground treatment to improve the bearing capacity of the soil. In the case of medium-rise buildings in and around Kolkata, a common practice has been to provide closely spaced 150–200 mm diameter timber piles of 5–6 m length. These timber piles have helped the structures to stand on soft clayey soil with little or negligible distress over the years. The present investigation includes the study of bearing capacity and settlement behaviour of model footings resting on small-diameter timber piles. Model footings were tested on an artificially consolidated kaolin bed with small-diameter timber piles inserted into the clay bed. To supplement the results of laboratory model tests, prototype tests were also carried out in the field. In order to obtain a parametric study, the length and spacing of timber piles as well as the extent of piles around and below the footings were varied. The study shows that the use of small-diameter timber piles produced beneficial effects for pile lengths greater than the width of the footing. An appreciable effect on bearing capacity is observed when area ratio (i.e. pile area/soil influence area) becomes more than 10%. The extent of these timber piles beyond twice the width of the footing results in a marginal increase in the bearing capacity. Considerable reduction in settlement occurred when timber piles were used. A semi-empirical method of calculating settlement has been presented for foundations with small-diameter timber piles.
Uplift of shallow foundations with cement-stabilised backfill
M. J. Rattley, B. M. Lehane, N. Consoli and D. J. Richards
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 2, May, 103–110, doi: 10.1680/grim.2008.161.2.103
This paper describes the results of a series of centrifuge model tests undertaken to investigate the effects of modifying a loose granular backfill using cement on the uplift performance of shallow anchors. These model tests, which involved a range of cement contents, are supported using a series of laboratory element tests and finite-element analyses. The study indicates that significant increases in uplift stiffness and peak capacity can be achieved by the addition of relatively small quantities of cement. Such increases are, however, limited to relatively low uplift displacements because of the brittle nature of the improved backfill shear strength characteristics.
Predicted and measured loads using the coherent gravity method
R. J. Bathurst, A. Nernheim and T. M. Allen
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 3, August, 113–120, doi: 10.1680/grim.2008.161.3.113
The paper investigates the accuracy of the coherent gravity method by using measurements reported in a large database of full-scale instrumented walls that was not available at the time the original method was developed. The new database includes data for bar mat, welded wire and steel strip soil reinforced walls. Measured reinforcement loads under operational conditions are compared with predicted values for bar mat and steel strip reinforced walls. The accuracy of the coherent gravity method as presented in the BS 8006 design standard is quantified by computing the mean and coefficient of variation of the ratio (bias) of measured to predicted loads. The paper shows that for steel strip walls the coherent gravity method is reasonably accurate for soils with friction angles less than 45°. For granular soils with higher friction angles and bar mat walls, the current coherent gravity method is shown to be less accurate and, on average, non-conservative for design.
Modifications to the method as currently described in BS 8006 are proposed to improve the accuracy of the method for bar mat reinforced soil walls and steel strip reinforced soil walls with high friction angle backfill soils.
Response of granular pile-anchors under compression
A. Srirama Rao, B. R. Phanikumar and K. Suresh
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 3, August, 121–129, doi: 10.1680/grim.2008.161.3.121
The granular pile anchor (GPA) is an innovative foundation technique, devised for mitigating heave of expansive clay beds and improving their engineering behaviour. It is a modification of the conventional granular pile wherein an anchor is provided in the pile to render it tension-resistant. This paper presents the results of a field-scale test programme conducted to study the response of anchors embedded in expansive clay beds. Plate load tests were conducted on unreinforced expansive clay beds and clay beds reinforced with GPAs to compare the compressive load response. The bulging pattern of the anchors was also studied. It was found from the field load tests that expansive clay beds reinforced with GPAs showed higher load-carrying capacity and improved compressive load response than unreinforced clay beds. Whereas the stress required to cause a settlement of 25 mm in the unreinforced expansive clay bed was 200 kN/m2, the clay bed reinforced with a GPA 1000 mm long and 150 mm in diameter (lgp/dgp = 6·67) required 500 kN/m2 to cause the same settlement, showing an improvement of 150%. Of the various single anchors with lgp/dgp values between 2·5 and 10, that of length 1000 mm and diameter 100 mm (lgp/dgp = 10) showed the best load response when tested alone, resulting in an improvement of 440%. The maximum bulge diameter increased with increasing diameter and length of the GPA. The effect of GPA length is more pronounced in the increase in maximum bulge length than that of diameter.
