Soil Mechanics Method Research Articles

Engineering geology and soil mechanics in project evaluation

A spects of project evaluation which come within the ambit of engineering geologists and soils engineers arise from the physical setting of the project. They are, firstly, concerned to observe and infer the distribution of soil and rocks and the ground water regime, secondly, to assess their implications for the construction of foundations for the structures which comprise the project. The first is geology and the second is foundation engineering. Foundation engineering is a subtle combination of geology and soil mechanics with the intuitive art of judgement and innovation where experience plays an important role. The role of the engineering geologist and the soils engineer is to ensure that the appropriate combination of geology and soil mechanics is used in the assessment of foundation engineering problems. In the early days, theoretical methods of soil mechanics combined with improved techniques of exploration promised to eliminate the necessity for depending on geological information. For a period engineers felt that it was only a question of time until all the problems in the field of sub-surface engineering, like those in steel and concrete design, could be solved by theoretical methods using values determined by laboratory tests. However, the practical application of soil mechanics, coupled with field observation of performance, often showed large discrepancies between the forecast and the actual events. Re-assessment of case histories invariably showed that such discrepancies were essentially dependent on the geological characteristics of the soils and rocks. Blind exploration of sites gave way to planned site investigations based on the

Investigation of the hydraulicked layer of a high-grade loess soil fill

1. The hydraulic fill layer of loess soil in its central section is characterized by the following features: a) It possesses complete water saturation over the whole depth; b) with increasing depth, the density of the poured soil increases and, accordingly, the moisture content diminishes; each depth has its corresponding density and moisture content; c) the top 3-m of the layer takes the form of a soil mass with moisture contents above the yield point (W=34–60%), being a small layer of poured soil with a low compaction pressure. This layer must be artificially dewatered if the hydraulic-fill structure has to be brought quickly into service. 2. The density and moisture content of the soil through the depth of the hydraulic-fill layer in its central section, starting from 3 m and lower, can be determined from the data of compression tests on the soil mass, assuming a depth of hydraulic fill numerically equal to the compaction pressure in accordance with Eqs. (9) and (10). 3. Prolonged stoppage of hydraulicking has an unfavorable effect on compaction of freshly poured layers and continuous pouring over the height favors more solid placing of the soil in the structure. 4. Stability of the outer slopes of the hydraulic fill during construction, starting from a depth of 3–4 m from the hydraulic-fill surface and lower, should be carried out by the usual methods of soil mechanics, disregarding fluidity of the core and pore pressure. For the layer from the hydraulic-fill surface to a depth of 4–5 m, where the freshly poured soil is unstable and presses on the side shells like a heavy liquid, the strength calculation has to be carried out for the dry embankments.

Principles of Soil-Vehicle Mechanics

The object of soil-vehicle mechanics is defined and existing theories briefly described. Experiments have shown that these theories work quite well for frictionless clay soils and for lightly loaded vehicles in firm soils where sinkage is small. For other soft soils performance cannot be predicted and the theory does not agree with experimental evidence.It is proposed that an improved and unified theory can be based on equations developed using the methods of soil mechanics, making appropriate allowance for the effect of compressibility. A resulting new pressure sinkage equation is shown to give better agreement with experiment.The complexities of the soil stress pattern imposed by vehicles are described and it is shown that these cannot be neglected. The effect of superimposing horizontal and vertical loads is to cause slip sinkage and experimental measurements show this to be important in sand but not in clay. An analysis is made which explains why this is so and gives insight into the physical nature of the phenomenon.The aims of current research are described and illustrated with results from recent measurements of wheel performance.

Effect of Site Conditions on Earthquake Intensity

A study is reported of over 100 strong motion earthquake accelerograms at seven sites based on interpretation of integrated and balanced strong motion acceleration records and 5% damped pseudo-velocity response spectra from these records; pertinent site properties computed from reported relationships; epicentral distances and earthquake magnitudes; and wave theory applied to bounded elastic media having constant or linearly variant properties. It is concluded that no strong preferential site period was amplified on a maximum pseudo-velocity response spectrum during strong motion earthquake; spectra velocities at soft layer sites average twice the base spectrum values; normal amplification near the earthquake mechanism period is approximately 3.2 times that of the base spectrum; an equation for the maximum velocity of the Helena earthquake is not recommended for design use because of the large number of unknowns. Further study is recommended into the influence of foundation rigidity and structural mass and stiffness related to base motion of the structures and the effective depth of impedence measurement; correlation, from standard soil mechanics methods, of shear strength to shear impedences; and transient random disturbances propagating up through energy absorbent elastic materials.

CORRIGENDA. APPLICATIONS OF SOIL MECHANICS IN THE DESIGN OF STABILIZING WORKS FOR EMBANKMENTS, CUTTINGS AND TRACK FORMATIONS.

