Progress from a qualitative to a quantitative science

It was in the seventeenth century that the quantitative science of hydrology was founded by Palissy, Pérrault and Mariotté in France, Ramazzini in Italy and the astronomer Halley in Britain.

Palissy, a sixteenth century potter and palaeontologist, stated that rain and melt snow were the only source of spring and river waters; and that rain water percolates into the earth, following ‘a downward course until they reach some solid and impervious rock surface over which they flow until they make some opening to the surface of the earth.’

Pérrault made rainfall run-off measurements and demonstrated the fallacy of the long held view that the rainfall was not sufficient to account for the discharge from springs. He also measured and investigated evaporation and capillarity. Mariotté verified Pérrault’s results and showed that the flow from springs increased in rainy weather and decreased in times of drought. Halley made observations of the rate of evaporation from the Mediterranean ocean and showed that this was adequate to supply the quantity returned to that sea by its rivers. His measurements of evaporation were conducted with considerable care but his estimates of stream flow were very crude.

Towards the end of the eighteenth century La Metherie extended the researches of Mariotté and brought them to the attention of meteorologists.

He also investigated permeability and explained that some rain flows off directly (surface water run-off), some infiltrates into the top soil layers only and evaporates or feeds plants, whilst some rain penetrates underground whence it can issue as springs (i.e. infiltration or groundwater recharge).

This is the first recorded mention of ‘permeability’ and so is the first link between hydrology and seepage to wells.

2.3.1 Seepage towards wells

The robust Newcomen engine greatly influenced mining practice during the eighteenth century but it was far too cumbersome for construction works.

Generally speaking, until the early nineteenth century civil engineers, by the use of timber caissons and other devices, avoided pumping whenever possible.

However, where there was no alternative pumping was usually done by hand – a very onerous task – using a rag and chain pump (Fig. 2.1), known also as ‘le chaplet’ (the rosary).

Some idea of the magnitude of the problem is given by de Cessart in his book Oeuvres Hydrauliques. Speaking of the foundations for the abutment of a bridge at Saumur in 1757, he says that forty-five chain pumps were in use, operated by 350 soldiers and 145 peasants. Work on this type of pump was, of course, most exhausting, and the men could only work in short spells.

Pryce, in his Mineralogea Cornubiensis in 1778, said that work on pumps of this sort led to a great many premature deaths amongst Cornish miners.

For permanent installations such as graving docks large horse-driven chaplet pumps were used. Perronnet, the famous French bridge builder, made use of elaborate pumping installations in the cofferdams for the piers of his larger bridges; for example, under-shot water wheels were used to operate both chaplet pumps and Archimedean screws.

According to Crèsy, writing in 1847, the first engineer to use steam pumps on bridge foundations was Rennie, who employed them on Waterloo Bridge in 1811. In the same year, Telford, on the construction of a lock on the Caledonian Canal at Clachnacarry, at first used a chain pump worked by six horses, but replaced it by a 9 horsepower steam engined Figure 2.1 Rag and chain pump, manually operated. 1556 (from Bromehead 1956). The balls, which are stuffed with horsehair, are spaced at intervals along the chain and act as one-way pistons when the wheel revolves.

pump. From then on steam pumps were used during the construction of all the principal locks on that canal. In 1825 Marc Brunel used a 14 horse-power steam engine when sinking the shafts for the Thames Tunnel. By this date steam pumping seems to have been the common practice for dealing with groundwater and so below ground excavations for construction were less problematical.

2.3.2 Kilsby tunnel, London to Birmingham railway

There appears to have been no important advance in pumping from exca-vations until the construction in the 1830s by Robert Stephenson, of the Kilsby Tunnel south of Rugby, on the London to Birmingham Railway. He pumped from two lines of wells sited parallel to and either side of the line of the tunnel drive (Fig. 2.2).

It is clear from Stephenson’s own Second Report to the Directors of the London, Westminster and Metropolitan Water Company, 1841, that he was the first to observe and explain the seepage or flow of water through sand to pumped wells. The wells were sited just outside the periphery of the con-struction so as to lower the groundwater level in the area of the work by pumping from these water abstraction points. This is most certainly the first

Figure 2.2 Pumps for draining the Kilsby tunnel (from Bourne 1839: courtesy of the Institution of Civil Engineers Library). A pumping well is shown in the fore-ground, with the steam pumphouse in the distance.

temporary works installation of a deep well groundwater lowering system in Britain, if not in the world. The following extract (from Boyd-Dawkins 1898, courtesy of the Institution of Civil Engineers Library) quotes from the report and shows that Stephenson had understood the mechanism of groundwater flow towards a pumping installation.

