Problems of making suitable foundations in waterlogged ground, of strengthening faulty foundations and of treating of leaky tunnel linings have all been solved by various ingenious processes
IN THE SEVERN TUNNEL a length of nearly 2,000 yards of tunnel lining was treated by the Francois Cementation process, to prevent the seepage of water. A special drilling carriage, running on the two inner rails of the double track, was used for drilling the holes for the injection of chemicals and cement. This process is frequently used for consolidating work in waterlogged ground.
IN recent years there have been many remarkable systems evolved for dealing with difficult foundation problems. In general the engineer’s greatest bugbear is the control of water, and many devices have been invented for surmounting the difficulty of making secure foundations in waterlogged ground.
One of the most remarkable of these systems is the freezing process, whereby the portion of ground through which it is desired to drive a shaft or tunnel is completely frozen. Excavation then proceeds through a solid mass of ice. Work in such material is no more difficult, sometimes much easier, than penetrating firm rock.
This method was adopted with great success for sinking the shafts in connexion with the cooling water tunnels of the Tir John Power Station (see the chapter “Britain’s Electric Power Supplies”) at Swansea. Two shafts were sunk by this method, one at Queen’s Dock and the other at King's Dock. A special plant was designed and built for freezing the ground and its principle of operation was exactly the same as that of an ordinary refrigerator, although on a much larger scale.
The freezing plant consisted of three large ammonia compressors driven by powerful electric motors, circulating ammonia through a huge refrigerator. This refrigerator was filled with brine solution, which was circulated through a number of pipes in boreholes sunk round the outside of the shaft. This gradually lowered the temperature of the ground until a complete ring of ice was formed through which the water could not penetrate.
The work was carried out with wonderful efficiency, the soft, unfrozen core in the centre of the shaft being excavated without any difficulty. The pipes through which the freezing solution was circulated were spaced about three feet apart, as this proved the most economical distance. If the pipes had been too close together the cost would have been higher, as much of the central core would have been frozen.
The process used is known as the Foraky process, and was invented and perfected in Belgium. It has been used for many different purposes, notably in coal mines for sinking shafts through water-bearing ground. It requires expert attention to give the best results and has proved its worth again and again.
Another method of dealing with water is known as the “deep well system of ground water lowering”, which is used extensively by John Mowlem & Co., Ltd. It consists of dealing with water in the ground by pumping from special bored wells fitted with filters. In other words, the object is to overcome the water problem before any excavation work is started. The first stage in applying this method is to bore a number of wells at certain definite intervals in the area where foundation work has to be carried out in the dry. These wells are equipped with special filters before the boring tubes are withdrawn, so that it is impossible for earth or sand to enter the boreholes and clog the pumps. Each pump is a complete unit in itself, a wonderful example of compact design, when it is considered that the borehole may have a diameter of not more than 12 in. Both pump and electric driving motor are situated at the bottom of the borehole.
The effect of pumping from a number of wells in this manner is to lower the general water level over the whole area keeping it at the new level for as long as the pumps are working. It is a simple and reliable method of dealing with water.
The same firm uses also a remarkable system of chemical consolidation, which is extremely useful for dealing with soft rock and bad ground. The invention of Dr. Hugo Joosten, it consists of the injection of two chemical solutions, one after the other, into gravel, ballast, sand or similar ground. These chemicals com bine immediately on contact, so that the ground is bound together in a solid mass.
It would be no exaggeration to say that in using this method man is doing instantaneously what Nature does in thousands of years. In other words, he is making artificial rock, as the chemical action is similar to that which forms sandstone and limestone. The chemicals are injected through perforated pipes driven into the ground to be treated, each pipe being driven to a predetermined distance in a number of short equal steps. A carefully measured quantity of the first chemical is then injected at each position of the pipe. As it is sticky, the chemical coats and flings to the sandy particles.
This process is then repeated with the second chemical. The two chemicals instantly combine to form the manmade rock which is impervious to water. Some amazing work has been carried out by this system in London and elsewhere. For instance, during the building of Monument Station (District Railway, London) it was necessary to drive a 26-feet diameter tunnel through bad ground. The loose ballast above the tunnel arch was therefore solidified by this means, and the excavation was carried out with the greatest of ease and in perfect safety.
