The Economic Aspect of Geology Part 44

This committee did useful work in correlating geological activities, mainly outside of Washington, and in cooperation with the War Department kept in touch with the geologic work being done at the front.

While the activities of geologists for government, state, and private organizations were for the most part in relation to mineral resource questions, this was by no means the total contribution. The U. S.

Geological Survey and other organizations, in cooperation with the War Department, did a large amount of topographic and geologic mapping of the eastern areas for coast-defense purposes. This work involved consideration of the topography for strategic purposes, as well as the stock-taking of mineral resources--including road materials and water supplies. The revision of Geological Survey folios, with these requirements in mind, brought results which should be of practical use in peace time. Studies were likewise made of cantonment areas, with reference to water supplies and to surface and sub-surface conditions.

Many geologists were engaged in the military camps at home and abroad, and in connection with the Student Army Training Corps at the universities, in teaching the elements of map making, map interpretation, water supply, rock and soil conditions in relation to trenching, and other phases of geology in their relation to military operations. The textbook on Military Geology,[61] prepared in cooperation by a dozen or more geologists for use in the courses of the Student Army Training Corps, is an admirable text on several phases of applied geology. The name of the book is perhaps now unfortunate, because most of it is quite as well adapted to peace conditions as to those of war. There is no textbook of applied geology which covers certain phases of the work in a more effective and modern way. The topics treated in this book are rocks, rock weathering, streams, lakes and swamps, water supply, land forms, map reading and map interpretation, and economic relations and economic uses of minerals.

Another book,[62] on land forms in France, prepared from a physiographic standpoint, was a highly useful general survey of topographic features and was widely used by officers and others.

GEOLOGY AT THE FRONT[63]

Perhaps the most spectacular and the best known use of geology in the war was at and near the front. This use reached its earliest and highest development in the German army, but later was applied effectively by the British and British Colonial armies, and by the American Expeditionary Force.

One of the first intimations to the American public of the use of geology at the front appeared in the publication of German censorship rules in 1918,--when, among the prohibitions, there was one forbidding public reference to the use of earth sciences in military operations. A leading American paper noted this item and speculated at some length editorially as to what it meant.

It was discovered that geologists to the number of perhaps a hundred and fifty were used by the Germans to prepare and interpret maps of the front for the use of officers. Features represented on these maps included topography; the kinds of rocks and their distribution; their usefulness as road and cement materials; their adaptability for trench digging, and the kinds and shapes of trenches possible in the different rocks; the manner in which material thrown out in trenching would lie under weathering; the ground-water conditions, and particularly the depth below the surface of the water table at different times of the year and in different rocks and soils; the relation of the ground-water to possibilities of trench digging; water supplies for drinking purposes; the behavior of the rocks under explosives, and the resistance of the ground to sh.e.l.l-penetration; the underground geological conditions bearing on tunnelling and underground mines; and the electrical conductivity of rocks of different types, presumably in connection with sound-detection devices and groundings of electric circuits. Some of the captured German maps were models of applied geology. They contained condensed summaries of most of the features above named, together with appropriate sketches and sections. During the Argonne offensive by the American army the captured German lines disclosed geologic stations at frequent intervals, each with a full equipment of maps relating to that part of the front. From these stations schools of instruction had been conducted for the officers in the adjacent parts of the front.

The British efforts were along similar lines, although they came late in the war, under the leadership of an Australian geologist. Their efforts were especially useful in connection with the large amount of tunnelling and mining done on the British front. Among the many unexpected and special uses of geology might be cited the microscopical identification of raw materials used in the German cement. It became necessary for certain purposes to know where these came from. The microscope disclosed a certain volcanic rock known to be found in only one locality. In the Palestine campaign, the knowledge of sources of road material and water supply based on geologic data was an important element in the advance over this arid region. Wells were drilled and water pipes laid in accordance with prearranged plans.

