Journal of Geotechnical and Geoenvironmental Engineering

Uncertainty and risk are central features of geotechnical and geological engineering. Engineers can deal with uncertainty by ignoring it, by being conservative, by using the observational method, or by quantifying it. In recent years, reliability analysis and probabilistic methods have found wide application in geotechnical engineering and related fields. The tools are well known, including methods of reliability analysis and decision trees. Analytical models for deterministic geotechnical applications are also widely available, even if their underlying reliability is sometimes suspect. The major issues involve input and output. In order to develop appropriate input, the engineer must understand the nature of uncertainty and probability. Most geotechnical uncertainty reflects lack of knowledge, and probability based on the engineer’s degree of belief comes closest to the profession’s practical approach. Bayesian approaches are especially powerful because they provide probabilities on the state of nature rather than on the observations. The first point in developing a model from geotechnical data is that the distinction between the trend or systematic error and the spatial error is a modeling choice, not a property of nature. Second, properties estimated from small samples may be seriously in error, whether they are used probabilistically or deterministically. Third, experts generally estimate mean trends well but tend to underestimate uncertainty and to be overconfident in their estimates. In this context, engineering judgment should be based on a demonstrable chain of reasoning and not on speculation. One difficulty in interpreting results is that most people, including engineers, have difficulty establishing an allowable probability of failure or dealing with low values of probability. The $F-N$ plot is one useful vehicle for comparing calculated probabilities with observed frequencies of failure of comparable facilities. In any comparison it must be noted that a calculated probability is a lower bound because it must fail to incorporate the factors that are ignored in the analysis. It is useful to compare probabilities of failure for alternative designs, and the reliability methods reveal the contributions of different components to the uncertainty in the probability of failure. Probability is not a property of the world but a state of mind; geotechnical uncertainty is primarily epistemic, Bayesian, and belief based. The current challenges to the profession are to make use of probabilistic methods in practice and to sharpen our investigations and analyses so that each additional data point provides maximal information.

The Tymfristos tunnel, a $1\text{,}365m$ long road tunnel with an $11m$ section diameter, was excavated through the highly tectonized, low strength Flysch formation of central Greece. About 1 year after the completion of the excavation in 1995, convergence of up to 20% followed by failure of the support system was recorded. A $501m$ long segment, in which the failure occurred, was reexcavated based on a more detailed study of the ground conditions; however, this second phase was also followed by an up to 6% convergence. The detailed geodetic study of the three-dimensional kinematics of the support shell in relation time, ring closure delays, and support measures reveals an unusual time-history of convergence occasionally continuing even after the ring closure and a nonuniform distortion of the lining. Furthermore, uncontrolled deformation of certain “weak” sections propagated bidirectionally to adjacent, previously stabilized sections producing a “chain effect,” responsible for destabilization and resumed convergence of long $(\sim 90m)$ segments.

The wetting-induced compression of compacted Oklahoma soils was investigated. One-dimensional oedometer tests were conducted on 22 Oklahoma soils that encompassed relative compaction and moisture contents within typical embankment specifications. Results show that factors directly related to the fines composition can be used for preliminary estimation of collapse potential. Statistical analysis of the oedometer test data indicates that variables having the most impact on collapse index were compaction moisture content, dry unit weight, plasticity index, and clay-size fraction. Charts were developed to facilitate the estimation of collapse settlement of fills for different conditions, including fill height, moisture content, and soil type. Three case histories involving embankments that experienced significant settlement are presented for comparison. The comparison shows a reasonable agreement between predictions and field estimates of collapse settlement at the embankment centerlines; the limited evidence suggests that predictions based on one-dimensional assumptions may underestimate actual settlements possibly due to the two-dimensional nature of embankments.

Micropiles are being applied in foundation rehabilitation projects to enhance the pullout capacity of the existing foundation system and minimize the vertical deflection of the structures. Consequently, the pullout load-displacement behavior is important for the design of micropiles used for the rehabilitation of foundations subjected to pullout. In this paper, we present and evaluate analytical relationships for micropile pullout load-displacement behavior, which explicitly considers the micropile–soil interaction. The analytical relationships are kept simple and accessible to designers by assuming the micropile–soil interface to be elastoplastic. The model parameters are combined into scaling factors and a normalization factor that are found to significantly influence micropile load-displacement behavior. The closed-form analytical relationships are used to study the effect of model parameters on the predicted micropile yield behavior. The micropile load transfer behavior calculated by the model is discussed. The model is shown to replicate the field measured load-displacement curves for two cases. A variety of cases are analyzed to study the effect of grout pressure on micropile–soil interface parameters.

The spectral analysis of surface waves (SASW) method is a nondestructive test for characterization of the variation with depth of the shear modulus of soils. While the testing procedure is well developed, only one preliminary study has investigated measurement uncertainty associated with SASW, and the methods utilized to quantify measurement uncertainty were prohibitive to routine assessment. Knowledge of this uncertainty, and ability to include its assessment in routine testing, would allow for inclusion of SASW results in reliability-based design and in assessment of the spatial variability of shear modulus. In this study, a large sample of test data was collected from two test sites. Characteristic statistics, statistical distribution, and measurement uncertainty were determined for each phase of SASW. Using the empirical statistical properties and measurement uncertainty results as validation criteria, an analytically based uncertainty assessment system was developed. Phase angle, inverse phase angle, and phase velocity data typically display a coefficient of variation (COV) of 2%, and the COV for combined phase velocity data is typically 1.5%. The COV for shear wave velocity is typically between $5$ and 10%, and thus the inversion appears to magnify measurement uncertainty. Phase angle, inverse phase angle, phase velocity, and combined phase velocity data are normally distributed. Shear wave velocity samples at a given depth are generally normally distributed. Using a small sample of experimental data and the analytically based process developed in this study, the measurement uncertainty of SASW test results can be assessed as part of routine testing.

