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The missing foundation

The owners of a house in Southern California decided to strengthen their home to better survive “the next big one”. They called a local earthquake retrofit contractor for a price. One of the improvements they were looking for was to bolt the wooden house structure to the concrete foundation.


The contractor arrived and, after taking exterior measurements, wiggled into the tight crawl space under the house to see what needed to be done there. When he later emerged, he reported to the home owners that a portion of their foundation was missing.

GEI was hired by their insurance company to inspect the site to identify and determine the predominant cause and approximate age of the missing foundation.

Our expert visited the residence and conferred with the owners. They showed him the report of estimated costs that had been prepared for them to install foundation bolting to comply with the voluntary earthquake preparedness provisions of their city codes. Their property was a single-family, timber-framed building, aligned principally north-south, with an integral garage to the north, built circa 1923, according to the County Tax Assessor’s office.

The living room to the residence was entered from the front door in the west wall. The floor of the southeast corner had some minor separation from the wall, but the separation was normal for a building of this age. There were no other sagging or developing cracks in existence that were the result of a missing foundation. The under-floor support was comprised of a series of 4-inch by 4-inch beams carrying 2-inch by 6-inch floor joists. The beams were supported on individual timber support posts bearing on concrete or concrete/brick amalgam pads. The property was built prior to the enactment of laws requiring permits for construction (permits were not required in the county unincorporated areas until 1933). As such, the building might not meet current standards for protection of public health and safety. The area under the front door, which was claimed to be without a foundation, did have the required support. It had stood for the last 88 years, apparently without damage. The support was provided by timber beams and concrete pads instead of the expected poured concrete foundation strip found elsewhere.

The foundation supports varied throughout the residence crawl space, but appeared to be solid. The exception to this was the support by the west end of the foundation beams on a variety of concrete/brick/stone pads and by burial in the front (west) wall foundation strip. None of the west end beam supports included any form of waterproofing that could be discerned without lifting the beams. While the beams were not currently deteriorating from dry rot, it was advised that they treat the buried and supported west ends of the 4-inch by 4-inch beams to prevent possible dry rot and/or insect attack.

Our expert also noted that the downspout by the front door was not functional and could be discharging rainwater near the foundation strip and beam-ends. Also noted was the fact that the crawl space did not meet current code requirements (most of us would describe the crawl space as claustrophobic). The current crawl space code requirements are not retroactive unless the crawl space is altered.

 

Based on the foregoing, it was the expert’s opinion that the predominant cause of the perceived missing foundation was a misinterpretation of the principles of foundation engineering and of framed construction. The approximate age of the residence was 88 years, and the foundation had been in existence since that time, with no apparent structural issues.

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Failed retaining walls

The insured had a slope failure behind his home.   Two retaining walls had collapsed and a water line was broken.   The client wished to know whether the walls failed first or whether the pipe failed first.

Our expert visited the insured’s site with the insured, his contractor, and our client. The property was a single level family dwelling, built on a steep slope with a rear timber deck.   The rear garden area had five retaining walls; the first at the highest level was still standing, but its foundation was compromised.   The next two were being demolished because of their failure.   The lowest two walls were intact.

Inspection of the walls showed that the uppermost wall, which supported the rear yard and the house, had a compromised foundation and was now exposed.

The concrete staircases that provided access to the lower levels of the rear garden of the residence also collapsed from a lack of support when the retaining walls collapsed, as did the rear deck.

The soil under the wall was clayey with small (4-inch) round rocks included.  Below this wall, the second wall had been built and below that a third and then a fourth and fifth wall.   The insured said that the contractor, when building the retaining walls, had dirt brought in as backfill.   The contractor agreed that this was the case.   The insured stated that the fill material was placed behind the wall and then compacted using a small machine tamper.   From our inspection of the soil, it was apparent that the fill material was not compacted in more than two layers for a total depth of fill of 4 feet.   Normal practice is to tamp the fill at every 12 inches of depth to achieve suitable fill for construction purposes and to provide a stable foundation.   Failure to compact the fill results in voids being left that subsequently fill with water and fine silty material, both of which are unsuitable for construction of retaining walls.

Engineered retaining walls are properly constructed of poured concrete, using reinforcing bars laid in a mesh pattern, which is tied in to a similarly reinforced foundation slab.   On a steep slope, such as that which existed at the site, it is also normal to provide a “heel” to the wall that penetrates below the level of the foundation slab for stabilization.  On these walls, no “heel” was provided, nor was a foundation slab provided.  The walls constructed at the site were of masonry block construction, not poured concrete.   The masonry block was reinforced with vertical bars tied in to a reinforced strip foundation.  This type of construction is typical of garden walling and is unsuitable for use as a retaining wall containing fill material with superimposed loads from a house and deck, constructed on a steep slope.

A properly constructed retaining wall also has “weep holes”, which are drainage outlets for the release of water that accumulates behind the wall. These were not seen in the walls being demolished.   Provisions should also have been made for properly draining water that accumulates behind the wall into those “weep holes,” by providing porous soil (gravel) behind the wall surface.   This also was not observed.

The reinforcement provided to the slab, on which the upper level deck was resting, was lacking in great part from proper reinforcement provisions and would only provide minimal crack prevention to the slab surface.

The damages to the two retaining walls, associated stairways, and the timber deck were caused by water flowing from a broken irrigation pipe unchecked for a period of several days.   This caused the wall foundation to be undercut and the wall subsequently to collapse.   The collapse of the higher-level wall, combined with the unchecked water flow, then overloaded the lower wall, causing it to fail also.

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Building cracks

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