In spite of the fact that the usefulness of geology had been clearly indicated by the experience of the German and British armies, the American Expeditionary Force was slow to avail itself in large measure of this tool; but after some delay a geologic service was started on somewhat similar lines under the efficient leadership of Lieutenant-Colonel Alfred H. Brooks, Director of the Division of Alaskan Resources in the U. S. Geological Survey. The work was organized in September, 1917, and during the succeeding ten months included only two officers and one clerk. For the last two months preceding the armistice there was an average of four geologic officers on the General Staff, in addition to geologists attached to engineering units engaged in road building and cement making, and plans had been approved for a considerable enlargement of the geologic force. The work was devoted to the collection and presentation of geologic data relating to (1) field works; (2) water supply; and (3) road material. Of these the first two received the most attention. Maps were prepared, based somewhat on the German model, for the French defenses of the Vosges and Lorraine sectors, and for the German defenses of the St. Mihiel, Pont-a-Mousson, and Vosges sectors. Water supply reports covered nearly 15,000 square kilometers. The following description of the formations, taken from the legend of one of the geologic maps, shows the nature of the data collected:

_Silt, clay and mud, with some limestone gravel_, usually more or less saturated, except during dry season (June to September), in many places subject to flooding. Surface usually soft except during Summer. These deposits are 1/2 to 2 meters thick in the small valleys, and 2 to 3 meters in the ---- Valleys. Unfavorable to all field works on account of ground-water and floods, and not thick enough for cave shelters.

_Silts with some clay and fine sands and locally some fine gravel and rock debris._ These deposits occur princ.i.p.ally on summits and slopes, and are probably from 1 to 2 meters thick.

Even during dry season (June to September) they retain moisture and afford rather soft ground. In wet season the formation is very soft and often muddy. In many places water occurs along bottom of these deposits. Favorable for trenches, but which require complete revetment, and ample provision for drainage, not thick enough for cave shelters; cut and cover most practical type of shelter.

_Clay at surface with clay shales below._ This deposit occurs in flats and is usually saturated for a depth of 1 to 2-1/2 meters, during wet season, for most of the year the surface is soft, but in part dries out in Summer. Deep trenches usually impossible, and even shallow trenches likely to be filled with water; defensive works will be princ.i.p.ally parapets revetted on both sides. Cave shelter construction usually impracticable, unless means be provided for sinking through saturated surface zone into the dry ground underneath. Cut and cover usually the most practical type of shelter in this formation.

_Clay at surface with calcareous clay shale and some thin limestone layers below._ This formation occurs in low rounded hills; surface saturated during wet weather, but terrain permits of natural drainage, and dries out during Summer; during wet season (October to May) the surface zone is more or less saturated, and ground may be muddy to a depth of a meter or more, ground-water level usually within two or three meters of surface. Trench construction easy, but requires complete revetment, and ample provision for surface drainage. Cave shelters can be constructed in this formation where the slope is sufficient to permit of drainage tunnels. The depth to ground-water level should always be determined by test shafts or bore holes in advance of dugout construction.

_Surface formation usually clay 1 to 2 meters in depth; below this is soft clay shales or soft limestone._ Surface usually fairly well drained, and fairly hard ground. In general, favorable for trenches and locally favorable for cave shelters. In some localities underground water prevents cave shelter construction. The presence or absence of underground water should always be determined by test shafts or bore holes in advance of dugout construction.

_Surface formation consisting of weathered zone 1/2 to 1-1/2 meters thick, made up of clay with limestone fragments and broken rock. Below is compact limestone formation._ The surface of this formation is usually fairly hard, and well drained except in wettest season. Trenches built in it require little revetting; very favorable for cave shelters, but requires hard rock excavation. Some thin beds of clay occur in some of the limestone, and at these a water bearing horizon will be found. Where a limestone formation rests on clay as near ---- a line of springs or seepages is usually found. Such localities should be avoided, or the field works placed above the line of springs or seepages. This formation is best developed in the plateau west of ----. Here it is covered by only a thin layer of soil, hard rock being close to the surface.

The limestones afford the only rock within the quadrangle which can be used for road metal.

_Quarries_ (in part abandoned).

_Limestone gravel pits._

_Locus of springs and seepages._ These should be avoided as far as possible in the location of field works, especially of dugouts. Field works should be placed above the lines of springs.

The water supply maps with accompanying engineer field notes are models of concise description of water supply conditions, with specific directions for procedure under different conditions. A few paragraphs taken from these notes are as follows:

Ground overlying rock, such as limestone, compact sandstone, granites, etc., which are usually fractured, is from the standpoint of underground water, most favorable for siting of field works. Clay shales and clay hold both surface and underground water, and are, therefore, unfavorable for field works. The contact between hard rocks resting on clay or clay shales is almost invariably water bearing, and should be avoided in locating field works.