Electrochemical stabilisation for offshore model caissons
E. Mohamedelhassan, J. Q. Shang, M. A. Ismail and M. F. Randolph
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 3, August, 131–141, doi: 10.1680/grim.2008.161.3.131
A laboratory-floor experimental study was conducted on the electrochemical treatment of calcareous sand for offshore foundations. A steel tube of diameter 200 mm and length 400 mm was used as a model caisson. Calcareous sand and seawater from the coastline of Western Australia were used in the study. Twelve electrodes made of perforated steel pipes, diameter 14 mm, length 450 mm and filled with soluble CaCl2 granules as a cementing agent, were installed around the caisson. The electrochemical treatment tests were carried out with two electrode layout configurations in parallel with a control test. The electric power was applied via pairs of the steel pipe electrodes in the first configuration whereas the caisson served as one electrode in the second configuration. The applied direct current (d.c.) voltage was 8 V for the first configuration and 6 V for the second. The pullout resistance of the caisson after the treatment was increased by 140% to reach a value of 304%, in comparison with the control. The cementation generated by the treatment was demonstrated by the formation of a soil plug inside the caisson, soil cemented to the surface of the electrodes and caisson, and the significant increases in pullout resistances. The cementation effects were confirmed by electron microscopy images, X-ray fluorescence analyses and X-ray diffraction analyses.
Prefabricated vertical drains: a simplified design procedure
I. Bellezza and R. Fentini
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 4, November, 173–178, doi: 10.1680/grim.2008.161.4.173
The available design procedures for prefabricated vertical drains are discussed and a simple-to-use design equation is derived by which the drain spacing ratio, n, is expressed as an explicit function of the degree of consolidation required time available, and geotechnical properties of the compressible layer. Both smear and well-resistance effects are included in the analysis. The proposed approach does not require the aid of a computer and the drain spacing is calculated avoiding any iterative process or interpolations. The design equation derived in this note can be considered an improvement over the design chart-based procedures as it provides a very accurate value of the drain spacing ratio instead of a graphical estimate. For smear and well-resistance parameters varying in a range of practical interest the results of a validation phase demonstrate that the proposed approach can be effectively used for practical applications. A numerical example is given to illustrate the design procedure.
Experimental study on sand and gravel columns in clay
P. Andreou, W. Frikha, R. Frank, J. Canou, V. Papadopoulos and J.-C. Dupla
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 4, November, 189–198, doi: 10.1680/grim.2008.161.4.189
Theoretical and experimental studies have shown that stone columns can be used for accelerating the consolidation rate of soft soils by providing a drainage path, reducing foundation settlements, improving the bearing capacity of the soil, and limiting the risk of liquefaction due to seismic activity. This paper studies the influence of the main controlling parameters in the design of stone columns through a series of laboratory experiments. The effects of (a) the drainage conditions, (b) the grain size of the stone column material, (c) the confining pressure of the soil, and (d) the rate of deformation are investigated. Triaxial compression tests are performed on composite soil specimens of soft kaolin clay, reconstituted from a slurry with a central compacted reinforcement column. Two cases of reinforcement material are studied: sand and gravel. The behaviour of the reinforced soil compared with that of the unreinforced soil under the same loading and drainage conditions is analysed. The experimental results show the beneficial effect of this ground improvement technique on the strength characteristics of the foundation soil, even with a relatively low replacement factor compared with that used in practice. As the confining pressure increases, the improvement in the specimen with the sand column is more pronounced.
Field behaviour of granular pile-anchors in expansive soils
B. R. Phanikumar, A. Srirama Rao and K. Suresh
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 4, November, 199–206, doi: 10.1680/grim.2008.161.4.199
Different innovative foundation techniques have been suggested for controlling the detrimental volume change behaviour of expansive soils. The efficacy of a recent innovation in the form of granular pile-anchors in controlling heave of expansive clay beds and improving the engineering behaviour of the ambient soil has also been reported, based on results obtained from laboratory model tests. Before suggesting granular pile-anchor foundations for installation in field practices, however, an in situ study of the behaviour of granular pile-anchors is also required as a support to the laboratory study. This paper describes an extensive field study of the behaviour of granular pile-anchor foundations of expansive clay beds. The field study was performed to study the rate of heave, reduction of heave and variation of heave with radial distance and with depth. Heave was found to be reduced by about 90% upon installation of granular pile-anchors. Final heave was reached in just about half the time required for final heave in the case of an unreinforced expansive clay bed. Improvement in the engineering behaviour of the ambient clay was studied through Proctor needle penetration tests and unconfined compression tests. It was found that a maximum increase of about 120% was observed in penetration resistance in the case of expansive clay beds reinforced with granular pile-anchors.
