Construction Dewatering and Groundwater Control: New Methods and The Third Edition of Construction Dewatering and G Show all. The Third Edition of Construction Dewatering and Groundwater Control, reflecting the most current technology and practices, is a timely and much- needed. INSTALLATION OF DEWATERING AND GROUNDWATER CONTROL SYSTEMS . 2. OPERATION .. slurry. (Construction techniques are still being developed.).
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Construction Dewatering and Groundwater Control J. Patrick Powers, Arthur B. Corwin, Paul C. Schm - Free ebook download as PDF File .pdf), Text File .txt) or . work. Construction dewatering. a. Need for groundwater control. Proper control of groundwater can greatly facilitate construction of sub-. Construction projects that extend below groundwater level present particular challenges. Construction dewatering is the range of techniques used to control .
The problems can vary from quite simple to extremely complex, depending on the nature of the beach deposit. When such a material is dewatered, the loss of buoyancy may cause the weakened cells to collapse when loaded, causing sudden and dramatic settlements. Amazon Rapids Fun stories for kids on the go. The pioneers in soil mechanicsTerzaghi, Arthur and Leo Casagrande, Taylor, Peck, and otherswere proposing theories and conducting laboratory investigations. Theoretically, a granular soil can exist in any state between the loosest possible orientation of its grains Fig. Characteristics of Natural Aquifers. Product details Hardcover:
Water in a conned aquifer can exist at pressures as high as its source, hence the owing well. Water trapped above the upper clay layer can become perched, and reappear as a small seep along the riverbank. Normally groundwater ows toward the stream, which is acting as a drain. However, if a dewatering system is operated as shown at left, the ow is reversed. The water in the stream, with its surface above the groundwater table, ows toward the ground.
Pumping, however, changes normal groundwater ow patterns; velocities increase sharply, sometimes approaching several feet per minute in the immediate vicinity of wells. Below the water table we say the soil pores are essentially saturated with water.
A more precise denition of the water table is difcult. Above the water table, soil moisture exists as disconnected droplets and capillary lms, while a substantial portion of the voids are lled with air. Below the water table, the water body is essentially continuous, except for an occasional bubble of air. Obviously, the transition from one to the other is not an abrupt plane, but a gradual zone.
An observation well placed in the soil will indicate a water level, sometimes referred to as the phreatic surface. In uniform aquifers the phreatic surface is a reasonable definition of the water table, provided that we understand its position can be modied by the effective size of the soil pores, by internal stresses in the soil, by the pattern of movement of groundwater particularly during periods of change, by the atmospheric pressure, and by the chemical and physical characteristics of the water itself.
So, much can be said for uniform aquifers. In the stratied soils that nature normally presents us with, the indicated phreatic surface in an observation well can be an average of several water tables and may have no physical signicance.
So we can see that the water table is far from a simple concept; its measurement, and the evaluation of its signicance to a construction project, can be complex.
Refer to Chapter 8 for a fuller treatment of water table measurement. An aquifer is a zone of soil or rock through which groundwater moves. A conned aquifer is a permeable zone between two aquicludes, which are conning beds of clay, silt, or other impermeable materials.
The development of a conned aquifer is illustrated in Fig. Water that inltrates the soil in the uplands gradually moves downward, eventually becoming trapped beneath an upper conning bed of clay. Depending on the elevation of the water source, and the hydraulic conductivity and rate of ow in the aquifer, the pressure in conned aquifers can rise to consid-.
Sometimes the head rises above ground surface so that artesian, or owing, wells can be constructed in the aquifer. The pressure in a conned aquifer will vary considerably depending on the rate of replenishment, the rate of discharge, and other factors, but the quantity of water stored in the aquifer changes only slightly. In a water table aquifer there is no upper conning bed.
The water table rises and falls with changing ow conditions in the aquifer. The amount of water stored in the aquifer changes radically with water table movements.
This storage effect is of great signicance to construction dewatering. A perched water table occurs when an impermeable layer of clay or silt blocks water seeping downward and saturates the sand above it, as shown in Fig. The sand below the clay is not saturated, so that the perched water is disconnected from the main ground water body. Perched water is typically of limited quantity, replenished or recharged very slowly. When encountered in an excavation, perched water will typically drain off very quickly, with limited continuous ow or bleeding, unless a source of recharge, such as a leaking utility, is present.
To summarize, we must conceive of groundwater as being in slow but constant motion; there is movement of water within aquifers and interchange of water between aquifers. There are continuing additions to the groundwater body by inltration from the ground surface and by recharge from lakes and inuent streams.
There are continuing subtractions of groundwater by evaporation and evapotranspiration, by seepage into efuent streams, and by pumping from wells. Patterns of groundwater movement change from time to time with changes in climate and with natural changes in topography due to erosion and deposition. And, of course, mankinds activities have been modifying the groundwater situation for millennia.
Land drainage projects lower the water table, dams and surface reservoirs encourage inltration, and when a river is conned within levees inltration is reduced. With mans wells for water supply and irrigation,. When mankind converts the land surface from woodland to farm, the recharge by inltration is reduced. When the farmland becomes covered with paved streets and buildings, recharge is reduced to very small levels.
Our activities in construction dewatering usually cause only temporary modication in groundwater patterns. But the structures created can make permanent changes. Human efforts to control water predate recorded history.
Amid the ruins of the great civilizations of Babylon and Egypt, we nd evidence of large aqueducts and even water tunnels. Many of the works were intended to supply water, but there were also land drainage projects to convert fetid marshes into arable land.
