Influence of the head and tip connection on the seismic performance of micropiles

This paper presents a study of the influence of connection conditions at the head and tip of micropiles on their response to seismic loading. The study is carried using a fully three-dimensional finite element modeling. The soil is assumed to be elastic with Rayleigh damping. The superstructure is modeled by a single degree of freedom system composed of a concentrated mass and a column. The study is carried out for both vertical and inclined micropiles. The results of analyses show that a pinned connection between the micropiles and the cap leads to a reduction in the axial force and bending moment in micropiles, in particular for inclined micropiles. They also show that the embeddement of the tip of micropiles in a stiff substratum layer leads to a dramatic increase in the internal forces in the micropiles.     

Multiscale adaptive estimation of the conductivity field from head and tracer data

Large scale fluctuations in the conductivity field are regionalized and estimated via a maximum likelihood, adjoint-state methodology. Small-scale fluctuations within each region are estimated adaptively via a Kalman-like stochastic filter. The variance and integral scale within each region are assumed to control the small-scale fluctuations. A Monte Carlo technique is used to examine the distribution of large-scale conductivity estimates.     

Grey degree and grey prediction of groundwater head

Boundary types and geologic conditions, which possess random and obscure characteristics, produce variations in groundwater heads. In this study, a groundwater flow system was regarded as a grey system and a grey degree defined and quantified those characteristics. Data were obtained from field records in Gao-Shu and De-Xie Stations, which are located in the upper and middle sites of Pingtung Plain, Taiwan, respectively. An estimated interval was used to represent the upper and lower groundwater head limits. The analyses showed that the grey degree in the wet season was smaller than in the dry season. This was due to the greater uncertainty in groundwater pumping and rainfall recharge in the dry season. With high confidence levels, the grey degree of the groundwater heads decreased. Under spatial distribution, the uncertainty of the groundwater pumping and rainfall recharge in the middle zone was greater than in the upper zone because the middle zone included residential districts and agricultural regions. Thus, the grey degree in the upper zone was less than the middle zone. The grey model can be used to predict a groundwater head based on the observed groundwater head data and the random degree in the groundwater head each month can be judged by the grey interval.     

Uniform head in horizontal and vertical wells

The steady-state head within a fully penetrating well may be estimated by evaluating the Thiem equation at the radius of the well. A method is presented here to extend results from the Thiem equation to horizontal wells and to partially penetrating wells. The particular model used in this investigation is based upon the analytic element method; it accurately reproduces a boundary condition of uniform head along the cylindrical surface at the perforated face of the well. This model is exercised over a representative range of parameters including the well's length, radius, and pumping rate, and the aquifer's hydraulic conductivity and thickness. Results are presented in a set of figures and tables that compare the well's drawdown to the drawdown that would have been obtained using the Thiem solution with the same pumping rate and radius. A methodology is presented to estimate the head within a horizontal or partially penetrating well by adding a correction term to results that can be readily obtained from computer models of vertical fully penetrating wells. This approach may also be used to contrast the differences in head between horizontal and vertical wells of various lengths, radii, and placement elevations.     

Modeling the meteor head echo using Arecibo radar observations

This paper reports results obtained using a combined set of models to determine meteoroid properties by comparing expected and observed meteor head-echo signal-to-noise ratio (SNR) and line-of-sight Doppler velocity as measured in high-power and large-aperture (HPLA) radar observations. For this task we model: (1) meteor ablation and ionization processes, (2) meteor head-echo radar cross-section (RCS), (3) the radar equation, and (4) the radar antenna gain pattern, together with an automated least-squares fitting procedure to estimate meteoroid and observation parameters (i.e. aspect angle, location within the radar beam, etc.). We compared our simulated results with 236 head-echo events observed using the Arecibo 430 MHz radar in Puerto Rico. We found good agreement between modeled and observed SNR versus meteor altitude profiles for a broad range of head-echo observations. We also find reasonable agreement between meteoroid mass distributions resulting from these models and estimated using dynamical arguments, with the dynamical mass generally resulting in lower values by about 1–2 orders of magnitude. A characteristic of our methodology is that we can trace back the original mass and velocity of the meteoroid “above” the atmosphere (∼150 km altitude) required to produce the observed meteors. We find that, the original mass is required to be, on average, 1–2 orders of magnitude larger than that at the time of observation, and 3 orders of magnitude larger than estimated using dynamical equations. These results suggest that many meteor head echoes are observed towards the end of the particle's life, which has significant implications for the use of these observations for the determination of meteoroid properties. The automated fitting procedure is very sensitive to the antenna pattern, and therefore allows for precise estimates of the location of the meteoroid's trajectory within the Arecibo radar beam. The results indicate a noticeable, but weak, dependence between the distance of the particle's trajectory from the center of the beam (i.e. maximum gain) and the mass and velocity of the meteoroid. This suggests that the Arecibo radar is not particularly biased toward a specific velocity population of meteoroids (i.e. high velocity) as has been suggested in previous work.     

