Temporary Props Research Articles

Construction of a deep basement with a single-level prop – prop forces and wall movements

This paper describes the construction of a deep basement in central London. The construction sequence for the basement was a combination of top-down and blue-sky excavation to enable phased delivery of the project. Temporary single-level props enabled blue-sky construction by managing ground movements. Ground movements associated with the basement construction and impact on third-party assets were monitored to validate the design using three-dimensional targets and inclinometers. The ground movements were predicted at each stage of construction using simple models with more complex three-dimensional soil–structure finite-element analysis used when examining the whole basement behaviour under the temporary and permanent loads. Temporary prop loads, and thermal loads on props were also monitored during the bulk digging. The small strain constitutive models for London Clay was shown to closely predict the observed movements.

Refurbishing the historic Old Tweed Bridge, Scotland – the challenges and innovations

The historic Old Tweed Bridge is a three-span listed masonry arch crossing the River Tweed. Structural integrity concerns resulted in the bridge being temporarily closed to the public and propped. This paper discusses the challenges associated with the construction works necessary to refurbish the bridge permanently. The works involved installing a temporary propping and encapsulation structure, excavating the bridge infill, taking down and rebuilding the walls, casting a concrete saddle above the arches, waterproofing and resurfacing. Other innovative works involved closing the gap between the voussoirs and arch barrel using a bespoke arch pulling system and repairing voussoir stones in situ with a lime-based material. There were significant environmental challenges on the project as the bridge is located in an ecologically sensitive area. The River Tweed is one of the UK's great salmon rivers, which restricted working in the river and required strict pollution controls. Alternative roosts for bats were provided and netting was required around the bridge as part of an advanced works contract to mitigate the risk of birds nesting in the bridge.

Benefits of pre-loading temporary props for braced excavations

Using soil profiles typical of excavations in London, UK and Dubai, UAE calculations have been carried out to compare the performance of temporary steel and proprietary hydraulic propping. Numerical modelling indicates that pre-loading allows the lower-stiffness hydraulic prop to match the performance (in terms of prop load and wall displacement) of a higher-stiffness steel prop, without pre-load. The low prop stiffness also helps to mitigate loading induced by thermal effects. Furthermore, where limiting the horizontal deflection of the wall is critical, the application of a calibrated pre-load can improve the economy of the temporary works by reducing the prop size. Pre-loaded props are sometimes associated with increased bending moment and shear force values in the retaining wall. The finite-element models produced for this paper prove that for the cases considered, the use of pre-loaded low-stiffness hydraulic props achieved the same performance as stiff steel props, with no impact on the wall’s reinforcement. Further work is suggested to explore how manipulating and varying the pre-loading values in multipropped basements might be used to improve further the performance and/or the economy of the retaining wall and how this might affect the load transfer from temporary to permanent supports in the longer term.

Temporary propping at Crossrail Paddington station

The construction of Crossrail Paddington station in London (between 2011 and 2018) required a large temporary propping system, which contained one of the most comprehensive monitoring schemes to date. This paper explores the design and operation of this system through the analysis of a data set collected during the construction period and the numerical modelling undertaken at Durham University. The effects of temperature variation on the partially exposed propping system are quantified, as well as the impact of events in the construction sequence including slab casting, adjacent prop removal and excavation. Several lessons were learned during the construction, in terms of prop installation and removal and the management of the monitoring system. These are discussed for the benefit of future projects using temporary propping and monitoring schemes. The findings from this research have led to a greater understanding of the behaviour of propping systems under different environmental conditions, and may therefore lead to more efficient and safer designs in the future.

Construction of the King's Cross northern ticket hall, London, UK

The recent completion of Network Rail's new western concourse at King's Cross station is the culmination of over 12 years of above and below ground station modernisation work that has transformed rail transport links in this part of north London, UK. The new concourse has been constructed directly above London Underground's northern ticket hall, which was the second and last phase of the redevelopment of its King's Cross St Pancras station. The construction of the northern ticket hall was a major undertaking and ultimately provided the support structure for the western concourse. The new ticket hall greatly enhances the overall capacity of the underground station and provides a second means of access to the Northern, Piccadilly and Victoria lines while also incorporating below ground links to both the King's Cross and St Pancras main line stations and to the enlarged Tube ticket hall. This paper describes the overall construction methodology of the main box forming the ticket hall with particular reference to the techniques used for bulk excavation, installation of plunge columns and the use of temporary propping.

