The quarry should be located in an area with sufficient proven deposits of good brickmaking soil and, preferably, a thin layer of overburden to minimise excavation work. The operation of mining clay from the clay pit or quarry is generally referred to as winning the clay.
I.1 Opening up the quarry
Access to the quarry from the bricks production plant should be quick and easy, preferably with no more than a slight gradient. A good route will minimise effort, time and expense in transporting clay, and will facilitate supervision of the pit. A track or roadway may need to be constructed, especially if wheeled vehicles are to be used to convey the clay to the brickworks. Trees and bushes must be cleared, and may be sold or kept for fuel.
Prospecting will indicate whether the clay stratum is horizontal or sloping. If it does slope down into the ground the worker should face that direction and remove the top soil. The top soil should be piled in two rows along the excavation. The trench thus formed will have a horizontal bottom, along the strike of the clay stratum(24). The angle at which the clay stratum slopes from the horizontal is a measure of the dip of the stratum. It must be borne in mind that more overburden will have to be removed as the clay winning proceeds. If there is no dip, the trench may be dug in any direction.
As a general principle it is unwise to start digging for clay at the lowest part of the ground(25) since surface water from rainstorms will then immediately flood the clay pit and stop the work. It is preferable to start digging at a higher point. This should be borne in mind whether the underlying clay has a dip or not.
A sufficient area of overburden should be removed to prevent any of it falling into the clay as winning proceeds (e.g. up to 10 m may be taken off to each side of the trench). If too much is cleared weeds may start to grow and will have to be cleared again. Clay may then be dug to a depth of a few metres, along the centre line of the exposed area. The actual depth will depend upon the adopted method of digging and the nature of the material extracted. Further material is then obtained by widening this deeper trench a small amount at a time. Eventually, it will be necessary to remove more overburden. This unwanted material may conveniently be used to fill the first-opened part of the trench once all the useful clay has been excavated. If good clay does extend lower down, it might be extracted at a much later date or by the method of benching or terracing (which is the working of two or more clay faces at different depths at the same time).
I.2 Operating the quarry
It is important to bear safety in mind in the clay pit. Clay is very slippery when wet. Thus steeply sloping paths and access routes should be avoided. Steep drops into the pit would be hazardous. Damp clay is not stable at a near vertical face. Consequently, a whole portion of the material may undergo a rotational slip, into the bottom of the pit. For this reason, it is advisable to slope back or batter the faces of the pit as they are dug. The latter should not be too high.
If flooding of the pit bottom becomes a problem, the water may be drained away through a downhill channel. If this is not possible, a sump must be dug in the pit bottom to collect the excess water. The latter may then be removed from the sump by pumping or with buckets. This water may be contaminated with soluble salts present in the ground. Thus, it would be unwise to use it in the subsequent brickmaking process, unless tests show that it does not contain salts. In countries having a wet season or monsoon, the quarry may need to be abandoned until the rainwater is drained off.
Rejection of impurities and reinstatement
As digging proceeds, the workers should discard any plant roots, stones, limestone nodules or harder clay inclusions since they would cause problems in subsequent processing. Any pockets of unsuitable soil should be removed rather than left in place and the pit should be kept tidy. The top soil should be returned ultimately, on top of discarded material into the worked-out part of the quarry. Crops may thus be grown again. This is the case, for example, in Madagascar where the most exploited clays for brickmaking are from the rice fields. The top soil is then reinstated for rice growing (26). One of the main sources of raw materials for the structural clay industry in Indonesia is also the rice fields.
Rate of extraction
The rate of extraction of clay from the pit must be sufficient to meet the demands of the brick-moulders. Alternatively, it may be slightly larger in order to guard against problems which may arise unexpectedly in the pit, such as temporary flooding, presence of an unsuitable pocket of material, contamination of the clay, etc. In some countries the onset of the wet season or the monsoon may halt operations in the clay pit. During these times, the natural drying of moulded bricks will become almost impossible, building of field kilns or clamps (Chapter VII) will be impracticable, and demand for bricks will fall due to adverse weather conditions restricting building and construction activities. In such cases, the pit will be closed and the whole brickmaking operation stopped. However, in other places, although the rain may prevent operations in the clay pit, some demand for bricks may continue, and it may be possible to carry on brickmaking and drying under cover. More permanent forms of kiln, also under cover, may still be in operation. In these cases, sufficient clay should be won from the pit during the dry season, and stock-piled, to meet the demand when no more can be mined. In some communities the workforce may wish to engage in agricultural activities during the harvesting season. This factor should be taken into account in designing the whole brickmaking process.
Working of the clay face
The depth of the top-soil may vary from a few centimetres in some arid climates to several metres in hot, humid areas. Frequently, a layer of sand may occur below the top-soil and over the clay layer. The best clay for brickmaking is likely to be that immediately below the sand, since it is likely to contain a proportion of sand itself. However, the depth of this good material may be small. Clay lower down may be too fat and will need addition of sand from above. Hence the best method of operation is to work a quarry face in such a way as to dig both clay and sand, taking shallow slices down the face, to obtain a suitable mixture (see Figure II.6). Another virtue of taking shallow slices of the face is that any embedded stones can be found more easily than if large cuts are taken. These stones can then be discarded(28).
If suitable soil containing the desirable proportions of sand and clay cannot be dug at one face only, it may be necessary to obtain a fat clay from one face and sand or sandy clay from another. This has often been done as, for example, near Mombasa in Kenya, where material from two faces has been mixed in the pit bottom prior to use. If the material varies horizontally (i.e. from one place to another) two separate faces in the pit, or two separate pits, might be worked simultaneously. If the material varies vertically (i.e. at different depths), two faces can be operated by benching (see section II.2).