A combination of chemical consolidation and ground-water lowering was adopted in the rebuilding of a large store at Kingston-on-Thames, Surrey. Here the problem to be solved consisted of underpinning the foundations of the building while the premises were in full business occupation. The walls and piers of the building were therefore supported by solidifying the ballast, after which the ground water was lowered by 12 feet over the site. The walls and piers were then underpinned and there was placed in position a huge foundation slab strengthened by stout steel The shopping area was available for five months longer than if demolition had taken place at the start.
Another wonderful system is known as “cementation”. Cementation is the injection of cement, lime, asphalt, bitumen, various chemicals and various fillers, separately or in conjunction, into the ground for the purpose of consolidating it, and rendering it impermeable. It includes grouting, petrification and silicatization. There is a method of treatment for any class of ground in any condition. The cementation process has been perfected by the Francois Cementation Company, of Doncaster, which carried out work on St. Paul’s Cathedral, London.
The silicatization process consists of the injection of chemicals — generally silicate of soda and sulphate of alumina — in conjunction with the injection of cement. The remarkable ingenuity of this system lies in the fact that the chemicals are used to seal the finest fissures in the rock, in addition to lubricating them and making easier the passage of cement. This means that the cement can be injected at a lower pressure than would otherwise be necessary.
PREPARATIONS FOR PRESSURE PILING at Pigeon House Power Station, Dublin. A borehole of 13-in. diameter is first made and lined with steel tubes. Steel reinforcement is introduced before concrete is admitted to the tubes. Air pressure then forces down the concrete pile and forces up the tubes.
The method has been used for the sinking of many colliery shafts in Great Britain, where great trouble has been experienced in dealing with considerable influxes of water. The extraordinarily searching effect of the chemicals, their partial consolidation of porous ground and the complete consolidation effected by the later cement injections reinforced the shafts and made sinking them a simple matter.
Another outstanding example of the application of this process is provided by the famous Severn Tunnel, which was one of the most difficult and dangerous engineering works ever carried out. The Francois process was used recently for consolidating the tunnel lining and preventing inrushes of the river water. A length of nearly 2,000 yards was treated, and general seepage and direct entry of water were completely sealed off. This remarkable piece of work occupied a period of twelve months and yet the train service was not affected during the whole of this time. A special drilling carriage was used for drilling the holes in the crown of the tunnel. The carriage travelled on the inner rails of the double track.
Gigantic Underground Barrier
One of the most troublesome types of cementation work is that required for the foundations of reservoirs, and here again the Francois process has been widely used. Apart from the loss of revenue due to lack of water, the insidious effects of leakage are well known to all engineers. Many of the major dam disasters of recent years have been due to this menace.
An excellent example of the process as applied to a large dam is afforded by the Camarasa Reservoir, in Spain. Here the concrete dam impounding the water is 500 feet long and 300 feet high. The reservoir is 14½ miles long and the greater part of it is in limestone. Before cementation was begun there was a leakage of 214 million gallons a day, equivalent to the requirements of a large city. By the Francois method there was built up a gigantic underground barrier, having an average depth of nearly 1,000 feet and a length of one mile. In addition, Francois methods of piling can be used in conjunction with piling or independently.
A great amount of foundation work is now carried out by piling, and many ingenious systems of pile driving have been invented and patented during recent years. The aim is piling without vibration, so that piles can be driven by pressure alone, without the use of a hammer.
One of the most ingenious of these systems is that used by the Pressure Piling Co., Ltd. In this system a boring with a diameter of 13 in. is made for each pile, the hole being lined with steel tubes as the boring proceeds. The boring apparatus is simple, being similar to that used in well sinking. The steel tubes sink into the ground by their own weight without the use of driving gear, thus avoiding vibration. The excavated ground is removed from the tubes in the same way as for well boring.