Since railways were built problems have arisen rectifying slips of earthworks and the breakdown of track formations. This Paper reviews the impact of soil mechanics on the solution of these problems and on works practice. It is based primarily on work done on the Southern and Western Regions of British Railways and deals mainly with the behaviour of clays, silts, sands and gravels and chalk. Methods are described for investigating slips and designing appropriate remedial works, while alternative theories used in the design of cutting slopes, retaining walls, etc., are discussed. Reference is made to the importance of drainage and the establishment of suitable vegetation on slopes. Since 1940, when the first attempt was made, on British Railways, to apply soil mechanics methods to investigate the breakdown of a clay track formation, much research effort has been directed by British Railways to this problem. The Paper reviews this research and its influence on remedial works to bad track formations and also...

APPLICATIONS OF SOIL MECHANICS IN THE DESIGN OF STABILIZING WORKS FOR EMBANKMENTS, CUTTINGS AND TRACK FORMATIONS

Since railways were built problems have arisen rectifying slips of earthworks and the breakdown of track formations. This Paper reviews the impact of soil mechanics on the solution of these problems and on works practice. It is based primarily on work done on the Southern and Western Regions of British Railways and deals mainly with the behaviour of clays, silts, sands and gravels and chalk. Methods are described for investigating slips and designing appropriate remedial works, while alternative theories used in the design of cutting slopes, retaining walls, etc., are discussed. Reference is made to the importance of drainage and the establishment of suitable vegetation on slopes. Since 1940, when the first attempt was made, on British Railways, to apply soil mechanics methods to investigate the breakdown of a clay track formation, much research effort has been directed by British Railways to this problem. The Paper reviews this research and its influence on remedial works to bad track formations and also the design of new roadbeds and drainage.

The superficial deposits of the buried valley of the River Devon near Alva, Clackmannan, Scotland

The buried valley of the River Devon, known from a small number ofdeep borings to underlie the present alluvial tract, was explored by means of a resistivity survey. Samples of the carse deposits, late-Glacial clays and boulder-clays in the area of the buried valley were obtained by augering and from a new borehole near Alva. The physical characters of the clays were determined by the methods of soilmechanics. Their mineral composition was estimated by differential thermal analysis and X-ray studies. The results suggest an estuarine origin for the late-Glacial clays (correlated with the ‘100 foot’ Raised Beach deposits), the estuary being characterised by fjord-like over-deepening in the neighbourhood of Alva.

REBOUND IN THE BEARPAW SHALE, WESTERN CANADA

The Bearpaw formation (Upper Cretaceous) which occurs over large areas of Western Canada presents problems in connection with engineering works. These clay shales were consolidated under high pressures, but the load has been removed. A rebound occurred which is a maximum at the shale surface and decreases with depth. The rebound is made up of elastic rebound which occurred immediately upon removal of load and a “time rebound” which is still occurring slowly and involves a softening of the material. The characteristics of the shale and the depth of the rebound zone are being studied by soil-mechanics methods. Routine tests were made for water content, density, and Atterberg limits; water content is the best indicator of consistency. The results of laboratory strength tests do not correlate well with observed field behavior, and this has led to the conclusion that observations of existing slopes will supply more reliable strength data for long-time stability studies. Field investigations involve piezometric measurements and observations of slides, particularly creep. The softened surface zone is unstable, and slides have occurred on flat slopes. Slickensides are common in this zone, but their intensity appears to decrease with depth. The underlying shale is firm and stable. Rebound is also caused by local excavations for structures. Observations are being taken on light concrete structures to correlate predictions from laboratory swelling tests with observed heave.

Large-Scale Superficial Structures in the Northampton Ironstone Field

In the discussion of this paper, Mr. F. G. H. Blyth suggests interpretation of the observed structural details by the methods of soil mechanics. In the writer's opinion, the prospects for the success of such an attempt are no better than those of an attempt to explain the tectonics of the Alps on the basis of the current concepts of the strength of materials. Both rocks and clay have a double personality. One of them represents the behaviour of these materials under laboratory conditions. It reflects the standard or technical strength. The other one represents the behaviour under small stress of long duration. It reflects what is sometimes referred to as the fundamental strength, which indicates the smallest stress which does not produce any flow. There is no known relation between these two types of strength. High standard strength may be associated with very low fundamental strength and vice versa. A stress in excess of the standard strength produces fractures which cut across the internal boundaries in composite bodies, whereas a stress above the fundamental strength of the major constituents of composite bodies leads to intricate patterns whose details reveal every local change of the mechanical properties of the material. Minor constituents such as thin, rigid layers whose fundamental strength is greater than the average stress gradually disintegrate, due to local stress concentrations, and the fragments are slowly dragged into a flow pattern. Intense deformation of this type leads to the clay pebble-beds mentioned on p. 11 of the paper.