Popular Science Monthly/Volume 81/July 1912/Trinidad and Bermudez Asphalts and their Use in Highway Construction I
|TRINIDAD AND BERMUDEZ ASPHALTS AND THEIR USE IN HIGHWAY CONSTRUCTION|
NEW YORK CITY
BITUMEN in its various forms is widely distributed in nature, as natural gas, petroleum, maltha, asphalt and other solid forms. Of the deposits of asphalt which are of great industrial importance there are two which have attracted world wide attention, the so-called Trinidad Pitch Lake and the Bermudez Pitch Lake, the name lake being applied to them very naturally, as they consist of a great expanse of more or less mobile character, covering many acres, and resembling in many ways a similar expanse of water. It is proposed, in the following pages, to give an account of these remarkable deposits, the manner of exploiting them and their industrial applications in highway construction.
The Trinidad Pitch Lake
The Island of Trinidad lies off the north coast of South America, between 10° and 11° of latitude and 61° and 62° longitude. It is bounded on the north by the Caribbean Sea, on the east by the Atlantic, on the south by a narrow channel, into which flow the waters of the northern and most westerly mouths of the Orinoco, and on the west by the Gulf of Paria, the two latter bodies of water separating it from the mainland of Venezuela.It is of an irregular rectangular shape, with promontories extending from its southwestern and northwestern corners which are several miles in length, between which and the mainland are the narrow straits known as the Dragon's and Serpent's Mouths. These promontories from a large portion of the northern and southern boundaries of the shallow rectangular Gulf of Paria, whose outlets to the ocean are through the Dragon's and Serpent's Mouths. The island has an average length of 48 miles and breadth of 36, containing about 1,750 square miles, and being about one fifth the area of the state of Vermont. It is, as a whole, a flat country, with a high and striking mountain chain descending abruptly into the sea along its northern shore, and with low central and southern ranges of less importance. Its coasts are naturally abrupt on the north, consist of low bluffs on the south and are flat on the east and west. The only harbors are on the western coast.
The shape and structure of the long promontories which have been mentioned reveal the fact that Trinidad is, structurally, intimately connected with the mainland. This is proved by the geology and fauna of the island, the latter corresponding closely to that of the mainland and the geological structure being a continuation of that of the continent. Its climate is entirely tropical and somewhat different from that of the remaining Antilles in this respect.
Trinidad was discovered by Columbus on his third voyage in 1498 and taken possession of in the name of Spain, which colonized it about ninety years after. In 1797 it was taken by Great Britain, and has remained since then one of her most important West Indian colonies, and the second in size.
The Island of Trinidad, while not directly connected with the chain of islands of volcanic origin known as the Windward or Caribbean Islands, is directly on the great line of volcanic disturbances running from these to the continent of South America and its volcanic regions. Many cf the Windward Islands are still possessed of active vents, so that Trinidad may be looked upon, with its thermal springs and pitch deposits, as being situated at the lowest point between the mountainous volcanic chains of the West Indies and those of South America. More than two thirds of the surface is of Tertiary or recent origin, including the entire southern portion, where the pitch deposits are located. The formations consist of clay, loose sand, shales, limestones, calcareous sandstones, indurated clays, porcelainites of brilliant colors, with pitch deposits here and there. The beds have been considerably disturbed and have at times a large dip. In a series of sands, clays and shales lies the pitch lake.
While there are deposits of pitch scattered all over the island, the only ones of commercial importance are those situated on La Brea Point, in the wards of La Brea and Guapo, in the County of St. Patrick, on the western shore of the island. They are about 28 miles in an air line from Port of Spain, the seat of government, the chief harbor and only port of entrance, and lie on the north shore of the southwestern peninsula, the point on which they are situated being apparently preserved from destruction by the sea, which is elsewhere rapidly wearing away the coast, by the bituminous deposits which exist along the shore and even some distance from it, and which from their toughness resist the action of the waves better than the soft rocks of this region. The pitch deposits are found scattered over the point, but can be divided conveniently into two classes, according to their source.
The main deposit is a body of pitch known as the pitch lake, situated at the highest part of the point. Between this and the sea, and more especially toward La Brea, are other deposits, covered more or less and mixed with soil. The pitch from these sources is classified as "lake pitch" and "land pitch."
By far the largest amount of pitch is found apparently in the pitch lake, a nearly circular area of 114.67 acres, 138-feet above sea level. From the lake the ground falls away on all sides, except, perhaps, a slight ridge to the east and southeast, in fact, it seems plain that this deposit lies in the crater of a large mud volcano which has been filled up with pitch.
