San Fran Bay Mud

[mitchell54]

In dealing with a project in the San Fran Bay area it was required to remove the top layers of vegatative growth and then an additional amount of earth. The grading and excavations removed 1' to 4' of overburben material to final grades that had a 0.1' tolerance that was closely adheared to. Before completion the rainy season set in and work was halted. Not being familiar with Bay Mud what kind of changes in grade elevations should I expect if any.

The reason I ask is because I have run into a similar situation before on the east coast, in which a a thick shale layer was excavated to unearth a clay layer. After a month the grade elevations (from GPS) changed almost 0.4' due to swelling of the clay.

 

[tmctim]

First, get your surveyor out to check this.

In construction on "Bay Mud", the most common problem is that an added overburden will "squeeze out" a layer from below--example, you think that you have added three feet of fill, but come back the next week and find that the elevation is only one foot higher than original grade.

The swelling type of problem that you ran into is not as common in Bay Mud over the short-term winter shutdown: usually, "Bay Mud" is already saturated, so it will not have a swelling problem.
You may be dealing with the black "adobe" clays that surround the Bay, sometimes these are also called "Bay Mud". These can be dried out, and so subject to swelling when they get wet (and subsequent shrinking next dry season). During the past few wet months of winter, there may not have been a whole lot of swelling that you can tell by a survey (the clay layer is pretty impervious, so the water may only penetrate a few inches down, so there's not much immediate swelling). The shrink and swelling is much more a long-term type of problem for any structure (or pavement) you try to put on top of the clay.

 

[mitchell54]

Thanks tmctim,
You put my concerns to rest, knowing that there shouldn't be signficant change in my elevations.