J. Erik Loehr

Sidewall leakage in hydraulic conductivity testing of Asphalt concrete specimens

Asphalt specimens with irregular sidewalls can require special testing considerations when measuring hydraulic conductivity in a flexible wall permeameter. The hydraulic conductivity of laboratory prepared, dense asphalt concrete with high asphalt cement content (6–8.5% asphalt cement) was measured in the laboratory. Hydraulic conductivities were found to be excessive and were shown to be caused by sidewall leakage. Applying a thin layer of silicone vacuum grease to the sides of the asphalt specimens arrested the sidewall leakage, thereby permitting more accurate measurement of the hydraulic conductivity of the asphalt concrete.

Live Load Effect in Reinforced Concrete Box Culverts Under Soil Fill

AbstractThe effects of live load in box culverts may be nearly negligible compared with the effects of dead loads when significant fill is placed above the crown of the culvert. The objective of this study was to determine the effects of live load (truck loads) on reinforced concrete box culverts classified as bridges [spans greater than 6 m (20 ft)] under soil fills of different thicknesses. The study considered the field testing of 10 existing reinforced concrete box culverts with fill depths ranging from 0.76 m (2.5 ft) to 4.1 m (13.5 ft). Instrumentation of the culverts consisted of 12 reusable strain transducers and 12 linear variable displacement transducers. The instrumentation was designed to be applied, used, and removed in one day of testing. Loaded trucks were driven over the culvert to provide live load. The results of the testing show that the live-load effect does diminish with increasing fill depth. The 2012 AASHTO LRFD bridge design specifications are overly conservative in predicting stra...

Sealed Double-Ring Infiltrometers for Estimating Very Low Hydraulic Conductivities

The goal of this paper is to document the issues relating to measurement of in situ hydraulic conductivity of very low hydraulic conductivity materials, particularly asphalt concrete using sealed, double-ring infiltrometers (SDRIs). A new flow volume measurement device known as a constant head board (CHB) was developed and used alongside with flexible bags to measure the infiltration rate of the asphalt concrete barrier test pad. The design, construction, and use of the SDRI are presented followed by the design and operation of the CHB. A method to determine the hydraulic gradient based on the total flow quantity and air void content of the liner are presented. Flow quantity measurement, infiltration rate, and calculated hydraulic conductivity based on flexible bag and CHB options are presented and discussed.

Shear Behavior of Compacted Silty Loess

Loess, a uniform-sized, wind-blown, cohesive soil can be problematic in geotechnical engineering applications. Natural deposits have high strength and exhibit stable vertical slopes; however, dramatic collapse can occur when loess becomes saturated. In remolded, compacted applications, the behavior of loess is less well known. Regionally (central USA), embankments of compacted loess can exhibit deformation and slope failure after years of service. The initial phase of a larger effort to characterize the strength behavior of compacted loess is reported herein. This phase is limited to assessing the short-term, undrained shear strength of compacted loess. Undrained strengths of compacted loess were measured to be up to 500 to 650 kPa. Higher compactive effort, dry of optimum water content, maximizes the undrained strength. Modified Proctor energy increased undrained shear strength by up to 2.5 to 3.0 times higher than loess compacted at the same water contents using standard energy. Loess compacted at or greater than optimum moisture content, as determined in the standard Proctor test, had similar undrained strength (100 to 200 kPa) regardless of the compactive effort. In order to maximize short term strength of compacted loess, use the greatest compactive effort and compact the soil well dry of optimum moisture content.

Effect of Number of Soil Strength Measurements on Reliability of Spread Footing Designs

One of the primary objectives of using load and resistance factor design (LRFD) is to produce designs that consistently reach some established target reliability. Achieving this objective is challenging in geotechnical applications because variability and uncertainty in design input parameters vary substantially from site to site and with different quantities of site characterization. Analyses were performed to simulate spread footing design by using different numbers of undrained shear strength measurements. Results revealed that the likelihood was high of over- or underestimating both the mean value of a design parameter and the uncertainty in the mean value when small numbers of measurements were considered. The analyses also revealed that LRFD methods that prescribed constant resistance factors frequently produced underreliable designs when small numbers of measurements were considered. The percentage of underreliable designs decreased when increasing numbers of measurements were used. However, this decreasing percentage of underreliable designs was accompanied by an increasing percentage of overreliable designs rather than an increasing percentage of designs that achieved the target probability of failure within practical limits. In contrast, LRFD methods prescribing resistance factors that depended on uncertainty in design parameters tended to produce a consistently low percentage of underreliable designs when more than five measurements were used. As greater numbers of measurements were considered, such methods produced increasing percentages of designs that practically achieved the target probability of failure and decreasing percentages of overreliable designs.