In past times the pitch very probably continued to collect until it overflowed the rim of the crater, particularly toward the north, and thus perhaps became the source of some or all of the land pitch deposits now found between the lake and the sea.
The surface of the deposit or lake is not a uniform expanse of pitch. It is grassy along the edges and becomes free from vegetation at some distance from the center. Shrubs and small trees occur in a few cases known as islands. These patches move from place to place with the movement of the pitch at the surface. The main mass of asphalt is a broad expanse of pitch made up of separate areas of irregular outline, but at times quite circular, which are separated by channels, filled with rain water, which prevents their coalescence. The boundaries are depressed and the center of the areas is always somewhat elevated above the edges, that is to say, they are mushroom-like. The origin of the separate areas evidently lies in the constant movement of the crude material, due
to the evolution of gas at the center, from which point the pitch rolls over toward the edges. This is shown by the fact that pieces of wood which emerge erect at the center are gradually carried to the circumference, their deflection from the perpendicular increasing as the distance from the center increases. At the channel they topple over and are again engulfed in the pitch. This illustrates very well the activity of the entire surface of the deposit, although it is much more active near the center of the lake.
As to the depth of the lake, borings made in 1894 at the center, were carried to a depth of 135 feet, by means of a wash drill, the entire distance being through asphalt of the same character as that at the surface. This result shows the great depth of the crater, and the uniformity
of the material which it contains. At the pitch lake, therefore, at a point 138 feet above the sea, there is a bowl-like depression, more than two thousand feet across, and over 135 feet deep, reaching to the sea level, and filled with a uniform mass of asphalt, a mass which must amount to many million of tons, making it the largest deposit of solid native bitumen in the world.
The material forming this deposit is an emulsion of water, gas, bitumen and mineral matter, the latter consisting largely of fine sand and a lesser amount of clay. It is in constant motion, owing to the evolution of gas, and for this reason whenever a hole is dug in the surface, whether deep or shallow, it rapidly fills up, and the surface resumes its original level after a short time. While sufficiently soft to accommodate itself to any change of level and to slow movement, it can be readily flaked out with picks, in large conchoidal masses weighing from 50 to 75 pounds. It is honeycombed with gas cavities, and resembles in appearance the structure of a Swiss cheese. The gas evolved by the pitch consists of hydrogen sulphide and carbon dioxide, but in such proportions that it burns readily when a match is applied, although the flame is not maintained for any length of time.
In the center of the so-called lake is a point where there is a continued influx of soft material, accompanied by a stronger evolution of gas, which gradually hardens and becomes like the remainder of the deposit. The point of evolution of the soft material moves about from
place to place along lines of least resistance. As that evolved at one point hardens the fresh material breaks out elsewhere. It is peculiar in that it is associated with so much free water which rises with it, that it can be handled freely and made into a ball without adhering to the hands. It is in an active state of change, since if it is sealed in a tin can the gas evolved will, in a few weeks, burst the containing vessel.The temperature of the pitch at the surface is no greater than that of the surrounding air except when it is exposed to the noon-day sun, when it may rise to 130° F. or over. That of the soft pitch is no higher than any other part of the deposit.
|Water and gas, volatilized at 100° C||29.0|
|Bitumen soluble in cold carbon disulphide||39.0||56.5|
|Bitumen retained by mineral matter||.3||.3|
|Mineral matter, on ignition with tricalcium-phosphate||27.2||38.5|
|Water of hydration in clay and silicate||3.3||4.2|
Composition of the Asphalt
An examination of the asphalt shows that it is one of extreme uniformity in composition. It consists of, according to the most refined methods of analysis, from whatever part of the deposit it is taken:
The average composition of samples taken on circles 200, 400, 600, 800, 1,000 and 1,100 feet from the center of the deposits, and for the average material from the entire depth of the boring, as determined by routine methods, was found, after drying the material to free it from the water which is present, to be as follows:
Average Composition of Trinidad Lake Pitch in Circles
|Bit. by CS2, Per Cent.||Mineral Matter, Per Cent.||Organic not Soluble, Per Cent.||Soluble in Naphtha, Per Cent.||Total Bitumen thus Soluble, Per Cent.|
|Circle 2, 200 ft. from center||55.02||35.41||9.57||31.83||57.85|
|Circle 4, 400 ft. from center||54.99||35.40||9.61||31.63||57.55|
|Circle 6, 600 ft. from center||54.84||35.59||9.67||31.85||58.26|
|Circle 8, 800 ft. from center||54.66||35.56||9.78||31.67||57.97|
|Circle 10, 1,000 ft. from center||54.78||35.44||9.78||31.58||57.64|
|Circle 12, 1,100 ft. from center||54.62||35.45||9.93||31.77||57.51|
|Circle 14, 14,000 ft. from center||53.86||36.38||9.76||30.52||56.66|
The great uniformity of the deposit is revealed by these figures.
