Reading Materil

Reading Materil
Reading Materil

Geologic Origins of Soils

It is a feature of earth’s geologic activity that the rocks forming the earth’s mantle are continually being pushed out of earth onto the surface , where they are then exposed to earthquakes , glaciation , freeze-thaw , water erosion , chemical attack , waterborne abrasion , wind erosion , and other forms of weathering . Under such attack , the rocks are broken progressively into huge boulders , these into smaller boulders , these into cobbles , these into pebble , and finally , the pebbles are reduced to grains . Deposits of these assorted particles of rock and rock grains are called soils .

There is at present no word in engineering geology that is used specifically to describe the evercontinuing redection of rock into smaller and smaller particles . For the sake of simplicity , it is termed degradation in subsequent discussions . Degradation of rock into soil grains is not something that happened only in some vague geologic past . It is a continuing process , as active today as it was a million years ago .

As the size of the particles become progressively smaller , the particles become progressively easier to transport . T ransported first by such things as glaciation and avalanche , the partcles might be deposited in a recognizable stratum for a few thousand years before something happens to cause them to be picked up , transported , and deposited in some new location along with particles transported similarly from a hundred other locations .

At every stage of deposition and stratification , water is the ever-present medium of erosion and transport . Water deep within the ground carries acids and bases that chemically attack the particles even when they are buried thousands of feet deep . At the surface , rain , snow , and sleet combine to combine to erode , freeze , thaw , and further reduce chunks of rock into smaller and smaller pieces .

Whenever exposed , the soil particles become subject to organic attack by vegetation , carbon dioxide , and atmospheric acids , changing them chemically into other compounds or even other minerals .Picked up again , redeposited , and exposed again and again , the particles might undergo thousands of yearws of changes before they come to rest for a few hundred years in relative quiet . It is at one of these quiet periods that the foundation engineer is given a stratum of these particles on which to place a foundation .

Because nature works on such a huge scale , the strata are usually ( but not always ) so large that the foundation can be located entirely on one stratum . But underlying this strtum could be another stratum having vastly different engineering properties , and underlying this , yet another . The material to be used to support a building foundation is thus a heterogeneous mixture of minerals coming from countless sources over the breadth of a continent , randomly deposited , irregularly stratified , and absolutely inconsistent . It is , in short , soil .

With such a description , defining the engineering properties of a material as variable as soil would seem to be hopeless . Within recent years , however , real progress has been made in defining the engineeting properties of soils . Although these properties are more

appropriate within large brackets rather than to refined exact details , the response of most soils to a beating load can now be predicted with some degree of confidence .

Insofar as the engineering properties of a soil are concerned , the mechanism of geologic transport , deposition , burial , and exposure is one of the more important influences . With few exceptions , all soils have been transported to their present locations from somewhere else . They have been blended , disturbed , chemically modified , reblended , mixed , crushed , restratified , picked up and redeposited , sometimes loaded by thousands of feet of overburden , and finally exposed when the overburden was eroded away . The mechanism by which the stratum was finally deposited and later exposed will be seen to be of profound importance when the soil is used as a foundation material .

A second major influence is the groundwater . The location of the water table , the , amount of its rise and fall throughout the year , and the chemistry of the groundwater can profoundly affect both the type of foundation and the ability of the soil to carry the foundation . In some circumstances , however , the water entering the soil from the surface can be far more important than in the water table below .

A third major inflence is the residue of vegetation . Even in the most barren deposits of soil , a few plants will manage to survive . The residue from their eventual death and decay will provide a somewhat more hospitable environment for the next generation of plants . The detritus of these plants in turn will deepen the fragile layer of organic material , providing a yet more fertile ground for other plants , and so on . Eventually , a gradient of organic mateial is developed , wich the high organic content at the surface diminshing steadily with depth . As always , water is the primary vehicle for transporting the organic material downward , aided in this case by disturbances from root penetration .

These combined effects of deposition , water percolation , vegetation , and other influnces on the soil enentually produce a typical distribution of soil and organic matter called a soil profile by agronomists . Such a profile , divided into soil horizon , is shown schematically in Fig . 1 for a temperate climate . The typical thicknesses shown in Fig . 1 are intended only as a broad indication ; the thicknesses of the various horizons would be significantly laer for the rich soils of the valleys and bottomlands and much shallower for the sparse soils of the mountain slopes .

The presence of humus or organic material in a soil can cause serious changes in engineering properties as the organic material continues to decay . Due to this variability , engineered foundations must be placed well below the organic agricultural soils and into the parent soils of the C horizon . The engineering properties of the soils in the C horizon are reasonably constant and are not subject to change due to the decay of oganic material .

Although soils have long been classified into broad groupings , the engineering properties of various classifications of soils cannot be determined once and used thereafter as constants ; a soil in Wyoming classified as clay might have vastly different properties from a soil in Georgia classified as clay . There are , however , certain characteristics that are common to all soils regardless of origin characteristics that are common to all soils regardless of origin or chemistry . Such characteristics can be used as a basis for soil classifications . One such characteristic is the size and shape of the individual particles that comprise the soil .

Karl Terzaghi

Ralph B Peck

Our first President and the founder of our profession is no stranger to any of us , because he wtote extensively of the development of his own understanding of soil mechanics and , especially in his Presidential Addresses , of his evaluation of the state of the art . Others , too , have not only written of his technical accomplishments but have recounted many anecdotes that illuminate his personality . Hence , the length of these remarks about T eraghi is by no means proportional to his overwhelming importance , for I do not wish to repeat what you already know .

