Other Civil Engineering Applications
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OTHER CIVIL ENGINEERING APPLICATIONS The use of iron in foundations Introduction Once cast iron had been established as a useful and practical structural material in the late eighteenth century, it was only going to be a question of time before an enterprising ironmaster, engineer or architect considered its application for substructures. Timber piles and platforms in combination with masonry were the traditional foundation materials, although other expedients such as rammed chalk and fascines had been employed, and in the early nineteenth century concrete began to be used . (Chrimes, 1996; Kerisel, 1956; 1985). Iron itself had been used for specialist applications such as rock foundations (below) and for pile shoes. The application of iron to foundations was a specialist area and even when iron was employed in superstructures, whether bridges, iron frames or roofs, its performance was generally governed by that of substructures built using traditional methods and materials. While the Leaning Tower of Pisa provides an enduring monument to the foundation problems faced by past generations, and towers in Bologna show similar signs of distress, others towers having collapsed completely, mediaeval and renaissance master builders were capable of erecting enduring structures on a scale not regularly surpassed before the twentieth century. The gothic cathedral is perhaps the most spectacular example, but in northern Italy and the Low Countries large civic buildings were erected, while military engineers designed successive generations of fortifications. More the province of the civil engineer were the hydraulic structures erected on the rivers and canals of the Netherlands and Lombardy from the fourteenth century onwards, using timber for bearing and sheet piles, lock walls, gates and floors, in combination with masonry and (pozzolanic) mortars. (Skempton, 1957 repr Chrimes 1998). Fascine work was traditionally used to stabilise coastal defence works. Our knowledge of the foundations of the medieval and early modern period is based on surviving documentary evidence (Brown, 1963; Parsons, 1939), including some specifications (Salzman, 1952) archaeological evidence, and the discoveries of those who have had to deal with surviving structures. Price, Willis (R. Willis 1972-1973) Architectural history of some English Cathedrals, 2 parts, Chicheley: P B Minet,), Viollet le Duc, F. Fox. Recent conferences have discussed some of the problems presented by older structures to modern geotechnical engineers. As an example at Amiens cathedral there was a raft of stones set in mortar on which a grid of masonry walls and stepped piers supporting the main structural columns rested. Appropriate good practice would have been adopted elsewhere although one suspects few modern engineers would accept such a definition for the foundations of the tower at Salisbury Cathedral. The main columns there rest upon stone slabs founded on medium dense gravel excavated to a depth of 5ft - just above the summer water table - the gravel rests upon chalk, and the load on the slabs is 10 tons/ft2. In a local context builders and artisans would have been aware of the limitations of local ground, and developed the necessary expertise. Although piling engines were rare, the London Bridge engines being lent for other work in the late mediaeval period, they were in use, and for some mediaeval projects there is considerable knowledge on how foundations were installed (Boyer, 1981-1985). In the early mediaeval period bridge pier foundations were generally built as artificial islands using loose masonry confined by piles on which a levelled platform could be formed above water level-London Bridge was erected on such ‘starlings’. On the continent cofferdams were in use by the fifteenth century as a means of excluding water and building up from the river bed in the dry; such techniques are known from early printed works such as Ramelli (1588), but it is unclear when they were first used in England. However, by the start of the eighteenth century in England the number of river improvement and land reclamation schemes was such that there were some craftsmen practising who described themselves as ‘water carpenters’, expert in the installation of sheet piles, locks and weirs. The Swiss engineer Charles Labelye introduced timber caissons at this time as an alternative to cofferdams for the foundations of Westminster Bridge (Walker, 1979) and in the second half of the eighteenth century this emerged as the most economical method of subaqueous foundations for bridge piers. At Westminster the masonry caisson was placed on a prepared dredged bed, and the masonry for the piers built up on the caisson bottom, but concerns over the performance of the foundations meant that at Blackfriars piles were driven beneath the site of the caisson before it was placed. In this technique the caisson sides would be reused for successive piers. By the end of the eighteenth century such techniques had become obsolescent as more efficient steam pumps became