This absence of vibration is an important feature in underpinning old buildings and ancient monuments, many of which may be in imminent danger of collapse. For forming the concrete pile in the ground, steel reinforcement is first lowered into the tubes. Then concrete is admitted to the tubes and air pressure is applied. The concrete is thus forced down into the ground and the tubes are forced upwards.
In this way the concrete under the air pressure forms “collars” at any weak point in the ground; thus the bearing power of the pile is increased and definite assurance of strength is given. As each section of tube is raised above ground it is unscrewed; then more concrete is added.
DAHREN PILING APPARATUS has been in general use for some years in Sweden. The pile is driven into the ground by the action of four screws (C) operated by an electric motor. The illustration shows the process of underpinning an existing building. The apparatus is bolted to the existing foundation at E. The pile (A) is built of 6-feet sections, connected by bands (one shown at B). A winch connected to the gearbox (D) operates the cable connected to the pile hammer over a pulley. The pile hammer, working inside the pile, strikes directly on the pile shoe. The hammer is used to assist the screws when harder strata are reached.
A striking adaptation of this method of piling is that which has been designed for piling in waterlogged ground. The general arrangement consists of a cylindrical container, with an opening in the top for admitting the concrete, and a large valve in the bottom for releasing it. The first procedure is to blow out the existing water in the tube with air, after which sufficient air pressure is maintained in the tube to keep the water at bay. Concrete is then filled into the container at the top and the top is closed. Compressed air is next admitted to the container until the pressure therein balances that in the pipe. Then the lower valve of the container is opened and the concrete is allowed to flow into the bore tube. By an increase in the air pressure the concrete is forced down and out as the tube is raised.
By this simple and effective means the concrete is thus formed in the dry. There is no danger of its being contaminated by the water in the subsoil. An interesting example of the use of this system of piling was afforded by the Knightsbridge Underground Station (London), where the piles were used for supporting a number of column bases. By the use of this method a great deal of unnecessary excavation and pumping in waterlogged ground was avoided.
Another ingenious system of piling has been in use for some years in Sweden. This is known as the Dahren system. The general arrangement consists of four powerful screws driven by an electric motor through gearing. The pile is forced into the ground by the action of these screws, which bear on the top of the pile. Each pile is built up of a number of sections, each 6 feet long.
The illustration of the Dahren piling apparatus (see above) shows a pile in process of being driven, the lower portion of the pile being indicated in section. The pile hammer is inside the pile and strikes directly on the pile shoe or foot. There is a winch connected to the gearbox D and a cable from this goes over a pulley at the top of the pile to be attached at its lower end to the pile hammer.
The whole apparatus is securely bolted down by the bolts E to the foundation of the building which is being underpinned, and through the initial soft material the pressure on the screws will probably be sufficient to drive the pile down. When the resistance becomes harder, the pile hammer is put into operation and the co-operation of blows and pressure forces the pile down. New sections are added after every six feet of driving, until the pile can bear a certain definite load. The amount of this load can easily be measured by placing a special gauge between the top of the pile and the screwing arrangement.
Triumphs of Underpinning
Some unusually interesting underpinning work has been carried out by this system in Sweden. For example, a sixteenth-century church in Stockholm showed imminent danger of collapse because of faulty foundations, and heavy cracks appeared due to uneven settlement of the building. The foundation walls of the church, consisting of sandstone, bear directly on sandy clay. The load of these walls and the horizontal forces of the vaults were transferred to the piles by a system of stout steel beams. The church was saved from collapse by 270 Dahren piles driven inside and outside the walls. The steel beams were embedded in concrete, which was about five feet thick and took the place of the old foundation of sandstone.
Several factory buildings have been underpinned by this method, without any interruption of production. There was, for instance, a large textile factory which suffered from foundation troubles. The piling machine, which occupies an area of only 20 square feet, was placed between the machines at work in the factory while the factory was in full operation.
When the pile has been driven to the required depth, the inside may then be filled with concrete; before the concrete is placed, reinforcement in the form of steel bars may be lowered into the tube. If necessary, the pile hammer may be used for ramming this concrete down the tube and thoroughly consolidating it.
