Mccoy
Geotechnical
- Nov 9, 2000
- 907
Hi everyone,
This is the case I submit to your attention: I am involved in a forensic reconstruction of events (occurred 10 years ago) regarding a residential building damaged allegedly by the collapse of the slope above an underlieing excavation.
Please note that I am still in the initial phases of assessment, so many parameters (such as soil properties) are uncertain.
The building is made up of a masonry (cinder blocks) structure, 2-3 storeys of elevation, continuos fondations (beams of poorly reinforced concrete, about 2.5 ft wide, extimated loading 200-250 Kpa max). On the downslope facing side there is a yard, level, about 36 ft wide beyond which the slope downgrades at approximately 30° from the horizontal.
Stratigraphy is 18-24 ft silty sand , phi about 30°, c’ close to nothing, with conglomerate boulders, underlain by OC silty clay (tipically phi = 27°, c’ = 5-10 Kpa). Groundwater has been found at the interface between the two layers. Soil parameters have been extimated.
After the owner of the estate located on the downslope side tried to cut the slope, an instability event occurred, with the creation of a landslide whose scarp reached up the slope to a distance of about 60 ft from the foundations of the damaged house. Also, a tension crack was observed further upslope, at about 40 ft from the foudations of the same house.
Now, my doubt is the following: the foundations appear not to have been reached by any failure surface; also, the stresses in the influence volume (the “pressure bulb”) propagate no more than 11 ft laterally from the foundation axis (10-20 Kpa residual stress), pretty far away from the tension crack. In such conditions, would you be ready to rule out the slope instability as the cause of the damage (decompression and foundations tilt and settlement)? Or is there something else I’m missing (related to slope instability propagating to the foundations)?
Please note the cracks are not of serious concern and are concentrated in the downslope-facing half of the building. Also, there are more factors to consider, which are hard to assess (because of disagreeing witnesses) and include possible settlements due to underexcavation of basement, to groundwater oscillation, to soil dishomogeneity and compressibility below foundation. In particular, it’s hard to evaluate wheter the cracks were already existing prior to the landslide (witnesses have a degree of unreliability). Settlement analysis in the conservative hypothesis (E=10 Mpa) yields 3 to 5 inches average settlement for a rigid foundation (loading 200 Kpa). Qamm is about 200 Kpa for foundation depth = 4 ft. But soil parameters have been extimated and are not known with certainty yet. Differential settlements may be significants due to the presence of boulders.
In conclusion, with which degree of confidence would you rule out (or rule in) the landslide as a cause of the cracks in the structure, and would you focus your soil investigation strategy mainly on alleged foundation problems or on slope problems?
![[worm]](/data/assets/smilies/worm.gif "[worm] [worm]")
This is the case I submit to your attention: I am involved in a forensic reconstruction of events (occurred 10 years ago) regarding a residential building damaged allegedly by the collapse of the slope above an underlieing excavation.
Please note that I am still in the initial phases of assessment, so many parameters (such as soil properties) are uncertain.
The building is made up of a masonry (cinder blocks) structure, 2-3 storeys of elevation, continuos fondations (beams of poorly reinforced concrete, about 2.5 ft wide, extimated loading 200-250 Kpa max). On the downslope facing side there is a yard, level, about 36 ft wide beyond which the slope downgrades at approximately 30° from the horizontal.
Stratigraphy is 18-24 ft silty sand , phi about 30°, c’ close to nothing, with conglomerate boulders, underlain by OC silty clay (tipically phi = 27°, c’ = 5-10 Kpa). Groundwater has been found at the interface between the two layers. Soil parameters have been extimated.
After the owner of the estate located on the downslope side tried to cut the slope, an instability event occurred, with the creation of a landslide whose scarp reached up the slope to a distance of about 60 ft from the foundations of the damaged house. Also, a tension crack was observed further upslope, at about 40 ft from the foudations of the same house.
Now, my doubt is the following: the foundations appear not to have been reached by any failure surface; also, the stresses in the influence volume (the “pressure bulb”) propagate no more than 11 ft laterally from the foundation axis (10-20 Kpa residual stress), pretty far away from the tension crack. In such conditions, would you be ready to rule out the slope instability as the cause of the damage (decompression and foundations tilt and settlement)? Or is there something else I’m missing (related to slope instability propagating to the foundations)?
Please note the cracks are not of serious concern and are concentrated in the downslope-facing half of the building. Also, there are more factors to consider, which are hard to assess (because of disagreeing witnesses) and include possible settlements due to underexcavation of basement, to groundwater oscillation, to soil dishomogeneity and compressibility below foundation. In particular, it’s hard to evaluate wheter the cracks were already existing prior to the landslide (witnesses have a degree of unreliability). Settlement analysis in the conservative hypothesis (E=10 Mpa) yields 3 to 5 inches average settlement for a rigid foundation (loading 200 Kpa). Qamm is about 200 Kpa for foundation depth = 4 ft. But soil parameters have been extimated and are not known with certainty yet. Differential settlements may be significants due to the presence of boulders.
In conclusion, with which degree of confidence would you rule out (or rule in) the landslide as a cause of the cracks in the structure, and would you focus your soil investigation strategy mainly on alleged foundation problems or on slope problems?
![[pacman]](/data/assets/smilies/pacman.gif "[pacman] [pacman]")
