Chapter VIII · TABLES AND MISCELLANEOUS INFORMATION.
Reading edition
The following tabulated form shows the order of succession of the various stratified rocks with their usual thicknesses.
Groups. Strata. Thickness in Feet.
RECENT 1 Modern Deposits.
PLEISTOCENE 2 Drift and Gravel Beds 20 to 100
3 Mammaliferous Crag 10 to 40
PLIOCENE 4 Red Crag 30
CAINOZOIC, OR 5 Suffolk (Coralline) Crag 30
TERTIARY. MIOCENE 6 Faluns (Touraine) Molasse Sandstones 6000
7 Hempstead Series 170
Upper 8 Bembridge Series 110
9 Headon Series 200
EOCENE Middle 10 Barton Beds 300
11 Bagshot and Bracklesham Series 1200
Lower 12 London Clay and Bognor Beds 200 to 520
13 Woolwich Beds & Thanet Sands 100
14 Maestricht Beds 110
15 Upper Chalk 300
16 Lower Chalk and Chalk Marl 400
CRETACEOUS 17 Upper Greensand 130
18 Gault 100
19 Speeton Clay 130
20 Lower Greensand 250
WEALDEN 21 Weald Clay 150
22 Hastings Sands 600
PURBECK 23 Purbeck Beds 150
UPPER 24 Portland Rock and Sand 150
OOLITE 25 Kimmeridge Clay 400
26 Upper Calcareous Grit 40
MESOZOIC, OR MIDDLE 27 Coralline Oolite 30
SECONDARY. OOLITE 28 Lower Calcareous Grit 40
29 Oxford Clay 400
30 Kellaways Rock 30
31 Cornbrash 10
32 Forest Marble and Bradford Clay 50
LOWER 33 Great Oolite 120
OOLITE 34 Stonesfield Slate 9
35 Fullers’ Earth 50 to 150
36 Inferior Oolite 80 to 250
37 Upper Lias Shale 50 to 300
LIAS 38 Marlstone and Shale 30 to 200
39 Lower Lias and Bone Beds 100 to 300
TRIASSIC, or 40 Variegated Marls or Keuper 800
NEW RED 41 Muschelkalk
SANDSTONE 42 Red Sandstone or Bunter 600
PERMIAN or 43 Red Sand and Marl 50
MAGNESIAN 44 Magnesian Limestone 300
LIMESTONE 45 Marl Slate 60
46 Lower Red Sandstone 200
47 Coal Measures 3000 to 12,000
CARBONIFEROUS 48 Millstone Grit 600
49 Mountain Limestone 500 to 1400
50 Limestone Shales 1000
PALÆOZOIC, OR DEVONIAN or 51 Upper Devonian
PRIMARY. OLD RED 52 Middle Devonian 3000 to 8000
SANDSTONE 53 Lower Devonian and Tilestones
54 Ludlow Rocks 2000
Upper 55 Wenlock Beds 1800
56 Woolhope Series 3050
SILURIAN Middle 57 Llandovery Rocks 2000
58 Caradoc and Bala Rocks 5000
Lower 59 Llandeilo Rocks 4000
60 Lingula Flags 8000
CAMBRIAN 61 Longmynd and Cambrian Rocks 20,000
METAMORPHIC Clay Slate, Mica-Schist.
AZOIC. Gneiss, Quartz Rocks.
IGNEOUS Granite.
The Quantity of Excavation in Wells for each Foot in Depth. (Hurst.)
Diameter of Excavation. Quantity.
ft. in. cubic yards.
3 0 ·2618
3 3 ·3072
3 6 ·3563
3 9 ·4091
4 0 ·4654
4 3 ·5254
4 6 ·5890
4 9 ·6563
5 0 ·7272
5 3 ·8018
5 6 ·8799
5 9 ·9617
6 0 1·0472
6 3 1·1363
6 6 1·2290
6 9 1·3254
7 0 1·4254
7 3 1·5290
7 6 1·6362
7 9 1·7472
8 0 1·8617
8 6 2·1017
9 0 2·3562
9 6 2·6253
10 0 2·9089
10 6 3·2070
11 0 3·5198
12 0 4·1888
The Measure in Gallons, and the Weight in Pounds, of Water contained in Wells, for each Foot in Depth.
Diameter. No. of Galls. Weight.
ft. in.
2 0 19·61 196·1
2 6 30·56 305·6
3 0 43·97 439·7
3 6 60·00 600·0
4 0 78·19 781·9
4 6 98·87 988·7
5 0 122·23 1222·3
5 6 147·96 1479·6
6 0 175·99 1759·9
6 6 206·59 2065·9
7 0 239·05 2395·0
7 6 275·49 2754·9
8 0 313·43 3134·3
8 6 353·03 3533·0
9 0 395·42 3954·2
9 6 441·71 4417·1
10 0 489·93 4899·3
Brickwork. The Number of Bricks and Quantity of Brickwork in Wells for each Foot in Depth. (Hurst.)
