Scott A. Ashford

New Automated Point-Cloud Alignment for Ground-Based Light Detection and Ranging Data of Long Coastal Sections

This paper presents new techniques with corresponding algorithms to automate three-dimensional point-cloud georeferencing for large-scale data sets collected in dynamic environments where typical controls cannot be efficiently employed. Beam distortion occurs at the scan window edges of long-range scans on near-linear surfaces from oblique laser reflections. Coregistration of adjacent scans relies on these overlapping edges, so alignment errors quickly propagate through the data set unless constraints (origin and leveling information) are incorporated throughout the alignment process. This new methodology implements these constraints with a multineighbor least-squares approach to simultaneously improve alignment accuracy between adjacent scans in a survey and between time-series surveys, which need to be aligned separately for quantitative change analysis. A 1.4-km test survey was aligned without the aforementioned constraints using global alignment techniques, and the modified scan origins showed poor ag...

Liquefaction Mitigation Using Stone Columns Around Deep Foundations: Full-Scale Test Results

The results presented were developed as part of a larger project analyzing the behavior of full-scale laterally loaded piles in liquefied soil, the first full-scale testing of its kind. Presented here are the results of a series of full-scale tests performed on deep foundations in liquefiable sand, both before and after ground improvement, in which controlled blasting was used to liquefy the soil surrounding the foundations. Data were collected showing the behavior of laterally loaded piles before and after liquefaction. After the installation of stone columns, the tests were repeated. From the results of these tests, it can be concluded that the installation of stone columns can significantly increase the density of the improved ground as indicated by the cone penetration test. Furthermore, it was found that the stone column installation limited the excess pore pressure increase from the controlled blasting and substantially increased the rate of excess pore pressure dissipation. Finally, the stone columns were found to significantly increase the stiffness of the foundation system by more than 2.5 to 3.5 times that in the liquefied soil. This study provides some of the first full-scale quantitative results on the improvement of foundation performance due to stone columns in a liquefiable deposit.

Liquefaction at Strong Motion Stations and in Urayasu City during the 2011 Tohoku-Oki Earthquake

The 2011 MW = 9.0 Tohoku-oki earthquake generated a large number of unique soil liquefaction case histories, including cases with strong ground motion recordings on liquefiable or potentially liquefiable soils. We have compiled a list of 22 strong motion stations (SMS) where surface evidence of liquefaction was observed and 16 SMS underlain by geologically recent sediments or fills where surface evidence of liquefaction was not observed. Pre-earthquake standard penetration test data and borehole shear wave velocity (Vs) profiles are available for some stations, but critical information, such as grain size distribution and fines plasticity, are often lacking. In the heavily damaged city of Urayasu, we performed post-earthquake cone penetration testing at seven SMS and Vs profiles, using surface wave methods at 28 additional locations to supplement existing geotechnical data. We describe the liquefaction effects in Urayasu, the available site characterization data, and our initial data interpretations.

Numerical Study of Shear Stress Distribution for Discrete Columns in Liquefiable Soils

AbstractDiscrete columns, such as stone and soil-cement columns, are often used to improve the liquefaction resistance of loose sandy ground potentially subjected to strong shaking. The shear stress reduction in the loose ground resulting from the reinforcing effect of these stiffer discrete columns is often considered as a contributing mechanism for liquefaction mitigation. Current design practice often assumes that discrete columns and soil deform equally in pure shear (i.e., shear strain–compatible deformation). In addition, because the discrete column is stiffer than the soil, it is assumed to attract higher shear stress, thereby reducing the shear stress in the surrounding soil. In this paper, shear stress reduction in liquefiable soils and shear strain distribution between liquefiable soil and discrete columns along with the potential of development of tensile cracks is investigated using three-dimensional linear elastic, finite-element analysis. Parametric analyses are performed for a range of geom...

TopCAT-Topographical Compartment Analysis Tool to analyze seacliff and beach change in GIS

This paper discusses the development of a new GIS extension named the Topographic Compartment Analysis Tool (TopCAT), which compares sequential digital elevation models (DEMs) and provides a quantitative and statistical analysis of the alongshore topographical change. TopCAT was specifically designed for the morphological analysis of seacliffs and beaches but may be applied to other elongated features which experience topographical change, such as stream beds, river banks, coastal dunes, etc. To demonstrate the capabilities of TopCAT two case studies are presented herein. The first case examines coastal cliff retreat for a 500m section in Del Mar, California and shows that large failures comprised a large portion of the total eroded volume and the average retreat rate does not provide a good estimate of local maximum cliff retreat. The second case investigates the alongshore volumetric beach sand change caused by hurricane Bonnie (1998) for an 85km section in the Cape Hatteras National Seashore, North Carolina. The results compare well (generally within 6%) with previous investigations. These case studies highlight additional information gained through performing a detailed, discretized analysis using TopCAT.