The water is probably of thermal origin, as it contains borates and iodides. Chlorides and sulphates of sodium are the predominating salts, sulphate of ammonia in marked amount, while chloride of potassium, lime and magnesium and ferrous iron are present. It is impossible to separate the water from the bitumen without change, but in the old methods of refining pools of it would collect on the surface of the asphalt, and this water, although somewhat concentrated and oxidized, had the following composition:
Analysis of Asphalt Water
In 1 kilogram
|I||.0008||C's and Rb||None|
The percentage of soluble mineral matter or salts as compared to that of the bitumen is extremely small and unimportant, and plays no part in the behavior of the material industrially, since the addition of the same salts to other asphalts has been found to produce no perceptible effect upon them.
For many years Trinidad asphalt was supposed to contain a considerable amount of organic matter not of a bituminous nature, but an investigation conducted by me in 1908 showed that this really consists
of the water of hydration of the clay forming part of the mineral matter, which was lost on ignition after the removal of the bitumen by solvents, and that, as a matter of fact, there is practically nothing of the nature of the organic matter not bitumen which has heretofore been attributed to Trinidad asphalt.
In direct contrast to this acid water is that which rises with the soft pitch in the center of the lake, which is alkaline in reaction, and has the following composition:
|Solids at 110° C||82||.100|
|Sulphuric acid, SO3||3||.207|
|Calcium oxide, CaO||Trace|
|Magnesium oxide, MgO||0||.506|
|Carbonic acid, CO2||3||.700|
The emulsified water can be removed from the crude Trinidad asphalt by grinding it to a fine powder and exposure to the air, or by heating the material to a temperature above the boiling point of water, and until the bitumen melts. Upon the latter fact is based the process of refining which is used industrially, and which will be described later.
The bitumen of Trinidad asphalt can be separated from the mineral matter by solvents, and thus prepared is a brilliant, glossy, pitchlike substance, which has a semi-conchoidal fracture when struck a sharp blow, but which yields to gentle pressure and slowly flows at summer temperatures. It softens at 76° C, flows quickly at 83° C, but is not liquid until a temperature is reached which is above 100° C. It has a specific gravity, when entirely free from mineral matter, of 1.032 at 25° C.
Its ultimate composition is
It is noticeable that this bitumen is characterized by the large percentage of sulphur which it contains, by the presence of nitrogen, and by the absence of oxygen.
It is to the sulphur which is present, as will be shown later on, that the valuable properties of the Trinidad bitumen are to be attributed.
It consists, like all the other more or less solid native bitumens, of components of two different classes, which can be separated by solvents;
those which are soluble in light petroleum naphtha, and the remainder which are insoluble, although the percentage will depend to a certain extent on the character of the solvents. The softer or oily portion which is soluble in naphtha is, for 88° naphtha at air temperatures, about 63 per cent. The components of Trinidad asphalt and of other bitumens, which are thus soluble, have been denominated by the writer "Malthenes" a name to be applied to this class of hydrocarbons, not as representing any homogeneous entity, but merely descriptive of their general character. The term petrolenes has also been used to cover this same class of material.
The hydrocarbons and their derivatives which are insoluble have been called asphaltenes.
The malthenes of Trinidad asphalt are distinguished by the fact that they are of an extremely sticky and cementitious nature, and not merely oily as is often the case with material of similar consistency prepared from petroleums. The value of any bitumen or combination of bitumens for highway construction depends on the character of the malthenes of which it is composed, and the relative proportion of these to the asphaltenes. Where the former are not present in sufficient amount, it is necessary to add to the asphalt material in which malthenes predominate to attain a proper consistency. This is known as fluxing the asphalt, and in the case of that of Trinidad, owing to the presence of the large amount of malthenes of a sticky nature, it can be accomplished
by the use of any of the fluxes that are available, even those of a paraffine nature, since the paraffine hydrocarbons, containing a considerable per cent, of paraffine scale, combine with the malthenes of the Trinidad asphalt to produce a satisfactory binding material. If the malthenes were not present to the extent in which they are found, it would be necessary to use an asphaltic flux, as for example in fluxing the harder bitumens, such as gilsonite and grahamite.
