When the bed of clay is covered by a quantity of foreign substance too great to be removed, or if it occurs in strata, it is cheaper to work it by shafts and galleries. Previous soundings will have informed us as to the thickness and probable extent of the bed. This is highly necessary, because the extraction by shaft being always expensive, it is evidently necessary that the outlay should be redeemed in a certain number of years.
The situation of the shaft is fixed by local circumstances, position of the bed, communication with the manufactory, etc. The size depends upon the method of extraction. In a large pit, a railway of 40 or 50 centimetres gauge will be used, hence the shaft must be square and large enough for two cars to pass one another; one ascending loaded, the other descending empty.
The square form also facilitates shoring up the shaft: for that purpose, by means of four beams, a square is formed, called "trousse," of the same size as the shaft.
The trousses rest against the earth; they are separated from one another by pieces of wood placed at the four angles, and if necessary in the middle of the bearing; these are called bearers. The distance from one trousse to another is more or less great according to the nature of the ground.
Shoring, properly so called, is carried out by introducing, between the trousses and the side of the shaft, oak planks of length equal to the vertical distance between the trousses. Behind these planks clay is heaped to prevent the entrance of water.
On reaching the bed of clay, a large reservoir is placed at the bottom of the shaft to collect the water coming from the galleries, which are bored through the whole thickness of the clay if it is not too great.
As a gallery is extended, it is lined by placing planks across it on the ground; on the ends of each plank two beams rest against the vertical sides of the gallery, and are joined at their upper ends by a strong oak plank, which thus forms a support to the roof.
These stays are placed more or less close together according to the consistence of the soil: in good ground they are placed at every metre, but in ground subject to landslip this distance is diminished, and the stays are joined by planks which support the soil.
The slope of the galleries will always be arranged so as to facilitate the flow of water towards the shaft reservoir. This reservoir will be emptied by pumps, or, better still, by using the water-tight bodies of waggons. The operation of emptying the reservoir is carried out every morning and evening before and after work.
A pit of this kind requires the same care as a mine: ventilation of the galleries, testing woodwork, etc. etc. The owner and manager must conform to the Mining Law of 21st April 1810, the articles of which referring to clay-pits we add below.
For the transport of clay through the galleries, a portable tramway will be used whenever possible, with waggons the bodies of which have rings riveted to the four corners. The loaded waggon is brought under the hoisting - gear, the hooks on the four cable extremities are attached to the rings, and the car is raised. The descending empty car is put on the waggon-frame in its place. The same operation takes place at the surface.
This is evidently the most economical system, and there are very few cases of large extraction to which it is not applicable.
Note. - Extracts from the Mining Law of 21st April 1810, modified by the laws of 9th May 1866 and 27th July 1880.
Masses of mineral or fossil substances, contained within the earth or on its surface, are classed, for purposes of regulation, under the titles of mines, ore-pits, and quarries.
Shall be considered as mines those known to contain in lodes, seams, or masses, gold, silver, platinum, mercury, lead, iron in lodes or seams, copper, tin, zinc, calamine, bismuth, cobalt, arsenic, manganese, antimony, molybden, plumbago, or other metallic substances, sulphur, earth or stone coal, fossil wood, bitumens, alum, and sulphates of metallic base.
Ore-pits comprise the ores of iron called alluvial, pyrite earths convertible into sulphate of iron, aluminous earths, and peats.
Quarries contain slates, sandstones, building and other stones, marbles, granites, limestone, plaster-stone, pozzalana, trass, basalts, lavas, marls, chalks, sands, flints, clays, kaolins, fuller's earths, potash earths, earthy matters and pebbles of all kinds, pyrite earths used as manure, all worked in the open air or with underground galleries.
The engineers of the mines shall, under the orders of the Minister of Public Works and of the Prefect, exercise police supervision for the preservation of buildings and security of the soil.
They shall observe the manner in which the workings are carried out, in order either to point out to the owners its inconveniences, and how it may be improved upon, or to inform the Government of defects, abuses, or dangers which may exist.
If the working is reduced or suspended in such a way as to affect the public safety or the needs of consumers, the Prefect shall, after hearing the views of the owners, make a report to the Minister of Public Works in order that proper measures may be taken.
If the experimental or after-working of a mine be such as to endanger public safety, the preservation of the mine, the safety of the miners, the preservation of the lines of communication or of the mineral waters, the solidity of dwellings, the use of springs which supply towns, villages, hamlets, and public institutions, the Prefect shall take proper measures.
Arts. 57 and 58 (replaced by Article 3 of the law of 9th May 1866).
The Articles 57 and 58 of the same Act are modified as follows: -
If the working of the pits is to be in the open air, the owner must, before commencing work, make a declaration to that effect to the Prefect. The Prefect gives an official certificate of such declaration, and the working proceeds without any other formality. - This arrangement is applicable to iron-ores in seams and lodes, in the case in which, in accordance with Art. 69, they are not concessible. If the working is to be underground, it can only proceed with the permission of the Prefect. The permit states the special conditions to which the manager must conform.
In the two cases foreseen in the preceding Article, the manager must observe the general or local regulations referring to public safety and health which are binding upon ore-pit managements. Articles 93 to 96 of the present Act apply to offences committed by managers of ore-pits, to the regulations of Art. 57, and to the general and local regulations referred to in the present Article.
The working of pyrites and aluminous earths will be subjected to the formalities prescribed by Arts. 57 and 58, whether it is carried on by the owners, or by others who have obtained their permission to do so.
If the pits are worked by non-owners, they will have to pay an indemnity to the proprietors, the amount of which shall be settled by agreement or expert arbitration.