Effects of Ground Failure on Bridges, Roads, and Railroads

The long duration and strong velocity content of the motions produced by the 27 February 2010 Maule earthquake resulted in widespread liquefaction and lateral spreading in several urban and other regions of Chile. In particular, critical lifeline structures such as bridges, roadway embankments, and railroads were damaged by ground shaking and ground failure. This paper describes the effects that ground failure had on a number of bridges, roadway embankments, and railroads during this major earthquake.

Comparison of Deep Foundation Performance in Improved and Non-Improved Ground Using Blast-Induced Liquefaction

The results presented in this paper were developed as part of a larger project analyzing the behavior of full-scale laterally loaded piles in liquefied soil, the first full-scale testing of its kind. This paper presents the results of a series of full-scale tests performed on deep foundations in liquefiable sand, both before and after ground improvement, where controlled blasting was used to liquefy the soil surrounding the foundations. Data were collected showing the behavior of laterally loaded piles before and after liquefaction. After the installation of stone columns, the tests were repeated. Based on the results of these tests, it can be concluded that the installation of stone columns can significantly increase the density of the improved ground as indicated by the cone penetration test. The stone columns were found to significantly increase the stiffness of the foundation system, by more than 2.5 to 3.5 times that in the liquefied soil. However, in non-liquefied ground, the improvement from stone columns could be more than compensated for by increasing the piles. In liquefied soil, however, more than doubling the number of piles or increasing shafts diameters by 50 percent did not nearly match the improved performance of the treated ground. This study provides some of the first full-scale quantitative results on the improvement of foundation performance due to stone columns in a liquefiable deposit.

Dynamic Centrifuge Tests to Evaluate Reinforcing Mechanisms of Soil-Cement Columns in Liquefiable Sand

AbstractFour centrifuge tests were performed to investigate the reinforcing mechanisms of soil-cement columns in liquefiable sand. Two unimproved baseline models and two models improved with soil-cement columns were subjected to sine sweep and earthquake base motions of varying intensities to observe acceleration, pore pressure, lateral displacement, and settlement responses. The dynamic records were processed to derive the effective natural frequency of the profiles and to obtain the dynamic stress-strain responses for unimproved and improved soil. It was found that the shear reinforcement mechanisms of columns was not effective in reducing cyclic stress ratios in the treated soil; liquefaction triggering occurred nearly at same time for both unimproved and improved soil cases and the magnitude of the resulting soil settlement was not significantly reduced. When the bases of the columns were free to rotate, the columns rocked within the soil and produced negligible shear stiffening of the soil profile. W...

Design of DSM Grids for Liquefaction Remediation

AbstractDeep soil mixing (DSM) to form in-ground shear walls has been used to remediate against the potential effects of earthquake-induced liquefaction on many projects. A grid pattern of soil-cement walls act as a confined shear box, which can provide additional shear stiffness and strength for sites to withstand liquefaction. Current design practice for DSM grids commonly relies on the strain compatibility assumption, where the DSM walls and confined soil are assumed to experience the same shear strain. In this paper, the distributions of shear stresses and strains in liquefiable soil deposits treated with DSM grids are investigated using three-dimensional linear elastic finite-element analyses of unit cells. Parametric analyses are performed for a range of geometries, relative stiffness ratios, and dynamic loadings. These linear elastic results provide a baseline against which future nonlinear modeling results can be compared, but they are also sufficient for demonstrating that shear stress reductions...

Pore Pressure Response of Liquefied Sand in Full-Scale Lateral Pile Load Tests

Several full-scale lateral pile load tests were performed at Treasure Island in the San Francisco Bay in sand liquefied by controlled blasting. Cyclic lateral loads were applied to the foundations with displacements up to 225 mm. Piezometers were installed adjacent to, and at a distance up to 4.2 m in front of, the deep foundations. Excess pore pressure ratios observed during testing are presented for the purpose of providing insight into the behavior of liquefied soil in response to laterally loaded piles. Observed excess pore pressure ratios show that soil in the loading path of the foundations experienced a phase transformation at distances as great as 4.2 m. The soil behavior in response to lateral foundation movement is presented based on interpretation of the measured excess pore pressure response.