‘Geotechnical’ Category

Spread Footing Foundation

Photo Credit: Alex Mead

Seen here are three rebar cages for the spread footings of a large building. Spread footing foundations are preferred to deep foundations. This is due to the increased work, and therefore increased cost, of a deep foundation. Deep foundations could use H-piles, micro-piles, or other options to support the building. For the design of a foundation, the bearing capacity of the soil or rock and the column loads of the proposed building must be understood. If the size of the footing can be made feasibly, given the loads and bearing capacity of the project, spread footings will be used.

Winter Construction – Mat Foundation

Photo Credit: Alex Mead

This picture illustrates many lessons for actual construction and also gives a perspective of the actual scales necessary for use in even moderate size concrete construction. To begin, it is clear the snow on the ground and in the air means the temperature is below freezing, single degree Fahrenheit temperatures to be specific. This means workers will be slower and must take more frequent breaks to warm their bodies. Also, the construction site is slippery and objects become covered in snow leading to increase fall and trip hazards. One final interesting fact is the size of the rebar cage the workers are building. Notice how the workers are standing fully upright in a volume that will be filled completely with concrete. This mat-foundation job was poured in several pours ranging from 2 000 to 6 000 yards in size.

Soil Resistivity Testing

Photo Credit: Nathan Shoemaker

Shown here is a field soil resistivity test. The resistance of the soil is measured by sending a current across the two outer probes and the voltage potential is measured across the inner two probes. From Ohm’s law the resistance of the soil can be found. The measured soil resistivity is the average resistivity of the soil to a depth down a distance as far as the probes are spaced apart. Soil resistivity can be used for different things; since different geo-materials have different inherent resistances, a change in soil resistivity can indicate a change in the soil type(or rock). Resistivity is also very sensitive to soil moisture such that the depth to groundwater can also be interpreted from soil resistivity data. The testing shown above is using the value of soil resistance directly to size the metal grounding rod for a high-voltage transmission line. The grounding rod is sized such that the tower can dissipate a lightning strike.

Dynamic Pile Testing

Photo Credit: Nathan Shoemaker

Shown here are the strain gauge transducer and accelerometer used for high-strain dynamic pile testing, there are two more gauges on the other side of the pile. The idea behind dynamic pile testing is that when the pile is struck with the hammer a wave propagates down the pile to the tip and back up. The wave is dissipated by dampening along the skin and toe. The strain data combined with the pile properties (modulus of elasticity and area) give axial force and the acceleration data can be integrated to find particle velocity, and since the wave speed of steel and concrete is reasonably known, the wave distance can be determined. Depending on the shape of the force and velocity graphs it can be determined if the pile is more of a friction or end-bearing pile and what the capacity of the pile is. The legitimacy of this testing can be proven by the reductions in factors of safety for deep foundations that AASHTO allows for when a high-strain dynamic pile testing program is implemented on a project.

Sinkhole from Sewer Failure

Photo Credit: Karl Jansen

This is a special kind of sinkhole probably caused by the failure of a storm sewer system connection. Sinkholes like this one form when and where a sewer pipe fails, allowing rainwater to wash earth material into the pipe and down the system. When the earth is washed away, the above material collapses down causing a hole to form. Due to the close proximity of this sinkhole to a catch basin, this sink hole was probably caused by a failure of the connection of the sewer pipe to the structure.

Leaning Tower of Pisa

Photo Credit: Jessie Benaglio

The Leaning Tower of Pisa in Pisa, Italy is the campanile, or freestanding bell tower, of the cathedral. The tower now leans at about 3.99 degrees. The top of the tower is displaced horizontally 12 ft 10 in from where it would be if the structure were perfectly vertical. Before restoration between 1990 and 2001, the tower leaned 5.5 degrees. The work involved to stabilize the tower was to extract 70 tons of earth from the northern side to encourage the tower to right itself which corrected the tower in a more upright position. The foundations only extend 3 meters below the surface. This was not deep enough to get past the soft soil. Hi-tech monitors are embedded in soil beneath its foundations and in the tower itself now show that it has stopped moving completely.

Spread Footings

Photo Credit: Alex Mead

Seen above are column footings being constructed in an excavation. These masses of concrete will eventually transfer the entire load of the building above to the soil. Also known as a shallow foundation or spread footings, column footings are always preferred to deep foundations due to their significantly small cost of construction. Unfortunately, shallow foundations are not always possible due to poor baring capacity of the soil.

Foundation Excavation

Photo Credit: Alex Mead

Seen above is a perfect example of building a structure right up to the property line. This close quarter construction is common in urban areas and produces challenges for engineers and construction crews. As seen here, special consideration must be taken not to disturb the foundation of the adjacent building to the construction site. In order to ensure this a worker carefully directs the operator of the excavator using hand signals to be sure the foundation is not damaged by an impact from the bucket.

Exploratory Drill Rig

Photo Credit: Alex Mead

Seen here is a mobile drill rig used to explore the subterranean world for resources such as petroleum and natural gas. Mobile drilling rigs such as this one are very capable of travel and can be found from pole to pole on planet Earth aiding humans in their search for more oil and natural gas to fuel the ever increasing demand of the world population.