Geotechnical Measurements: Lab and Field

The present paper summarizes the details of a testing system developed for performing combined axial torsional loading on hollow cylinder cohesive specimens. The axial-torsional loading system used in this research employs a MTS loading frame with Testware-SX control. The frame is operated hydraulically through the use of a servovalve that feeds actuators which can independently control the axial load/displacement and/or torque/angular rotation. Stress paths ranging from triaxial compression to pure shear to triaxial extension can be obtained using this testing apparatus by rotating the fixed inclination of the major principal stress (β) with the axis of rotational symmetry. The ratio between the torsional shear stress increment and the vertical stress increment is maintained a constant value to achieve a desired β value. A new approach using PID control was implemented to achieve precise control. To implement this into the control procedure, calculated control channel was used throughout the test using updated specimen dimensions. Undisturbed hollow cylinder specimens were obtained using high water content kaolinite slurry, and custom built consolidometer. Typical test results for stress path corresponding to β = 30 degrees were presented for isotropically consolidated kaolin clay under undrained conditions.

Undrained Triaxial compression tests were performed on reconstituted samples of Ardebil sand to evaluate the effect of the specimen preparation procedure on the position of the steady state line under monotonic loading. Tests were performed on saturated samples by three distinctive methods of sample preparation, namely, “wet tamping”, “water sedimentation”, and “undercompaction”. In all cases, the loosest state of structure are tried to be obtained. The weakest samples were those formed by water sedimentation method because of non-uniformity of the sample. The steady state line of specimens prepared by water sedimentation method lies under those of wet tamping and undercompaction methods. The test results indicate that the normalized steady state strength of a given material tends to vary to some extent depending upon the fabric formed by different modes of deposition. Based on the data presented, it is found that the liquefaction behavior of specimens tested in triaxial equipment may be significantly affected by the method of sample preparation.

A series of displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses in a microgravity environment aboard the Space Shuttle during the NASA STS-89 mission. Post-flight analysis included studying the internal fabric and failure patterns of these specimens using Computed Tomography (CT). The CT scans of three specimens subjected to different compression levels (uncompressed specimen, a specimen compressed to 3.3% nominal axial strain (ε_a), and a specimen compressed to 25% &eh;_a) are presented to investigate the evolution of instability patterns and to quantify void ratio variation. The progress of failure is described and discussed. Also, specimens' densities were calibrated using standard ASTM procedures and void ratio spatial variation was calculated and represented by contour maps and histograms. The CT technique demonstrated good ability to detect specimen inhomogeneities, localization patterns, and quantifying void ratio variation within sand specimens.

This paper presents the results of a series of investigations performed with peat soils to determine the modulus of deformation, E, and the modulus of subgrade reaction, K, using an elastic foundation model. The experiments were performed both in situ and under laboratory conditions using concrete plates placed over peat. Experiments were carried out under both static and dynamic loads. Based on the results of the investigation, empirical equations for the modulus of subgrade reaction for sustained and resilient deformation are proposed and related to peat soil properties.

The effects of two structural factors — grain crushing and bonding — were investigated using model materials — granulated silica gel (for the compression) and cement-sand mixture (for the shear resistance). Grain crushing correlates well with kinking of the (oedometric) compression curves. It is suggested that this is the reason for a garlandlike shape of compression and creep curves. Ratio of compression indices C_α/C_c is a constant. Bonding finds place within a specific bonding surface where the behavior is brittle. Outside of this surface, debonded material (sand) behaves in a standard manner, e.g. as ductile (if loose). Both structural factors — grain crushing and debonding — deeply affect the constitutive behavior of geomaterials.

Time domain reflectometry (TDR) has been used for many years in the telecommunication industry to locate damage or faulting in communication cables. More recently TDR has been applied in the geotechnical/geologic field in order to detect movement and locate shear planes in rock or soil slopes by grouting coaxial cables in boreholes. TDR can provide significant cost savings over traditional inclinometers in many applications as well as be remotely accessed. This paper presents five case histories in which TDR has been successfully used to monitor and analyze unstable rock slopes and soil embankments. Case histories cover a range of applications which include an open pit rock slope, a small rock slope, two embankment/levees over soft/loose soil, and a native soil slope. In addition to monitoring for soil movement, the last case history also presents the innovative employment of TDR to measure changing groundwater levels in an economical remote sensing configuration. This paper also summarizes testing equipment, cable selection, and cable installation methods for each case history.

As TDR technology grows in acceptance, its use stimulates further innovative applications and comparison with slope inclinometer measurements. This paper presents cases in which the opportunity arose to compare these two technologies to detect and measure subsurface deformation in slopes. Among the main points addressed are (1) the comparison of TDR reflection magnitude and inclinometer incremental displacement to help quantify deformation with TDR technology, and (2) the comparison of the accuracy of the two technologies in detecting and measuring shear deformation in localized versus general shear. Case histories are presented that involve monitoring movement in soil and rock slopes and embankments as well as retrofitting deformed inclinometer casing with coaxial cables. This paper describes installation details. When monitoring to detect narrow shear zones in soils, it is best to use small ratios of hole-to-cable diameter, and prudent use requires that larger diameter, solid, metallic coaxial cables be installed in separate holes. Grout strength should be (1) low enough to fail before bearing capacity of the surrounding soil is reached, and (2) high enough to deform the cable it encapsulates. It is recommended that other users publish cases in which theses two technologies are compared in order to expedite continued assessment.

