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Engineering
geology
The application of education and
experience in geology and other geosciences to solve geological problems posed
by civil engineering works. The branches of the geosciences most applicable are
surficial geology, petrofabrics, rock and soil mechanics, geohydrology, and
geophysics, particularly exploration geophysics and earthquake seismology. This
article discusses some of the practical aspects of engineering geology.
Terminology
The terms engineering geology and
environmental geology often seem to be used interchangeably. Specifically,
environmental geology is the application of engineering geology in the solution
of urban problems; in the prediction and mitigation of natural hazards such as
earthquakes, landslides, and subsidence; and in solving problems inherent in
disposal of dangerous wastes and in reclaiming mined lands.
Another relevant term is
geotechnics, the combination of pertinent geoscience elements with civil
engineering elements to formulate the civil engineering system that has the
optimal interaction with the natural environment.
Engineering
properties of rock
The civil engineer and the
engineering geologist consider most hard and compact natural materials of the
earth crust as rock, and their derivatives, formed mostly by weathering
processes, as soil. A number of useful soil classification systems exist.
Because of the lack of a rock classification system suitable for civil
engineering purposes, most engineering geology reports use generic
classification systems modified by appropriate rock-property adjectives. See also: Rock; Rock mechanics; Soil
mechanics
Rock sampling
The properties of a rock element can
be determined by tests on cores obtained from boreholes. These holes are made by
one or a combination of the following basic types of drills: the rotary or core
drill, the cable-tool or churn drill, and the auger. The rotary type generally
is used to obtain rock cores. The rotary rig has a motor or engine (gasoline,
diesel, electric, or compressed air) that drives a drill head that rotates a
drill rod (a thick-walled hollow pipe) fastened to a core barrel with a bit at
its end. Downward pressure on the bit is created by hydraulic pressure in the
drill head. Water or air is used to remove the rock that is comminuted (chipped
or ground) by the diamonds or hard-metal alloy used to face the bit. The core
barrel may be in one piece or have one or two inner metal tubes to facilitate
recovery of soft or badly broken rock (double-tube and triple-tube core
barrels). The churn-type drill may be used to extend the hole through the soil
overlying the rock, to chop through boulders, occasionally to deepen a hole in
rock when core is not required or to obtain drive samples of the overburden
soils. When the rock is too broken to support itself, casing (steel pipe) is
driven or drilled through the broken zone. Drill rigs range in size from those
mounted on the rear of large multiwheel trucks to small, portable ones that can
be packed to the investigation site on a person's back or parachuted from a
small plane. See also: Drilling,
geotechnical
The rock properties most useful to
the engineering geologist are compressive and triaxial shear strengths,
permeability, Young's modulus of elasticity, erodability under water action, and
density (in pounds per cubic foot, or pcf).
Compressive
strength
The compressive (crushing) strength
of rock generally is measured in pounds per square inch or kilograms per square
centimeter. It is the amount of stress required to fracture a sample unconfined
on the sides and loaded on the ends (Fig. 1). If the load P of 40,000 lb is
applied to a sample with a diameter of 2 in. (3.14 in.2), the compressive stress
is 40,000 ÷ 3.14 = 12,738 lb/in.2 (177,920 N ÷ 0.00203 m2 = 87,645 kN/m2). If
this load breaks the sample, the ultimate compressive strength equals the
compressive stress acting at the moment of failure, in this case 12,738 lb/in.2.
The test samples generally are cylindrical rock cores that have a
length-to-diameter ratio (L/D) of about 2. The wide variety of classification
systems used for rock results in a wide variation in compressive strengths for
rocks having the same geologic name. The table gives a statistical evaluation of
the compressive strengths of several rocks commonly encountered in engineering
geology.
