Jose L. Llopis

Comprehensive geophysics investigation of an existing dam foundation; engineering geophysics research and development

Part 1 of this paper (TLE, August 1989) reviews recent geotechnical investigations conducted at Beaver Dam, Arkansas. The problem addressed was anomalous seepage beneath Dike 1, adjacent to the main embankment dam. The paper presents a summary of the site geology, seepage history, and foundation grouting programs. Figure 17 is a plan map showing the north and south bounding fault zones of a graben structure beneath Dike 1. The foundation is a down‐faulted block of severely weathered limestone/dolomite of the Boone formation. Figure 4 of part 1 shows a simplified geologic cross‐section through Dike 1 showing the graben structure. Overall objectives of the geotechnical investigations were to assess the anomalous seepage and plan remedial measures to eliminate or significantly abate the seepage. The engineering geophysics investigations discussed in part 1 were designed to detect, map, and monitor anomalous seepage paths and delineate geologic structure beneath Dike 1.

Comprehensive geophysical investigation of an existing dam foundation

Engineering geophysics has long played a role in geotechnical site investigations, although the significance and acceptance of the results have varied considerably. There has been a tremendous surge in acceptance and applications of engineering geophysics within the past 10 years. This improved status has resulted from a number of factors, not the least of which are improved instrumentation and microcomputers, better trained personnel, and the recognition of classes of geotechnical problems for which engineering geophysics is not only ideally suited but in many cases the only viable option. One such class of problems is the investigation of existing structures and their foundations, such as earth and rockfill dams. This class of problems has emerged in the United States, for example, because of an aging and decaying infrastructure; the key words in this effort are evaluation, repair, maintenance, and rehabilitation. Remediation efforts are directed to increasing the useful life of structures and insuring ...

Selected short stories on novel applications of near‐surface geophysics

Applications of near‐surface geophysics can be grouped under engineering and geotechnical, environmental, ground water, archaeological, forensic or law enforcement, military, and miscellaneous. However, the distinction is often arbitrary. For example, cavity or tunnel detection might be classified as engineering/geotechnical, archaeological, military, or forensic/law enforcement although the geophysical methods and approach are likely the same in each case. Near‐surface geophysics, for most applications, has three distinguishing characteristics not shared with regional geophysics or oil and gas exploration: (1) very high resolution surveys; (2) public health and safety concerns; (3) “near real‐time” validation or “ground truthing” of the interpretations.

Repeat Gravity Surveys For Anomaly Detection In an Urban Environment

Applicability of high-resolution potential field methods to anomaly detection and mapping in an urban environment can be limited due to cultural noise. Another problem which limits the general applicability of high-resolution potential field methods, particularly microgravimetry, is the difficulty in correcting for the effects of rugged topography in the vicinity of the surveys. This paper presents the results of a novel field study which addresses these two limitations. Two survey lines were selected which crossed the centerline of a tunnel being advanced by a tunnel boring machine. Surveys were conducted on three occasions: (1) prior to tunnel construction: (2) tunnel advanced to a location directly under a survey line; (3) after tunnel construction. The field study assesses the detectability of the tunnel anomaly ( < 10 nanoteslas; < 40 microgals) in the presence of the cultural noise level. The gravity survey results are examined for single-pass detection of the tunnel anomaly without topographic corrections. Differences in the repeat gravity survey data are also examined for anomaly detection; the difference gravity profiles in principle should eliminate all topographic effects (including the need for free-air and Bouguer corrections) and should contain only anomalies due to changed subsurface conditions. The gravity difference profile results demonstrate the successful detection of the tunnel gravity anomaly.

Applied geophysical techniques to evaluate earth dams and foundations

Mill Creek Dam, near Walla Walla, Washington has experienced anomalous seepage since its first filling in 1941. Various attempts to abate and control the seepage, including construction of a concrete wall, have not been completely successful. Construction of the cutoff wall reduced the seepage by about 30 percent, from 33 cubic feet per second to 22 cubic feet per second, and downstream saturated farmland was reduced by 56 percent. However, there are indications of increased seepage pressures in a conglomerate formation in the right abutment. A comprehensive, integrated geophysics investigation of the right abutment area of the dam was conducted to detect and map anomalous conditions and assist in the evaluation of remedial measures. The geophysics program consisted of microgravity, ground penetrating radar, seismic reflection, electromagnetic conductivity, and electrical resistivity surveying. Results of the program indicate anomalous conditions extending from the reservoir area through the right abutment. The aspects of the program planning leading to technique selection and field procedures are emphasized, as well as the role of different geophysical techniques in defining the nature of anomalous condition.

Remote Sensing and Monitoring of Earthen Flood-Control Structures

Abstract : The purpose of this study was to identify and review technologies that are applicable in locating weaknesses and poor performance within flood-control structures from extreme loading events. The focus of this study was to assess current technologies and state-of-practice techniques involving remote sensing, testing, and real-time monitoring of earthen structures. Advancements in satellite and sensor technology combined with high-speed internet and telecommunication capabilities and smart decision-making software permits real-time monitoring of earthen flood-control structures such as dams and levees. Technologies evaluated included both active and passive sensing methods. These technologies included satellite, airborne, and ground-based sensor systems to identify surface and subsurface characteristics of the watershed, as well as point sensors typically embedded in hydraulic structures to monitor the health of the structure. Point sensors typically record water loading, soil pore pressures, soil movements, and other important properties to evaluate global stability of the water control structure. Geophysical-based methods are typically used in mapping, monitoring, and detection of subsurface stratigraphy, seepage, and any changes in subsurface conditions through time within flood-control structures and their foundations.

Crosshole seismic methods to assess the seismic stability of earthen dams

An in situ geophysical investigation consisting of crosshole and downhole shear wave (S-wave) and surface vibratory tests was performed at Sardis Dam, located on the Tallahatchie River in northwest Mississippi. Tests were conducted in a pile test section at the downstream toe of the dam. The purpose of the investigation was to determine changes in S-wave velocities, which are related to soil strength, in the test section due to pile driving activities. Comparison tests were conducted three times: prior to driving the piles, immediately after, and three months after the piles had been driven. The S-wave crosshole and surface vibratory tests indicated a significant velocity increase, averaging more than 20 percent, due to the driving of piles in the test section. Of the geophysical tests conducted, crosshole testing was the most sensitive in denoting velocity changes. The downhole S-wave test did not detect any significant velocity increases. The scope of this paper is limited to results of crosshole testing.