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Digging up Boston: The Big Dig Builds on Centuries of Geological Engineering
Bradford A. Miller


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Urban Fill: Everything Including the Kitchen Sink



The “Big Dig” in Boston, Mass., is one of the most challenging and complex heavy construction projects ever undertaken. Formally called the Central Artery/Third Harbor Tunnel, or CA/T, project, the Big Dig takes on a formidable challenge: to construct a below ground superhighway beneath an active, but aging, six-lane elevated highway that must remain in service 24/7 — keeping the city open for business — and to build a third tunnel beneath Boston Harbor; all while working within complex subsurface geology and applying advanced engineering solutions.

The view southeast toward Boston Harbor highlights the Zakim Bridge, roadways and ramps under construction across the Charles River. The soon-to-be replaced, elevated I-93 interstate highway is on the left. Downtown Boston is at the right-center of the image. In the distance, the twin hills of Spectacle Island rise to form new harbor parkland. Supplemented with excavated Big Dig soils as landfill cover, Spectacle Island was once the site of a municipal waste dump and rendering plant. Image courtesy of CA/T Project and Bechtel/Parsons Brinckerhoff

However, the challenge faced by the Big Dig constructors is only a recent example of how the subsurface conditions and complex geology in the Boston area have influenced and controlled the development of the city and its built environment; and also contributed to the development of the disciplines of engineering geology, geotechnical engineering and several aspects of modern construction practice.

Boston’s blessing has been its extraordinary coastal setting. The Town of Boston was originally founded on the small, 789-acre Shawmut Peninsula, a highland connected by a narrow neck to the mainland that high tides often breached. By 1623, early English settlers recognized that the area hosted defensible promontories, a deepwater harbor, abundant shellfish and the potential for marine trade.

In his 1989 article “History of Boston: The Impact of Geology,” engineering geologist David Woodhouse summarized several geologic factors affecting the founding, and growth, of the city. First, the indented coastline of Massachusetts was the result of glacial erosion of the underlying softer bedrock, generally formed of the Cambridge “argillite,” a weakly metamorphosed, laminated, late-Proterozoic mudstone. The erosion provided the natural topographic basin that was later flooded following the end of the Wisconsinan glacial period and subsequent rise in eustatic sea level. Second, prominent glacial landforms, principally drumlins, exposed marine clays and granular glaciofluvial “outwash” features — as well as complex, puzzling moraines exhibiting deformed and folded sediments, such as Beacon Hill — created features that could function as natural defenses from seagoing enemies. Third, natural springs and good quality water in shallow dug wells sustained the town, providing enough for the initial settlements until the population substantially increased.

Colonial streetscapes developed around the irregular topography and coastline. As the city grew, so did the harborfront and coastline. The shallow tidal bays, coves and marshlands became logical areas to fill in, creating “made land.” By the early 1800s, the geography and settlement patterns began to expand toward the surrounding areas as the city’s growth accommodated the geological setting.

The confluence of education, geology and engineering

As the city of Boston grew in national influence, the educational institutions of Massachusetts and New England attracted 19th century geologists such as Edward Hitchcock (1830s), William Otis Crosby (1880s), Nathaniel Shaler (1890s) and J.B. Woodworth (1900). But the academic and practical understanding of the interaction between geology and engineered structures remained relatively unrefined until the late 1890s.

Through the 1870s, early area buildings, bridges and other structures utilized comparatively simple soil-supported foundations. These were usually constructed on brick or mortared stone footings. Later, tapered wood-piles driven into the desiccated, stiffer marine clays commonly supported waterfront structures (Parkhill, 1998).

Many of these 19th-century buildings remain in downtown Boston, often reused for elegant office or residential space. A significant construction challenge facing today’s geotechnical engineers, particularly along the route of the Big Dig, is anticipating and mitigating potential construction impacts to these delicate masonry structures. Sophisticated instrumentation installed along the new construction route allow engineers to measure and respond to potential ground movements, utility conduit deflections, and vibration impacts to other structures well before damage can occur.

In the early 20th century United States, the discipline of foundation engineering grew along with rapid growth in urban areas, including New York, Chicago and San Francisco. In Boston, emigration, industrial advances, desire for larger buildings and the economic expansion occurring between 1880 and the mid-1920s fueled demands for new construction on filled lands outside the old urban core. The core then was suitably underlain by typically dense, stable glacial soils. The “made land” — marginal by itself even for simple construction purposes — was often a combination of unstable fill soils, soft organic peats and silts, and thick compressible marine clay. If loaded beyond a certain point, these materials could potentially deform, leading to settlement of the structure with consequent shifting and likely damage.