Fig. 1 Unconfined compression test. (a) Shear
failure, showing failure planes QS. (b) Tension failure. (After D. P. Krynine
and W. R. Judd,Principles of Engineering Geology and Geotechnics, McGraw-Hill,
1957)
Most laboratory tests show that an
increase in moisture in rock causes a decrease in its compressive strength and
elastic modulus; what is not generally known, however, is that the reverse
situation shown in Fig. 2 has been encountered in certain types of volcanic
rocks. In sedimentary rock the compressive strength is strongly dependent upon
the quality of the cement that bonds the mineral grains together (for example,
clay cement gives low strength) and upon the quantity of cement (a rock may have
only a small amount of cement, and despite a strong bond between the grains, the
strength is directly related to the inherent strength of the grains). Strength
test results are adversely affected by microfractures that may be present in the
sample prior to testing, particularly if the microfractures are oriented
parallel to the potential failure planes.
Fig. 2 Increase in Young's modulus caused by
saturation of dacite porphyry. (After J. R. Ege and R. B. Johnson, Consolidated
tables of physical properties of rock samples from area 401, Nevada Test Site,
U.S. Geol. Surv. Tech. Letter Pluto-21, 1962)
The value of compressive strength to
be used in an engineering design must be related to the direction of the
structure's load and the orientation of the bedding, discontinuities, and
structural weaknesses in the foundation rock. This relationship is important
because the highest compressive strength usually is obtained when the
compressive stress is normal to the bedding. Conversely, the highest Young's
modulus of elasticity (E) usually results when the compressive stress parallels
the bedding. When these strength and elastic properties apparently are not
affected by the direction of applied load, the rock is described as isotropic;
if load applied parallel to the bedding provides physical property data that are
significantly different than those obtained when the load is applied normal
(perpendicular) to the bedding, the rock is anisotropic or aeolotropic. If the
physical components of the rock element or rock system have equal dimensions and
equal fabric relationships, the rock is homogeneous; significant variance in
these relationships results in a heterogeneous rock. Most rocks encountered in
foundations and underground works are anisotropic and heterogeneous.
Shear in
rocks
Shearing stresses tend to separate
portions of the rock (or soil) mass. Faults and folds are examples of shear
failures in nature. In engineering structures, every compression is accompanied
by shear stresses. For example, an arch dam compresses the abutment rock and, if
the latter is intersected by fissures or weak zones, it may fail in shear with a
resulting tensile stress in the dam concrete that may rupture the concrete. The
application of loads over long periods of time on most rocks will cause them to
creep or even to flow like a dense fluid (plastic flow). See also: Structural geology
Ambient
stress
This type of stress in a rock system
is actually potential energy, probably created by ancient natural forces, recent
seismic activity, or nearby human-caused disturbances. Ambient (residual,
stored, or primary) stress may remain in rock long after the disturbance is
removed. An excavation, such as a tunnel or quarry, will relieve the ambient
stress by providing room for displacement of the rock, and thus the potential
energy is converted to kinetic energy. In tunnels and quarries, the release of
this energy can cause spalling, the slow outward separation of rock slabs from
the rock massif; when this movement is rapid or explosive, a rock burst occurs.
