Engineering Geology

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For over 40 years, our experts have partnered with the US Army Corps of Engineers (USACE) and the Diamond State Port Corporation (DSPC) to enhance the Port of Wilmington— the heart of Delaware’s blue collar jobs. Specifically, when these groups have needed assistance planning improvements and managing the site, they have consistently turned to our team for support.

Over the years, our staff has provided extensive environmental, civil, geotechnical, marine, and materials testing engineering services for projects including environmental maintenance and larger projects such as a $10 million expansion at Wharf 7, the addition of a $30 million automobile berth in the Delaware river, the design of a multiple dry and conditioned warehouses, container crane staging and support and monitoring and reconstruction on over 150 acres of reclaimed land adjacent to the Delaware River. Here, our rich history of institutional knowledge at the site, enabled the Port to continue its successful work as an economic driver.

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The North County Transit District/Amtrak route is a scenic one as it travels between the surf and coastal bluffs. Beautiful for the rider, unless the train is stopped due to the coastal bluff deterioration, slope failures and landslides that occurred adjacent to the railway, and prevented the trains from passing through.

In April 2001, a 40-foot section of concrete retaining wall collapsed, a wall that had lined the top of the Del Mar Bluffs at 7th Street. The failure, which had been identified in the completed Del Mar Bluffs Geotechnical Study as one of the highest risk areas, consisted of three wall sections approximately 60 feet above the beach.

To keep trains running atop the bluffs, the North County Transit District (NCTD), which owns the tracks, declared an emergency and approved spending $1,050,000 to design both immediate and long-term repairs to the portion that gave way. In addition, funding for mid-term measures, that could cost as much as $8.2 million on other high-risk portions of the bluff, were also approved. These measures are intended to ensure that the Amtrak and Coaster trains that run along the 1.6-mile-long section will be able to do so for the next 20 years.

In response to the emergency, our experts assessed the geological conditions and provided emergency repair alternatives for local agency review. With the selection of a shear-pin stabilization method, a “fast-track” analysis, the design and review period was successfully completed within 30 days. Construction of the stabilization system, twelve 60-foot-deep shear pins with tie-backs, was then successfully completed within 45 days.

Prior to this emergency, our geotechnical report had informed the NCTD that saturation from groundwater is/was the biggest problem facing Del Mar’s bluffs. The report predicts that, without reinforcing measures along at least half of their length, the bluffs will erode another 12 feet within the next 20 years, making them even more fragile. Specifically, our report had identified the area (at 7th Street) that failed as a high-risk zone.

The railroad tracks were first built along the bluffs in 1910. Because of population growth, runoff from irrigation has increased tremendously, saturating Del Mar’s bluffs with groundwater and causing them to erode faster than they otherwise would. (Leighton’s study showed that normal rainfall in the San Diego County would dump about 10 inches on Del Mar Bluffs, but with the increased runoff, water reaching the bluffs each year is equivalent to between 100 and 200 inches of rain.)

Improvements constructed included additional shear pins, soil cement improve­ments, sea walls at the base of the bluffs, reinforcing the bluffs, and improving surface and sub-surface drainage, all with site aesthetics as a key element.

The selected stabilization method included additional shear pins connected at the top by a reinforced grade beam with tie backs. Where improvements are exposed they have been boulder-scaped to match the adjacent bluffs. Site stabilization also included the installation of a deep cut off subdrain which was outletted by boring through the bluff to the beach below and exiting through a boulder-scaped outlet.

Leighton has continued to provide geotechnical services since that first report, including the Thanksgiving 2019 failure.

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The Goldstone Deep Space Communications Complex, located in the Mojave Desert, is the site for the Mars Deep Space Station 14 (DSS 14) a 229 foot diameter antenna that tracks spacecraft. Its most eye-catching element is its parabolic dish, weighing nearly 4,000,000 pounds. For the STEM minded, tours are available through the Goldstone website: www.gdscc.nasa.gov.

Repairs to the hydrostatic bearing supporting the antenna, required lifting the dish raising it and then dropping it down onto three temporary, 40-foot-tall support legs.

Our team was contracted by the structural engineer to assess if three existing temporary support foundations, used to support the antenna when it was 64-meters in diameter, could now be used to support the enlarged antenna while repairs were made.

The performance of the antenna was critical, and height movement was restricted within 1/8”. We recommended that the temporary support foundation movement be measured at all three supports during full dead loading. Linear Variable Differential Transformer (LVDT) displacement transducers or similar instrumentation was suggested to be installed during initial loading up to the full dead weight, to measure temporary support foundation displacement relative to the DSS 14 permanent reinforced concrete base. This instrumentation is expected to have a relatively low cost compared to the cost of the antenna and risk of this operation.

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