Half-Brick Thick. One Brick Thick.
Number of Bricks. Number of Bricks.
Laid Dry. Laid in Mortar. Cubic Feet of Brickwork. Laid Dry. Laid in Mortar. Cubic Feet of Brickwork.
1·0 28 23 1·6198 70 58 4·1233
1·3 33 27 1·8145 80 66 4·7124
1·6 38 31 2·2089 90 74 5·3015
1·9 43 35 2·7979 112 92 6·4795
2·3 53 44 3·0926 122 100 7·0686
2·6 58 48 3·3870 132 108 7·6577
3·0 68 57 3·9760 154 126 8·8357
3·6 79 65 4·5651 174 142 10·0139
4·0 89 73 5·1541 194 159 11·1919
4·6 100 82 5·7432 214 176 12·3701
5·0 110 90 6·3322 234 192 13·5481
5·6 120 98 6·9213 254 209 14·7263
6·0 130 107 7·5103 276 226 15·9043
6·6 140 115 8·0994 296 242 17·0825
7·0 150 123 8·6884 316 260 18·2605
7·6 160 131 9·2775 336 276 19·4387
8·0 170 140 9·8665 358 292 20·6167
8·6 180 148 10·4556 378 308 21·7949
9·0 191 156 11·0446 398 326 22·9729
10·0 212 174 12·2227 438 360 25·3291
Good bricks are characterized as being regular in shape, with plane parallel surfaces, and sharp right-angles; clear ringing sound when struck, a compact uniform structure when broken, and freedom from air-bubbles and cracks. They should not absorb more than one-fifteenth of their weight in water.
After making liberal allowance for waste, 9 bricks will build a square foot 9 inches thick, or 900, 100 square feet, or say 2880 to the rood of 9-inch work, which gives the simple rule of 80 bricks = a square yard of 9-inch work.
The resistance to crushing is from 1200 to 4500 lb. a square inch; the resistance to fracture, from 600 to 2500 lb. a square inch; tensile strength, 275 lb. a square inch; weight, in mortar, 175 lb. a cubic foot; in cement, 125 lb. a cubic foot.
Compressed bricks are much heavier, and consequently proportionately stronger, than those of ordinary make.
Storing Well-water.
The reservoirs for storing well-water should be covered with brick arches, as the water is generally found to become rapidly impure on being exposed to the sunlight, principally owing to the rapid growth of vegetation. Various methods have been tried, such as keeping up a constant current of fresh water through them, and a liberal use of caustic lime; but so rapid is the growth of the vegetation, as well as the change in the colour of the water, that a few hours of bright sunlight may suffice to spoil several million gallons. These bad results are completely prevented by covering the reservoirs.
Hints on Superintending Well-work.
The engineer who has to superintend the construction of a well should be ever on the watch to see whether, in the course of the work, the strata become so modified as to overthrow conclusions previously arrived at, and on account of which the well has been undertaken.
A journal of everything connected with the work should be carefully made, and if this one point alone is attended to it will be found of great service both for present and future reference.
Before commencing a well a wooden box should be provided, divided by a number of partitions into small boxes; these serve to keep specimens of the strata, which should be numbered consecutively and described against corresponding numbers in the journal. At each change of character in the strata, as well as every time the boring rods are drawn to surface, the soil should be carefully examined, and at each change a small quantity placed in one of the divisions of the core box, noting the depth at which it was obtained, with other necessary particulars. A note should be made of all the different water-levels passed through, the height of the well above the river near which it is situated, as well as its height above the sea. The memoranda in the journal relating to accidents should be especially clear and distinct in their details; it is necessary to describe the effects of each tool used in the search for, or recovery of, broken tools in a bore-hole, in order to suit the case with the proper appliances, for without precaution we may seek for a tool indefinitely without being sure of touching it, and perhaps aggravate the evil instead of remedying it. It is by no means a bad plan to make rough notes of all immediate remarks or impressions, in such a manner as to form a full and detailed account of any incidents which occur either in raising or lowering the tools. At the time of an accident a well kept journal is a precious resource, and at a given moment all previous observations, trivial as they may have often seemed, will form a valuable clue to explain difficulties, without this aid perfectly inexplicable.
When an engineer has a certain latitude allowed him in the choice of the position for a well, he should not, other things being equal, neglect the advantages which will be derived from the proximity of a road for the transport of his supplies; of a well, if not a brook, from which to obtain the water necessary for the cleansing of the tools; and of a neighbouring dwelling, to facilitate his active supervision. This supervision, having often to be carried on both day and night, should be the object of particular study; well carried out, it may be effective, while at the same time allowing a great amount of liberty; badly carried out, however fatiguing it may be, it will be incomplete.