The latter is a different phenomenon from a rock bump, which is a rapid upward
movement of a large portion of a rock system and, in a tunnel, can have
sufficient force to flatten a steel mine car against the roof or break the legs
of a person standing on the floor when the bump occurs. See also: Rock burst; Underground mining
One of two fundamental principles
generally is used to predict the possible rock load on a tunnel roof, steel, or
timber supports, or a concrete or steel lining: (1) The weight of the burden
(the rock and soil mass between the roof and the ground surface) and its shear
strength control the load, and therefore the resultant stresses are
depth-dependent, or (2) the shear strength of the rock system and the ambient
stresses control the stress distribution, so the resultant loading is only
indirectly dependent upon depth. The excavation process can cause rapid
redistribution of these stresses to produce high loads upon supports some
distance from the newly excavated face in the tunnel. The geometry and span of
the opening also influence the stress distribution. Lined tunnels can be
designed so that the reinforced concrete or steel lining will have to carry only
a portion of the ambient or burden stresses. See also: Tunnel
Construction
material
Rock as a construction material is
used in the form of dimension, crushed, or broken stone. Broken stone is placed
as riprap on slopes of earth dams, canals, and river banks to protect them
against water action. Also, it is used as the core and armor stone for
breakwater structures. For all such uses, the stone should have high density
(±165 lb/ft3 or 2650 kg/m3), be insoluble in water, and be relatively nonporous
to resist cavitation. Dimension stone (granite, limestone, sandstone, and some
basalts) is quarried and sawed into blocks of a shape and size suitable for
facing buildings or for interior decorative panels. For exterior use, dimension
stone preferably should be isotropic (in physical properties), have a low
coefficient of expansion when subjected to temperature changes, and be resistant
to deleterious chemicals in the atmosphere (such as sulfuric acid). Crushed
stone (primarily limestone but also some basalt, granite, sandstone, and
quartzite) is used as aggregate in concrete and in bituminous surfaces for
highways, as a base course or embankment material for highways, and for railroad
ballast (to support the ties). When used in highway construction, the crushed
stone should be resistant to abrasion as fine stone dust reduces the
permeability of the stone layer; the roadway then is more susceptible to
settling and heaving caused by freezing and thawing of water in the embankment.
Concrete aggregate must be free of deleterious material such as opal and
chalcedony; volcanic rocks containing glass, devitrified glass, and tridymite;
quartz with microfractures; phyllites containing hydromica (illites); and other
rocks containing free silica (SiO2). These materials will react chemically with
the cement in concrete and release sodium and potassium oxides (alkalies) or
silica gels. Preliminary petrographic analyses of the aggregate and chemical
analysis of the cement can indicate the possibility of alkali reactions and thus
prevent construction difficulties such as expansion, cracking, or a strength
decrease of the concrete. See also:
Cavitation; Concrete; Petrography; Stone and stone products
Geotechnical
significance of soils
Glacial and alluvial deposits
contain heterogeneous mixtures of pervious (sand and gravel) and impervious
(clay, silt, and rock flour) soil materials. The pervious materials can be used
for highway subgrade, concrete aggregate, and filters and pervious zones in
earth embankments. Dam reservoirs may be endangered by the presence of
stratified or lenticular bodies of pervious materials or ancient buried river
channels filled with pervious material. Deep alluvial deposits in or close to
river deltas may contain very soft materials such as organic silt or mud. An
unsuitable soil that has been found in dam foundations is open-work gravel. This
material may have a good bearing strength because of a natural cement bond
between grains, but it is highly pervious because of the almost complete lack of
fine soil to fill the voids between the gravel pebbles. See also: Delta; Floodplain
Concrete or earth dams can be built
safely on sand foundations if the latter receive special treatment. One
requirement is to minimize seepage losses by the construction of cutoff walls
(of concrete, compacted clay, or interlocking-steel-sheet piling) or by use of
mixed-in-place piles 3 ft (0.9 m) or more in diameter. The latter are
constructed by augering to the required depth but not removing any of the sand.
At the desired depth, cement grout is pumped through the hollow stem of the
auger, which is slowly withdrawn while still rotating; this mixes the grout and
the sand into a relatively impervious concrete pile. The cutoff is created by
overlapping these augered holes. Some sand foundations may incur excessive
consolidation when loaded and then saturated, particularly if there is a
vibratory load from heavy machinery or high-velocity water in a spillway. This
problem is minimized prior to loading by using a vibrating probe inserted into
the sand or vibratory rollers on the sand surface or by removing the sand and
then replacing it under vibratory compaction and water sluicing.