Behaviour of lime–slag-treated clay
R. James, A. H. M. Kamruzzaman, A. Haque and A. Wilkinson
Proceedings of the Institution of Civil Engineers, Ground Improvement, 161, No. 4, November, 207–216, doi: 10.1680/grim.2008.161.4.207
This paper describes the strength and microstructural behaviour of lime–slag-treated clay. The microstructure was investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the strength of the treated clay was measured using the unconfined compressive strength (UCS) test. Untreated clay was prepared in the laboratory by mixing commercially available kaolin and bentonite, while hydrated lime and ground granulated blastfurnace slag (ggbs) were used as binders. XRD analyses of lime–slag-treated clay showed the existence of numerous reaction products such as calcium silicate hydrate (C–S–H), calcium aluminium silicate hydrate (C–A–S–H), and hydrotalcite (HT), which facilitated the strength increment. For a fixed proportion of lime, the relative intensity of pozzolanic reaction products was found to increase with increasing slag content. Slag was found to be very active to promote the pozzolonic reaction with lime, as evidenced from the presence of crystalline reticular (C–S–H) and platy (C–A–S–H) cementitious products in the SEM images. The role of lime in the activation of slag was also seen to have an upper limit whereby excess proportions failed to provide significant additional benefit. The UCS of lime–slag-treated clay was found to be higher in comparison with lime or slag alone with the same replacement ratio. This was due to the formation of more crystalline cementitious products, which accelerated the bridging (cementation) effect between lime–slag and clay particles.
Challenges in underground construction
Y. Yuan
Proceedings of the Institution of Civil Engineers, Municipal Engineer, 161, No. 1, March, 25–34, doi: 10.1680/muan.2008.161.1.25
Infrastructure construction in mega-cities poses great challenges. Most transportation systems, energy and water supplies, shopping facilities and other daily necessities require underground spaces. However, underground construction in soft ground is demanding and must be carefully undertaken with great attention to temporary works design. Additionally, in areas with a high water table, watertight construction is a primary requirement in the whole-life maintenance strategy. This paper deals with key aspects in tackling such problems. Several recent construction projects in Shanghai are used to illustrate practical solutions to these problems. In dealing with long-span underground spaces, the use of reinforced concrete is compared with a composite construction. The design procedure for supporting deep excavations is based on constructing ‘cut-and-cover’ structures. Early-age cracking in underground structures is prevented by simulation-based design and on-site monitoring. This paper also provides an example of sustainable development for infrastructure through planned maintenance, and inspection and appraisal techniques for existing infrastructure. All of the examples show the technical aspects of the work of an efficient municipal engineer.
Design for a large underground space
Y. Yuan, H.-L. Zhao and X.-F. Bao
Proceedings of the Institution of Civil Engineers, Municipal Engineer, 161, No. 1, March, 35–41, doi: 10.1680/muan.2008.161.1.35
Large underground spaces are required when constructing multifunctional buildings. These constitute one of the major engineering projects for the World Expo 2010 in Shanghai, China. Restrictions caused by a building's function, and requirements for arranging municipal pipelines, mean that options for structural systems are limited. This paper presents a scheme design for a large underground space. To achieve a spacious underground passage and to resist the heavy load of the soil that holds pipelines, major structural systems are taken into consideration. Performance criteria for a scheme design involve safety, serviceability and economics. Subsequently, comparisons of a conventional reinforced concrete (RC) system, a prestressed RC system, and a steel–concrete composite (SCC) system are made with regard to these criteria. It is found that a conventional RC structure cannot provide adequate resistance to heavy load at a large column span. Correspondingly, prestressed RC slabs can replace conventional RC structures with the required structural capacity, but suffer drawbacks because construction space is confined. A composite system is desirable for an underground space with a large span and heavy load. Specifically, it has a convenient construction procedure and high economic efficiency which takes advantage of industrial manufacturing of steel members and reduction of concrete formwork.
