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MAIN REPORT
Summary Of Findings
- The primary cause of the collapse was a break in the water supply line on the outside of the wall, discharging an estimated 11,000 to 12,000 gallons of water into the soils behind the wall and overloading the wall capacity.
- The underlying cause of the collapse was the faulty construction of the basement retaining wall. Failure to properly grout the wall introduced a fatal structural flaw in the wall. The water line probably broke as a result of gradual yielding or movement of the defective wall, leading to the final collapse.
- The building structure is completely compromised. No portions of the structure are salvageable.
Site Description
The subject property is a one story single family residence with stone veneer and vinyl siding on the exterior walls and a composition shingle roof surfacing. There is a full basement under this home. The home generally faces west and was approximately 11 months old a the time of collapse. The home was approximately 1.5 years old at the time of our inspection.
The home is presently unoccupied.
Due to the failure of the basement foundation wall, significant portions of the wall itself and those portions of the basement slab which are now buried were not accessible for inspection. Other portions of the basement area were reasonably accessible. The main floor of the home is unsafe and was not entered.
Exterior Observations
E-1 This inspection was conducted on Saturday, August 30, 2001 at 4:00 pm under sunny skies and with temperatures in the mid 80's.
E-2 The lot slopes gently from the front left (at the road) towards the front of the home and to the right. There is an approximately 10 foot high retaining wall on each side of the home, which is continuous with the front foundation wall. The lot behind this wall is generally flat across the width of the home and then slopes steeply behind the home.
E-3 There are cantilever retaining walls extended from the front corners of the basement on each side of the building. These walls are basically intact. There is no damage to these walls except at the interface between the right sidewall and the basement foundation wall.
E-4 With the front foundation wall collapsed, the front of the home has fallen inward. The rear wood-framed basement wall is leaning outward approximately 2 feet at the top.
E-5 We observed that a drainage swale has been cut across the yard parallel to the front of the house to direct the water which drains off the road around the end of the right side retaining wall.
E-6 An approximately 8 feet wide area of the yard adjacent to the front foundation wall has slid down onto the fallen wall and front portions of the home. We observed no other indications of slope failure or movement.
E-7 The basement area is divided into two sections. The undamaged area is a smaller room along the left side of the home which has a dirt floor and is covered by metal bar joists and a metal pan. Presumably, there is a concrete slab above the metal pan. The main basement area is toward the right. The is the area which has suffered the collapsed wall.
Interior Observations
I-1 The basic construction of this home consists of a wood-framed superstructure supported by 12 inch thick concrete masonry block foundation walls on the front and left sides and wooden foundation walls on the rear and right sides. The right end wall of the basement contains a standard sized (7'x9') garage door opening. The floor framing above the right side basement area consists of prefabricated 4x2 wood trusses. Presumably, the roof framing is also prefabricated wood trusses. This is a fairly common method of construction in this area. The floor framing on the left side is of metal bar joists supporting a metal pan and raised concrete floor slab. This is an unusual construction feature in a single family residence.
I-2 We observed that the entire right side foundation wall has collapsed into the basement area. Where visible, the wall consists of 12 inch concrete block construction with reinforcing at 4 feet on center with bundles of three No.3 reinforcing bar. The basement foundation wall and the left and right side retaining walls were approximately 14 courses high (9'-4"). The reinforcing bar and grout was stopped 3 courses from the top of the basement wall. There is an interior front-to-rear masonry wall located at approximately one-fourth the width of the home from the left side. There appears to be a gap in the basement wall reinforcement for approximately 8 feet to the right of this interior wall.
I-3 There was standing water and mud on the basement slab during our inspection. There had been a very hard rain which occurred several hours prior to our inspection.
I-4 There is a opening near the base of the front foundation wall in the small left side basement area. This opening is packed with gravel and is reported to be the outlet for a drain system along the front of the wall. There were no signs of erosion or washing of this gravel, nor was there any indication of mud moving through this area.
Discussion
It was reported to us that the water supply line for the home ruptured and repaired approximately 1 week prior to the collapse of the wall. Water meter readings show that an estimated 11,000 to 12,000 gallons of water were discharged behind the basement wall. A moderate rain storm occurred 12 to 18 hours prior to the collapse of the wall. For the purposes of our discussion, we have assumed that the above information is correct.
The proximate reason for the collapse was the ruptured water line However, the underlying reasons for the collapse are the inadequate design and defective construction of the wall. We believe that the yielding of the poorly constructed wall caused the water line to rupture. The water from the break increased the effective pressure against the wall and softened the soils at the footing level, permitting the footing to rotate and causing the wall to move beyond the equilibrium point and collapse. A preliminary design check of the wall shows that the does not have enough reinforcing steel. The wall should have contained roughly twice the amount of reinforcing steel we observed.
