Innovative Applications of Geophysics in Civil Engineering

The D-Day invasion site at Pointe du Hoc, Normandy, France is an important WWII battlefield and cultural resource but is at risk from chalk cliff collapse. The American Battle Monuments Commission seeks to evaluate the geohazard to the observation post and other cliff-side buildings. Geophysical multi-electrode resistivity profiling is used to study cliff stability and the condition of the observation-post foundations. Preliminary 2-D geological interpretations are provided of individual profiles. The copious steel, concrete and void spaces at the site renders hydrogeological interpretation challenging but tractable. The cliff face appears to be relatively intact and well-drained. Several routes taken by groundwater into fractures within the chalk were identified mainly on the western side of the site. The observation post may be moving with the soil and not securely anchored to bedrock. Resistivity tomography is an appropriate geophysical technique to study geological stability of the cliffs at Pointe du Hoc.

A new method for estimating the permeability of soils using an acoustic technique is proposed in this paper. Biot's coupled theory mixtures for elastic wave propagation is used. According to Biot, the permeability of saturated soils is related to the characteristic frequency of elastic waves. The characteristic frequency is determined from attenuation vs. frequency curve obtained by an acoustic sweep test in the field. This study presents the testing procedure for measuring the characteristic frequency of soils, the estimation of the permeability from the characteristic frequency and the comparison of the acoustic soil permeability with the in situ permeability test results. The measured permeability of soils using the acoustic technique was comparable to but generally higher than that obtained from in situ permeability test.

The utilization of reclamation methods to expand coastlands and create artificial islands is becoming more and more popular recently. Reclaimed clay, however, is generally weak, and thus effective prediction of its in-situ strength and consolidation status is of critical importance. In-situ reclaimed clay sequentially follows sedimentation and consolidation processes under self-weight. In the case of kaolinite clay, the free fall settling process is short; thus consolidation is a more dominant process for kaolinite-type reclaimed deposits. Conventionally, large tank sedimentation tests, settlement monitoring, and CPT test methods are widely used for characterization of reclaimed sites. However, these methods are often inefficient and disagree with the existing consolidation theories. The present study suggests a laboratory procedure to characterize reclaimed clays using shear waves. Because it is difficult to obtain undisturbed soil samples, small scale sedimentation tests are performed in the similar environmental conditions as in the field so as to reproduce the in-situ soil fabric in the laboratory. Consolidation tests are also performed to simulate the field consolidation process. During the consolidation test, shear wave velocities of soil are continuously measured. The experimental results show that for a given normally consolidated clay, the shear wave velocity is uniquely related to the effective stress and the void ratio. Based on the relationships, the shear wave velocity is used to estimate design parameters such as the degree of consolidation, and undrained shear strength. Finally, the in-situ consolidation state and shear strength are estimable by comparing the obtained relationships with the in-situ shear wave velocity.

Estimation of seismic surface wave phase velocities is important to dynamic and earthquake engineering problems. Several methods have been proposed to estimate the Rayleigh seismic surface wave velocity, including the recent applications of array-based processing algorithms. This paper will discuss the importance of appropriate geometric spreading weights for wavenumber and amplitude estimates. In particular, the article will introduce the forward-backward cylindrical beamformer and compare it with the forward-only cylindrical beamformer performance for wavenumber and amplitude estimation. The algorithm performance will also be compared for two candidate weighting functions, which are the square-root of distance far-field weights and the cylindrical wavenumber weights. The results show that the square-root weighting tends to yield better wavenumber estimates, while the cylindrical weighting tends to yield better amplitude estimates. Additionally, the forward-backward algorithm shows better mainlobe and sidelobe characteristics compared to the forward-only algorithm.

The Refraction Microtremor (ReMi) method has recently been introduced to estimate the Rayleigh seismic surface wave phase velocity with passive sources. The ReMi method utilizes the p-τ algorithm to estimate wave velocity with a linear array of sensors. Relying on the observation that, in a wavefield with equal energy impinging from all directions, 40.9% of the wave velocity will be within 100 to 125% of the expected phase velocity, ReMi attempts to determine the fundamental mode phase velocity in a passive energy environment with a linear array. This paper will show that, due to material attenuation, multiple modes and varying direction of arrivals, ReMi does not yield reliable results. Using simulated data, the fundamental performance of ReMi in different realistic wavefield environments is studied.

