Compacted Glacial Till Research Articles

Modeling Behavior of Friction Pile in Compacted Glacial Till

AbstractExperimental and numerical modeling studies related to the behavior of pile foundations are conventionally undertaken based on the assumption that the surrounding soils are in a state of saturated condition. Such an assumption may contribute to erroneous estimations of the stability and settlement behavior of pile foundations installed in unsaturated soils due to neglecting the influence of soil suction. In this paper, the behavior of a single friction pile loaded in drained and undrained conditions in a compacted glacial till is modeled using finite-element analyses (FEAs). The influence of the soil suction on (1) the shear strength and the modulus of elasticity of the soil and (2) the shear strength of the pile-soil interface derived from the experimental studies is discussed and considered in the FEA. There is reasonable agreement between the load-settlement behavior and the bearing capacity of the friction pile modeled using the FEA and the measured results from experimental studies. This stud...

Modelling the Mechanical Properties of a Compacted Glacial Till

This paper summarizes comprehensive experimental studies on the soil–water characteristic curve (SWCC) and the variation of several mechanical properties with respect to suction for a compacted glacial till collected from Indian Head, Saskatchewan, Canada (referred to as Indian Head till). The summarized mechanical properties include the shear strength, unconfined compression strength, undrained elastic modulus, and resilient modulus which were either directly measured or back-calculated from the experimental results. A unified model is used to predict the variation of the mechanical properties with respect to suction. This model only requires the information of the mechanical property at saturated condition and one reference unsaturated condition and the SWCC for prediction. The advantage of this model is that it uses one constant model parameter that does not need to be calibrated. It is demonstrated that the model is capable of reliably predicting all the mechanical properties of the compacted Indian Head till. The mechanical properties are then used in numerical software Plaxis to model the load-settlement behavior of model footing tests performed on compacted Indian Head till. There is good agreement between the measured and modelled load-settlement behavior and the bearing capacity at various suction levels. Studies presented in this paper using the unified model would be helpful for implementing the mechanics of unsaturated soils in geotechnical practice.

Simple Techniques for the Estimation of Suction in Compacted Soils in the Range of 0 to 60,000 kPa

Abstract Two simple techniques are proposed in this Technical Note to estimate suction values over a range of 0 to 60,000 kPa on compacted glacial till specimens. The first technique uses a conventional pocket penetrometer in the estimation of matric suction values lower than 300 kPa. This technique is developed based on the assumption that there is a strong relationship between matric suction and the compressive strength measured using a pocket penetrometer. The second technique uses conventional tensiometer to estimate relatively high suction values in the range of 1200 to 60,000 kPa. In this technique, the tensiometer response versus time (TRT) behavior for a suction range of 0 to 50 kPa is used in a hyperbolic model to estimate the high suction value. This technique is proposed based on the assumption that each suction value has a unique initial tangent of TRT behavior. The equilibrium suction values of the compacted glacial till specimens are respectively measured using axis-translation technique and a psychrometer for low and high suction values and compared with those estimated using the techniques proposed in this study. There is a reasonably good comparison between the measured and estimated suction values both in the low and high suction range.

Shallow lateral flow from a forested hillslope: Influence of antecedent wetness

A 5-m-wide soil pit at the base of a forested hillslope in coastal British Columbia was instrumented for studying subsurface flow processes during rainstorms. Three typical, low-intensity autumn rainfall events with different antecedent moisture conditions are assessed. Outflow from the organic horizon was captured and measured by a single trough, and outflow from the mineral horizon (above compact glacial till and bedrock) was measured separately for three adjacent sections of the soil pit. For two storms that were preceded by dry conditions, lateral outflow from the organic horizon occurred, although the water table did not rise up to the organic horizon. However, the calculated effective contributing area was small (<0.7 m 2) and the effective contributing slope length was short (<0.15 m). Furthermore, volumes of outflow from the organic horizon during these storms were more than 400 times less than during a later storm with wet antecedent conditions. During this later storm, a portion of the outflow from the organic horizon may have been generated as saturated overland flow due to the rising water table. The calculated effective contributing area (>170 m 2) and the effective contributing slope length (>36 m) were substantially greater for the “wet” antecedent storm compared to the “dry” storm events. Shallow lateral flow over unsaturated soil is therefore unlikely to be a significant contributor to storm runoff at such forested sites. Flow at the organic horizon–mineral soil interface may contribute to interconnected preferential flow pathways during wetter antecedent conditions.