Application of observational method at Crossrail Tottenham Court Road station, UK

Excavation of the western ticket hall box at Tottenham Court Road station represented a critical path activity of the Crossrail railway project in London, UK. Base slab construction and preparatory sprayed concrete lining works needed to be completed before the arrival of the two tunnel boring machines. A full observational method (OM) design was implemented to eliminate the lowest level of temporary propping, resulting in a 13 m prop-free excavation at the bottom of the 30 m deep diaphragm wall station box. This paper describes the design and successful implementation of the OM, which eliminated the need to fabricate, install and remove the fifth level of temporary propping. It covers the comprehensive review of the original design input parameters, back analysis of the most probable geotechnical design parameters and comparison of wall deflection and forces in the temporary props from early stages of the construction works. The robust strategy formulated as part of the observational method design to control the subsequent excavation is described: this included trigger criteria, a review process and predetermined contingency measures to ensure safe execution of the excavation works. A genuinely collaborative effort between client, permanent and temporary works designers, independent design checker and the contractor is elaborated. Cost and programme savings achieved by implementation of the observational method approach are outlined.

Concise and informative title: innovative geotextile materials for the extension of urban green space—contribution to urban sustainability

Modern Movement architecture employed new materials to deal with social challenges and industrial production. Today environmental problems are the challenge. Resilience to natural hazards, including climate change, is such one. Geotextiles are employed in large landscape (ash/garbage deposits, river sides), but planting on building surfaces is possible. We focus on green walls/roofs for emergency housing, including reshaping of temporary propping systems. Green walls allow integrating parcels remaining empty after the collapse of buildings through Pocket Parks. The earthquake impact becomes urban restructuration occasion. A special support skeleton for the earth&seeds&geotextile in green wall installations will be designed, and later pattented.

Design and performance of retaining walls

Embedded retaining walls for 1·8 km of cut-and-cover tunnels and earth-retaining structures were redesigned during the construction period to optimise construction methods and temporary propping. The design approach included many of the developments now recommended in Ciria report C580, and the paper summarises the experience gained on this contract. The site team made detailed observations of the performance of all 14 structures during construction, and this paper gives an overall summary of the wall movements and prop loads that were measured. The walls were surcharged, and none of the props was preloaded. Despite this, the maximum movements were within those estimated from published correlations. Wall movements were time dependent, and occurred at rates of up to 0·2 mm/day. Prop loads were generally about 40% of the values obtained from moderately conservative calculations. Reducing the prop stiffness assumed in calculations improves agreement, and measurements are reported that provide a basis for closer appraisal of this aspect in future designs.

Contributions to Géotechnique 1948–2008: Retaining structures

Retaining structures and earth support systems form an important part of geotechnical engineering. In the first 60 years of Géotechnique, some 110 publications have appeared that related to such structures. These are discussed in this review under five headings: case histories and full-scale observations; ground movements associated with retaining structures; numerical analyses; compaction stresses acting on structures; and Symposia in Print. Individual comments are made on approximately half of the papers published, highlighting key contributions to the advancement of the understanding of retaining structures. Many of the papers have provided important information subsequently incorporated into standard reference documents for geotechnical engineers, notably three reports published by the Construction Industry Research and Information Association (CIRIA): Temporary propping of deep excavations: Guidance on design; The observational method in ground engineering: Principles and applications; and Embedded retaining walls: Guidance for economic design. In this way, and through the contributions of individual papers to other aspects of performance, Géotechnique has made a valuable contribution to the improved analysis, engineering design, construction and monitoring of retaining structures.

Open-ended participatory design as prototypical practice

This article argues in favour of seeing co-design as an open-ended exploration where prototypical practices are explored that engender favourable conditions for ongoing negotiation of meaning. Participatory design approaches to designing for specific practices are reviewed with particular focus on how to handle constantly evolving practices, where some design researchers argue for creating open and flexible technical systems while others emphasise design as primarily concerned with questions of changing practices. By discussing an extended participatory design project in which new ways of engaging in informal learning through self-produced videos were explored in an intensive care unit, I argue first and foremost for viewing co-design as prototypical practice which is explored through an open-ended exploration of possibilities. Second, I argue that a focus on practice necessarily requires in situ explorations to see if the proposed design explorations invoke relevant prototypical practices in the midst of work. Third, I argue that a focus on practice entails viewing tools as temporary props for various settings, rather than as central features that define the settings of learning, knowing and working.

Reducing damage to concrete stitches in bridge decks

In bridge widening projects, it is a common practice to provide a monolithic connection between the existing and new decks by casting an in situ concrete stitching slab. Due to the difficulties associated with complete closure of the bridge, traffic flow has to be maintained on the existing deck. As a result, the existing deck will deflect under traffic load and the concrete stitching slab will be subjected to differential deflection, which is cyclic in nature. Such cyclic differential deflection hinders strength development of the concrete and may affect the structural integrity of the stitching slab. Some possible methods of reducing the differential deflection so as to mitigate damage to the curing concrete stitches have been proposed, but no systematic comparative studies have been carried out. These methods include traffic restriction, provision of temporary shear connections, provision of temporary propping and segmental concrete stitching. This paper evaluates the relative effectiveness of these mitigation measures as well as their different combinations by conducting grillage analysis of the widening bridge deck at various stages of construction. Based on the results, it is advocated that among these, the combination of temporary shear connection and segmental concrete stitching should be the most practicable and effective.