While the malthenes give to the asphalt its cementitious property, the asphaltenes impart cohesiveness as distinguished from adhesiveness, and supply body and stability to the binding material. It has been found that asphalt cement, that is to say, a solid asphalt combined with a suitable flux, must contain not less than 15 per cent, of asphaltenes or will otherwise lack cohesiveness and stability, while on the other hand, if it contains less than 70 per cent, of malthenes it will not be sufficiently adhesive. Even with the proper proportions of malthenes and asphaltenes a bitumen may still be valueless as a cement, if the malthenes are not of a proper character, that is to say, not sticky and adhesive.
Trinidad asphalt, owing to the character of and proportions in which its components are present, has been found to possess to the highest degree, the properties which are necessary for adhesiveness and stability, and it is to this that we may attribute the fact that it has proved itself to be a standard material with which to compare other bitumens.
In what has been said hitherto reference was had to the asphalt taken from the lake deposit itself. As has been mentioned, there are deposits of bitumen between the lake and the sea, and the material taken therefrom is known as land pitch. It has been subject to atmospheric weathering and by contact with the soil for years, as a result of which its physical properties are inferior to those possessed by the lake deposit. The greater the distance from the lake at which the land pitch is found, the more it shows signs of age and weathering and the more inferior it is.
There has recently been developed on the Island of Trinidad, in the neighborhood of the pitch lake, a new source of binding or cementing material of a very desirable character for surface application to roads, and for the productions of a residual pitch of a high grade. It has been called a liquid asphalt, and must have been the source of the asphalt found in the main deposit, and of the material which now forms the soft spot in the middle of the lake. It was recognized in the earliest days of the Trinidad asphalt industry that the millions of tons of asphalt existing in the old crater must be associated with some mother
source of bitumen. Long before the asphalt was used on any industrial scale, attempts to reach this and obtain petroleum were made. A comparatively shallow well was sunk not far from the lake, and a heavy liquid asphalt was discovered, but the facilities available at that time for sinking a deep well, and the lack of demand or means of utilizing this material, caused development to be abandoned. Success has now crowned the efforts to obtain this liquid asphalt in commercial quantities, and it is now available for road construction. It has proved to be a most remarkable and unique form of bitumen. Primarily it is, of course, a petroleum, being a liquid form of bitumen coming from a depth of about 900 feet below the surface. Its characteristics as determined by the usual form of distillation proposed by Engler, and generally followed by oil experts, appears from the following data:
|Specific gravity at 60° F||.965|
|Beaume at 60° F||15||.1°|
|Bitumen sol. in CS2||99||.9 per cent.|
|Bitumen insol. in 88° naphtha||8||.3 per cent.|
|Loss 5 hours 325° F. (20 grams)||24||.6 per cent.|
|Condition of residue||Fluid|
|Viscosity—Engler—212° F.—50 c.c.||34||.5 sec.|
|Below 302° F||4.43||per cent.|
|302-520° F||20.74||per cent.|
|Residue above 520° F||74.53||per cent.|
This petroleum is truly asphaltic and carries no solid or heavy liquid paraffine hydrocarbons. It is distinguished by the fact that it yields a high percentage of light distillates or "tops" for an oil of such low gravity. The intermediate distillates, those of the lubricating type, are small in amount, while the residue is truly asphaltic resembling that found in the lake deposit, but of course free from mineral matter and water. On this account the oil is peculiarly adaptable to road surfacing work, the light oil carrying the heavier asphaltic portion into the surface and afterwards, on its evaporation under the sun, leaving it there in a most desirable form as a binding agent. If the lighter fraction or "tops" are removed, we have at once an asphaltic oil which is of the most desirable character for hot application, and has the following characteristics:
|Specific gravity at 60° F||.994|
|Beaume at 60° F||10||.8°|
|Bitumen sol. in CS2||99||.9 per cent.|
|Bitumen insol. in 88° naphtha||10||.8 per cent.|
|Loss 5 hours 325° F. (20 grams)||17||.5 per cent.|
|Condition of residue||Fluid|
|Viscosity—Engler—212° F—50 c.c.||102||.2 sec.|
The oil is further distinguished by the fact that it carries a very considerable percentage of sulphur, in the neighborhood of 3 per cent, and it is evident that the sulphur found in the Trinidad crude asphalt is derived, at least in part, from this source. Owing to the presence of this sulphur the oil possesses those desirable characteristics, as a road oil, which the refined Trinidad asphalt possesses as a paving material, and it is for this reason that a road carpet prepared with this liquid asphalt does not become unpleasantly soft when exposed to the sun.
(To be concluded)