Mexico City, which was about 2,240m above sea-level in 1974, had a population of some 6M inhabitants. It stood on a small plain which occupied the south-western part of a depression known as the Valley of Mexico (El Valle de Mexico), and lay on the western shore of Lake Texcoco. At one time, the waters of Lake Texcoco covered most of the area on which the city now stood.
From its earliest recorded history, Mexico City has had severe drainage problems and was rated as one of the worst-drained cities in the world. The city was only 2m above the level of the lake. During the year 1629 the city was inundated by floodwaters, which rose to a height of approximately 1m and did not recede again until five years later.
Some effort was made to drain the city by the Spaniard, Maartens. As Lake Zumpango was the nearest natural drainage basin in the area, Maartens proposed a drainage channel through the northern hill of Nochistonga so that the overflow from Lake Zumpango could be directed into the river Tula, a tributary of the Pánuco. The cutting (21km in length) was commenced in the year 1607 and was finally completed in 1789. It was named the Tajo de Nochistongo.
As these steps only partly alleviated the problem, tenders for further drainage works were called for by President Ignacio Comonfort. The proposal was for a 69km long drain which was to commence as an open canal at a depth of 12m below Mexico City on its eastern side and then run north towards Zumpango, at which point it would enter a tunnel 94m under the mountain rim and veer eastwards for 9.5km, until it eventually emptied into a tributary of the river Páncho.
In 1900 the work was completed and water began flowing down the canal. However, the drainage system was not entirely satisfactory as it created other more serious problems. In effect, the city was built on a vast lake of mud, which had collected in and filled the crater of a volcano. Though this mud had dried on the surface, the subsoils were still permanently saturated and contained as much as 80% water.
In 1929 there were 11 public, and 1,375 private, artesian wells operating in the city. As the population expanded so did the number of wells. The result was that greater and greater quantities of water were daily being drawn up from the subsoil for use in households, or extracted and channelled away via the canals and tunnel for irrigation etc. As the water was removed, the subsoils began drying out and contracting and the city commenced sinking at the alarming rate of approximately 300mm per annum.
According to the Encyclopaedia Britannica of 1929, the city was some 2,260m above sea-level in 1911. By 1974 the level of the city was about 2,240m above sea level, which showed a drop of approximately 20m in 63 years. But the problem was not generally taken very seriously until buildings began leaning over. By 1935 the Palace of Fine Arts had actually sunk a complete storey. Over the succeeding years citizens and officials watched helplessly as various parts of their city sank lower and lower. By 1950 it was 6m and more below the original level of the canal tunnel. Eventually it became necessary to pump all the sewage uphill, and dire warnings were issued by eminent engineers that unless something was done soon, a severe rainstorm could flood their sewers and overflow into the city to the height of about 1m carrying with it the vile-smelling effluent. Nevertheless it was only after the experience of the 1951 floods that this advice was heeded. Many of the artesian wells were permanently closed and the city’s water was drawn from the hills nearby. In addition, plans were laid for the construction of a 6m diameter central interceptor drainage tunnel 30m below the city surface. Its purpose was to draw off the majority of the storm waters in the event of a bad flood.
A part (48km) of the proposed tunnel route lay through the mud under the city, and the remainder (a distance of some 64km) travelled through the rock strata which lay beneath the mountain ridge.
It was for the mud section of the tunnel that engineers from Markham were asked to submit designs for three shields based on the combined design concepts of engineers from Kinnear Moodie Tunnelling Machines Ltd (a member of the Mitchell Construction Kinnear Moodie Group) and the Federal District Department of Mexico City. Work on the first machine was started in July 1967. By 14 February 1968 the prototype had successfully completed its work tests.
The slurry machine – Universal Softground TBM
The machine, built by Markham to tunnel through Mexico City’s unstable subsoil under soil-water pressures nearing 300kPa, was basically a mechanized shield, but of an unusual type. The head, or excavating section of the machine, was separated from the main body by a heavy steel bulkhead that was designed to withstand the full hydrostatic head of the ground water. The excavator cutting head, consisting of six radially arranged cutting arms was mounted on a central drive shaft, which transmitted an oscillatory movement to the arms. This particular type of movement was chosen in order that the unstable ground ahead of the shield should not be unduly disturbed and also in order to counteract any tendency for the machine to rotate within the bore. Two pairs of hydraulically powered rams attached to radial arms at the rear end of the main drive shaft and reacting against the invert and crown sections of the shield, respectively, provided the necessary oscillatory action to the cutting head. When in operation the arms described an arc of some 60°-70° across the tunnel face.
Twenty-two powerful jacks, placed circumferentially at the rear of the unit and reacting against the recently constructed tunnel lining, provided a total forward thrust of 1,300t to the shield. (In addition, a spline on the main drive shaft permitted a 400mm axial movement of the head within the shield. This movement was effected by eight rams capable of a total thrust of 500t.)
The space between the bulkhead and the tunnel face (containing the cutterhead mechanism) was filled with water under pressure. In operation, the cuttings from the face were turned into a slurry by three hydraulically driven agitators situated behind the cutterhead at the bottom of the bulkhead. The drive for the agitators was placed behind the bulkhead. The resulting slurry was then sucked from the excavation compartment through two 150mm diameter outlet pipes placed near the bottom of the bulkhead. These pipes conveyed the slurry to a tank and slurrifying unit mounted on a sledge, which formed part of the ancillary equipment behind the shield itself. From this point the spoil was pumped away from the working area and out of the tunnel. However, some of the slurry was redirected back into the digging compartment through a pressure-actuated bypass valve that maintained the hydraulic pressure at the face.
Concrete lining segments (designed by Kininear Moodie (Concrete) Ltd) were positioned by a hydraulic erector arm, situated in the tail section of the shield.
While the remainder of the shield and, of course, the tunnel, remained in free air, access to the face for maintenance purposes, etc. was provided through a small airlock built into the bulkhead. The airlock was 1.2m in diameter and 1.8m long.
Soon after the universal soft ground shields reached Mexico City, Markham lost contact with their progress, as the contractor to whom the units had been supplied ceased to be involved in the actual tunnelling process. During the intervening years the author made various attempts to discover the fate of these machines but to no avail, as all letters sent to Mexico were ignored. Finally some news filtered through via a group of Japanese delegates (who had visited Mexico). They advised that two of the units had been converted to conventional shields and the third had been abandoned. Then in October 1976 the author received a letter from D H Scotney of Kinnear Moodie, which is quoted above.
On reading through the case histories of new machines which have been introduced over the years, one cannot but be struck by the fact that most of these units succeeded – not because of the perfection of the prototype model, though naturally it was essential that all engineering principles involved be sound – but because of the ingenuity, courage, perseverance, faith, and financial backing of one man or a group of men who had a personal and dedicated interest in the success of the unit.
The story of the old McKinlay entry driver is a classic example of this and there are, of course, many others. Under the circumstances one is left with the thought that the ultimate fate of those three machines might very well have been different had the project been undertaken in England where both the designers and manufacturers of the units would have been available if required.