Aeolian (windblown)
deposits
Loess is a relatively low-density
(±0.044 ton/ft3 or 1.4 metric tons/m3) soil composed primarily of silt grains
cemented by clay or calcium carbonate. It has a vertical permeability
considerably greater than the horizontal. When a loaded loess deposit is wetted,
it rapidly consolidates, and the overlaying structure settles. When permanent
open excavations (“cuts”) are required for highways or canals through loess, the
sides of the cut should be as near vertical as possible: Sloping cuts in loesses
will rapidly erode and slide because of the high vertical permeability. To avoid
undesirable settlement of earth embankments, the loess is “prewetted” prior to
construction by building ponds on the foundation surface. Permanently dry loess
is a relatively strong bearing material. Aeolian sand deposits present the
problem of stabilization for the continually moving sand. This can be done by
planting such vegetation as heather or young pine or by treating it with crude
oil. Cuts are traps for moving sand and should be avoided. The failure of Teton
Dam in 1976 indicated, among other factors, that when loessial or silty soils
are used for core materials in dam embankments, it is important to take special
measures to prevent piping of the silts by carefully controlled compaction of
the core, by using up- and downstream filters, and by extraordinary treatment of
the foundation rock. See also:
Loess
Organic
deposits
Excessive settlement will occur in
structures founded on muskeg terrain. Embankments can be stabilized by good
drainage, the avoidance of cuts, and the removal of the organic soil and
replacement by sand and gravel or, when removal is uneconomical, displacement of
it by the continuous dumping of embankment material upon it. Structures imposing
concentrated loads are supported by piling driven through the soft layers into
layers with sufficient bearing power.
See also: Muskeg; Tundra
Residual
soils
These soils are derived from the
in-place deterioration of the underlying bedrock. The granular material caused
by the in-place disintegration of granite generally is sufficiently thin to
cause only nominal problems. However, there are regions (such as California,
Australia, and Brazil) where the disintegrated granite (locally termed DG) may
be hundreds of feet thick; although it may be competent to support moderate
loads, it is unstable in open excavations and is pervious. A thickness of about
200 ft (60 m) of DG and weathered gneiss on the sides of a narrow canyon was a
major cause for construction of the Tumut-1 Power Plant (New South Wales) in
hard rock some 1200 ft (365 m) underground. Laterite (a red clayey soil) derived
from the in-place disintegration of limestone in tropical to semitropical
climates is another critical residual soil. It is unstable in open cuts on
moderately steep slopes, is compressible under load, and when wet produces a
slick surface that is unsatisfactory for vehicular traffic. This soil frequently
is encountered in the southeastern United States, southeastern Asia, and South
America.
Clays supporting structures may
consolidate slowly over a long period of time and cause structural damage. When
clay containing montmorillonite is constantly dried and rewetted by climatic or
drainage processes, it alternately contracts and expands. During the drying
cycle, extensive networks of fissures are formed that facilitate the rapid
introduction of water during a rainfall. This cyclic volume change of the clay
can produce uplift forces on structures placed upon the clay or compressive and
uplift forces on walls of structures placed within the clay. These forces have
been known to rupture concrete walls containing 10.75-in.-diameter (19-mm) steel
reinforcement bars. A thixotropic or “quick” clay has a unique lattice structure
that causes the clay to become fluid when subjected to vibratory forces. Various
techniques are used to improve the foundation characteristics of critical types
of clay: (1) electroosmosis that uses electricity to force redistribution of
water molecules and subsequent hardening of the clay around the anodes inserted
in the foundation; (2) provision of adequate space beneath a foundation slab or
beam so the clay can expand upward and not lift the structure; (3) belling, or
increasing in size, of the diameter of the lower end of concrete piling so the
pile will withstand uplift forces imposed by clay layers around the upper part
of the pile; (4) treatment of the pile surface with a frictionless coating (such
as poly(tetrafluoroethylene) or a loose wrapping of asphalt-impregnated paper)
so the upward-moving clay cannot adhere to the pile; (5) sufficient drainage
around the structure to prevent moisture from contacting the clay; and (6)
replacement of the clay by a satisfactory foundation material. Where none of
these solutions is feasible, the structure then must be relocated to a
satisfactory site or designed so it can withstand uplift or compressive forces
without extensive damage. See also:
Clay
Silt may settle rapidly under a load
or offer a “quick” condition when saturated. For supporting some structures
(such as residences), the bearing capacity of silts and fine sands can be
improved by intermixing them with certain chemicals that will cause the mixture
to “set” or harden when exposed to air or moisture; some of the chemicals used
are sodium silicate with the later application of calcium chloride, bituminous
compounds, phenolic resins, or special cements (to form “soil cement”). The last
mixture has been used for surfacing secondary roads, for jungle runways in
Vietnam, and as a substitute for riprap of earth dams. Some types of silt
foundations can be improved by pumping into them soil-cement or clay mixtures
under sufficient pressures to create large bulbs of compacted silt around the
pumped area.