A comparison between the single ring pressure infiltrometer and simplified falling head techniques

Testing the relative performances of the single ring pressure infiltrometer (PI) and simplified falling head (SFH) techniques to determine the field saturated soil hydraulic conductivity, K_fs, at the near point scale may help to better establish the usability of these techniques for interpreting and simulating hydrological processes. A sampling of 10 Sicilian sites showed that the measured K_fs was generally higher with the SFH technique than the PI one, with statistically significant differences by a factor varying from 3 to 192, depending on the site. A short experiment with the SFH technique yielded higher K_fs values because a longer experiment with the PI probably promoted short‐term swelling phenomena reducing macroporosity. Moreover, the PI device likely altered the infiltration surface at the beginning of the run, particularly in the less stable soils, where soil particle arrangement may be expected to vary upon wetting. This interpretation was supported by a soil structure stability index, SSI, and also by the hydraulic conductivity data obtained with the tension infiltrometer, i.e. with a practically negligible disturbance of the sampled soil surface. In particular, a statistically significant, increasing relationship with SSI and an unsaturated conductivity greater than the saturated one were only detected for the K_fs data obtained with the PI. The SFH and PI techniques should be expected to yield more similar results in relatively rigid porous media (low percentages of fine particles and structurally stable soils) than in soils that modify appreciably their particle arrangement upon wetting. The simultaneous use of the two techniques may allow to improve K_fs determination in soils that change their hydrodynamic behaviour during a runoff producing rainfall event. Copyright © 2013 John Wiley & Sons, Ltd.     

Sensitivity and moment analyses of head in variably saturated regimes

A numerical approach for approximating statistical moments of hydraulic heads of variably saturated flows in multi-dimensional porous media is developed. The approximation relies on a first-order Taylor series expansion of a finite element flow model and an adjoint state numerical method for variably saturated flows to evaluate sensitivities. This approach can be employed to analyze uncertainties associated with predictions of head of steady-state or transient flows in variably saturated porous media, with any type of boundary and initial conditions. Limitations of stochastic analytical methods such as spectral/perturbation approaches and the time-consuming Monte Carlo simulation technique are thus alleviated. An example is given to demonstrate the utility of the approach and to investigate the temporal evolution of head variances in a variably saturated flow regime. Results show that the fluctuation of the water table can have significant impacts on the propagation of the head variance.     

Hydraulic head applications of flow logs in the study of heterogeneous aquifers

Permeability profiles derived from high-resolution flow logs in heterogeneous aquifers provide a limited sample of the most permeable beds or fractures determining the hydraulic properties of those aquifers. This paper demonstrates that flow logs can also be used to infer the large-scale properties of aquifers surrounding boreholes. The analysis is based on the interpretation of the hydraulic head values estimated from the flow log analysis. Pairs of quasi-steady flow profiles obtained under ambient conditions and while either pumping or injecting are used to estimate the hydraulic head in each water-producing zone. Although the analysis yields localized estimates of transmissivity for a few water-producing zones, the hydraulic head estimates apply to the far-field aquifers to which these zones are connected. The hydraulic head data are combined with information from other sources to identify the large-scale structure of heterogeneous aquifers. More complicated cross-borehole flow experiments are used to characterize the pattern of connection between large-scale aquifer units inferred from the hydraulic head estimates. The interpretation of hydraulic heads in situ under steady and transient conditions is illustrated by several case studies, including an example with heterogeneous permeable beds in an unconsolidated aquifer, and four examples with heterogeneous distributions of bedding planes and/or fractures in bedrock aquifers.     

