Quarrying a Hillslope in Waltham: Considerations and Lessons Learned from Stabilizing Rock in an Open Excavation-PPT Presentation

PPT Presentation

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The presentation reviews the investigation, design, and construction of major rock cuts for a school development in Waltham, Massachusetts. The project involved excavating about 200,000 cubic yards of soil and 800,000 cubic yards of rock, processing 300,000 cubic yards of rock fill, and constructing approximately 1,530 linear feet of rock slopes—up to 120 feet high. The development includes interconnected school buildings totaling 420,000 square feet, with a parking garage and athletic field.<br /><br />From 2019 to 2020, the team conducted a broad subsurface investigation, including 19 borings, 35 test pits, groundwater testing, geophysical surveys, geologic and joint mapping, and laboratory testing. Slope analyses assessed planar, wedge, and toppling failure risks across different slope angles. Design and construction measures included controlled blasting, benches, wire mesh, rock dowels, and soil stabilization.<br /><br />The presentation highlights several lessons. The distinction between geologic rock and sound rock can affect excavation limits and slope design. Property boundaries may prevent grading back weathered rock or may be exceeded by rock-dowel embedment, requiring careful coordination and potentially easements or land acquisition. Shear zones—altered joints that may contain soil or silt and be affected by weathering—can become evident as excavation exposes new rock faces and must be addressed in stabilization plans. Groundwater, rainwater, and ice-related effects can also influence slope and foundation conditions.<br /><br />Overall, the project demonstrates the importance of detailed investigation, adaptable design, and close coordination among owners, engineers, and contractors during large-scale excavation.

Keywords

Waltham Massachusetts

rock cuts

school development

subsurface investigation

controlled blasting

rock slopes

slope stability

geologic mapping

rock dowels

groundwater effects