Our professionals have been practicing in the local area since 1974. This is especially important in the local area because of highly variable geologic conditions.
Washington D.C. is located on a fall-line where major changes in geology occur in relatively short distances. West of the fall-line, sedimentary and metamorphic rock is shallow and the cost of excavating hard rock can be problematic. In the Frederick Valley, limestone pinnacles, sinkholes and caverns are overlain by very soft soils. Shallow foundations, roads and pavements have been impacted by sinkholes and settlement. Deep foundations may be difficult to construct due to variable groundwater, soil and rock conditions.
East of the fall-line, rock is over 1000 feet deep in the Coastal Plain. These soils are highly variable. The deposits include very loose wind deposited sand, very soft marine clays, inter-bedded layers of alluvial soft to stiff clay and loose to dense sand. The high plasticity Marlboro Clays are known to be expansive and landslides are common. The miocene age deposits are known to be problematic and they extend very deep. Higher elevations in the Coastal Plain are sometimes capped with high quality deposits of dense sand and gravel. Many different types of shallow and deep foundations and soil stabilization are in use in these variable soil conditions.
There is a wide band of miocene age high plasticity clay locally referred to as "Marine Clay" along the fall-line, which roughly parallels Interstate 95. These older alluvial soils were deposited by ancient rivers that no longer exist. Many structures have been damaged by landslides and by shrink-swell in these problematic soils. Both shallow and deep foundations are being used for support of structures.
The geology of Washington, D.C. is further complicated by the fact that is located at the intersection of two major rivers and built on top of numerous tributaries that once flowed through the area. Many of these old streams, and at least a few canals, have been filled. Buried channels, poor quality existing fill and soft river deposits add to the complexity of the underground conditions. Multiple groundwater levels are common. Shallow foundations, mat foundations, and many types of deep foundations are in use.