Geotechnical
investigation
For engineering projects, these
investigations may include preliminary studies, preconstruction or design
investigations, consultation during construction, and the maintenance of the
completed structure.
Preliminary
studies
These are made to select the best
location for a project and to aid in formulating the preliminary designs for the
structures. The first step in the study is a search for pertinent published
material in libraries, state and federal agencies, private companies, and
university theses. Regional, and occasionally detailed reports on local geology,
including geologic maps, are available in publications of the U.S. Geological
Survey; topographic maps are available from that agency and from the U.S. Army
Map Service. Oil companies occasionally will release the geologic logs of any
drilling they may have done in a project area. Air photos and other remote
sensing techniques such as pulsed or side-looking radar or false color can be
used to supplement map information (or may be the only surficial information
readily available). The U.S. Geological Survey maintains a current index map of
the air-photo coverage of the United States. The photos are available from that
agency, the U.S. Forest Service, the Natural Resources Conservation Service, and
commercial air-photo companies; for some projects, the military agencies will
provide air-photo coverage. The topographic maps and air photos can be used to
study rock outcrop and drainage patterns, landforms, geologic structures, the
nature of soil and vegetation, moisture conditions, and land use by humans
(cultural features). Airborne geophysical techniques using magnetometers or
gravimeters also may be useful to delineate surface and subsurface geologic
conditions. See also: Aerial
photography; Literature of science and technology; Remote sensing; Topographic
surveying and mapping
Field reconnaissance may include the
collection of rock and soil specimens; inspection of road cuts and other
excavations; inspection of the condition of nearby engineering structures such
as bridges, pavements, and buildings; and location of sources of construction
material. Aerial reconnaissance is essential at this stage and can be performed
best in helicopters and second-best in slow-flying small planes.
Preconstruction
Surface and subsurface
investigations are required prior to design and construction. Surface studies
include the preparation of a detailed map of surficial geology, hydrologic
features, and well-defined landforms. For dam projects, a small-scale geologic
map (for example, 1:5000) is made of the reservoir area and any adjacent areas
that may be directly influenced by the project; in addition, a large-scale
geologic map (for example, 1:500) is required of the specific sites of the main
structures (the dam, spillway, power plant, tunnels, and so on). [This preferred
means of designating map scales can be used for either customary or metric
units. It means 1 unit of measurement on the map is equal to 5000 similar units
on the ground; for example, 1 cm measured on the map is equal to 5000 cm
measured on the ground.] These maps can be compiled by a combination of field
survey methods and aerial mapping procedures. They should have a grid system
(coordinates) and show the proposed locations for subsurface investigations.
Subsurface investigations are
required to confirm and amplify the surficial geologic data. These may include
test pits, trenches, short tunnels (drifts or adits), and the drilling of
vertical, horizontal, or oblique (angle) boreholes. Geologic data obtained by
these direct methods can be supplemented by indirect or interpreted data
obtained by geophysical methods on the surface or in subsurface holes and by
installation of special instruments to measure strain or deformation in a
borehole or tunnel.