Indeed, the construction of the water supply works must have entailed some form of what we call dewatering. The biblical well of Jacob required excavation below the water table, and presumably some means to control the water during digging was developed. The ancient waterworks depended on gravity for transportation where possible. Lifting water, when unavoidable, was done manually with buckets until mechanical devices were gradually developed Fig.
The Dutch polders are great stretches of fertile land below sea level protected by dikes. The inhabitants of the Rhine delta have struggled with the North Sea for many centuries; the early dikes predate the Romans. When water is resisted by a dike, seepage through the dike and rain falling inside its protection must be pumped away. There is evidence that in what is now the Netherlands the work was Figure 1.
Then people learned to harness the wind with devices so successful that picturesque windmills dot the countryside to this day, although few are still in dewatering service behind the dikes. The search for gold, silver, and precious stones, and for useful materials such as copper and iron, sent people burrowing into the earth, and into direct conict with groundwater. By the eighteenth century, with the dawn of the Industrial Revolution, the demand for coal was justifying elaborate efforts to recover it.
The British coal mines pushed deeper and into more difcult water conditions. Endless rope conveyors powered by horses on treadmills removed water in buckets. In the s, James Watt set in motion a train of events that was to result in our modern pumping systems. Many of Watts early steam engines were used in mine dewatering.
They were clumsy devices by modern standards; the cylinder was made of wooden staves and the piston was wood with canvas packing. Steam in the cylinder was condensed by water injection. Vacuum moved the piston and a wooden linkage transmitted the power to the bucket conveyor. Watts economic studies convinced owners that the cost of the engine, plus the cost of the coal it consumed and the men who tended it, was less than buying and feeding horses.
Naturally, Watt rated each engine by the number of horses it replaced. The term horsepower persists to this day in both the English and metric systems. The practical inventions of Watt and his contemporaries came about because of a fundamental change in mans con-. Ancient beliefs were challenged, as exemplied by Galileo and da Vinci in the Renaissance, and Descartes and Newton in the Age of Enlightenment. No longer were natural phenomena to be accepted as mysterious and unknowable, but questioned, observed, and studied until the laws governing natural forces could be understood.
When the philosophers and scientists had made progress in the understanding of natural laws, the engineers and technologists of the Industrial Revolution made use of those laws to meet the needs of a burgeoning civilization. While the scientists were making discoveries in mechanics, chemistry, physics, and electricity, and the engineers were achieving great progress in construction, manufacturing, transportation, and communication, the understanding of groundwater remained dim.
Well into the twentieth century, our laws reected the common belief that underground seepage was unknowable, and the courts refused to intervene in groundwater disputes. As recently as , a book was published purporting to be a serious treatment on dowsing or water witching.
Clever people still collect fees for locating underground streams by the manipulation of forked sticks, brass rods, or pendulums. Explanations for the sluggish progress in understanding hydrology come readily to mind.
In the simplest aquifer situations, the mathematics of groundwater ow are complex. And most natural aquifers are far from simple, as will be seen in Chapter 5. Observation of groundwater levels is difcult, expensive, and often confusing. Orderly patterns are not easy to discern.
We cannot see the groundwater moving until it emerges into a stream or an excavation. And, so, the subject remained generally shrouded in mystery although some progress was being made.
Darcy stated his law of uid ow through porous media in But this science of hydrology did not reach maturity until determined people, faced with problems of major economic signicance, demanded a reasonable explanation for the observations they were making.
Robert Stephenson, the great British bridge and railroad builder, drew some strikingly pertinent conclusions during his work on the Kilsby Tunnel of the London and Birmingham Railway in the s. Stephenson made careful observations of the groundwater level in shafts, in boreholes, and in the tunnel face itself.
He concluded that there was a slope to the groundwater table created by his pumping and the slope was related to the resistance of the sand to water ow. The Kilsby tunnel was a very early application of predrainage, that process of removing water from the soil by wells, wellpoints, or other devices in advance of the excavation. No doubt there were earlier applications.
But in his work, Stephenson made observations in an effort to understand the process more clearly. His conclusions seem overly. Predrainage with wells continued to be applied in the nineteenth century, especially in Europe. But wells are normally successful only in favorable aquifer situations and no doubt there were many failures. It would be decades before wells with submersible electric pumps would be utilized for dewatering work.
At the end of the century, wellpoints began to appear. These small-diameter wells, driven into the ground and connected to a common suction manifold, were suitable for shallower aquifers where conventional wells had difculty functioning.
Wellpoints were used successfully in clean, ne to medium sands in Gary, Indiana, in , and in similar soils in Atlantic City, New Jersey, in succeeding years. In , Thomas Moore, a builder of trench machines, encountered difcult water conditions on a sewer project in Hackensack, New Jersey.
The soil was a very ne silty sand to sandy silt and driven wellpoints clogged up immediately. Moore introduced several innovative concepts: The ne-grained soils were effectively stabilized.
Moores success in New Jersey demonstrated that predrainage under very difcult conditions was practical, and dewatering techniques began to develop rapidly Fig. Self-jetting wellpoints with ball valves and rugged screens capable of repeated installation were introduced. The original wellpoint pumps were diaphragm or piston-type positive displacement units. These were replaced with highercapacity centrifugal pumps, continuously primed by positive displacement vacuum pumps.