Characteristics of head wave in multi-layered half-space

Introduction Head wave is one of the main phases of local seismic arrivals, and it is important in the study of the crustal structure and the physical characteristics of the transition-zone between the crust and the mantle. So far, the kinetic characteristics of head wave such as arrival time have been widely used, while the dynamic characteristics of head wave like amplitude were usually ignored al-though it includes more information of the structure. In this article, we shall investigate the…     

Transience of channel head locations following disturbance

We used annual re‐surveys of two populations of channel heads affected by a severe wildfire in 2012 to monitor changes in channel head location with time following disturbance. Relative to channel heads in surrounding unburned areas, the median contributing drainage area of burned channel heads decreased by two orders of magnitude immediately after the fire, but then returned to values comparable to unburned areas within four years. We distinguish three types of channel heads. Permanent channel heads, which constitute 4% of the total population, occur in well‐developed swales in association with stable features such as bedrock outcrops: these channel heads appear to have been unaffected by the fire. Persistent channel heads, which are 40% of the total population, also occur within hillslope concavities, but the exact location of the channel head moves upslope and downslope through time in response to varying inputs of water and sediment. Transient channel heads form on straight and convex slopes immediately following disturbance, but disappear as regrowth of ground cover limits overland flow and sediment movement. The majority of the position changes for persistent and transient channel heads occurred abruptly when viewed as annual time steps. Copyright © 2017 John Wiley & Sons, Ltd.     

Geometrical spreading factor of head waves in cased and open boreholes

Factors (coefficients) of geometrical spreading of compressional and shear head waves are calculated for an impulse multipole source of elastic oscillations in boreholes. It is shown that the length of the logging tool (i.e., the distance between the source and the nearest receiver) used for sonic measurements and the velocities of elastic waves in the medium both contribute to the factor of geometrical spreading. For a high-velocity formation (the shear wave velocity in the rock is higher than the compressional wave velocity in the fluid that fills the borehole) and a sufficiently long sonic tool with a monopole source, the coefficient of geometrical spreading is approximated by asymptotic formula 1/Z [Roever et al., 1974; Krauklis and Krauklis, 1976], where Z is the length of the tool; i.e., the amplitude of the compressional head wave decreases proportionally to the distance between the source and the receiver. In acoustically soft formations, this approximation is inapplicable even for long tools with length Z > 4 m. Waveforms in cased boreholes have a significant frequency dispersion even in case of good-quality cementing, and the factor of geometrical spreading there depends considerably on the length of the tool and the elastic properties of the rocks.     

Transmission of pressure head through the zone of tension saturation in the Lisse effect phenomenon

ABSTRACT

The problem of transmission of pressure head through the zone of tension saturation in the Lisse effect (LE), i.e., the rapid response of groundwater level to pressurized pore air in the unsaturated zone, is investigated theoretically and experimentally. From the law of conservation of energy and the continuity equation, a one-dimensional diffusion equation is derived for transmission of pressure head through the zone of tension saturation. The solution to the equation is the pressure head at any point below the upper boundary of the zone of tension saturation and at any time after the compressed pore air pressure is imposed on the boundary. The key parameter, which determines the behaviour of transmission of pressure head, is the newly proposed pressure head diffusivity coefficient. The theoretical results agree with the experimental results, obtained from laboratory column experiments in three physically different soils.

Editor Demetris Koutsoyiannis Associate editor Xi Chen     

A method for assessing kinematic bending moments at the pile head

The conventional design methods for seismically loaded piles still concentrate in providing adequate resistance from the pile to withstand only the inertial bending moments generated from the oscillation of the superstructure, thus neglecting the effect of kinematic interaction between pile and soil. By contrast there has been extensive research on kinematic effects induced by earthquakes and a number of simplified methods are available for a preliminary evaluation of kinematic bending moments at the interface between two soil layers. Less attention has been paid to the effects of kinematic interaction at the pile‐head. The paper summarizes recent research work on kinematic response analysis of fixed‐head piles aimed at the performance evaluation of a piled foundation. Results from an extensive parametric study, undertaken by means of three‐dimensional FE analyses, suggest a new criterion to predict kinematic bending effects at the pile head, where the combination of kinematic and inertial effect may be critical. Copyright © 2010 John Wiley & Sons, Ltd.     