Throughflow variability on a forested hillslope underlain by compacted glacial till

This study examined the spatial and temporal variability of throughflow in a shallow forest oil in terms of the controls exerted by topography, soil properties and the shape of the water table. Hillslope outflow was measured at nine troughs installed at a road cut, and hydraulic head was measured at 37 piezometers installed to the confining till surface at the base of the soil. At the lowest flows, the distribution of throughflow across the hillslope conformed reasonably to the distribution of upslope contributing area computed from the topography of the confining basal till layer. At higher flows, however, the distribution of throughflow shifted, partly as a result of changes in the orientation in flow tubes as the piezometric surface changed. At high flows, surface topography appeared to provide a reasonable proxy for the shape of the water table. Shunting of water through discrete macropores, however, can apparently overwhelm topographic controls on throughflow, at least for slope widths less than about 10 m. Effective hydraulic conductivities computed at the scale of the troughs varied over two orders of magnitude and displayed no consistent relationship with the thickness of the saturated layer. Estimates at the plot scale (c. 10 m hillslope width) were far less variable, and for higher flows displayed an approximately linear relationship with saturated zone thickness. Copyright © 2000 John Wiley & Sons, Ltd.

Throughflow variability on a forested hillslope underlain by compacted glacial till

This study examined the spatial and temporal variability of throughflow in a shallow forest oil in terms of the controls exerted by topography, soil properties and the shape of the water table. Hillslope outflow was measured at nine troughs installed at a road cut, and hydraulic head was measured at 37 piezometers installed to the confining till surface at the base of the soil. At the lowest flows, the distribution of throughflow across the hillslope conformed reasonably to the distribution of upslope contributing area computed from the topography of the confining basal till layer. At higher flows, however, the distribution of throughflow shifted, partly as a result of changes in the orientation in flow tubes as the piezometric surface changed. At high flows, surface topography appeared to provide a reasonable proxy for the shape of the water table. Shunting of water through discrete macropores, however, can apparently overwhelm topographic controls on throughflow, at least for slope widths less than about 10 m. Effective hydraulic conductivities computed at the scale of the troughs varied over two orders of magnitude and displayed no consistent relationship with the thickness of the saturated layer. Estimates at the plot scale (c. 10 m hillslope width) were far less variable, and for higher flows displayed an approximately linear relationship with saturated zone thickness. Copyright © 2000 John Wiley & Sons, Ltd.

EVALUATION OF EPIC FOR A SHALLOW NEW ENGLAND SOIL: II. SOIL NITRATE

The Erosion-Productivity Impact Calculator (EPIC) model was evaluated for the prediction of nitrate (NO3-N) concentrations in a fine sandy loam underlain by a compact glacial till in Connecticut. Predicted soil NO3-N levelsfrom EPIC simulations were compared with measured levels from maize (Zea mays L.) and fallow plots with five nitrogen(N) application rates for three years. Soil NO3-N concentrations were measured at 15-cm increments to a total depth of60 cm. Initial simulations showed that use of measured soil parameter values versus EPIC-default values produced muchbetter results. The accuracy of EPIC predicted versus observed NO3-N using measured soil parameters values varied,with better predictions for fallow than maize and for deeper than shallow soil depths.

EVALUATION OF EPIC FOR A SHALLOW NEW ENGLAND SOIL: I. MAIZE YIELD AND NITROGEN UPTAKE

Predictions of maize (Zea mays L.) silage yields and nitrogen (N) uptake from the Erosion-ProductivityImpact Calculator (EPIC) model were compared with observed values from plots located in Connecticut. The soil was afine sandy loam underlain by a compact glacial till. The influence of soil parameters was assessed by comparing resultsusing EPIC default versus measured soil parameter values. This article addresses the comparisons for maize yield and Nuptake under five rates of N fertilization across three different growing seasons. Statistical analyses showed that the EPICmodel in general under-estimated both maize yield and N uptake. The accuracy of predicted values for both yield and Nuptake were affected by both growing season precipitation and N application rate. Possible reasons for the influence ofthese factors on EPIC predictions are given.