Installation methods began to include holepunchers, casings, higher-pressure jetting pumps, and air compressors. As the equipment improved, engineers and contractors attempted bigger and deeper excavations, under increasingly difcult conditions. Much experimentation was done at the jobsite, on projects already under way. But it was soon recognized that the art of dewatering had to be reduced to a more scientic basis if predictable success was to be assured.
By the end of the s, engineers in the growing dewatering industry, like Thomas C. Gill and Byron Prugh, were recording and analyzing their observations. The pioneers in soil mechanicsTerzaghi, Arthur and Leo Casagrande, Taylor, Peck, and otherswere proposing theories and conducting laboratory investigations.
As early as the s, Meinzer was organizing relationships that could be used to understand groundwater ow. In the s, impelled by the growing economic signicance of groundwater for water supply and irrigation, hydrologists like Muskat, Theis, Jacob, Hantush, and others were developing practical techniques for aquifer testing and analysis.
These methods were later adapted to the solution of dewatering problems. Some dewatering problems deed solution by analytic techniques until powerful personal com-.
Courtesy Moretrench. Now approximate numerical solutions are available. New equipment and techniques for deep well construction, developed for oil exploration and for water supply wells, made wells a more practical tool for dewatering. Improved well screens and better understanding of gravel pack criteria made wells more efcient and suitable for less favorable soils. Improved drilling methods, such as the rotary, the reverse rotary, the down-the-hole drill, and the bucket auger, became available.
The submersible electric motor,. As will be seen in Chapter 18, todays improved well equipment and well construction techniques, together with better methods of aquifer analysis, make possible the dewatering of many projects with wells where the method would have been unsuccessful only a few decades ago.
The ejector system sometimes referred to as an eductor system for dewatering was adapted from the domestic jet. As discussed in Chapter 20, it is a most effective tool in certain job situations. Coincident with improvements in dewatering technology and equipment, other methods of groundwater control have been developed.
Cast-inplace slurry walls, jet grouting, and ground freezing have been used successfully to both cut off groundwater and support the sides of an excavation. Slurry trenches can cut off water ow. Electro-osmosis can reduce the moisture content of, and strengthen, ne-grained silts and clays. Sand drains and wick drains have proven useful in relieving pore pressure in ne-grained compressible soils during consolidation.
Each of these methods has had some degree of success in the specic job conditions to which they are suited. With the advances that have been achieved in the more than 80 years since Thomas Moore rst jetted his wellpoints to control quicksand in New Jersey, much of the mystery that once enshrouded groundwater has dissipated.
But con-. It is doubtful that it will ever be. The soil materials, the sources of water, and the demands of the project are too variable to be precisely analyzed. Any conclusions we base on theory must always be tempered by judgment and experience.
The successful practitioner in dewatering will be the person who understands the theory and respects it, but who refuses to let theory overrule judgment. When theoretical conclusions coincide with judgment, the dewatering engineer can proceed with the program with condence.
When there is disagreement, caution should be used until the discrepancy is understood. With appropriate regard to both theory and practical judgment, effective dewatering can be accomplished under almost any eld conditions Fig. However, because of the uncertainties of the underground, any proposed dewatering program must be exible, with provisions for modication if unexpected conditions are encountered.
In the experience of the authors of this book, it is atypical that a dewatering system, installed as it is designed, is successful without any modication. Flexibility is a key element in success. In succeeding chapters we will see how samples recovered from test borings using accepted procedures can enable us to learn much about how the ground might behave.
Knowledgeable observation and manipulation of the samples, followed by laboratory tests where appropriate, help us predict not only the behavior of the soil but how we might control groundwater so that the behavior of the soil in and around an excavation will be acceptable for our purposes. Other chapters will show us how to predict the movement of water within the soil and how we might manage that movement. We will discuss how to plan a geotechnical investigation to attempt to get the clearest possible picture of the soil and water conditions.
But even with the best-planned geotechnical investigation, we must acknowledge that we are sampling a minute fraction of the total body of soil that might affect the work.
This is where an understanding of geology can be most useful. If we recognize the geologic forces that created the soil deposits, the value of our judgmental interpolations, both vertically and horizontally between the samples we have observed, will be enhanced. If we understand the forces that formed the land we can see, and especially the land beneath our feet that we cannot see, our dewatering designs will be improved.
This chapter briey summarizes some geologic principles that are of particular interest to dewatering specialists. But understanding how the ground will behave can be improved by further reading in geology. We recommend to our readers the following works for more complete coverage. Press and Siever  is a broad overview of the geologic discipline. It is not written specically for geologists, so others can comprehend it more readily. Driscoll , Fetter , and Freeze and Cherry  are useful because they emphasize the impact of geology on groundwater occurence.
Leggett  and Krynine and Judd  are also recommended. Some geologic processes are going on in our own time; rivers erode their headwaters and deposit their deltas as we watch and then adjust our topographic maps accordingly.
Barrier islands along our coasts can change dramatically in a few stormy hours. Skilled geologists observing these phenomena can be of great help to us in predicting how the soils comprising these landforms will behave. We are well advised to make use of this knowledge. We must caution the reader, however, that with some exceptions, such as those mentioned, geologists cannot see the processes they are describing; many of their conclusions are inferred.
Those conclusions must be used with judgment, after comparing them with eld data. For example, one investigator we have read describes the plutonic rocks under northwestern North America as having a limited number of ssures which are small in dimension. Therefore, the hydraulic conductivity and specic yields of the rocks are low.