For later reference, a note-book should be kept for recording progress, and any significant happenings in the quarry. A map should be made of the quarry, showing the position of original test holes or pits, the depth of clay, and other major features such as streams, tracks, large trees and the brickworks if adjacent. The position of the clay-pit face should be drawn on the map every few months, and the date written on the line representing the face. If the floor of the pit is dug a second time or if benching is used, a second colour could be used to update the map. This will assist in an orderly exploitation of the reserve: haphazard digging is wasteful of material and effort(27). The rate of ingress into the reserve should be clearly visible, and if problems or complaints arise with the finished bricks, the fault may be traceable to a cause in the pit. The extent of any problem materials in the pit should be marked on the map. The supervisor should check constantly the work at the clay face and inspect the material being won to ascertain that it is suitable and does not contain deleterious materials.
Two basic methods are available: mechanical winning and hand-digging. These are briefly described below.
Mechanical methods such as the use of the drag-line and multi-bucket excavator are mostly appropriate for the largest-scale brickmaking operations. It is most unlikely that even a face shovel (figure III.1) could be justified in works of the size considered in this memorandum, unless it is available on hire from a nearby depot for a short period of time each year, (e.g. in order to build a stock pile). It seems unlikely that mechanical winning could be economical for output of less than 14,000 bricks per day(22).
On the other hand, the more commonly available and versatile bulldozer could have a place in the laborious task of clearing overburden on infrequent occasions. It might be brought in on hire, or when available from nearby road construction or other civil engineering works (e.g. against payment of a fee).
Most of the clay resources utilised by the small-scale manufacturer are likely to be of the soft plastic type. In some areas, when only hard shales are available, blasting might be undertaken occasionally to loosen material from the quarry face.
Hand-digging has been widely used even for medium-size production plants, because of its versatility in dealing with all clays from soft muds to shales or even with ant hills. Hand-digging can also be adjusted to various types of work, and allows workers to sort out unwanted stones, limestones, roots, etc. It also avoids large amounts of capital investment, the stocking of spares and the organisation of maintenance of machinery. In many situations, hand-digging may be the only possible means of winning clay.
The rate of winning clay will depend upon the type of clay, the nature of the pit and the productivity of labour. Productivity rates for one man digging enough clay for the production of approximately 3,500, 1,500 and 4,000 bricks per day have been estimated(5,25,8). However, these estimates are not strictly comparable as some of them include an element for the transport of clay over a short distance. Measurement of shovelling rates in the American mines(29) indicated an optimum working day of 6.5 hours. Longer working hours result in lower outputs.
Once clay has been dug, there will be a natural reluctance to reject any which may prove unsuitable, especially after the hard work of winning it. In particular, the workers paid according to quantity excavated may be reluctant to reject unsuitable material. Hence the importance of supervision, inspection and quality control.
If the face is benched, the separate levels need be only 1 m different(25) and 0.5 m wide(5), especially if materials from two or more levels are to be mixed. This can be done by throwing all materials down to the lowest level for mixing, and subsequent transportation away to the works.
The details of working the pit must be decided locally. For example, at Asokwa in Ghana (figure III.2) the clay was hand-dug from a face which was approximately 2.5 m high in places(30).
Steel bladed, medium-weight spades are well suited for digging plastic clays. Preferences in blade design vary from country to country. It is, however, recommended to use narrow and slightly conical blades for the digging of this type of clay. The handle of the shovel should be shorter whenever the foot is used on the top of the blade. In places where this is not done, the handle is traditionally very long.
Figure III.1 - Face shovel or single bucket excavator
Figure III.2 - Clay-pit at small brickworks (Ghana)
If hard, dry clays are to be won (figure III.3) it may be necessary to loosen them from the face with a pick, then shovel the material away.
In many countries, the hoe and mattock are more generally used and are suitable for winning clay.
In large works, clay is conveyed from the pit in various ways, including the use of lorry or truck, large dumper truck, small-gauge railway systems, aerial ropeway or belt conveyor. Capacity, capital cost, maintenance and repair militate against the use of these methods for the smaller works.
While heavy transport equipment may not be suitable, the use of a small diesel-powered dumper or a front-end loader may become feasible on a hire basis. Similarly, an agricultural tractor may be used to haul a loaded trailer of clay. Alternatively, a draught animal may be useful for pulling a trailer if the road is not too steep and muddy.
The wheelbarrow is a versatile and low-cost device for moving clay. It need not be all-steel or specially imported and can be locally produced from available materials. It can be taken from the clay face to any desired point at the plant site, on a narrow path or a plank on muddy ground (figure III.3). The larger the wheel, the more easily the barrow will pass over irregularities in the ground. The wheel should be as close to the load as possible in order to take the weight off the hands. The handles should also be approximately 50 mm lower than a standing persons palms when the barrow is at rest on the ground. Thus, the arms are just slightly crooked when the barrow is wheeled.
A simple aid for carrying clay and other materials is the litter (figure III.4). Its use by two people avoids twisting the body. Large loads may be carried over rough terrain, or up steep slopes. It may be fabricated easily by unskilled labour using cheap, locally available materials.
The most simple devices for transporting clay are the basket and headpan, both of which will be available in many communities.
Figure III.3 - Digging and transporting of dry hard clay (United Kingdom)
Figure III.4 - Litter for carrying wet clay (Sudan)