From 1942 to 1948 , T erzaghi and I worked intensively on Soil Mechanics in Engneering Practice . Somewhat to our surprise , the writing did not go well . It is enough to say that even to T erzaghi the book presented for the first time the necessity for a critical evaluation of the relevance of each facet of soil mechanics to practice ; moreover , gaps appeared where soil mechanics was inadequate to cope with some of the most commonplace problems and , if possible , these gaps had to be filled . Y et , this was not his fitst book . Erdbaumechanik and Theoretical Soil Mechanics had already appeared and had been remarkably influential . How did T erzaghi regard them ?

As to Erdbaumechanik,I vividly recall one incident while T erzaghi was visting the University of Illinois . He wished to refer to some information he had included in the book , but after thumbing thtough the pages and fruitlessly consulting the index , he exploded in frustration .”When I wrote that book ,” he exclaimed , “I thought I was doing my duty if I merely laid out the bill of fare . It was up to the reader , poor devil , to grub though it to find what nourihment he could . Even the index is no good ; not even I can find anything !”

Theoretical Soil Mechanics was quite a different matter . It cost him several years of intense concentration and is still a model of clarity and orgnization . Indeed , it was so well done that , much to his dismay , it gave many engineers and academicians the impression that soil mechanics is primarily a theoretical subject – an idea the very opposite of his own convitions . Why , then ,did he write such a book ?

It was his overwhelming interest and purpose to write a book for the practitioner , a book on applied soil mechanics . he feared , however , that introducing the necessary theory into the discussions would divert the rader’s attenion from the main thrust of hia approach , so he deciddto precede the applied book by one containing all the necessary theory to which he could then refer , In spite of his full realization that he was not a theorist – he admired elegance in theory and regarded his own efforts as clumsy – he nevertheless felt the necessity to take upon himself the responsibility of examining the available stock of theories , of evaluating their assumptions , and of making the judgments concerning their utility and shortcomings for practice .

He did the job remarkably well . Moreover , the companion volume on applied soil mechanics never appeared ; the section on applied soil mechanics in Soil Mechanics in Engineering Practice in effect took its place . Y et , because Theoretical Soil Mechanics is

complete within itself , many engineers have been led to the false impression that theory is the essence of soil mechanics . T erzaghi considered the extract of theory in the second part of Soil Mechanics in Engineering is Practice to be ample for most practioners , and the continued sale of Theoretical Soil Mechanics at a volum comparable to that of Soil Mechanics in Engineering Practice was a source of concern rather than astisfaction to him .

Unfortunately , the reader of Theoretical Soil Mechanics would never get even the slightest hint that T erzaghi’s avocation and hobby were geology , or that he considered soil mechanics to be a quantified aspect of applied or engineering geology . Y et , in his lectures to students and in his consideration of individual jobs , his emphasis was always on the in which nature had created the deposit , on what variations in properties might be expected as a consequence of the natural events during and after deposition , and on what the physical properties of the geologically differentiable parts of the deposit might be . Boring programs and soils tests were always laid out to illuminate the geology ; his intetpretation and interpolation of the findinga always had a geological basis . Until he understood in detail the geology og a site , he had little confidence in predictions based on tests , theories or statistical analyses . He would have taken a dim view of today’s trend to decrease the time a civil engineering student devotes to geology in order to add corresponder to add corresponding studies of computer science .

T erzaghi’s almost instinctive use of geology appeared in the first major consulting job we shared after our work on the Chicago Subway – the iron ore storage yard at Cleveland , Ohio . Y ou may recalls his Final Report on this project , reprinted in the Terzaghi Anniversary V olume . It happened that I was the first to visir the job . The immediate questions were the length and bearing capacity of the piles beneath the retaining walls that would enclose the iron ore to be stockpile , involving the retaining wall piles as well , was almost certain . Indeed , on the basis of my estimate of strength , the factor of safety was na more than 0.7 . When the officials of the steel company asked me what could be done about the prophesied failure , I had no answer except to suggest they call T erzaghi . They did , without telling him they had already asked my advice , and on his arrival explained , exactly as they had to me , their concern about the retaining wall piles . He also watched the test boring in progress and , like myself , made a quick mental calculation of the bearing capacity of the storage area .His calculation , like mine , indicated failure . But he noticed one failure I had overlooked . The ore yard was to be located in valley surrounded by steep banks in sand extending to an upland over 100 feet above the valley , whereas the ore pile was to be only 70 feet high . The valley fill , a fluvio –glacial deposit , must once have extend across the entire valley and subsequently have been eroded by the ancestor of the stream now flowing by the ore storage area . Hence , the glaciolacustrine clays underlying the valley must have been overconsolidated and quite likely had a greater strengh than that suggested by the disturbed drive samples . Furthermore , Terzaghi also noticed that there was no sign of failure beneath or in front of the toes of the steep sand bluffs , even though the geology clearly indicated that the lacustrine clays extended beneath the toes . These favorable indications geology clearly indicated that the lacustrine clays extended beneath the toes . These favorable indications prompted T erzaghi to judge that the lacustrine clays extended beneath the toes . These favorable indications prompted T erzaghi to judge that the lacustrine clays extended beneath the toes . These favorable indications prompted T erzaghi to judge

that the chances were actually quite good that the ore pile would be stable , and that referred , shows that his optimism was justified .

It would be misleading not to complete this sketch without recalling the many discussions after dinner , when we had spent the day over a manuscript or in the field on one of our jobs , and when T erzaghi inevitably turned to the future of soil mechsnics . He feared for it , because he feared it might depart from reality . He feared the consequences of theory for its own sake , of soil tests as an exercise in increasingly sophisticated laboratory expertise , of routine calculation as a substitute for the impliacations of geology . Above all , he feared that workers in soil mechanics would take for granted that Nature would behave in accordance with their predictions , and that they would fail to take every opportunity to determine how Nature had actually reacted to their designs and construction activities .

相关主题
相关文档
最新文档