Another investigator tells us that the basalt under the Snake River valley in Idaho contains some of the most prolic aquifers ever encountered. Both geologists can be correct; one is speaking regionally, the other of a specic local area. It behooves the dewatering engineer to know the context in which the geology is discussed and the specic condition that exists at the site. Geologists differentiate formations and describe soil conditions based on geologic originhow and when the soils were deposited.
Engineers, on the other hand, separate soil strata and describe soil conditions based on soil classication and engineering characteristics. While there is purpose in providing detailed geologic descriptionsincluding the era of deposition and so forthin the geotechnical report, this information is often of limited value to the practicing geotechnical or dewatering engineer.
When possible,. The geologic forces that have formed and re-formed the earth with which the dewatering engineer must be concerned have been proceeding for a very long time, measured in millions of years.
Table 2. In the authors experience, construction dewatering projects are more frequent in deposits formed during the Recent, Pleistocene, or Cretaceous epochs.
But this pattern is not universal. A major drydock project in the late s took place in shelly sandstone of the Pliocene age. Soil formation begins with the breakdown of massive rock by weathering and erosion. The processes are many.
Rock can split from internal stresses or be split by tectonic movements of the earth. Rock surfaces exposed to the atmosphere, or rock close to the earths hot core, can crack under thermal expansion and contraction. At the surface, water seeps into the joints and in cold climates freezes there, forcing the joint to open further.
Water owing over the rock surface erodes it, assisted by the cutting action of sands and gravels moving with the water. The massive ice sheets we call glaciers override the rock, crushing, grinding, tearing, and plucking. Windborne sands cut and abrade. Natural acids and alkalis cause chemical disintegration. All these processes are very slow in human terms, but for geologic events there has been ample time. When the rock has disintegrated into ne particles, it may remain in place; we call such material residual soil.
More frequently, however, the soil particles are transported by water, ice, or wind and deposited in another location. Time millions of years 0. Large particles break into smaller ones as a result of being dragged and tumbled along; angular particles that originally fractured along crystalline planes become rounded and smooth.
In the transportation process, soils may be sorted into different sizes, with sands and gravels deposited in one area and silts and clays in another. Such soils are called uniform. Well-graded soils on the other hand contain a mixture of sizes, because they have not been waterborne far enough to complete the sorting. Well-graded soils can range from clean sands and gravels with moderate to high porosity and hydraulic conductivity, to glacial tills with very low hydraulic conductivity.
Soils once deposited may be scoured away and redeposited in a new location, undergoing further change in grading, grain size, and shape. Under certain conditions, soil deposits can become sedimentary rock. With proper moisture and the necessary overburden pressure, well-graded soil with clay can become hardpan; under compaction and with cementation, clay becomes shale; with cementing agents, sand and silt become sandstone and siltstone; by a complex biological and chemical process, limestone forms.
After soils or sedimentary rocks have formed, tectonic movement of the earths crust can shift them. Sedimentary rocks can be converted back to soil by weathering. Under the necessary conditions of temperature and pressure, sedimentary rock can become metamorphic rock; limestone, for example, can become marble. The mineral composition of most granular soils we encounter is some form of silica. This hard, durable, chemically inert mineral is best able to survive the processes of weathering and transportation.
In many soils, softer or more soluble grains have been eroded or leached away. However, we should not assume that all granular soils are silica. Oolites, a carbonate particulate material that may be encountered in Florida, for example, is subject to erosion and solutioning during lengthy pumping periods.
Soft coral limestone will sometimes erode quite rapidly. Volcanic soil particles may be vesicular; the grains themselves are porous, and low in specic gravity. Such soils are more sensitive to seepage pressures than silica soils of equivalent particle size. The clay mineralskaolinites, montmorillonites, and illiteshave a molecular structure that results in the platelike particle arrangement and distinctive properties of clays.
Clay properties are discussed at length in Chapter 3. Clay minerals can be a fascinating study . Organic constituents can markedly alter soil properties. Peat is saturated, partly decomposed wood and other vegetation that may retain a cellular structure. When such a material is dewatered, the loss of buoyancy may cause the weakened cells to collapse when loaded, causing sudden and dramatic settlements. Organic silts and silty clays in estuaries are sometimes quite compressible. Organic materials can af-.
The river is a conduit, moving water to the sea. In the process it is both a constructive and a destructive force on the land. On balance, the river erodes material in its headwaters, where the velocity is rapid, and creates deposits in its delta, where it debouches at lower velocity into the sea. But the processes of erosion and deposition take place throughout its length.
Alluvial deposits are the soils formed by rivers. The science of river sedimentation is quite complex. The fundamental relationship is Stokes law, which tells us that the particle size transported is a function of the water velocity. Hence the sorting action of rivers: Along the Mississippi River, we expect to nd alluvial sands and gravels in Minnesota and clays and silts in Louisiana. There is such a pattern, but there are variations throughout the valley.
We nd clays in Minnesota and sands in New Orleans. The basic velocity is determined by the fall of the river bed sometimes expressed in feet per mile meters per kilometer. The fall is not uniform; consider the Niagara River, which ows at reasonable velocity until it tumbles over a ft m cliff.
So, the base velocity varies and the actual velocity at any point along the river, and across it, is affected by the width, shape, and meanderings of the channel. In an oxbow, for example, eddy currents generated on the outside of the curve as the river changes direction create deposits of coarse particles. In extreme cases, these deposits are what we call openwork gravels gravel with little or no sand , the most permeable of natural soils.