At localities where impervious formations (clay, etc.) occur at or near the surface, they hold the water and form a superficial zone of saturation. This condition makes trench construction and maintenance difficult, and cave shelters can usually only be made by providing means of sinking through the saturated zone. The surface saturated zone often dries out in summer.

In pervious, or almost pervious rocks, the zone of saturation, or ground-water level, lies at much lower depth, and may permit of the construction of field works as well as cave shelters above it.

Underground water bearing horizons and water bearing faults should be avoided in locating field works.

Wherever there is any uncertainty about the underground water conditions, test shafts or bore holes should always be made in advance of the construction of extensive deep works.

EFFECT OF THE WAR ON THE SCIENCE OF ECONOMIC GEOLOGY

In general, the war required an intensive application of geology along lines already pretty well established under peace conditions. Much was done to make the application more direct and effective, and a vast amount of geologic information was mobilized. The general result was a quickened appreciation of the possibilities of the use of geology for practical purposes. Perhaps the most important single result was a wider recognition of the real relations of mineral resources to human activities, and of the international phases of the problem. More specifically, there was a most careful stock-taking of mineral resources and a consideration of the "why" of their commercial use. Many new resources were found, as well as new ways to utilize them.

FOOTNOTES:

[60] Now known as Economic Liaison Committee.

[61] _Military geology and topography_, Herbert E. Gregory, Editor.

Prepared and issued under the auspices of Division of Geology and Geography, National Research Council, Yale Univ. Press, New Haven, 1918.

[62] Davis, W. M., _Handbook of Northern France_, Harvard Univ. Press, Cambridge, 1918.

[63] For more detailed description of this subject the reader is referred to The use of geology on the Western Front, by Alfred H.

Brooks, _Prof. Paper 128-D_, _U. S. Geol. Survey_, 1920.

CHAPTER XX

GEOLOGY AND ENGINEERING CONSTRUCTION

Economic applications of geology are by no means confined to mineral resources (including water and soils). The earth is used by the human race in many other ways. Human habitations and constructions rest on it and penetrate it. It is the basis for transportation, both by land and water. Its water powers are used. In these various relations the applications of geology are too numerous to cla.s.sify, much less to describe. While only a few of these activities have in the past required the partic.i.p.ation of geologists, the growing size of the operations and increasing efficiency in their planning and execution are multiplying the calls for geologic advice. The nature of such applications of geology may be briefly indicated.[64]

FOUNDATIONS

The foundations of modern structures such as heavy buildings, especially in untried localities, require much more careful consideration of the substrata than was necessary for lighter structures. In planning such foundations, it is necessary to know the kinds of rocks to be excavated, their supporting strength, their structures, the difficulties which are likely to be caused by water, and other geologic features. Failure to give proper attention to these factors has led to some disastrous results.

The planning of foundations and abutments of bridges requires similar geologic knowledge. In addition, there must be considered certain physiographic factors affecting the nature and variation of stream flow and the migration of sh.o.r.e lines.

SURFACE WATERS

Construction of great modern dams is preceded by a careful a.n.a.lysis of sub-surface conditions, in regard to both the rocks and the water. It is necessary to know the supporting strength of the rocks in relation to the weight of the dam; to know whether the rocks will allow leakage around or beneath the dam; and to know whether there are any zones of weakness in the rocks which will allow shearing of foundations under the weight of the dam in combination with the pressure of the ponded water.

It is necessary to know whether the valley is a rock valley or whether it is partially filled with rock debris; if the latter, how deep this debris is, and its behavior under load and in a saturated condition.

Here again physiographic factors are of vital importance, both in relation to the history of development of the valley, and to questions of stream flow and reservoir storage.[65]

Construction of dams is only an item in the long list of engineering activities related to surface waters. River and harbor improvements of a vast range likewise involve geologic factors. Problems of wave action, sh.o.r.e currents, shifting of sh.o.r.es, erosion, and sedimentation, which are of great importance in such operations, have long occupied the attention of the geologist. They belong especially in the branch of the science known as physiography.

Geology in relation to underground water supplies is discussed in Chapter V.