Construction monitoring of cut and cover tunnels

Over 700 instruments were installed to monitor the performance of the cut and cover tunnels that carry the Channel Tunnel Rail Link through Ashford. Careful monitoring confirmed design assumptions and provided a sound basis for changes to the construction sequence that accelerated the works. This paper highlights key features of the monitoring system. It describes the instruments that were installed, reviews their performance, and identifies practical issues that were addressed during the contract. The results help to quantify toe movements of embedded retaining walls. They identify variations in temporary prop loads that are explained by reference to the method of excavation and changes in prop stiffness during initial loading. Prompt reporting and open access to the monitoring data developed confidence and trust between the project partners, and led to closer interaction and greater efficiency.

Up–up construction takes off in Korea

An innovative deep-basement construction technique pioneered in Korea is proving highly beneficial and looks set for adoption worldwide. Experience on over 40 deep-basement building projects shows that it cuts construction times and costs by 15–20% and significantly reduces ground movements and personal injuries. The technique eliminates the need for installing and removing traditional temporary propping systems by using a permanent steel basement framework to support lateral soil loads during excavation. When the excavation and framework reach the bottom, upward construction of the basement and superstructure start simultaneously—hence the description ‘up–up’. This paper describes the first project on which the technique was used and sets out its many advantages over temporary propping systems as well as alternative top–down construction techniques.

The Copenhagen Metro—observational method at Norreport station

The Copenhagen Metro includes six cut and cover stations located in the centre of Copenhagen, the deepest of which is Norreport. The observational method was used to eliminate a level of temporary propping at Norreport station in order to maintain the station excavation programme to suit the arrival of the tunnel-boring machine. This paper describes the development of the monitoring and control system associated with the use of the observational method by a team including the client, the permanent and temporary works designers and construction team members.

Discussion: Measurement of temporary prop loads at Mayfair car park

Discussion: Measurement of temporary prop loads at Mayfair car park

The Copenhagen Metro?observational method at Norreport station

The Copenhagen Metro includes six cut and cover stations located in the centre of Copenhagen, the deepest of which is Norreport. The observational method was used to eliminate a level of temporary propping at Norreport station in order to maintain the station excavation programme to suit the arrival of the tunnel-boring machine. This paper describes the development of the monitoring and control system associated with the use of the observational method by a team including the client, the permanent and temporary works designers and construction team members.

Measurement of temporary prop loads at Mayfair car park

Measurement of temporary prop loads at Mayfair car park

Measurement and analysis of temporary prop loads at Canary Wharf underground station, east London

The loads developed in tubular steel temporary props during the construction of the London Underground Jubilee Line Extension station at Canary Wharf were monitored using vibrating wire strain gauges. Prop temperatures were also measured to investigate the fluctuation of load with temperature. In this paper, the measured prop loads are related to temperature, the excavation process and certain construction details. They are then compared with the loads calculated using modified limit equilibrium calculations and a series of parametric finite-element analyses. Prop loads reasonably close to those measured can be calculated using either technique, but certain discrepancies between the two calculations in terms of the detail of the earth pressure distributions acting on the wall require further research.

Comparison of measured and calculated temporary-prop loads at Canada Water Station

The loads developed in tubular-steel temporary props during the construction of the London Underground Jubilee Line Extension (JLE) station at Canada Water were measured using vibrating-wire strain gauges. Prop temperatures were monitored and their influence on the prop loads assessed. Wall movements were also measured, by means of inclinometers. In this paper, the temperature-normalized prop loads are compared with the results of modified limit equilibrium calculations. Prop loads and wall movements are compared with the results of a series of parametric finite-element analyses carried out using the program CRISP. On the basis of the finite-element analysis results, the design assumptions giving the closest correlation between measured and calculated prop loads and wall movements are identified.

Measurement of temporary prop loads at Mayfair car park

The construction of an underground car park in Park Lane, London using diaphragm walls has provided a further opportunity to investigate the development of loads in temporary props during construction of a braced excavation. Two transverse and two corner props in one half of the symmetrical excavation were monitored using vibrating wire strain gauges incorporating thermistors. The prop temperatures were also monitored independently using an additional set of thermistors attached directly onto the steel prop. The prop loads are presented both with and without correction for temperature effects and are compared with the design loads, which significantly underestimated the effects of temperature on prop loads. The horizontal force parallel to the wall generated by the corner props is assessed and its implications for the design of waling systems are discussed.