The Washington Park Station is a key element of the Tri-County Metropolitan Transportation District's (Tri-Met's) Westside Light Rail Transit System in Portland, Oregon. The Station platforms are located 79 meters (260 feet) below the ground surface, with sole access provided through two 9.5 meter (31 foot) diameter concrete-lined vertical shafts. Within theses shafts are high- speed elevators for transferring passengers between the surface and the Station, fixed stairways, ventilation ducts, electric power conduits, signal and communication system conduits, and a fire suppression standpipe system. The lower 55 meters (180 feet) of each of the shafts is constructed in bedrock and the upper portions are within a large ancient landslide. In addition, the drilled pier foundation elements for the headhouse structures extend through the landslide and into the underlying bedrock. The discovery of creep movements within the ancient landslide resulted in the need to incorporate special design features in the shafts. These design features allow for a total of 5 centimeters (2 inches) of horizontal shear movement within the shafts, at the location of the basal failure plane of the landslide. Five centimeters (2 inches) was selected as the design criteria based on an evaluation of the creep movements measured in the inclinometer instrumentation and the extrapolation of these movements over the 50-year design life of the structure. The beneficial impacts of the remediation measures installed during construction of the Station were also considered in the selected 5-centimeter (2-inch) design criteria. Additional design considerations included the guide rail supports for the high-speed elevator: they can tolerate only 0.64 centimeters (0.25 inches) of movement without adjustment to maintain safe operation. With these design criteria, both Tri- Met and the City of Portland Building Department considered it essential to adopt a long-term monitoring program. If performance monitoring showed rates of movement in the landslide that would result in the design criteria being exceeded, then additional landslide remediation measures would need to be implemented. To achieve the long-term performance monitoring objectives, a monitoring system was designed and installed. The system provides real-time monitoring of the elevator shaft movements, and the movements and ground water conditions within the landslide. The monitoring system consists of: 1) instrumentation located at the slip joints within the shaft structures and in the landslide mass near the Station, 2) both manual and automated data acquisition for monitoring the instrumentation, and 3) an integrated warning system and associated response plan if detected movements exceed predetermined threshold levels.

Movements of a pile-supported wharf due to tidal fluctuations acting on unstable sediment mounds are described and analyzed. A 244 meter long by 61.5 meter wide general cargo wharf (Berth 14) at Port Klang, Malaysia has moved approximately 135 millimeters horizontally since its construction in 1963. Results of a detailed investigation are summarized, along with an analysis conducted for the structure. A detailed geotechnical study was completed to characterize the soil, both within and underlying the sediment mounds down to firm bearing horizons for deep foundations. Additionally, a short-term tidal fluctuation movement- monitoring program was completed to measure the movements of the wharf during the peak tidal cycles. The gaps at the expansion joint between the access trestle and the wharf were measured at high and low tides for correlation purposes. The data gathered in the field confirmed movement of the wharf and supporting piles. Secondly, it allowed determination of maximum permissible dredge depth for the berth. Thirdly, it provided a determination of the cause of the movement. A correlation between the measured movements in the berth and empirical relationships for lateral load on piles was established using the procedure documented by C. Vigianni (1981) and a finite element model of the berth.

Landslides are frequently remediated by constructing engineered fills, improving drainage, or other more specialized construction methods. Monitoring instruments, such as surface monuments and inclinometers, are sometimes installed to evaluate the performance of the remedial measures. Where remediation involves engineered fills, the engineer should recognize that compacted fills undergo an equilibration process that can take years. This process can involve heave caused by expansive soil and consolidation due to the weight of the fill and imposed structural loads. Additionally, fills placed upon hillsides can be subjected to differential settlement consistent with fill thickness and development changes, surface creep, and lateral extension. Monitoring data can indicate various subsurface movements which are a product of settlement of the fill mass and lateral extension, and not related to movement of the remediated landslide. Some of these conditions were encountered at a site in Northern California. Misinterpretation of the gathered data could easily have occurred, possibly initiating unwarranted remedial measures or preventing development. However, the use of certain analytical and data presentation methods clearly showed that the remediated landslide was performing as designed.