Correlating fracture trends and hydraulic head using semivariogram cloud analysis

Fracture trends (defined as kilometer-scale linear features interpolated between field observations of fractures along their strikes) often have a dominant orientation. Finding a correlation between this orientation and hydraulic data could shed light on their hydraulic influence. A significant correlation between head residuals from first-order regional drift and the orientation of 2- to 4-km-long fracture trends was found in a study site in the Negev, Israel, using the semivariogram cloud analysis. Correlation of head residuals rather than the head itself implies that the orientation of the fracture trends controls the anisotropy and heterogeneity at this scale, mainly because the fracture trends define the orientation of blocks, which differ in their hydraulic properties. Preferential transmissive pathways are probably shorter than the full extent of the fracture trends, causing a relatively high head difference along the trends on the 2- to 4-km scale. Fracture trend density and additional data from short-range hydraulic tests helped characterize two blocks separated by a fault zone. The identification of hydraulic features on a kilometer scale is necessary for better modeling of regional ground water flow and transport. Hydraulic tests at this scale are not feasible, thereby rendering combined analyses of head and structural data, such as the one presented here, essential.     

Ray path of head waves with irregular interfaces

Head waves are usually considered to be the refracted waves propagating along flat interfaces with an underlying higher velocity. However, the path that the rays travel along in media with irregular interfaces is not clear. Here we study the problem by simulation using a new approach of the spectral-element method with some overlapped elements (SEMO) that can accurately evaluate waves traveling along an irregular interface. Consequently, the head waves are separated from interface waves by a time window. Thus, their energy and arrival time changes can be analyzed independently. These analyses demonstrate that, contrary to the case for head waves propagating along a flat interface, there are two mechanisms for head waves traveling along an irregular interface: a refraction mechanism and transmission mechanism. That is, the head waves may be refracted waves propagating along the interface or transmitted waves induced by the waves propagating in the higher-velocity media. Such knowledge will be helpful in constructing a more accurate inversion method, such as head wave travel-time tomography, and in obtaining a more accurate model of subsurface structure which is very important for understanding the formation mechanism of some special areas, such as the Tibetan Plateau.     

Stochastic analysis of flux and head moments in a heterogeneous aquifer system

This study investigates the behavior of flux and head in a strongly heterogeneous three-dimensional aquifer system. The analyses relied on data from 520 slug tests together with 38,000 one-foot core intervals lithological data from the site of the General Separations Area in central Savannah River Site, South Carolina, USA. The skewness in the hydraulic conductivity histograms supported the geologic information for the top two aquifers, but revealed stronger clay content, than was reported for the bottom aquifer. The log-normal distribution model described adequately the hydraulic conductivity measurements for all three aquifers although, other distributions described equally well the bottom aquifer measurements. No apparent anisotropy on the horizontal plane was found for the three aquifers, but ratios of horizontal to vertical correlation lengths between 33 and 75 indicated a strong stratification at the site. Three-dimensional Monte Carlo stochastic simulations utilized a grid with larger elements than the support volume of measurements, but of sub-REV (representative elementary volume) dimensions. This necessitated, on one hand, the use of upscaled hydraulic conductivity expressions, but on the other hand did not allow for the use of anisotropic effective hydraulic conductivity expressions (Sarris and Paleologos in J Stoch Environ Res Risk Assess 18: 188–197, 2004). Flux mean and standard deviations components were evaluated on three vertical cross-sections. The mean and variance of the horizontal flux component normal to a no-flow boundary tended to zero at approximately two to three integral scales from that boundary. Close to a prescribed head boundary both the mean and variance of the horizontal flux component normal to the boundary increased from a stable value attained at a distance of about five integral scales from that boundary. The velocity field 〈q_x〉 was found to be mildly anisotropic in the top two aquifers, becoming highly anisotropic in the bottom aquifer; 〈q_y〉 was anisotropic in all three aquifers with directions of high continuity normal to those of the 〈q_x〉 field; finally, 〈q_z〉 was highly anisotropic in all three aquifers, with higher continuity along the east–west direction. The mean head field was found to be continuous, despite the high heterogeneity of the underlying hydraulic conductivity field. Directions of high continuity were in alignment with field boundaries and mean flow direction. Conditioning did not influence significantly the expected value of the flux terms, with more pronounced being the effect on the standard deviation of the flux vector components. Conditioning reduced the standard deviations of the horizontal flux components by as much as 50% in the bottom aquifer. Variability in the head cross-sections was affected only marginally, with an average 10% reduction in the respective standard deviation. Finally, the location of the conditioning data did not appear to have a significant effect on the surrounding area, with uniform reduction in standard deviations.     