The velocity also varies with the seasons as ow rate rises and falls in response to precipitation. High transient velocities occur in periods of heavy rainfall as the river level rises, until the increased volume can dissipate.
Previously deposited soils are scoured out and transported further downstream. Figure 2. The ood plain is the at, low-lying land through which the channel meanders. Tributaries cross the ood plain, feeding the channel. Terraces are remnants of an ancient ood plain, most of which has been scoured away in some later event.
During periods of high ow, the river may rise enough to cause inundation of the ood plain. During major oods, the inundating water may gouge out a new channel. When the ood recedes, the old channel becomes a quiet backwater that gradually lls with ne sediments and becomes invisible from the surface.
The original bed of the old channel remains beneath the ne sediments, however, and if the original bed material is clean sand and gravel it may continue to be the major conduit for groundwater ow down the valley. Such buried channels are quite common in.
Note in the plan view, Fig. The main channel is the trunk, its major tributaries the limbs, and the minor tributaries seem like branches and foliage. This is called a dendritic pattern. It is quite common for buried channels to have a similar shape.
There can be channels of clean sand and gravel, fed by lesser tributary veins. One such underground system, inferred from drilling and pumping observations of a river valley in Colorado, is shown by the dashed lines in Fig. Well A, located in the main buried channel, will also receive water from tributary channels, and through the tributaries. Well A has access to water stored in the siltier soils surrounding the clean sand veins. Well B, in a tributary, is not so favorably located and will have considerably less capacity.
The impact of such an underground channel system on dewatering operations is discussed in Chapter 7. If we consider the entire prole, from headwaters in the mountains to the mouth, we usually see certain patterns in deposition. Supercial deposits tend to be coarse and clean upriver, with silts and clays in the delta. Deep deposits at the delta may, however, be clean and coarse because of the situation early in the life of the river. It was common at that time for clean sands to be carried further downstream.
As the delta gradually builds up, the fall of the river is reduced, the velocity grows less, and the sands do not carry so far. The Mississippi delta is an excellent example of this situation.
A very large deposit of clean sand exists in the delta at a depth of some hundreds of feet m , and is as much as ft m in thickness. It forms a major aquifer for municipal and industrial water supplies and for irrigation.
When a river overows its banks and inundates the ood plain, its velocity decreases and ne-grained soils are deposited. These river bank deposits obstruct vertical recharge during subsequent inundations and can affect dewatering in the ood plain. It is typical for alluvial deposits to be stratied: Sometimes the strata are quite thin, perhaps a few inches 10 cm.
During a geotechnical investigation, careful observation is necessary when the split spoon sample is opened, otherwise the stratication may be missed. When we excavate in alluvium, we are cutting through the geologic history of the river that formed the deposits.
The better the understanding of the mechanisms of river deposition, the more accurate will be the predictions of dewatering behavior. When a rapidly running stream debouches into the quiet waters of a lake, its suddenly diminished velocity results in.
Bottom Section through river valley. Lakes begin when a stream is dammed by landslides, or by upheavals of the earth. Depressions in the surface of a river valley caused by tectonic earth movements can result in lakes. The lake is a transient phenomenon. It begins to die as soon as it is born, lling gradually with sediments eroded from its watershed. Organic and inorganic nutrients borne by the feeding streams create an environment suitable for the fascinating ecosystem of a mature lake: At the entrances, deltas form of coarse, clean sands.
Further down the lake the ner sediments are deposited. In mature lakes, the debris from all the biological activity becomes organic constituents in the resulting soils, markedly affecting their properties.
During periods of heavy ow, extremely ne sediments are ushed through the outlet and the soils deposited are somewhat cleaner; when ow dimin-. In cold climates, when the surface of the lake freezes, water motion virtually stops and the very nest particles settle to the bottom.
These variations create a varved structure, with very thin lenses varves of ne sand alternating with layers of silt or silt and clay. The varved structure of lake clay has a signicant effect on its properties. The horizontal hydraulic conductivity along the clean sand varves is much higher than the vertical hydraulic conductivity through the silt and clay layers.
Such a structure, if it is identied during the geotechnical investigation, can be used to advantage during dewatering. The presence of the sand varves accelerates drainage, improving the effectiveness and reducing the necessary time of dewatering. Given time, a lake will ll entirely and become a marsh. The marsh may later be covered with river or sheet sediments and disappear entirely until it becomes a problem to be solved by some later construction engineer.
When a river reaches the sea, its velocity slows and its sediments are deposited. There are some similarities between a lake and an estuary, but many signicant differences.
In an estuary, the river encounters tidal currents; near the mouth the ow reverses completely four times a day. The tidal effect can also extend many miles upstream, causing variable levels and velocities.
The wave action and storm currents can be violent in an estuary and constant shifts in deposition are common. Salt water has an effect on the deposition of clays and organic silts. The electro-chemical action occulates the clays, causing soft, compressible deposits, sometimes of considerable depth.
Foundations in such deposits can suffer severely from these conditions. Dewatering above or below a compressible silt or clay can cause ground settlement which may cause damage to adjacent structures unless appropriate measure are taken, as discussed in Chapter 3. Biologic activity in an estuary is different from that in a freshwater lake. We encounter buried tidal marsh deposits, sometimes called meadow mat because of the stringy remnants of vegetation that hold the material together.