The Stonepoint Sweetwater Reconstruction Project consists of a 172 unit residential development of single family homes on shallow concrete slab foundations built in 1987 near San Diego, California. The project has experienced various levels of distress and damage from geotechnical related causes attributed to unstable fill slopes and associated lateral fill slope extension, volcanic rock fills with voids and inadequately compacted clayey fill soils, and soil creep on fill slopes as high as 18 meters and as steep as 1 1/4 to 1. Differential building settlements of up to 20 cm and horizontal movements of building and slope improvements of several centimeters forced immediate action and the expenditure of several million dollars to solve the problem over a 5 year design/construct period from 1995 to 2000. Considerable investigation and evaluation of numerous options resulted in the selection of a unique but economic solution of tied back individual anchor blocks as the primary slope restraint system to maintain the homes from further movement. Over 100 wire tendon tiebacks were installed and pressure grouted into the volcanic bedrock using down hole percussion drilling techniques. The tiebacks were drilled to depths as deep as 61 meters and tested to capacities of 1423 kN. Concrete and steel anchor blocks measuring as much as 2.4 meters square and 0.6 meters thick were then constructed for each tieback on the rock and clay fill slopes below the homes. When tested and locked off these tied back anchor blocks formed the primary line of defense to mitigate the shallow building foundation and slope improvement failures. This paper will discuss the design of the tieback restraint system, the difficult construction constraints encountered on the steep slopes, and the current successful results of slope and building monitoring.

Foundation systems in expansive soils within the western US commonly utilize drilled concrete piers and grade beams. A field test site was established to investigate the behavior of full scale drilled concrete piers. Five concrete piers were instrumented and installed to depths ranging from 3 to 7.6 m in a known expansive soils deposit on the Colorado State University campus. Four reinforced concrete piers were instrumented with vibrating wire strain gauges at 1.8, 3, 4.3 and 5.5 m depths. One pier was not instrumented. Two of the piers were installed beneath a simulated slab-on-grade and three were placed in an uncovered area. The uncovered area was irrigated beginning a year after installation of the piers. A vibrating wire strain gauge was installed in a laboratory test cylinder. The laboratory cylinder was used to compare baseline readings with the field cylinders, and to monitor environmental factors (e.g. temperature and shrinkage) influencing strain. Strain in the concrete piers was monitored over a two year period and compared with soil water content changes, free field heave and displacement of the piers at the ground surface. Performance of the drilled concrete piers was compared with that of a patented helical pier system installed in the same environment. Skin friction mobilized in the heaving soil zones has not been sufficient to overcome the anchoring capacity of the deep piers, but the 3 m pier heaved approximately 30 mm over the 2 year period. Significant tensile strains have been measured, and have continued to increase. It is believed that strain in the concrete piers can be correlated with movement of the wetting front downward through the soil.

The three earth-core rockfill dams of the Diamond Valley Lake Project were instrumented with vibrating wire piezometers during their construction. The vibrating wire piezometers were buried within the central earth core of the dams and as additional embankment was constructed the pore pressures within the core material were recorded. The primary purpose of the piezometers is to monitor pore water pressures in the core during first filling and long term operation. During construction the piezometers provide baseline readings with which to compare readings taken during first reservoir filling and long term operation. The construction data also provided information on the development and dissipation of pore pressures within partially saturated soil due to fill placement. The earth-core rockfill dams range in height from 58 to 114 meters (190 to 375 feet).The central earth-core material generally consists of clayey sand and sandy lean clay excavated from the upstream alluvial valley. Immediately after compaction, the material has a degree of saturation of approximately 85 percent. Vibrating wire piezometers with coarse porous tips are installed at regular intervals within the core material by placing them in shallow trenches and compacting core material directly around the piezometer. Measurements were taken immediately. For this paper the term "pore pressure" refers to the measurements of the coarse porous tipped vibrating wire piezometers. This type of piezometer measures approximate pore air pressure in partially saturated soil, and pore water pressure in saturated soils. Initially after compaction the pore air pressure is about atmospheric pressure and the pore water pressure is below atmospheric pressure due to capillary suction. As additional fill is placed and the soil compresses the difference between the pore air and pore water pressure decreases. Once the reservoir is filled the core will become saturated and the piezometers will measure pore water pressures. This paper presents a general description of the earth-core rockfill dams with details provided on the core material properties and construction methods. A general discussion of the instrumentation program with details on the vibrating wire piezometers and their installation are also provided. Finally, the pore pressure measurements are presented.

Port of Freeport has built a new General Cargo Dock and Transit Shed. The project included a 12m high-anchored sheet-pile wall and a relieving platform supported by auger cast-in-place piles. Detailed instrumentation of the dock system was undertaken. The instrumentation included inclinometers, strain gauges and total earth pressure cells on the sheet-pile wall and strain gauges on the anchor rods. Measurements were made over a period of six years to determine deflection, moment and soil pressure on the sheet-piles and tension on tie backs at various stages. The deflections measured in the sheet-pile wall were surprisingly larger than originally calculated. A parametric study was conducted to determine what factors might have been responsible for the large and unexpected deflections. It was concluded that the high deflections were due to the manner in which the backfill was placed and the reduced section modulus: It was determined that the manner in which the sheet-pile was installed did not provide the section modulus as expected. Alternative methods of construction should be examined for future dock construction, which will eliminate the high construction stresses observed in the present project.