Generally, basement foundation walls are designed to withstand an effective soil pressure of 35 to 45 pounds per cubic foot against the wall. A safety factor of from 1-1\2 to 6 may then be applied, depending upon local conditions and the relative importance of the particular structural element. A factor of 1-1/2 is commonly used in residential construction. Due to the rupture of the water line, the actual load on the wall became closer to 100 to 110 pounds per cubic foot. Under these conditions, a properly designed and constructed wall would be overloaded but would be very unlikely to fail in this catastrophic manner.
The front basement wall is seriously flawed in at least 2 ways. A critical error in the construction of the wall occurred when the grout was stopped 3 courses from the top of the wall. More than any other factor, this is the weak link in the wall. Without the grout to engage the steel, only the bond strength of the mortar on the face shell of the hollow concrete block was available to resist the lateral soil pressure loads on the wall. Once the bond strength in the mortar joint was overcome, the joint cracked and failure of the wall became a function of rotating of the wall footing or yielding of the steel at the wall-footing joint.
The failure of this wall does not follow the patterns of classic retaining wall failure. In the classic failure mode, a wall gradually bulges inward over time, and distress in the wall is clearly evident. This is a ductile failure mode. Even unreinforced walls will usually exhibit gross distortions and may bulge significantly for many years prior to collapse. The subject wall remained essentially intact from the toe of the wall to the top of the reinforcing steel, indicating that failure of this wall occurred as a catastrophic brittle failure that than a gradual ductile failure.
It is interesting to note that the retaining wall to the right of the failed basement area does not appear to have under gone significant movement. Based upon the topography of the site, a large portion of the storm water runoff would be retained against this wall, rather than the front basement wall.
The second significant flaw is the lack of adequate reinforcing steel in the wall. A preliminary design check shows that the wall contains approximately half of the reinforcing steel necessary to resist the normal design loads imposed on basement retaining walls.
The construction of the wood-framed right end wall of the basement also appears to be flawed. This wall must functioned as a shear wall. This wall contains a large garage door. Although we have not performed design calculations, our experience with such walls suggests that this wall does not have sufficient size or rigidity to adequately resist the shearing forces.
Following our inspection, several other possible causes of the collapse were advanced. One theory is deep-seated slope failure of the soils. Another is that the water deposited by the rain storm in the hours prior to the collapse is the culprit.
The conditions and the topography of this site do not support a conclusion that deep-seated slope failure caused the collapse of the building foundation walls. Such slope failures in mountainous regions tend to occur where homes have been constructed on silt deposits at the foot of a slope. The topography of this site did not indicate such a condition exists. Deep-seated slope failure would also tend to cause an upward heaving of the foundation slab. Lastly, once a soil mass of this size became dynamic, it would tend to exhibit landslide characteristics and it is very unlikely to have only caused the wall to fall in. We would expect much more catastrophic damages from such an event. Only about 8 feet of the soil right at the edge of the failed wall has fallen or eroded into the basement area.
With respect to the rain storm, preliminary calculations indicated a maximum of 700 to 800 gallons would have been deposited along the front foundation wall of the home. Based upon site topography, this water would probably drain against the right side retaining wall rather than the foundation wall. Although the additional water may have been the proverbial “straw that broke the camel’s back”, it does not represent the primary cause of the failure.
It was also suggested that the water line was not a culprit because the water deposited by the broken water line would have already drained out through the drain opening in the left side basement wall before the collapse occurred. Conditions at the site do not support this conclusion. The soils in front of the wall consist of silty clays, which have notoriously poor drainage characteristics. 11,000 to 12,000 gallons of water draining through the small opening would have not only washed all of the stone away, but would also have deposited a large quantity of silt in this area. Under this scenario the rain water should also have drained away.
Conclusion
In conclusion, we find that the primary cause of the failure of this wall is the discharge of 11,000 to 12,000 gallons of water behind the wall. This discharge severely over-stressed the capabilities of the wall. If not for this extreme event, it is likely that the wall, though under reinforced and improperly constructed, may well have lasted many years before repair or replacement became necessary. Furthermore, similar deficiencies in other walls generally manifest themselves for a period of years or even decades prior to a catastrophic collapse.
The most significant contributory factor and the primary reason for the catastrophic nature of the failure, is the defective construction of the wall. Failure to fully grout the wall introduced a fatal structural flaw. Adequate reinforcing steel would have made no difference without proper grouting. Proper grouting would have made even the under-reinforced wall serviceable for many years and any ultimate failure would have been ductile instead of catastrophic. A ductile failure would have permitted the structure to be salvaged.
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