Building cracks, soil subsidence, storm drains

The case of the month concerns a commercial two-story building with cracks in some of the exterior walls. We were called in to identify the structural damage, and to determine what was due to recent events, long-term conditions, construction defects or deficient maintenance.   This turned out to be a complex case with a lot at stake.   As the case developed, we assembled a team that included a structural engineer, a geotechnical engineer, and a PhD hydrogeologist to answer the question of “What really happened?”

To begin with, the structural engineer surveyed the structure and documented the damages.  The structure was built in the 1920s on what was now a busy commercial street in southern California.   Exterior walls were concrete on the first level and un-reinforced brick on the second level.   There were vertical cracks in the concrete basement east wall.   There were diagonal cracks in the un-reinforced masonry east wall.   The base of the west wall was immediately adjacent to the neighboring building on that side.   The gap between the adjacent building and the west wall varied in width along the height of the two buildings and the west wall appeared to be leaning towards the east.   The damage patterns told the structural engineer that it was not all recent.   The causation was loss of soil support.   In addition, the damage was not a sudden event (such as an earthquake), but had been progressively happening over an extended period of time.

The loss of soil support in the vicinity of the southeast corner of the structure caused the structure to “break” somewhere along the south and east wall.   These breaks were evident in the vertical and diagonal cracks in the brick and concrete walls.   Once the breaks occurred, the southeast portion of the structure settled until it again found supporting soil.   As far as remediation goes, once the soil was stabilized to mitigate any future settlement of the structure, the cracks could be repaired and the integrity of the structure could be returned to its previous condition before the onset of the soil subsidence.

The next question was, what was under the building?

Our geotechnical engineer surveyed the floor levels and found that the center of the lower floor slab sloped more or less uniformly 7 inches to the rear alley.   The sides sloped down one to two inches to either side of the building indicating more-or less uniform long-term fill settlement under the sidewall foundations.

The front upper floor at street grade was on a shallow foundation over +/- 20 feet of fill.   The lower floor was set back about 20 feet from the front of the building and was also founded on fill that was about 10 feet deep, per a soil report done for the owner on adjacent properties for another project.   The southeast corner of the building was +/- 35 feet above a diagonally running large diameter (60 inch) storm drain that dated back to the early 1900’s.

Recommended repairs were compaction grouting of the fill and soil under the southeast corner and nearby sides of the building to mitigate future subsidence and possibly mechanical jacking of the shallow foundations under the front and side of the building to bring the front back to a more uniform slope, and then patching the cracks.

This lead us to the third phase of the assignment.  The building owner hired his own engineer, who asserted that the subsidence was due to the presence of the storm drain.   The owner of the storm drain (the city) was then sued for the cost of the repairs to the building that they allegedly caused.   Our hydrogeologist examined the merits of this claim.

To lay the foundation for the discussion, (pardon the pun) arid and semi-arid areas of the world frequently contain collapsible soils.   These can be caused by alluvial soils (soils transported by water) that are moved by short bursts of intense precipitation.   When they are moving, they are saturated with water and have a high void ratio.   When they stop moving, they dry quickly by evaporation, and capillary action draws the pore water toward the particle contact points, bringing clay, silt particles, and soluble salts with it.    Once the soil becomes dry, the smaller materials bond the sand granules together at their points of contact, forming a cemented honeycomb structure.   As long as the soil remains dry, it produces a strong soil that is capable of carrying large loads, such as building foundations.   If the soil becomes wet, the cementing agents soften and the honeycomb structure can collapse.

We knew that the soils under the foundation subsided, but was it caused by the storm drain?

The opposing engineer argued that since the storm drain went under the corner of the building that had dropped, therefore it was the cause, because only that corner of the building dropped.   However, association is not causation.   If you see firemen at fires, does that mean firemen cause fires? (Ok, sometimes!)

In this case, the presence of the storm drain superposed a pattern on the subsidences, but they did not cause the subsidences.   Just as a fifty ton rock in the middle of a river causes disturbances in the water flow, it does not pull the water out of the hills, into the river bed.

In this case, there had been heavy rains that had caused major flooding in the neighborhood, as well as flooding in front of the building.   Over the past nine decades, there had been about six storms of similar magnitude.  The cause of the subsidence was wetting of the normally dry fill.   The depth of fill under the front of the structure was not equal to the depth of fill under the rear of the structure.   Accordingly, after wetting, they would not shrink the same amount, nor (due to paving and sidewalks) was there evidence to presume that all of the soil was uniformly wetted.

The opposing expert argued that the storm drain was probably broken and the accompanying leaks were the source of water that caused the fill wetting.   A video survey of the storm drain showed water moving into the pipe through small cracks and joints.   This showed that the water table was higher than the storm drain, so that eliminated that argument.   Another argument was that the storm drain sank due to inadequate compaction.   Again, the video survey disproved this theory, on the contrary, the storm drain actually contributed some positive support to the foundations above it.

Our final finding was that the fill embankment that constituted the soils under the street in front of the building was a complex shape, actually a saddle.   This was demonstrated by leaning utility poles, heaving sidewalks, cracks in the roadway, and tilting block walls up and down the street, as well as the subsidence of the building in question.   The city-owned storm drain did not cause the building distress