The Relationship Between the Soil-Water Characteristic Curve and the Unsaturated Shear Strength of a Compacted Glacial Till

Abstract Soils compacted at various “initial” water contents and to various densities should be considered as “different” soils from a soil mechanics behavioral standpoint even though their mineralogy, plasticity, and texture are the same. The engineering behavioral change from one specimen to another will vary due to differences in soil structure or aggregation. The shear strength of an unsaturated soil and the soil-water characteristic curve are dependent on soil structure or the aggregation, which in turn is dependent on the “initial” water content and the method of compaction. The laboratory preparation of specimens must, therefore, closely represent the physical conditions and the stress state conditions likely to occur in the field if a proper assessment of the shear strength parameters is to be achieved. This paper is primarily concerned with the study of the relationship between the shear strength of an unsaturated soil and its soil-water characteristic curve. Consolidated drained direct shear tests were conducted on statically compacted glacial till specimens, both under saturated and unsaturated conditions, representing three “initial” water contents and densities. The “initial” water contents and densities of the specimens were selected to represent the dry, optimum, and wet of optimum water content conditions with reference to the compaction curve. Multistage, unsaturated, direct shear tests were conducted under three different net normal stresses with varying matric suction values for each case. The soil-water characteristic curves were also developed on specimens with “initial” conditions similar to those used for the unsaturated shear strength tests. The shear strength variation with respect to matric suction was found to be nonlinear for all the tests. The rate of increase in the shear strength contribution due to matric suction, however, was found to be related to the rate of desaturation of the soil. The desaturation characteristics are a function of the “initial” water content of the compacted specimens. For any particular net normal stress and matric suction, specimens compacted wet of optimum water content offered more resistance to desaturation and exhibited a higher shear strength when compared to specimens compacted at dry of optimum or at optimum water content conditions. Under similar “initial” conditions, the soil-water characteristic curve bears a close relationship to the unsaturated shear strength behavior of the soil.

Construction and monitoring of a composite soil cover on an experimental waste-rock pile near Newcastle, New Brunswick, Canada

A 130 cm thick composite soil cover was constructed on an experimental waste-rock pile at the Heath Steele mine site near Newcastle, New Brunswick. The cover consisted of a 30 cm thick sand base, a 60 cm thick compacted glacial till, a 30 cm thick granular layer, and a final 10 cm thick gravel layer for erosion protection. The till was compacted on the sand base in three finished lifts each of 20 cm thickness. Results of a preconstruction pad test indicated six passes of a 5-t vibratory compactor were required to attain the design specifications of 95% of the Modified Proctor maximum dry density at a moulding water content of 2–3% wet of the optimum. These compaction specifications also ensure that the till has a degree of water saturation of at least 95%, which is required to reduce oxygen and acid fluxes in the underlying pile. Quality control measures were taken during the construction to ensure the specifications were followed. Monitoring instrumentation was installed during the construction of the cover. Results indicate a reduction in gaseous oxygen concentrations in the pile from 20% before cover to about 3% after cover placement. The decreased oxygen penetration implies reduced oxygen flux and acid production. Volumetric water contents averaged about 32% in the till both immediately following cover installation and 7 months later. The water-content data are corroborated by soil-suction measurements. Temperatures in the pile have decreased following cover installation but appear to be more influenced by climatic variability than by a decrease in heat production and hence sulphide mineral oxidation. Observed discharge from two lysimeters, installed below the cover, indicates infiltration of 2–2.5% of precipitation during a 55-day period when rainfall was heavy. The quality of seepage from the pile has not changed since cover installation. Further monitoring will be required to confirm the reduction in acid production. Key words : waste-rock pile, acid generation, soil cover, suction, oxygen flux, percolation.

Volume change indices during loading and unloading of an unsaturated soil

The paper presents the volume change theory and the designation of associated soil properties that must be measured for an unsaturated soil. The equipment required for the measurement of each of the relevant volume relationships is described. Several testing procedures for obtaining the volume change indices during loading and unloading of an unsaturated soil are presented. Typical results from loading and unloading tests on compacted silt and compacted glacial till specimens are presented and analyzed. The analysis is given in order to illustrate the application of the volume change theory to practical problems. Key words : unsaturated soil, volume change indices, constitutive relations, coefficients of volume change, oedometer tests.