Although not alone in recognizing the influence of geology on urban foundation construction at the time, geologist Irving B. Crosby published engineering geology maps in the early 20th century, including possibly the first detailed seismic studies of an urbanized area (in 1903 and 1932) that depicted relative stability of the local terrain to earthquake-induced ground motions. And as twists of fate often turn, differential settlement observed in the foundations of what was then the new Massachusetts Institute of Technology (MIT) campus in Cambridge attracted the interest of scientist Karl Terzaghi following his arrival at MIT in 1925. By the early 1930s, Terzaghi and Arthur Casagrande examined the relationship between laboratory experiments on the strengths and properties of undisturbed cohesive soils (among them the Boston Blue Clay) and the predictions for how these soils would behave under construction conditions. Their careful analyses of cohesive soil strengths and characteristics of other soils identified general soil properties, behaviors and limitations that were formative to the discipline of soil mechanics. Many of the early theories of soil mechanics are still applicable to major constructed works and deep excavations worldwide, including the Big Dig.

These challenging subsurface conditions, directly or indirectly, made Boston one of the educational centers for the nascent sciences of soil mechanics and engineering geology, offshoots of the established disciplines of civil engineering and “traditional” geology.

Foundations go deeper

With the construction of larger buildings in the 1940s and 1950s, foundations had to stretch from the soil, or overburden, down into the deeper glacial soils and bedrock. Geologic investigations were limited to exploratory soil borings and diamond core drilling methods performed for geotechnical design purposes. Drilling typically evaluated the characteristics of only the upper five to 10 feet of the bedrock.

A comprehensive understanding of the structure of the Boston Bay Group bedrock — which primarily hosts Precambrian argillites, sandstones, conglomerates, “tillites,” volcanic materials, and younger crosscutting mafic dikes — remained elusive. Most of the bedrock surface was masked beneath thick overburden deposits, as outcrops near the center of the Boston basin were nonexistent.

However, in the 1950s, further complexities in the deep bedrock basin structure were revealed and examined in several bedrock water supply and sewer infrastructure tunnels constructed under the supervision of the Metropolitan District Commission. Most of the Dorchester Tunnel was reportedly driven employing the first application of a mechanized, tunnel-boring machine (TBM), now a commonly used construction technique for both soil and bedrock tunnels.

According to Woodhouse, less than ideal bedrock conditions were observed in sections of almost all of these tunnels studied. To differing degrees, the areas included softer, kaolinized, and decomposed bedrock resulting from likely hydrothermal alteration, which were often affiliated with major shear zones, breccias, fault gouge or fault zones. The structurally weaker rock was accommodated within the tunnels by concrete lining, steel supports, rock bolts or other construction methods. Again, the construction techniques responded to the problematic ground conditions.

In the 1960s and 1970s, presaging the future building boom to come in Boston, foundations became more complex and were constructed by a variety of methods: machine-excavated straight shaft and belled caissons replaced the dangerous hand-excavation methods. Precast-prestressed concrete piles replaced the more costly steel pipe piles, and, in 1976, the advent of “slurry wall” excavation support method arrived in Boston. With the downtown real estate development boom in the early 1980s, the need for deeper excavations and foundation systems yielded additional insight into the soils and bedrock below the city. And the extensive subsurface explorations for the publicly funded CA/T and Deer Island Outfall Tunnel provided rock cores that are available for examination outside the engineering and engineering geology community.

Now: the Big Dig

In a league by itself for magnitude, the Big Dig consists of almost 168 total lane-miles of highway and ramp access, about half of that in tunnels. As in the past, CA/T construction designers and engineers contend with complex geological conditions that vary in both lateral extent and with increasing depth. And while the range of anticipated subsurface conditions identified by early explorations and historical foundation construction experience can be beneficially applied to the CA/T project, geotechnical designers still require detailed geologic information at the locations of each engineered structure.

Important engineering attributes of the soils include the thickness and content of the fill materials, presence of compressible organic soils, and strength and stability of the often thick cohesive marine and glaciomarine clays. Also important is identifying the presence of dense glacial tills that might provide suitable deep structural support, and determining the depth, overall competency and strength of the bedrock. Designers must accommodate the presence of groundwater and minimize the effects that construction dewatering in a deep excavation could have on adjacent ground stability. Understanding the characteristics and engineering behaviors of the individual stratigraphic units is critically important for large infrastructure works such as the Big Dig.

Dramatic changes in roadway configurations leading northeast toward Boston demonstrate the complex construction required at the interchanges of interstate I-93, the Massachusetts Turnpike (I-90) and the access ramps leading to the Third Harbor Tunnel beneath the Fort Point Channel (just off the images to the right). The image of the left reflects an approach to Boston before the Big Dig; on the right, after. Image courtesy of CA/T Project and Bechtel/Parsons Brinckerhoff


Building the Big Dig means working in tight spaces that have low headroom and are close to fragile, early 19th century masonry buildings. As construction of Interstate I-93 below the city and I-90 access roads for the Ted Williams Tunnel to Logan International Airport advances, workers are close to huge steel and glass office towers and below-grade subway lines. Reportedly, former CA/T Director Peter Zuk once likened the construction of the CA/T to “conducting heart surgery on a patient actively playing tennis.” Several construction approaches demonstrate the technical challenges the project teams face, as summarized in an article prepared by the CA/T projeect for Southeast Asia Construction magazine in January/February 2002:

Building on the past

The contributions of early local geologists, engineers and researchers laid the groundwork for our current understanding of several problematic geologic conditions that can exist throughout the Boston area. Many of these discoveries and their subsequent engineering and construction solution were transferable to similar conditions elsewhere. But even with our knowledge of historical conditions and local construction experience, careful examination and analyses of site-specific ground conditions are still performed for virtually all engineered structures, including the tremendous Big Dig effort. As in the past, geologists and engineers continue to gather more data and information about the subsurface. By applying both well-established and advanced engineering construction techniques, they learn to work with the existing “urban geology” beneath the city.

