Function and Limitations of the Soil-Nailed Wall
“[Loess) can turn into soup” when wet according to John Wolosick, area manager for Hayward Baker, Inc, member of the Design Build team for the soil nailed wall.
Bluff Stabilization, Natchez, Mississippi.
Expanded Shale, Clay and Slate Institute (July, 2002)
Many citizens have a false sense of security about the soil-nailed wall, believing that the wall was designed to hold back the bluff or to somehow “anchor” the bluff to more stable ground.
The general purpose of the soil nailed wall is to prevent rainwater and surface drainage from washing over the side and eroding the face of the loess bluff. It does not address the seepage of water into the flat surface of the bluff top itself.
The function of the spear-like SOIL NAILS that were sunk into the face of the bluff is to anchor the wall to the side of the bluff. Therefore, it is the bluff that holds up the soil nail wall, not vice versa.
The soil nails in the Natchez Bluff range in length from 20 to 40 to 160 feet.
- In a typical soil nailed wall, the average distance between nails (viewed from the top down) is normally 1 nail per five feet.
- However, because of the unique and unstable loess soil conditions, the frequency of nails at Clifton Avenue Wall in Natchez (Phase I of the Stabilization Project) was 1 nail per 8 inches when viewed from the top of the wall. (550 soil nails per 450 linear feet). (Source: Innovative Stabilization of Loess Bluff along Mississippi River. D’Appolonia.)
(Click for larger view)
Unlike the diagram above, large sections of the soil-nailed wall at Natchez only cover the top of the 200-ft Bluff. They do not reach the ground. Basically, they hang on the side of the bluff.
- The wall does not protect the lower section of the bluff or address the “toe” of the bluff, where mudflow occurs.
- It will not stop a problem that originates more than 30 or 40 feet back from the bluff’s edge, which is where most MAJOR collapses originate.
Finally, the Historic Natchez Bluff Stabilization Project was the first time soil nails have been used in the United States in windblown LOESS soils. It is not proven to be a permanent solution.
Soil nails do not add weight bearing strength to support loads UNLESS they are in very strong soils, which LOESS is NOT.If the loess turns to SOUP, soil nails will not prevent collapse of the bluff, the wall and any structures above.
A 2002 FEDERAL HIGHWAY ADMINISTRATION (FHA) STUDY warns that SOIL NAILED WALLS in LOESS CAN FAIL where water activity leads to dissolution of the loess. The failures can occur as a rotational slump or collapse marked by
- heaves at the toe of the wall and
- fractures parallel to, and often about 30-40 feet back from the wall
The following diagram shows the toe of the wall heaving upward, as the soil above weakens and slumps:
Soil nail failure is exacerbated by
- WATER activity,
- CONSTRUCTION VIBRATION
- REMOVAL of previous slide debris at the toe of the cliff, WHICH HAS BEEN DONE since the wall was completed.