The geology disclosed by subsurface
investigations is “logged” on appropriate forms. Tunnel logs display visual
measurements of features and joint orientations (strike and dip); rock names and
a description of their estimated engineering properties; alteration, layering,
and other geologic defects; the location and amount of water or gas inflow; the
size and shape of blocks caused by fracturing or jointing and the width of
separation or the filling material between blocks; and the irregularities in the
shape of the tunnel caused by the displacement of blocks during or after
excavation (rock falls, rock bursts, chimneying, and overbreakage). Geophysical
seismic methods may be used to define the thickness of loosened rock around the
tunnel; geoacoustical techniques that detect increases in microseismic noise
during tunneling may be used to determine if the excavation is causing excessive
loosening in the tunnel rock. This detection of “subaudible rock noise”
occasionally is used to detect the potential movement of rock slopes in open
excavations.
The borehole data can be logged on a
form such as shown in Fig. 3. These data can be obtained by direct examination
of the core, by visual inspection of the interior of the borehole using a
borehole camera (a specially made television camera) or a stratoscope (a
periscopelike device), or by geophysical techniques. Direct viewing of the
interior of the hole is the only positive method of determining the in-place
orientation and characteristics of separations and of layering in the rock
system. The geophysical techniques include use of gamma-gamma logging that
evaluates the density of the rock surrounding the borehole or at depths as great
as 150 ft (45 m) beneath the gamma probe; neutron logging to determine the
moisture content of the rock system by measuring the depth of penetration of the
neutrons; traversing the borehole with a sonic logger that, by calibration,
measures differences in the velocity of wave propagation in different strata
(and thus can determine in place Young's modulus of elasticity and the thickness
of each stratum encountered by the borehole); and electric logging that uses
differences in the electrical resistivity of different strata to define their
porosity, moisture content, and thickness.
Occasionally a hole is drilled
through a talus deposit containing the same type of rock as the underlying rock
in place (bedrock). Because of the similarity in rock types, the talus-bedrock
contact sometimes is best identified by determining the orientation of the
remnant magnetism in the core: the magnetic lines in the core will have a
regular orientation, but the talus magnetism will have random directions. This
method is useful only in rocks that contain appreciable remnant magnetism such
as some basalts.
Geophysical seismic or electrical
resistivity methods also can be used on the ground surface to define the
approximate depth of bedrock or various rock layers. The results require
verification by occasional boreholes, but this is an inexpensive and
satisfactory technique for planning and design investigations. The seismic
methods are not useful when it is necessary to locate soft strata (wherein the
seismic waves travel at relatively low velocity) that are overlain by hard
strata (that have higher wave velocity); the latter conceal and block the signal
from the soft strata. Also, difficulties may occur when the strata to be located
are overlain by soil containing numerous large boulders composed of rock having
higher velocities than the surrounding soil, or when the soil is very compact
(such as glacial till) because its velocity characteristics may resemble those
of the underlying bedrock. Another problem is that the seismic method seldom can
identify narrow and steep declivities in the underlying hard rock (because of
improper reflection of the waves).
Construction
Geotechnical supervision is
desirable during construction in or on earth media. The engineering geologist
must give advice and keep a record of all geotechnical difficulties encountered
during the construction and of all geological features disclosed by excavations.
During the operation and maintenance of a completed project the services of the
engineering geologist often are required to determine causes and assist in the
preparation of corrective measures for cracks in linings of water tunnels,
excessive settlement of structures, undesirable seepage in the foundations of
dams, slides in canal and other open excavations, overturning of steel
transmission-line towers owing to a foundation failure, and rock falls onto
hydroelectric power plants at the base of steep canyon walls. The engineering
geologist also is considered an important member of the team assigned to the
task of assessing the safety of existing dams as now required by federal
legislation.
Legal
aspects
An important consideration for the
engineering geologist is the possibility of a contractor making legal claims for
damages, purportedly because of unforeseen geologic conditions (generally
referred to legally as charged conditions) encountered during construction.