Variability and Uncertainty in Consolidation and Settlement Parameters from Different Sampling and Testing Methods

A comprehensive research program was recently completed to evaluate variability and uncertainty in geotechnical design parameters, including those used for consolidation and settlement analyses. Numerous one-dimensional consolidation tests were conducted following incremental load and constant rate of strain loading procedures for specimens obtained from four different sites using state of the practice and state of the art sampling techniques. The four sites included two soft clay sites, one soft silt site, and a stiff silt/clay site. Laboratory consolidation tests were conducted on specimens from each site to determine compression and recompression indices, preconsolidation stress, and coefficients of consolidation. Results from different sampling and testing methods are compared to draw conclusions regarding the variability and uncertainty associated with consolidation parameters obtained following different sampling and testing protocols. State of the art boring, sampling, and testing methods were found to produce lower values of preconsolidation stress, greater values of compression index, and similar values of recompression index and coefficient of consolidation compared with results from state of the practice methods for the three soft soil sites. Variability and uncertainty in consolidation parameters determined from state of the art methods were also found to be notably less than the variability and uncertainty established using state of the practice methods for the soft soil sites while the variability and uncertainty in consolidation parameters for the stiff soil site were similar for both state of the art and state of the practice methods.

DESIGN METHODOLOGY FOR STABILIZING SLOPES USING RECYCLED PLASTIC REINFORCEMENT

Surficial slope failures, or nuisance slides, constitute a significant economic and manpower burden for many transportation agencies due to the frequent and recurring nature of the slides. A new method for stabilizing surficial slides using reinforcement manufactured from recycled plastics is being developed to provide agencies with a cost-effective alternative for stabilizing these slopes. As a part of this development, a design procedure has been established that draws upon previous experience with more conventional reinforcing materials such as concrete and steel, but with modifications to account for the reduced strength and stiffness of plastics. The design method follows a limit state design approach wherein a number of different limit states are considered, including failure of the reinforcing members, to establish the resisting force provided by the reinforcement. In this paper, the general design method is presented followed by more detailed coverage of each of the specific limit states that are considered in the design. Several design issues that remain to be addressed are also discussed.

GEOTEXTILE SEPARATORS FOR HIKE AND BIKE TRAIL

Intrusion of surface aggregate into the underlying soil of hike and bike trails creates undesirable conditions for trail users and exacerbates the problem of trail rutting. A study is underway to evaluate the effectiveness of geotextile separators for rails-to-trails applications. Non-woven needle-punched and spunbonded geotextiles were placed between the subsoil and the aggregate surface in three test sections along a trail. The test sections are being monitored for visible intrusion of aggregate into the subsoil, drainage of surface water off the trail, and enhanced performance and durability of the aggregate surface compared to control sections. Destructive samples of the geotextile separator and aggregate were taken and tested periodically to evaluate field performance of the geotextiles. The characteristics of the surface aggregate and geotextile separators, installation of the three test sections, and results of performance monitoring and laboratory testing activities are described herein. The findings indicate the geotextile separators to be performing well and the test sections have maintained a well-drained, smooth surface after one year. These sections vastly exceeded the performance of the control (no separator) sections.

Influence of Geotechnical Investigation and Subsurface Conditions on Claims, Change Orders, and Overruns

Subsurface conditions are frequently considered to represent significant elements of technical and financial risk for highway construction projects. Unfortunately, information quantifying these risks is rare. This report documents the extent and type of claims, change orders, and cost overruns from subsurface conditions for state departments of transportation (DOTs). Information used in this study was gathered through a literature review and a survey of state DOTs. Follow-up interviews with agencies that have experience with reducing claims, change orders, and cost overruns from subsurface conditions provided additional information. The Synthesis addresses: the scope of subsurface investigation required by transportation agencies; causes of claims, change orders, and cost overruns attributed to subsurface conditions; ranges of costs and prevalence of claims, change orders, and cost overruns attributed to subsurface conditions; and successful practices to reduce claims, change orders, and cost overruns.

Field Performance of an Asphalt Barrier Test Pad

A field study of asphalt concrete and fluid applied asphalt - geotextile (FAA/GT) was undertaken to evaluate their suitability as a barrier (liner/cover) material in waste containment applications. A test pad consisting of 2-3 mm thick FAA/GT and 10-15 cm thick asphalt cement concrete was designed, constructed using full-scale asphalt paving equipment and its performance monitored. The in situ hydraulic conductivity of the barrier was measured using sealed, double-ring infiltrometers. Core and large block samples were taken from the test pad and analyzed in the laboratory under several conditions including: immediately after compaction, after aging for up to three years and after deformation. The in situ hydraulic conductivity of the barrier was 1x10 -9 cm/s to 1x10 -10 cm/s. The conductivity range represents the lower limit of quantifiable conductivities with the in situ testing equipment. The laboratorymeasured hydraulic conductivity of the specimens sampled immediately after compaction was 1x10 -10 cm to 1x10 -11 cm/s. The hydraulic conductivities of the same specimens were re-evaluated twice after exposing them to the ambient temperature in the laboratory for a period up to three years. No change in hydraulic conductivity was observed during this period. Beams of aged asphalt concrete were subjected to three point bending to impart distortions in the range from 1/500 to 1/25. Though observable cracks developed in the beam that underwent the maximum distortion of 1/25, there was no appreciable effect on the hydraulic conductivity. The hydraulic conductivity of the asphalt barrier design used in this test pad was uncompromised by aging of 3 years and distortions up to 1/25. Based on the results of the testing performed, asphalt concrete combined with fluid applied asphalt - geotextile proved to be an effective barrier when designed and constructed with low hydraulic conductivity as a goal.