Urban Fill: Everything Including the Kitchen Sink

Many Bostonians know that the city's shoreline footprint has grown through history. The waterfront expanded outward into Boston Harbor and the city grew onto land "reclaimed" along the Charles River estuary, perhaps most famously in the Back Bay and South End areas. However, not all of the residents give much thought to how much area actually was created, and where the filling material originated. Nor do they consider how this unique soil profile affects construction and underground engineering.

Cumulatively, an urban area far larger than the Back Bay was filled to accommodate the growth and settlement patterns of the vibrant port and commercial hub of Massachusetts. In their book Historical Atlas of Massachusetts (1991), editors Richard Wilkie, Roy Doyon and Jack Tager estimated that the general urban land area increased due to filling by approximately 3,930 acres (6.1 square miles) between 1803 and 1988, very roughly doubling Boston's geographic size. This estimate is qualified somewhat, because it includes former towns later annexed by Boston (Charlestown, Roxbury, East Boston, South Boston); portions of Cambridge and Brookline adjacent to the Charles River; and Logan International Airport.

From an engineering and site characterization perspective, these urban fills create problematic subsurface conditions that raise several yellow flags to engineering geologists and geotechnical engineering professionals. The soil properties of the fill materials and their engineering behavior, particularly in response to surcharges, loads and excavations, are almost exclusively unfavorable -- a situation that demands fairly comprehensive evaluation and analysis prior to most excavation and construction activities.

Urban fills are likely to be heterogeneous over short distances, are compressible due to their inconsistent manner of placement (or because they often contain overly soft organic deposits), and are often unstable in proximity to excavations. Because of their nature, these fills cannot be used as foundations because due to the continued and variable settlement of the structure that occurs, requiring support on deeper, more stable soils. In some areas of Boston, fills have been documented to be more than 30 feet thick.

Creating particular problems for subsurface construction efforts are the presence of former granite seawalls and wharves. These robust waterfront structures, typically constructed from hundreds of massive, dry-laid granite blocks and an underground "forest" of wood piles, were often infilled with all manner of stone rubble, debris and a startling array of other materials. Once buried and forgotten under an advancing urban shoreline ("Progress!" the city's fathers must have shouted), these structures lie in wait for the unsuspecting construction project.

The source of the fill soils is variable and identified in only limited documentation. Several area drumlins, and the former topographic highs on the original Shawmut Peninsula (including portions of Beacon Hill, which was likely formed by localized ice thrusting and deformation of earlier glacial tills, clays and sands during Pleistocene deglacial events), were cut down and transported as borrow material to fill in low-lying coves, bays and the developing waterfront. Land modification efforts were so substantial that two of the original three major hills, or "trimounts" (reflected in the name of Tremont Street), were completely removed and leveled.

As described in Harl Aldrich's 1970 article in the journal of the Boston Society of Civil Engineers, the Back Bay and Fenway areas were filled between 1856 and 1890 using sand and gravel transported by train from Needham, a residential community located about nine miles southwest of Boston. Hydraulic fills were also used to provide material for certain areas of Boston, where soft, organic marine muds and clays were dredged to deepen channels or waterways, and then pumped or sluiced as slurry to open coastal tracts rimmed by crude wooden barriers. Once exposed, the hydraulic slurry desiccated, consolidated and provided the newest "made land" along the waterfront.

Urban soils can also host a wide range of hazardous chemical contaminants as well, often with no discrete or identifiable source. In a 360-year-old city, a given parcel of land may have experienced uncountable building and construction cycles, or several generations of undocumented commercial or industrial uses. As byproducts of heating with wood, and later coal, discarded cinders and ashes are common components of urban soils. Typical fill contaminants may include polycyclic aromatic hydrocarbons (PAHs) or other combustion byproducts, volatile organic compounds (VOCs) including chlorinated solvents and lighter-fraction gasoline components, numerous metals (from arsenic to zinc), and a slew of petroleum constituents. Contaminated soil characterization and soil management is a significant cost consideration for underground construction and development projects in an urban setting, a cost exacerbated by the erratic subsurface extent, often high concentrations, and variety of chemical compounds potentially present. To this end, one environmental professional, tongue-firmly-in-cheek, succinctly described the urban Boston soils as "Downtown Brown."

Bradford A. Miller


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Miller is a senior environmental geologist at Haley & Aldrich Inc. in Boston, a company that works on the Big Dig. He is also past president of the New England Section of the Association of Engineering Geologists. E-mail

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