Legal support for such claims can be diminished if the engineering geologist
supplies accurate and detailed geologic information in the specification and
drawings used for bidding purposes. These documents should not contain
assumptions about the geological conditions (for a proposed structure), but they
should show all tangible geologic data obtained during the investigation for the
project: for example, an accurate log of all boreholes and drifts and a drawing
showing the boundaries of the outcrops of all geological formations in the
project area. The engineering geologist should have sufficient experience with
design and construction procedures to formulate an investigation program that
results in a minimum of subsequent uncertainties by a contractor. Numerous
uncertainties about the geologic conditions not only can result in increased
claims but also may cause a contractor to submit a higher bid (in order to
minimize risks) than if detailed geologic information were available.
Special
geotechnical problems
In arctic zones, structures built on
permafrost may be heaved or may cause thawing and subsequent disastrous
settlement. The growth of permafrost upward into earth dams seriously affects
their stability and permeability characteristics. Obtaining natural construction
materials in permafrosted areas requires thawing of the borrow area to permit
efficient excavation; once excavated, the material must be protected against
refreezing prior to placement in the structures. Permafrost in rock seldom will
cause foundation difficulties. In planning reservoirs, it is essential to
evaluate their watertightness, particularly in areas containing carbonate or
sulfate rock formation or lava flows. These formations frequently contain
extensive systems of caverns and channels that may or may not be filled with
claylike material or water. Where extensive openings occur, grouting with cement
slurry or chemicals can be used as a sealant; however, as demonstrated by the
1976 failure of the Teton Dam, such measures are not always successful.
Sedimentation studies are required for the design of efficient harbors or
reservoirs because soil carried by moving water will settle and block or fill
these structures. In areas with known earthquake activity, structural design
requires knowledge of the intensity and magnitude of earthquake forces. The
prevention and rehabilitation of slides (landslides) in steep natural slopes and
in excavations are important considerations in many construction projects and
are particularly important in planning reservoirs, as was disastrously proved by
the Vaiont Dam catastrophe in 1963.
See also: Earthquake engineering; Permafrost
Geohydrologic
problems
In the foundation material under a
structure, water can occur in the form of pore water locked into the interstices
or pores of the soil or rock, as free water that is moving through openings in
the earth media, or as included water that is a constituent or chemically bound
part of the soil or rock. When the structure load compresses the foundation
material, the resulting compressive forces on the pore water can produce
undesirable uplift pressures on the base of the structure. Free water is
indicative of the permeability of the foundation material and possible excessive
water loss (from a reservoir, canal, or tunnel); uplift on the structure because
of an increase in hydrostatic head (caused by a reservoir or the like); or
piping, which is the removal of particles of the natural material by flowing
water with a consequent unfilled opening that weakens the foundation and
increases seepage losses.
The possibility of excessive seepage
or piping can be learned by appropriate tests during the boring program. For
example, water pressure can be placed on each 5-ft (2-m) section of a borehole,
after the core is removed, and any resulting water loss can be measured. The
water pressure is maintained within the 5-ft section by placing an expandable
rubber ring (packer) around the drill pipe at the top of the test section and
then sealing off the section by using mechanical or hydraulic pressure on the
pipe to force expansion of the packer. When only one packer is used, because it
is desired to test only the section of hole beneath it, it is a “single-packer”
test. In a double-packer test, a segment of hole is isolated for pressure
testing by placing packers at the top and the bottom of the test section. The
best information on the permeability characteristics of the rock can be obtained
by the use of three or more increments of increasing and then decreasing water
pressure for each tested length of hole. If the water loss continues to increase
when the pressure is decreased, piping of the rock or filling material in
fractures may be occurring or fractures are widening or forming. The
water-pressure test can be supplemented by a groutability test in the same
borehole. This test is performed in the same way as the water test except,
instead of water, a mixture of cement, sand, and water (cement grout) or a
phenolic resin (chemical grout) is pumped under pressure into the test section.
The resulting information is used to design cutoff walls and grout curtains for
dams. The pressures used in water-pressure or grouting tests should not exceed
the pressure exerted by the weight of the burden between the ground surface and
the top of the test section. Excessive test pressure can cause uplift in the
rock, and the resulting test data will be misleading.