Estimation of co-conditional moments of transmissivity,hydraulic head,and velocity fields

An iterative co-conditional Monte Carlo simulation (IMCS) approach is developed. This approach derives co-conditional means and variances of transmissivity (T), head (φ), and Darcy's velocity (q), based on sparse measurements of T and φ in heterogeneous, confined aquifers under steady-state conditions. It employs the classical co-conditional Monte Carlo simulation technique (MCS) and a successive linear estimator that takes advantage of our prior knowledge of the covariances of T and φ and their cross-covariance. In each co-conditional simulation, a linear estimate of T is improved by solving the governing steady-state flow equation, and by updating residual covariance functions iteratively. These residual covariance functions consist of the covariance of T and φ and the cross-covariance function between T and φ. As a result, the non-linear relationship between T and φ is incorporated in the co-conditional realizations of T and φ. Once the T and φ fields are generated, a corresponding velocity field is also calculated. The average of the co-conditioned realizations of T, φ, and q yields the co-conditional mean fields. In turn, the co-conditional variances of T, φ, and q, which measure the reduction in uncertainty due to measurements of T and φ, are derived. Results of our numerical experiments show that the co-conditional means from IMCS for T and φ fields have smaller mean square errors (MSE) than those from a non-iterative Monte Carlo simulation (NIMCS). Finally, the co-conditional mean fields from IMCS are compared with the co-conditional effective fields from a direct approach developed by Yeh et al. [Water Resources Research, 32(1), 85–92, 1996].     

Tide-induced head fluctuations in coastal aquifers of variable thickness

In this work, a new analytical solution to describe tide-induced head fluctuations in aquifers of variable thickness is presented. The proposed model assumes a finite and confined aquifer with a thickness that increases or decreases quadratically with the distance to the coast. A closed-form analytical solution is obtained by solving a boundary-value problem with both a separation of variables method and a change of variables method. This solution is a generalization of the solution obtained by Cuello et al., Hydrogeological Journal, 2017, 25, 1509–1515. The analytical solution is expressed in terms of the wedging parameter, a parameter that depends on the length and thicknesses at the coast and at the inland edge of the aquifer. Positive values of the wedging parameter describe aquifers with increasing thickness towards land and negative values describe aquifers with a decreasing thickness in the inland direction. The comparison of the new solution and the solution for a finite aquifer with constant thickness indicates that the sign of the wedging parameter enhances or decreases the amplitude of the tide-induced signal. However, the differences in time-lag between both solutions are negligible near the coast. The slope factor, which quantifies the inconsistencies between aquifer diffusivities estimated from attenuation and time-lag data, is computed and analysed. Near the coast, slope factor values greater than one are obtained for negative wedging parameters while slope factor values less than one are obtained for positive wedging parameters. The analysis of the new solution also indicates that more reliable estimates of the hydraulic diffusivity can be obtained from time-lag data.     

Worth of head data in well-capture zone design: deterministic and stochastic analysis

In this study, we examine the effects of conditioning spatially variable transmissivity fields using head and/or transmissivity measurements on well-capture zones. In order to address the challenge posed by conditioning a flow model with spatially varying parameters, an innovative inverse algorithm, the Representers method, is employed. The method explicitly considers this spatial variability.

A number of uniform measurement grids with different densities are used to condition transmissivity fields using the Representers method. Deterministic and stochastic analysis of well-capture zones are then examined. The deterministic study focuses on comparison between reference well-capture zones and their estimated mean conditioned on head data. It shows that model performance due to head conditioning on well-capture zone estimation is related to pumping rate. At moderate pumping rates transmissivity observations are more crucial to identify effects arising from small-scale variations in pore water velocity. However, with more aggressive pumping these effects are reduced, consequently model performance, through incorporating head observations, markedly improves. In the stochastic study, the effect of conditioning using head and/or transmissivity data on well-capture zone uncertainty is examined. The Representers method is coupled with the Monte Carlo method to propagate uncertainty in transmissivity fields to well-capture zones. For the scenario studied, the results showed that a combination of 48 head and transmissivity data could reduce the area of uncertainty (95% confidence interval) in well-capture zone location by over 50%, compared to a 40% reduction using either head or transmissivity data. This performance was comparable to that obtained through calibrating on three and a half times the number of head observations alone.