The Kilsby Tunnel, near Rugby, completed in the year 1838, presented extreme difficulties because it had to be carried through the water-logged sands of the Inferior Oolites, so highly charged with water as to be a ver-itable quicksand. The difficulty was overcome in the following manner.

Shafts were sunk and steam driven pumps erected in the line of the tun-nel. As the pumping progressed the most careful measurements were taken of the level at which the water stood in the various shafts and bore-holes; and I was soon much surprised to find how slightly the depression of the water-level in the one shaft, influenced that of the other, not with-standing a free communication existed between them through the medium of the sand, which was very coarse and open. It then occurred to me that the resistance which the water encountered in its passage through the sands to the pumps would be accurately measured by the angle or incli-nation which the surface of the water assumed towards the pumps, and that it would be unnecessary to draw the whole of the water off from the quicksands, but to persevere in pumping only in the precise level of the tunnel, allowing the surface of the water flowing through the sand to assume that inclination which was due to its resistance.

The simple result of all the pumping was to establish and maintain a channel of comparatively dry sand in the immediate line of the intended tunnel, leaving the water heaped up on each side by the resistance which the sand offered to its descent to that line on which the pumps and shafts were situated.

As Boyd-Dawkins then comments:

The result of observations, carried on for two years, led to the conclusion that no extent of pumping would completely drain the sands. Borings, put down within 200 yards [185 m] of the line of the tunnel on either side, showed further, that the water-level had scarcely been reduced after twelve months continuous pumping and, for the latter six months, pumping was at the rate of 1,800 gallons per minute [490 m³/h]. In other words, the cone of depression did not extend much beyond 200 yards [185 m] away from the line of pumps.

In this account, … it is difficult to decide which is the more admirable, the scientific method by which Stephenson arrived at the conclusion that the cone of depression was small in range, or the prac-tical application of the results in making a dry [the authors would have

used the word ‘workable’ rather than ‘dry’] pathway for the railway between the waters heaped up [in the soil] … on either side.

It is astonishing that neither Robert Stephenson, nor any of his contem-poraries, realized the significance of this newly discovered principle. That by sinking water abstraction points, and more importantly placing them clear of the excavation so that the flow of water in the ground will be away from the excavation rather than towards it – stable ground conditions were created. For many decades this most important principle was ignored.

2.3.3 Early theory – Darcy and Dupuit

In the 1850s and early 1860s Henri Darcy made an extensive study of the problems of obtaining an adequate supply of potable water for the town of Dijon. He is famous for his Darcy’s law (Darcy 1856) postulating how to determine the permeability of a column of sand of selected grading, know-ing the rate of water flow through it but this formed only a small part of his treatise. He compiled a very comprehensive report (two thick volumes) in which he analysed the available sources of water from both river and wells – some of them artesian – and how economically to harness all these for optimum usage.

Darcy investigated the then current volume of supply of water per day per inhabitant for about ten municipalities in Britain – Glasgow, Nottingham and Chelsea amongst others – as well as Marseille and Paris, and many other French towns. He concluded that the average water provision in Britain was 80–85 l/inhabitant/day and more than 60 l/inhabitant/day for Paris. Darcy designed the water supply system for Dijon on the basis of 150 l/inhabitant/-day – no doubt his Victorian contemporaries this side of ‘la manche’ would have applauded this philosophy.

In the mid-1860s Dupuit (1863), using Darcy’s law to express soil perme-ability, propounded his equations for determining flow to a single well positioned in the middle of an island. Dupuit made certain simplifying assumptions and having stated them (i.e. truly horizontal flow) then dis-counted their implications! For this Dupuit has been much castigated by some later purists but most accept that the Dupuit concept, latter slightly modified by Forcheimer, is acceptable and adequate in many practical situations.

Exchange of information was not as simple in Darcy’s and Dupuit’s time as it is now. Much of the fundamental work of these two French engineers was duplicated by independent developments shortly afterwards in Germany and Austria and a little later, in the United States.

About 1883 Reynolds demonstrated that for linear flow – that is, flow in orderly layers – commonly known as laminar flow, there is a proportional-ity existing between the hydraulic gradient and the velocproportional-ity of flow. This is in keeping with Darcy’s Law but as velocity increased the pattern of flow

becomes irregular (i.e. turbulent) and the hydraulic gradient approached the square of the velocity. Reynolds endorsed the conclusion that Darcy’s Law gives an acceptable representation of the flow within porous media – that is the flow through the pore spaces of soils will remain laminar, save for very rare and exceptional circumstances. However, this may not always be true of flow through jointed rocks (e.g. karstic limestone formations).