Included or pore water generally is
determined by laboratory tests on cores; these are shipped from the borehole to
the laboratory in relatively impervious containers that resist loss of moisture
from the core. The cores with their natural moisture content are weighed when
received and then dried in a vacuum oven at about 110°F (45°C) until their dry
weight stabilizes. The percentage of pore water (by dry weight) is (wet weight −
dry weight) × 100 ÷ dry weight.
Temperatures up to 200°F (about
90°C) can be used for more rapid drying, provided the dried specimens are not to
be used for strength or elastic property determinations. (High temperatures can
significantly affect the strength because the heat apparently causes internal
stresses that disturb the rock fabric or change the chemical composition of the
rock by evaporation of the included moisture.)
Protective
construction
Civilian and military structures may
be designed to minimize the effects of nuclear explosions. The most effective
protection is to place the facility in a hardened underground excavation. A
hardened facility, including the excavation and its contents, is able to
withstand the effects generated by a specified size of nuclear weapon. These
effects include the amount of displacement, acceleration, and particle velocity
that occurs in the earth media and the adjacent structure. Desirable depths and
configurations for hardened facilities are highly dependent upon the shock-wave
characteristics of the surrounding earth media, for example, the type of rock,
discontinuities in the rock system, free water, and geologic structure.
Therefore, prior to the design and construction of such facilities, extensive
geotechnical field and laboratory tests are performed, including an accurate
geologic map of the surface and of the underground environment that will be
affected by the explosion. The map should show the precise location and
orientation of all geologic defects that would influence the wave path, such as
joints, fractures, and layers of alternately hard and soft rock. See also: Explosive; Nuclear explosion
Application of
nuclear energy
The use of nuclear energy for the
efficient construction of civil engineering projects has been investigated in
the Plowshare Program. Examples include rapid excavation, increasing production
of natural gas by opening fractures in the reservoir rock, expediting production
of low-grade copper ore by causing extensive fracturing and possible
concentration of the ore, and by the underground “cracking” of oil shale. The
production of electrical energy by nuclear fission requires engineering geology
inputs during the planning and design of the power plant; for example, a major
question to be answered is the presence or absence of faults and an estimate of
when the last movement on the fault occurred. This question of “active” faults
also is of increasing concern in the siting of dams.
Waste
disposal
Another geotechnical problem occurs in the use of nuclear energy for generation of power or radioisotopes: safe disposal of the radioactive waste products. These products can be mixed with concrete and buried in the ground or ocean, but geohydrologic or oceanographic conditions must not be conducive to the deterioration of the concrete. One proposed solution is to excavate large caverns in rock or salt a thousand or more meters deep; however, such a solution must consider possible contamination of ground water in the event that the waste products' containers leak. The disposal of toxic chemical or biological products in deep wells no longer is considered safe.
- F. G. Bell, Engineering Properties of Soil and Rocks, 4th ed., 2000
- B. M. Das, Principles of Geotechnical Engineering, 5th ed., 2001
- R. E. Goodman, Engineering Geology: Rock in Engineering Construction, 1993
- M. E. Harr, Groundwater and Seepage, 1992
- R. B. Johnson and J. V. DeGraff, Principles of Engineering Geology, 1989
- R. F. Leggett and A. W. Hatheway, Geology and Engineering, 3d ed., 1988
- R. F. Legget and P. K. Karrow, Handbook of Geology in Civil Engineering, 3d ed., 1982
- P. H. Rahn, Engineering Geology: An Environmental Approach, 2d ed., 1996
- Q. Zaruba, Landslides and Their Control, 2d ed., 1982
این وبلاگ تمامی موضوعات و مقالات و اطالاعات تخصصی زمین شناسی را که از سایتهای علمی جهان برگرفته شده در اختیار بازدیدکنندگان محترم قرار می دهد.گفتنی است که مطالب موجود در این وبلاگ در نوع خود بی نظیر بوده و از هیچ وبلاگ ایرانی ای کپی برداری نشده است و اگر هم شده منبع آن به طور کامل ذکر شده است.