Electrical resistivity methods are active geophysical exploration techniques with a very wide range of applications in geology, civil engineering, geotechnical engineering, geoenvironmental engineering, archaeology, water resources engineering, among many others. The premise of the technique is on how the applied electric current flows through geomaterials of different electrical properties in the subsurface, resulting in the measured electrical potential across two points on the ground surface. If the geomaterials can be idealized as an electric circuit composed of analogous circuit elements, a well-developed circuit simulator will become a readily available and indispensable tool to analyze and interpret the field data. The results of an attempt to adopt the approach are presented in this paper. Advantages and potential future developments of the approach to expand the applicability of the electrical methods in geophysical investigation are also discussed.

Micro-meter dynamic crack responses in a two story structure to rock blasting- and wind gust-excitation are compared to those induced by long term climatological effects. These measurements substantiate the conservancy of the 12.5 mm/s (0.5 in./s) blasting vibration control to protect residential structures against cracking. The test structure was instrumented with both velocity transducers to measure superstructure motions as well as special micro-meter displacement sensors to measure crack expansion and contraction. Crack responses to 48 to 64 km/hr (30–40 mph) wind gusts were equal to those generated by blasting vibrations of 11.5 mm/s (0.45 in./s) or 90% of the control limit. There are few reports of wind responses in the rock blasting literature because the long period of the wind "drift" motions produces low structural velocity responses, which are below the noise (and thus trigger levels) of most standard seismographs. The paper closes with an assessment of the frequency of occurrence of significant wind gusts during an average weather year.

The Spectral Analysis of Surface Waves (SASW) method is a nondestructive in situ seismic testing method used in evaluation of shear wave velocity profiles of layered systems. The method has been successfully used for twenty years in evaluation of layered media. The assumption of the SASW test is that the system tested consists of a set of homogeneous layers of constant thickness. However, results of theoretical and experimental studies point to significant effects of underground obstacles on wave propagation patterns in soils in general, and surface waves in particular. Specifically, obstacles cause strong fluctuations in the dispersion curve. Increase in the apparent phase velocity was observed with an increase in the obstacle rigidity. This paper focuses on the influence of composite underground obstacles on propagation patterns of surface waves. Composite obstacles are described as objects that combine rigid and soft materials. In order to investigate effects of composite obstacles on apparent phase velocity values and dispersion curve patterns, a field experimental study, complemented by numerical simulations, was conducted. The results pointed out that composite obstacles cause, contrary to the expected, a decrease in the apparent phase velocity. This conclusion is of particular interest in detection of underground utilities buried within loose-backfilled trenches. Compacting backfill to the level that will provide the shear wave velocity equal to the one of the surrounding virgin soil is essential for the successful detection of underground utilities by means of SASW.

The application of the Impact Echo (IE) test in long term condition monitoring of concrete bridge decks was investigated through a comprehensive numerical study. The potential use of the IE technique for detection of various stages of delamination progression was evaluated. It was assumed that the condition of a delaminated deck worsens according to two probable scenarios. The first scenario assumes the growth of a single small delamination. The second one involves progressive expansion of a number of small delaminations and their ultimate linking to make a large delaminated area. Both deterioration scenarios were numerically simulated by analyzing a large number of finite elements (FE) models of concrete decks containing expanding delaminations (Scenario 1) and delamination segments of increasing size (Scenario 2). Changes in the IE test results due to the expansion of a delaminated zone according to either scenario were studied. The numerical results indicate that the worsening condition of the deck has distinct effects on the IE test results which can be easily identified. Therefore, IE test can be employed for condition monitoring of concrete bridge decks.

The principle and process of the parallel seismic test for the integrity evaluation of in-service piles are introduced in this paper. A 3D finite element model for the pile-soil system is developed for impulse responses. For both saturated and unsaturated soil conditions, velocity-time histories at different depths were obtained using the numerical model. It is shown that the length and one-dimensional P-wave velocity of a pile are easy to determine from the velocity-time histories. By examining the slopes of the first arrival times plotted versus depth or by examining the depth where the amplitude of the first arrival decreases significantly, the bottom or the defect location of the pile can be estimated. The results indicated that the parallel seismic test is a suitable method to evaluate the integrity of in-service piles.

The conventional dynamic tests of composite foundation are reviewed and the velocity or mechanical admittance method for a block on the single-piled composite foundation is then recommended in this paper. The formulation for the mechanical admittance of block-pile-soil system is established by using the axisymmertic coupled finite-infinite element method. Dynamic response of a block on single-piled composite foundation is analyzed with the model. The influences of the pile length, pile diameter, pile defects, and pile-soil stiffness ratio are subsequently studied. The sensitivity analysis for the shear wave velocity of soil, pile length, and pile defects are also presented. The study will be of guide significance on further research of the proposed mechanical admittance method as well as on design optimization for dynamic machines with composite foundations.