Structures are only as stable as the ground on which they stand. Virtually all land development involves modification of the ground surface by grading—the excavation, hauling, and placement of rock or soil material—to create building pads, roadways, and earthen structures. Grading can improve soil conditions, such as when landslide masses or zones of collapsible soil are removed and replaced with engineered fill, but poor grading practices can create soil stability problems where none existed before. Earthwork includes not only the creation of graded surfaces and slopes but also the construction of dams, levees, landfill liners and caps, and other earthen structures.
The post-World War II building boom coincided with the development of heavy earthmoving equipment that facilitated rapid and extensive residential expansion into hillside areas. The first grading codes—the equivalent of building codes for earthwork—were developed in the 1950s, in response to widespread slope failures in new hillside subdivisions during heavy rains in southern California. Grading codes and practices have been improved over the years, reducing the incidence of slope failures and fill settlement, but instances of grading-related damage to structures, pavements, and buried pipelines still occur.
Typical grading-related problems include cut and fill slope failures; landsliding associated with filling at the head or cutting at the toe of a natural slope; soil and rock
creep; settlement, differential settlement, seismic compression, and heave of fills; heave of bedrock cuts; and diversion of surface drainage. Grading a site can exacerbate existing stability problems and mask natural topographic indications of potential hazards such as existing landslides or sinkholes. Unexpected subsurface conditions on an excavation project—such as unrippable rock, shallow groundwater, or an old slide plane—can cause expensive delays, necessitate design changes, and pose hazards to construction workers. Cut slopes that are too steep for the soil, rock, or groundwater conditions at a site can fail and undermine properties or structures at the top or threaten persons or property at the bottom. Cuts can also intercept zones of groundwater, which can cause problems ranging from nuisance seepage to slope instability. Construction of fills without proper subgrade preparation, drainage, keying, benching, moisture conditioning, and compaction can result in fill settlement or slope failures. Use of unsuitable soil such as expansive clay in near-surface fills can result in cracking of structures and pavements constructed on the fill. Uncontrolled (dumped) fills typically are susceptible to severe settlement, erosion, slumping, and mudflows or debris flows and can impose unanticipated surcharges on buried pipelines or downslope cuts. Such fills can block watercourses, causing sedimentation and posing a risk of upstream and downstream flooding.
Exponent’s geologists and engineers are specialists in characterizing subsurface conditions, distinguishing fill from native soil and cut slopes from natural slopes, determining the history of site grading, and evaluating the performance of slopes and earthwork structures. Mapping the geometry of cuts and fills, determining the sources and discharge points of underground drainage pipelines, and developing a timeline for site grading and drainage changes typically are among the first tasks in an investigation of the causes of soil and foundation problems in graded areas. Exponent has multidisciplinary scientific and engineering capabilities that allow us to supplement the expertise of our geologists and geotechnical engineers with that of our building technology and structural engineers, mechanical and materials engineers, metallurgists, and construction consulting staff to assess the effects of grading on the built environment.
Our services include:
- Field investigation of failures of graded sites
- Analysis of ground movement or slope failure causes
- Determination of relative contribution of factors controlling settlement or slope failure
- Emergency response and evaluation of slope failure hazards and damage to buildings and structures
- Analysis of dam and levees performance and failure
- Development of conceptual repair recommendations
- In-house geotechnical laboratory testing for measuring engineering properties of fill and natural soil, including expansivity and hydrocollapse potential