The fundamental parameters such as after-curing void ratio $\left(e_{\mathrm{ot}}\right)$ and cement content $\left(A_w\right)$ have been found sufficient to characterize the strength and compressibility of cement-admixed clay at high water contents. From analyses performed on the results of unconfined compression tests, the ratio $e_{\mathrm{ot}}/A_w$ has been proven to combine together the influences of clay water content, cement content, and curing time on the strength of cement-admixed clay. Moreover, the results of oedometer consolidation tests revealed that while $A_w$ governs the position of the postyield compression line, the magnitude of $e_{\mathrm{ot}}$ determines the magnitude of the one-dimensional vertical yield stress ${\sigma }_{v_y}^{\prime }$ at particular $A_w$. The value of $e_{\mathrm{ot}}$ reflects, primarily, the clay water content and, secondarily, the cement content and the curing time. Normalizing the after-curing unit weight, after-curing water content, and after-curing specific gravity were incorporated in an empirical relationship of $e_{\mathrm{ot}}$.

Aging effects in sand, such as increases in cone penetration resistance with time after deposition and/or densification, are known to occur in the field, but the causes of these effects are not fully understood. A laboratory testing program was designed to study mechanisms responsible for aging effects under controlled conditions. The testing program included measurements of the small strain shear modulus, electrical conductivity, pore fluid chemistry, and minicone penetration resistance after different periods of aging. Two different sands were tested, and aging effects were evaluated for different combinations of relative density, temperature, and pore fluid composition. Increases in the small strain shear modulus were observed throughout most of the tests, and chemical analyses suggest that precipitation of carbonate and silica occurred in two tests. Despite these changes, there was no corresponding increase in the minicone penetration resistance with time in any of the tests. It is unlikely that precipitation of carbonate or silica is responsible for aging effects in sands; other possible mechanisms include arching due to dissipation of blast gases and redistribution of stresses through the soil skeleton. An additional possibility is that the boundary conditions imposed by the laboratory tests obscure changes in penetration resistance that would be measured had the volume of sand tested been much larger. The implications of these findings in terms of other published field and laboratory studies are discussed.

Application of the effective stress principle in unsaturated geotechnical engineering problems often requires explicit knowledge of the stress acting on the soil skeleton due to suction pore water pressure. This stress is defined herein as the suction stress. A theoretical formulation of suction stress profiles, based on the soil water characteristics curve, the soil permeability characteristic curve, and previous shear strength experimental verification, is developed. The theory provides a general quantitative way to calculate vertical suction stress profiles in various unsaturated soils under steady flow rate in the form of infiltration or evaporation.

The influence of particle shape on granular material response is examined by using the discrete element method. Triaxial drained and undrained tests were performed on specimens of ellipsoids of two sizes. The triaxial test boundary conditions were simulated with a recently developed boundary mechanism. Different loading paths including axial compression, axial extension, lateral compression, and true extension were employed. The specimens were composed of 1,170 ellipsoids having two types of particles. The specimen is made up of 50% by weight of Type I particles that have an aspect ratio of 1.2. The aspect ratio of the Type II particles varies between 1.5 and 2. The specimens were consolidated isotropically before shearing. Comparing with the behavior of specimens of mono-size particles, a higher friction angle and a more complex particle shape effect were observed. The friction angles from the drained tests (axial extension, true extension, and lateral compression tests) were similar and the values are higher than that of the axial compression test. All simulated results are in good agreement with laboratory observation of sands.

Unsaturated and saturated soil hydraulic properties were determined and compared for three sandy soils at adjacent field sites. Drying soil–water retention curves were measured on soil specimens using a pressure plate apparatus. Saturated hydraulic conductivities ($K_s$) were measured with a Guelph permeameter and falling head tests. Parameter optimization was used to simultaneously estimate the drying and wetting soil–water retention and hydraulic conductivity curves from cone permeameter and multistep inflow/outflow data. $K_s$ values from all test methods were within an order of magnitude of each other at each site and, as expected, trended with bulk density. The Guelph permeameter generally yielded the highest $K_s$ values. The soil–water retention curves were similar in shape, except for the cone permeameter curves, which had steeper slopes due to rapid flow of water into the soil. Relative hydraulic conductivity curves were similar in character to the soil–water retention curves. Each method provided important information about the soil hydraulic properties. No one method provided the entire range of information provided by all of the tests combined, and no one method was found to be superior to the others.

Understanding of undrained shear behavior of cement admixed clay is of utmost importance for strength and deformation analyses in composite soft clay under short-term condition. From the critical analysis, the distinct difference in the responses of the same clay at uncemented and induced cemented states is brought out. The undrained shear behavior of uncemented clay is mainly dependent upon the clay fabric. The dismembering of the clay clusters in the fabric brings about the interlocking when the clay is in overconsolidated state. For the cement admixed clay, the clay is in meta-stable state. Hence, the strength and deformation characteristics are controlled by the clay fabric and cementation. The shear resistance is the sum of the shear resistance due to cementation $q_b$ and due to fabric $q_f$. The term $q_b$ is practically constant with the increase in effective confining pressure at preyield state. The contribution from the clay fabric to the shear resistance $q_f$ comes into the picture at postyield state.