A major part of Aspect’s analysis focused on the location, thickness, and nature of the weak and shallow soils draped above the strong glacial soils, intertwined with the historical changes that occurred as Seattle developed its waterfront. The Pioneer Square area was a small upland Native Americans had lived on for millennia. Occidental Square was a shallow coastal lagoon, and most of the area south of King Street was tide flats. As timber and shipping industries flourished in the 1800s, much of the waterfront was developed with piers and trestle bridges, then filled with sawdust and wood waste from waterfront mills and soft muck sluiced from nearby hills.
On top of all that, roads, sewer lines, water lines, and buildings were constructed on the new ground. Not surprisingly, these weak fills have settled over time, resulting in bumpy roads, tilted sidewalks, and tall curbs in the Pioneer Square neighborhood. But these same soils were also highly susceptible to further consolidation when the water table was rapidly lowered by the Bertha rescue shaft dewatering.
Historical Record Sleuthing
To assess the areas of vulnerable underground utilities, we dug into the historical records—maps dating from the original land surveys of Seattle to locate the original shorelines, and previous drilling investigations going back decades to reveal the subsurface data. Aspect compiled soil records of hundreds of borings and wells to develop a database that could be mapped in three dimensions and used to identify the areas where weak soils were present. When combined with the City’s utility maps, Aspect’s weak-soils map allowed the City to easily spot areas where weak soils and vulnerable infrastructure overlapped.
Aspect also assisted the City in pursuing remote surface elevation surveying techniques including use of synthetic aperture radar (InSAR), a satellite-based radar distance measuring technique that permits detection of precise changes in ground surface elevation over time. These studies looked at time-series analysis of data beginning well before tunneling started through shaft dewatering and continuing to the time of the study. They showed an unmistakable correlation between rescue shaft dewatering and broad areas of ground settlement.
Dewatering: The Science of Making a Wet Excavation Dry
Dewatering the excavation required pumping, but at a rate and depth to keep water pressure low (too high could risk “blowing out” the bottom of the 130-foot-deep shaft). There are two aquifers beneath the rescue shaft site and Pioneer Square area – one shallow and one deep. Because the shaft bottom sat in the “deep” aquifer, pumping targeted that aquifer, which was thought to be confined from the shallow aquifer and the vulnerable utilities sitting above it.
However, because settlement happened in the shallow aquifer area, Aspect’s forensic analysis looked at answering several questions: how susceptible are the utilities to pumping a deep aquifer? Does pumping the deep aquifer cause dewatering of the much shallower aquifer? And if so, does pumping the deep aquifer or dewatering the shallow aquifer cause most of the settlement that the City was observing? Understanding the hydrogeologic connections between pumping the two aquifers and the net effects of dewatering on soil behavior required multidisciplinary consideration of many factors.