Rate of Progress of Boring. (André.)
There are probably no engineering operations in which the rate of progress is so variable as it is in that of boring. That such must necessarily be the case will be obvious when we bear in mind that the strata composing the earth’s crust consist of very different materials; that these materials are mingled in very different proportions, and that they have in different parts been subjected to the action of very different agencies operating with very different degrees of intensity. Hence it arises not only that some kinds of rocks require a much longer time to bore through than others, but also that the length of time may vary in rocks of the same character, and that the character may change within a short horizontal distance. Thus it is utterly impossible to predicate concerning the length of time which a boring in an unknown district may occupy, and only a rough approximation can be arrived at in the case of localities whose geological constitution has been generally determined. Such an approximation may, however, be attained to, and it is useful in estimating the probable cost; and to attain the same end, for unknown localities, an average may be taken of the time required in districts of a similar geological character. The following, which are given for this purpose, are the averages of a great number of borings executed under various conditions by the ordinary methods. The progress indicated represents that made in one day of eleven hours.
ft. in.
1. Tertiary and Cretaceous Strata, to a depth of 100 yards, average progress 1 8
2. Cretaceous Strata, without flints „ 250 „ „ 2 1
3. Cretaceous Strata, with flints „ 250 „ „ 1 4
4. New Red Sandstone „ 250 „ „ 1 10
5. New Red Sandstone „ 500 „ „ 1 5
6. Permian Strata „ 250 „ „ 2 0
7. Coal Measures „ 200 „ „ 2 3
7. Coal Measures „ 400 „ „ 1 8
General Average 275 1 9
When the cost of materials and labour is known, that of the boring may be approximately estimated from the above averages. Should hard limestone or igneous rock be met with, the rate of progress may be less than half the above general average. Below 100 yards, not only does the rate of progress rapidly increase, but the material required diminishes in like proportion, so that for superficial borings no surface erections are needed, and the cost sinks to two or three shillings a yard.
Cost of Boring.
The cost of boring when executed by contract has already been treated of at page 80. The following formula will furnish the same results as the rule there given, but with the least possible labour of calculation;
x = 0·5d(·187 + ·0187d);
x being the sum sought, in pounds, and d the depth of the boring in yards.
Example. Let it be required to know the cost of a bore-hole 250 yards deep.
Here 125{·187 + (·0187 × 250)} = £607·75.
Tempering Boring Chisels.
1. Heat the chisel to a blood red heat, and then hammer it until nearly cold; again, heat it to a blood red and quench as quickly as possible in 3 gallons of water in which is dissolved 2 oz. of oil of vitriol, 2 oz. of soda, and 1⁄2 oz. of saltpetre, or 2 oz. of sal ammoniac, 2 oz. of spirit of nitre, 1 oz. of oil of vitriol: the chisel to remain in the liquor until it is cold.
2. To 3 gallons of water add 3 oz. of spirit of nitre, 3 oz. of spirits of hartshorn, 3 oz. of white vitriol, 3 oz. sal ammoniac, 3 oz. alum, 6 oz. of salt, with a double handful of hoof-parings, the chisel to be heated to a dark cherry red.
Gases in Wells.
The most abundant deleterious gas met with in wells is carbonic acid, which extinguishes flame and is fatal to animal life. Carbonic acid is most frequently met with in the chalk, where it has been found to exist in greater quantity in the lower than in the upper portion of the formation, and in that division to be unequally distributed. Fatal effects from it at Epsom, 200 feet down, and in Norbury Park, near Dorking, 400 feet down, have been recorded. At Bexley Heath, after sinking through 140 feet of gravel and sand and 30 feet of chalk, it rushed out and extinguished the candles of the workmen. Air mixed with one-tenth of this gas will extinguish lights; it is very poisonous, and when the atmosphere contains 8 per cent. or more there is danger of suffocation. When present it is found most abundantly in the lower parts of a well from its great specific gravity.
Sulphuretted hydrogen is also occasionally met with, and is supposed to be generated from the decomposition of water and iron pyrites.
In districts in which the chalk is covered with sand and London clay, carburetted hydrogen is occasionally emitted, but more frequently sulphuretted hydrogen. Carburetted hydrogen seldom inflames in wells, but in making the Thames Tunnel it sometimes issued in such abundance as to explode by the lights and scorch the workmen. Sulphuretted hydrogen also streamed out in the same place, but in no instance with fatal effects. At Ash, near Farnham, a well was dug in sand to the depth of 36 feet, and one of the workmen descending into it was instantly suffocated. Fatal effects have also resulted elsewhere from the accumulation of this gas in wells.