Determination of the shear strength parameters of an unsaturated soil using the direct shear test

Multistage direct shear tests have been performed on saturated and unsaturated specimens of a compacted glacial till. A conventional direct shear apparatus was modified in order to use the axis-translation technique for direct shear tests on unsaturated soils. The soil can be subjected to a wide range of matric suctions. The testing procedure and some typical results are presented. Nonlinearity in the failure envelope with respect to matric suction was observed. Suggestions are made as to how best to handle the nonlinearity from a practical engineering standpoint. Key words: shear strength, unsaturated soils, negative pore-water pressures, soil suction, direct shear.

Saturation, Aeration, and Color Patterns in a Toposequence of Soils in North‐central Indiana

AbstractThe purposes of this study were to determine how geomorphology and stratigraphy influence soil water and O2 regimes, how water and O2 regimes are reflected in soil color patterns, and how soil color patterns can be used to predict soil behavior and suitability for various uses. We measured water table depths, soil‐water O2 levels, and saturated hydraulic conductivity (Ksat), and described soil color patterns in soils formed in silty loess and loam glacial till in two forested toposequences. In both sequences, Aqualfs and Aquolls are on the swells and swales, respectively, of the till plain summit, and Udalfs occupy hillslope shoulder and backslope positions along drainageways cutting back into the till plain. Compact glacial till C horizons had very low Ksat values, which forced subsurface water to flow laterally, through more permeable B horizons, to lower landscape positions. In this manner, stratigraphy and shape of the land surface controlled water movement in the landscape. Udalfs had deeper water tables, shorter saturation periods, higher soil‐water O2 levels, and higher chroma colors than Aqualfs and Aquolls. Udalfs, however, were periodically saturated above a depth of 1 m. Soils with dominant chroma of 2 or less in B horizons had water‐oxygen regimes of saturation and reduction consistent with the aquic moisture regime. Soils with no chromas ≤2 but with some three or four chroma colors had water‐oxygen regimes of saturation and oxidation. Such chromas should be recognized as indicators of soils that are occasionally saturated but not reduced. These saturated‐oxidized soils are not properly recognized in soil interpretation records and published soil surveys. They can be farmed without artificial drainage but they have definite limitations for many nonagricultural uses such as on‐site waste disposal.

Estimation of Saturated Hydraulic Conductivity from Soil Morphological and Genetic Information

AbstractSaturated hydraulic conductivity, Ksat, was measured below the water table by the piezometer method for subsurface horizons of several Indiana soils. Lowest Ksat rates were in compact glacial till, paleosols formed in till and fragipan horizons formed in loess. Camblc and argillic horizons had intermediate rates. Highest rates were in B3 horizons formed in water‐reworked glacial till materials. For prediction purposes, data were grouped by soil texture and standard soil structure classes in one classification and by texture, origin of parent material, and kind of genetic horizon in another classification. The second grouping resulted in more homogeneous classes, but some of these classes spanned more than one order of magnitude. The proposed method for determining Ksat and using the results to estimate it for other horizons can be used by field soil scientists to obtain better estimates for many soils.

Soil‐Water Regimes in Brookston and Crosby Soils

AbstractHydraulic conductivity and seasonal patterns of water content were measured and related to soil formation in associated very poorly drained Brookston soils (Typic Argiaquolls) and somewhat poorly drained Crosby soils (Aeric Ochraqualfs) in central Indiana. In the Brookston soil the water table was near the surface for only a short time during periods of heavy rainfall in the winter and spring. The water table then stabilized near 125 cm deep, where it was controlled by tile drains. In Crosby, also tile‐drained, the water table stayed near the surface longer in the spring, then became deeper than in Brookston. The hydraulic conductivity is relatively high in the Brookston B and C and Crosby B horizons but very low in the Crosby C horizon, compact glacial till. Since downward movement is restricted in the Crosby C horizon, water tends to move laterally through the B horizon into the Brookston soil where the tile are in more permeable horizons than they are in Crosby.Available water capacities of soils can be estimated by measuring the field water contents in a profile at the beginning of the growing season and during dry periods for several years using a soil under perennial vegetation. In these two soils, however, it appears that water movement from Crosby to Brookston and the profile storage capacity are both important in determining the water‐supplying capacity of the soils.