Groundwater pumped from estuarine areas can contain gases, such as methane marsh gas , free carbon dioxide CO2 , and hydrogen sulde H2S with its easily recognizable smell of rotten eggs. Methane, being explosive, can be hazardous, particularly in conned spaces such as tunnels.
Free carbon dioxide is corrosive. Suldes can be toxic, are highly corrosive to dewatering equipment, create an obnoxious nuisance from the odor at the discharge, and can be destructive to sh.
The nature of a given shoreline is determined by a series of factors, including the composition of the soil or rock behind the high-water mark which is usually providing the material of which the beach is formed.
The formative mechanisms include runoff from the land, wave action and energy, especially during storms, abrasion, transportation and redeposition by wind, and the littoral currents in the sea parallel to the beach. Wharves, sea walls, intakes, outfalls, and other structures along the beach can be built by marine construction methods, but some degree of groundwater control is frequently required. The problems can vary from quite simple to extremely complex, depending on the nature of the beach deposit.
In the clean, ne, sand beaches that predominate along the mid-Atlantic coast of the United States, dewatering for structures to modest depth is routine. But on the rocky coasts of Maine installation problems can be severe. In sections of Florida, the Caribbean Islands, Hawaii, and the Middle East, coral formations can cause difcult installation and high volumes to be pumped.
Some weakly cemented sandstones in or near subtropical beaches have a porosity and hydraulic conductivity much higher than would be expected from the grain size distribution. On the Alexandria project, a controlled study revealed that the density of the sandstone was signicantly lower than the density of the same sand grains when disaggregated and measured in a very loose state.
The in situ sandstone had a higher porosity and pore size and a higher hydraulic conductivity. The familiar sand dune found behind ocean beaches is one type of Aeolian, or wind-deposited, soil. Dune sands tend to be clean and very uniform because of the sorting action of the wind and they are usually rounded in grain shape.
These characteristics result in moderately high hydraulic conductivity, despite the relatively ne grain size. Because of the combination of moderately high hydraulic conductivity and ne, rounded grains, dune sands are sensitive to seepage pressure.
Natural quicksand occurs readily with such materials, although a quick condition can occur in any granular soil. Loess is wind-deposited silt. It is usually of glacial origin, the result of erce windstorms coming off the ice sheet. It can occur in massive beds many tens of feet in thickness.
Its properties are complex, and construction problems in it can be severe [, ]. Aeolian soils are, by denition, surface deposits. But the earths surface changes, so it is not uncommon to encounter wind-deposited soils below the water table, sometimes at considerable depth.
Because of their sensitivity to seepage pressures, aeolian soils and dune sands in particular can rarely be dewatered by open pumping; predrainage with wells or wellpoints is usually essential to successful excavation.
Between three million and 10, years ago, the earth, or at least its northern hemisphere, was from time to time much colder than it is today. This climatic vagary had enormous effect on formation of some of the soils with which the dewatering engineers must be concerned.
Huge masses of ice accumulated in the polar region. As the weight increased, the ice began to squeeze out and ow southward in broad sheets we call the continental glaciers. The glaciers spread over the plains, lled in the valleys, and pressed against the mountainsides; sometimes the mountains and ridges themselves were overtopped.
As the face reached a warmer climate, the rate of melting increased; when melting rate equaled the rate of ice movement, the glacier was stationary. In colder periods the face advanced, in warmer periods it retreated. Geologists have found evidence of four major glacial advances over three million years. The last advance, called the Wisconsin, drew to a close about 10, years ago. Conditions during the Pleistocene epoch defy imagination. The mass of ice, up to many thousands of feet or meters thick, crept slowly southward, grinding and tearing at the surface.
The crust of the earth sagged under the weight, creating folds, faults, and large depressions. Soils that survived under the ice became overconsolidated, with densities sometimes approaching that of concrete. Great quantities of soil and rock were picked up by the glaciers and carried along, to be modied and re-deposited further south.
It is helpful to understand these processesthe effect on surviving soils in glaciated areas and the myriad forms in which soils transported by glaciers have been redeposited. Pre-Pleistocene soils in glaciated areas tend to be dense to very dense from the weight of ice bearing down on them.
The degree of consolidation depends on many factors: Material transported and deposited by the glaciers varies greatly, depending on the source materials and how they were deposited. Glacial till is material that has been deposited by the ice itself. Without the sorting effect of water or wind, till tends to be very well graded, often containing all particle sizes from boulders and cobbles down to the nest silts and clays.
Sometimes such materials have been termed boulder clay. Some tills are gap graded, with gravel and cobbles in a matrix of silt or clay, the intermediate sizes missing. If the till is deposited and then overridden by a subsequent advance, it can become extremely compact.
Glacial till is among the densest soil encountered. Glacial outwash is material that has been transported by melt water and sorted into relatively uniform deposits. Outwash can range from clean sand and gravel to ne silts and clays.
Its distinguishing characteristic is the uniformity of an individual deposit or a layer within the deposit. Layering is not uncommon in outwash, since changes in Pleistocene weather caused increase or decrease in the velocity of the melt water.
Outwash sands and gravels can be extremely permeable. Geologists have deduced that Pleistocene rivers carrying the melt water to the sea during warm periods were very large in comparison to even our greatest rivers of today.
Indeed, before the phenomenon of glaciation was better understood, some early investigators attributed the coarse outwash de-. The terms diluvian and antediluvian, meaning during Noahs deluge or before it, appear frequently in nineteenth-century geologic literature.