During recent years a great deal of research has been devoted towards the development of a remarkable piling method known as the Rotinoff system. The pile is in the form of a hollow tube, built up from a number of concrete shells. These are threaded on a steel mandrel (cylindrical rod), which passes through them and rests on the pile shoe. The blows of the pile hammer are delivered on top of this mandrel, the force passing directly to the pile shoe without imposing any shock on the concrete shell.
By an ingenious patented system sufficient force is applied to the concrete shells to overcome the friction between them and the ground. Thus they pass into the ground at exactly the same rate as the pile shoe penetrates. After the necessary “set” has been obtained, the mandrel is withdrawn from the shells, taking with it any shells which remain above ground. This means that the shells which are withdrawn may be used again in another pile; thus waste is avoided.
DEEP WELL PUMPS ready to be lowered into a series of wells on the site of the King George V Graving Dock, Southampton. The principle of this system is to lower the level of the ground water before foundation work is begun. Wells are bored and ingenious self-contained pumps are lowered which keep the ground water from rising above the level of the bottom of the borehole. The wells are equipped with special filters.
The interior of the concrete tube can then be inspected throughout the whole of its depth, after which it is filled with concrete reinforced with steel bars. A Rotinoff pile under test with a test load of 110 tons on a 42-feet pile showed a settlement of only 3/64 in.
A striking example of the application of this system to the foundations of a large multi-story building was the work on the new postal sorting office in Bristol for H.M. Office of Works. On this contract 440 piles, each of 17½-in. diameter, were driven some 42 feet through filling and clay to an underlying stratum of firm ballast. The work was somewhat unusual because the building has a deep basement and the tops of the piles were to be left 16 ft. 6 in. below ground level. This entailed the driving of piles 42 feet in length to obtain for them a final length of 25 ft. 6 in.
Special arrangements had therefore to be made so that the top unfilled portions of the piles could be recovered. Their presence in the ground would have hampered excavation work and added considerably to the cost of the scheme.
An amazingly simple and effective device was used to achieve this end. The lowest concrete shell in each pile to be recovered was replaced by a 17½-in. steel shell fitted with two anchor pins, to which were fixed the ends of a pair of wire ropes. On withdrawal of the mandrel the top shells were thus removed automatically without the slightest difficulty.
Piledriving Through Concrete
Another remarkable piece of foundation work was carried out by this method for the West Kent Main Sewerage Board. This contract involved the driving of 1,416 piles inside twenty-four sewage- precipitation tanks — of concrete with concrete floors. The piles, each of which had a diameter of 14 in., had to be driven through holes cut in the floor, and then a new floor slab was built above the old. This was a remarkable method of underpinning.
An interesting feature of this work was a special lorry piledriving outfit, designed for the purpose of overcoming the difficulty of transporting the pile gear from one position to another. The winch actuating the pile hammer was driven by electricity from the town main, or by the lorry engine when current was not available. As many as twenty piles were driven in a single day. As time goes on more and more specialized methods are introduced for surmounting piling and foundation problems. These methods are remarkable for their simplicity and for the ease with which they can be carried out; furthermore, they are witnesses to the spirit of co-operation which exists between one branch of science and another. All this has been brought about by linking up the various activities of research workers, and combining their results to achieve final solution of a problem.
The use of new materials in building construction and the advent of entirely new methods will allow the height of buildings to be increased considerably in the next few years. The height of buildings is largely governed by the security which can be guaranteed by firm and well laid foundations.
Methods of piling, cementation and underpinning are of the greatest possible importance in the realm of engineering science. In these days we see the full application of scientific knowledge to foundation problems. In an age of speed, foundation work must be carried out rapidly and with absolute safety. Research into foundation problems never ceases and future generations may see even more remarkable developments in this field.
ROTINOFF PILING SYSTEM in operation at Bristol. The pile is in the form of a hollow tube built up from a number of concrete shells. These are threaded on a steel mandrel which rests on the pile shoe. The pile hammer strikes the top of the mandrel and the shells pass into the ground at the same rate as the pile shoe.