Educational Resources > Introduction to Bridge Foundations
Introduction to Bridge Foundations
By Dylan Felty and Richard Haight
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Foundations are an important design consideration for any bridge project.
Although not always visible after construction, the foundation of a bridge is responsible for supporting all loads a bridge experiences and distributing them into the earth. As a result, choosing the correct type of foundation depends on a couple key factors; site conditions and the bridge superstructure.
Site conditions refer to the soil and geologic features at the location of the bridge. Soft soils typically require deep foundations. Solid, well compacted soils or high bedrock, offer the opportunity for shallow foundations. Furthermore, large superstructure loads resulting from long-span bridges may result in deep foundations being required.
As their names imply, the primary difference between deep and shallow foundations is the depth that the foundation extends into the ground. Shallow foundations commonly distribute loads from the bridge across a spread-out area of soil not far below the surface of the ground. Shallow foundations rely on soils with large bearing capacity to support the loads from the bridge. Alternatively, deep foundations extend deep below ground level. Certain types of deep foundations utilize friction along their depth to support loads from the bridge. Other types of deep foundations simply extend deep enough below the ground to reach soils or bedrock with great enough bearing capacity to support the bridge loads.
The most common type of shallow foundation are spread footings. Spread footing foundations are almost exclusively constructed from concrete and have two primary components; the stem and the footing. The stem is a solid, vertical section of concrete on which the bridge directly rests. It’s primary function is to transfer loads from the bridge down into the footings, which are typically located several feet below the ground. Footings are the large base of the foundations and serve to evenly distribute loads over a large area.
Driven piles are a common type of deep foundation that utilize side friction to support bridge loads. As the name suggests, this type of foundation consists of long shafts of material - or piles - being driven deep into the ground. Piles are commonly composed of concrete, timber, or steel and can extend several hundred feet below the ground depending on soil conditions and bridge loads. Large machinery is used to drive piles deep into the ground by repeatedly hammering them down until they reach the desired depth.
Also referred to as caissons, drilled shafts are another commonly used type of deep foundations. Drilled shafts involve drilling a large hole deep into the ground and filling it with reinforcing and concrete. Drilled shafts extend deep into the ground to reach strong soil or bedrock that has adequate bearing capacity to support the bridge loads. Drilled shafts are frequently much larger in diameter than driven piles to distribute loads across a greater area. The larger diameter of drilled shafts helps to compensate for the lack of friction resistance present with driven piles.
Shelburne Falls/Deerfield Geologic History
The Bridge of Flowers was built to bridge over the Deerfield River to connect the railroad in Shelburne Falls with the valley community of Colrain. The existing Iron Bridge through Shelburne Falls did not have the capacity to carry the heavier freight loads from trolleys so it was decided that a new bridge would be built. Hence, the Bridge of Flowers was born. The engineers now had the challenge of designing the bridge. The conditions the engineers were dealing with have been millennia in the making.
For several million years New England has experienced cycles of glacial advances and retreat. The last period of glaciation, the Wisconsin, ended only 11,000 years ago and is responsible for the subsurface conditions found at the Bridge of Flowers. The glaciers were responsible for scouring the landscape, and hence, bedrock is commonly found close to the surface, as is the case in Shelburne Falls. When the glaciers finally retreated it started a period of violent flooding. Evidence of these floods can be seen several hundred yards down river from the Bridge at the Falls, where rocks caught the water from melting glaciers carved giant “potholes”. By 15,000 years ago the glaciers had retreated from the area and the Deerfield River valley slowly transformed into what it is today.
The geological history of the area has very important ramifications for geotechnical engineering. In New England bedrock is close to the surface. You can see this when you drive down a road or walk on a trail and see outcroppings. Even in the Deerfield River it is possible to see the bedrock sticking out of the water. For the bridge of flowers engineers this meant they could use shallow foundations and take advantage of the hard layer of bedrock right beneath the bridge.
Bridge of Flowers Foundation Specifics
Just beneath the water within the Deerfield River, sits solid expansive bedrock upon which the Bridge of Flowers rests. For most conventional bridges, high or even exposed bedrock acts as a feature that offers great benefit yet also requires careful attention.
The concrete arches that comprise the Bridge of Flowers bear on large concrete abutments at either approach and rest on 4 intermediate concrete piers located within the streambed of the Deerfield River. The original construction of the Bridge of Flowers took advantage of the large bearing capacity offered by bedrock by founding the bridge directly on the exposed bedrock, or, “ledge”. As seen in the record drawings of the original structure (Figure 7) the foundations for each abutment as well as each pier were constructed from cast-in-place concrete, poured directly on ledge.
Historically, incorporating existing bedrock into foundation design is a technique that dates back hundreds of years. Compared to typical soil, bedrock offers an average of 10-20 times greater bearing capacity to potential bridge designers. This means a significantly larger amount of force can be supported per square foot by bedrock as opposed to soil. As mentioned previously, the large bearing capacity of bedrock is a great benefit that has encouraged bridges to be founded on bedrock for decades.
Utilizing cast-in-place concrete for foundations on bedrock is still a common practice to this day. Cast-in-place foundations allow for wet concrete to be poured directly onto exposed bedrock and perfectly match the irregular profile of the rock. This method prevents the need for a time-consuming analysis of the existing ledge profile and incredible amount of precision necessary to construct a precast concrete foundation to perfectly match existing bedrock.
Further investigation of the record drawings shows reinforcing bars were socketed into the ledge prior to pouring cast-in-place concrete during the 1983 repairs to the Bridge of Flowers (Figure 8). The addition of reinforcing bars socketed into bedrock is a more recent development in foundation design for high ledge conditions. Today, it is a common detail for foundations bearing on bedrock. These additional reinforcing bars help to secure bridge foundations in place and resist global sliding of the foundation.