Outwash and till can occur in many ways, depending on the glacial action at the time of deposition and subsequent to it. It is common, for example, for the ice to have plowed out a valley down to bedrock, or perhaps to a rm cretaceous soil, and then put down a till deposit.
Later when the glacier recedes, melt water will deposit outwash on top of the till. We might, therefore, expect outwash above till, but we cannot rely on such a universal pattern.
On a project in New York City, the character of the dewatering problem was completely different from that expected because a major aquifer of glacial outwash existed under the till through which the tunnel was being driven. Till is normally cohesive and resistant to erosion, but the torrential ows from a rapidly receding glacier occasionally scoured channels, which subsequently lled with outwash.
Such a channel in till can have a major impact on construction operations, particularly if its existence is unexpected. Ice contact deposits, materials dumped at or near the ice face, may contain zones and layers of both outwash and till. Geologists have further subdivided various types of glacial deposits and it is helpful to understand their signicance. Terminal moraine is a ridge of soil pushed in front of the ice before its nal retreat. Terminal moraine is till-like in character, although it can be interngered with channels and layers of permeable outwash.
Ground moraine is a relatively thin cap of till deposited during the nal retreat. It can reduce surface inltration to aquifers of outwash beneath it. An esker is a ridge of alluvium deposited by a stream owing in a tunnel through the ice. A kame is a conical hill that forms where the stream escapes through the ice face. Eskers and kames are frequently surface features but they can become buried channels or zones of very permeable soil.
Another form of kame occurs at the edges of valleys, where reection of the suns rays off the ridges caused the glacier to melt more rapidly at the sides than in the center. Transient lakes or pools formed along the sides, and streams entering off the ice or from the ridges deposited materials that can be sorted or unsorted, depending on the distance transported.
A drumlin is a smooth, streamlined hill composed of till. A kettle is a depression formed by the melting of a detached, stationary mass of ice. An erratic is a large isolated boulderfor example, in an otherwise uniform deposit of outwash sand and gravel. One possible explanation is that the boulder was embedded in an ice oe that broke off from the glacier in a period of rapid melting and oated downstream. After the ice oe ran aground and melted, the boulder gradually became buried in outwash, to be discovered eventually by a startled excavation contractor.
Glacial lakes can form, as the Great Lakes did, in depressions created by the gouging action of the ice or by its sheer weight. Glacial lakes can also form when the advancing ice dams the channel of a northward owing river, such.
Glacial lake deposits have characteristics similar to other lake deposits, as described in Section 2. We nd deltas of clean sands and gravels, and thick deposits of ne grained soils, with the varved structure often pronounced. The Pleistocene epoch was probably characterized by erce storms.
In cold periods of low melting, the land surface south of the glaciers became quite dry, and ne-grained soils were picked up by the turbulent winds and re-deposited as loess. The authors have seen medium to large size beds of very uniform dune sand within or at the edges of glacial outwash deposits, suggesting aeolian deposition on some ancient beach.
From the discussion above it is apparent that glacial deposits are extremely variable, containing dense impermeable till, clean outwash sands and gravels, clays that range from stiff and overconsolidated to relatively soft and varved, and uniform wind-deposited dune sands and loess.
Occasionally, glacial deposits are very extensive, such as the great outwash plain that forms the south shore of Long Island, New York. More commonly, the glacial materials change within very short distances. Soils of Pleistocene age, even when deposited far south of the active glaciers, have nonetheless been affected by them. So much of the earths waters had accumulated in the great ice sheets that the sea level was at various times as much as several hundred feet lower than it is today. The mouths of the rivers were far out on the continental shelf compared to their present positions.
This situation, combined with the greater ow in the rivers during periods of rapid melting, affected the properties of the deeper soils beneath our coastal plains. Note that pressure on the left is higher than on the right. If a tunnel approached from the right, a sudden inrush of water might occur when the fault was breached. In Section 2. During the Pleistocene era the beach levels were, of course, much lower and the coral was formed at lower elevations.
The authors have encountered deep limestone and coral deposits of Pleistocene age on the coasts of Hawaii, Florida, and Spain that signicantly affected dewatering. We have seen how bedrock provides the raw material from which, by the processes of weathering, transportation, and deposition, soils are formed.
The bedrock itself can be of signicance to dewatering. Most rock is low in hydraulic conductivity. However, all rock is jointed and ssured to some extent Fig.
Such rock has the characteristic called secondary permeability. The transmissivity of the rock depends on the number, size, and degree of interconnection of the ssures. If the rock is relatively soluble, the ssures can be enlarged from solution activity. A fault is a vertical shift between adjacent blocks of rock.
A fault is sometimes a conduit for water, but under other conditions it can develop into a dam in the path of groundwater ow. When excavation takes place in rock, the water owing in through the ssures does not usually create an unstable ground condition but presents only the problem of pumping it away. But there are certain geologic situations where. The upper zone of rock immediately under the soil mantle is frequently the most weathered.
Sometimes this zone is a very permeable aquifer, more permeable than the soil above it. If so, experience shows that dewatering wells or wellpoints must penetrate the weathered rock or the soil above can be dewatered only with great difculty. Drilling into the rock can be costly, particularly since the weathered zone may be more difcult to drill than sound rock.
Some rock has large ssures, but they are partly lled with sand, clay, or chemical precipitates. Water ow through rock is concentrated, and velocities can be much higher than are normal in soils. If the material lling the joints is soft or chemically soluble, such as gypsum, then the concentrated ows may open up cavities.