available, cofferdam construction techniques improved, and the need for economy was less pressing (Ruddock, 1979). Caissons were also used in harbours as instanced by Smeaton at Ramsgate in the 1790s. With the changing nature of warfare retaining wall design became a major consideration for the military engineer. Although this aspect of military architecture is most commonly associated with work of the French engineer Vauban at the end of the seventeenth century, one of the earliest English military treatises, by Paul Ive, discusses proportions for retaining walls, as well as the use of piling (Ive, 1589). If foundation engineering was essentially a practical science down to 1700, from then onwards, particularly in France there was increasing consideration given to providing a more theoretical foundation for the design of arch bridges, including their abutments, piers, and retaining walls. These developments have been summarised by Heyman (1972). They were accompanied by some practical experiments on earth pressure, which continued into the early nineteenth century (Field, 1948; Skempton, 1977). More general reviews of the advances in the understanding of soil mechanics are provided by Skempton (1977, 1985). While one can doubt the influence of theory on the local contractor at a time when contracts where still generally given on a trade basis, by the end of the eighteenth century, when iron was being introduced, engineers like Telford and Rennie are known to have possessed a number of continental textbooks, which would also have been available to military engineers. Published works like those of Meyer (1685), Perronet (1782-1789) and De Cessart (1806) provide illustrations of foundation techniques of the time. Jensen ( 1969) was able to draw on these to provide a useful summary. Perronet and De Cessart also provide detailed records of construction experience. For some idea of British practice one can refer to Smeaton’s reports (Smeaton, 1812), Cresy’s Encyclopaedia of civil engineering (Cresy, 1847), and Hughes’ Essay on bridge foundations for Weale. Hughes’ work is of particular interest as he was a second generation civil engineering contractor whose family had worked for Telford and Rennie. For ordinary masonry walls, and column piers stepped brickwork would be normal to help spread the load [fig.__], accompanied as appropriate by piling and/or a timber platform. Between the pile heads it was normal to ram layer of rubble. It was important the steps were not too broad or there was a danger the concentrated load would shear off the step below. Such methods were carried over into iron supported structures such as that illustrated here at the Tobacco Dock warehouse London Docks (Mitchell). Such methods were not always successful as a brick footing capped with Yorkshire stone failed, according to a London iron founder George Cottam, c.1830. The iron column it was supporting passed through the stone and on through its brickwork support [fig. ___]. The most likely explanation could be a weakened slab, and a footing of brick encased ‘rubbish’, with no solid bonded brick core. London Bridge (Nash, 1973; 1981) can be seen as indicative of best practice in bridge foundations at the start of the nineteenth century and can be compared with that of the iron bridges at Southwark and Tewkesbury [figs. ____]. By that time, in contrast to half a century earlier, major contractors existed such as Hugh McIntosh (Chrimes) and Jolliffe and Banks (Dickinson). Such organisations would have had considerable expertise in construction, and, allowing for commercial pressures and the occasional incompetent agent, they were unlikely to install inappropriate foundations unless instructed by the engineer. Foundation engineering seems to have remained their province, if one can judge by the lack of textbooks which appeared on the subject through the nineteenth century, Dobson being a solitary exception. Site investigation was still, however, in its infancy in terms of instrumentation, and it was difficult to obtain uncontaminated samples in soft ground [fig. ____]. Use of iron in foundations and substructures Turning to the introduction of iron, timber’s susceptibility to decay, particularly in exposed marine locations meant any economical alternative was likely to be considered seriously. Availability of reliable quality ironwork at economical prices, facility of fabrication and installation, perceived advantages of durability, and relative strength of the material will all have played a part in the adoption of iron for foundations. Perhaps the most obvious application would be the use of hollow circular castings for piles, but plate iron could be used for sheet pile work - accurate driving of timber dovetail piles which had been used for centuries must have been very difficult - and iron could also be used for ties and anchorages. Timber piles were unsuitable for hard driving, and iron offered a possible practical alternative - assuming the use of piles was appropriate at all. Larger diameter ‘cylinders’ could be fabricated and be used for bridge piers and as caissons, later making use of compressed air work.