Prolonged pumping time results in steadily increasing water volume. In some cases, the foundation properties of the rock can be impaired. In sedimentary rocks, such as sandstones and some siltstones, there may be uncemented or weakly cemented zones and layers which are usually more permeable than the main body of rock. Flow tends to be concentrated in the uncemented zones, eroding the sand and undermining the sound material. This has been particularly troublesome in the St.
Some rocks are so highly permeable that the large volume of water to be pumped becomes a major problem.
Basalt is an igneous rock with a high coefcient of thermal expansion. When it cools as it solidies, the network of shrinkage cracks can develop into a major aquifer. Basalt, scoria, and other deposits from some recent volcanoessuch as are found on the island of Oahu, Hawaii, Tenerife in the Canary Islands, and Icelandcan be extremely porous.
Scoria is a porous rock that formed as a slag on top of the lava ow. Sometimes successive eruptions cause a sandwich of very high hydraulic conductivity. The scoria may also roll under the molten basalt during rapid downhill ow. A lava tube can form when molten rock continues to ow within a partially solidied mass; on Oahu, lava tubes exist within which one can walk upright. Sedimentary rocks, by their nature, contain bedding layers frequently with quite variable hydraulic conductivity for example, sandstones alternating with claystones.
If such a condition exists below subgrade of a deep excavation, it may be necessary to install deep wells to relieve pressure in the more permeable layers. The water volume pumped may be quite small, but without the pumping the unrelieved pressure in permeable beds may heave and crack the overlying material, impairing the foundation properties of the rock. At the time of formation, the bedding layers of sedimentary rock are usually horizontal, or approximately so.
Subsequent tectonic movements may fold or tilt the rock.
The Coastal Range of California is an extreme example of rocks that have been heaved up a considerable distance from their level at the time of deposition. We can nd bedding planes in the area that are horizontal or vertical or anywhere.
An inclined bedding layer is a plane of weakness, subject to sliding on the updip slope of an excavation or highway cut Fig. Water can aggravate the situation in two ways.
Flow through a permeable layer can lubricate the adjacent surfaces. Worse, if pressure builds up in the permeable layer it reduces the effective weight of the overlying mass on which the stability of the slope depends.
Slopes of sedimentary rock have been stabilized by dewatering with horizontal drains or with vertical wells. Limestone presents such special problems to dewatering that it demands a separate discussion. Its principal constituent is calcite calcium carbonate , a mineral that in geologic terms is soluble in water.
In its various forms, limestone is abundant in nature; it occurs in massive beds, thin layers, and delicate coral skeletons; it is the hardness in water and the cementing agent in many sandstones. The common mechanism for limestone deposition begins with shellsh. Over geologic ages, the shells accumulate and gradually dissolve until the water becomes supersaturated with calcium carbonate, which then precipitates out to form the limestone.
The chemical processes of precipitation and solution are reversible, depending on the concentration of carbonates in the water, pH, temperature, and other factors. For example, the great limestone caverns at Luray, Virginia were created and are still being enlarged by solution action. But the stalactites and stalagmites we see in the caverns are forming from calcite precipitation. Thus, the mineral is both dissolving and precipitating in the same place at the same time.
Many factors affect solutionization. Water that has recently inltrated the ground is slightly acidic from dissolved carbon dioxide and solution is relatively rapid.
As the concentration of carbonates in the water increases, the pH rises and solution slows. Thus, the volume of water ow is signicant. Temperature has a pronounced effect, as does the presence of minerals other than calcite in the limestone. Dolomite, a rock containing magnesium as well as calcium carbonate, is more resistant to solution action than limestone but still susceptible to the same process.
In massive limestone beds, the solutionization tends to be concentrated in the upper zones. Weathered limestone. Sometimes a cavern forms and then collapses, causing a sinkhole in the overlying soil at the surface. However, badly solutionized limestone has also been encountered at considerable depth. Karst topography is a term used to describe an area that has experienced considerable solutionization near the surface, resulting in sinkholes and hidden caverns. The transmissivity of the solutionized limestone can be very high and dewatering in such areas can be difcult because of the large volume of water that may need to be pumped.
Dewatering in karstic terrain can increase the sinkhole activity. Groundwater movement will increase the solutioning of the rock and lowering of the water table will increase the effective stress near the rock surface that promotes sinkhole formation. Dewatering required for quarrying and phosphate strip mining in the southeastern United States has been linked to sinkhole activity. Kaeck P. First published: Print ISBN: About this book The most up-to-date guide to construction dewatering and groundwater control In the past dozen years, the methods of analyzing and treating groundwater conditions have vastly improved.
Discussion includes: Dozens of case histories demonstrating various groundwater control practices and lessons learned in groundwater control and work performed Detailed methods of controlling groundwater by use of conventional dewatering methods as well as vertical barrier, grouted cutoff, and frozen ground techniques Contracting practices and conflict resolution methods that will help minimize disputes Alternatives and effective practices for handling and treating contaminated groundwater Innovations in equipment and materials that improve the performance and efficiency of groundwater control systems Practices and procedures for success in artificial recharge Groundwater modeling to simulate and plan dewatering projects Inclusion of dual U.
Reviews "Following an introduction to the origins and developments of dewatering technology, they offer chapters discussing, among other topics, the geology of soils; hydrology of the ideal aquifer; characteristics of natural aquifers; groundwater modeling, measurement, and monitoring; pumping tests; pump theory; groundwater chemistry, bacteriology; contaminated groundwater; and piping systems.
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