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Technical Note (National Research Council of Canada. Division of Building Research), 1970-03-01
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Land Subsidence in Built-Up Marshland
Bozozuk, M.; Penner, E.
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NRC Publications Record / Notice d'Archives des publications de CNRC: https://nrc-publications.canada.ca/eng/view/object/?id=051119f2-ed67-45a9-8651-dcfdb2217d9d https://publications-cnrc.canada.ca/fra/voir/objet/?id=051119f2-ed67-45a9-8651-dcfdb2217d9d
DIVISION OF BUILDING RESEARCH
No.
NATIONAL RESEARCH COUNCIL OF CANADA
547
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PREPARED BY M. Bozozuk and
E. Penner
CHECKED BY L. W. G. APPROVED BY C.B.C.
March 1970
PREPARED FOR Inquiry and Record Purposes
SUBJECT LAND SUBSIDENCE IN BUILT-UP MARSHLAND
Marshland often consists of saturated peat or organic matter sup-]Drted by weak and compressible mineral soil or marl. When drained for use as a building site, physical and chemical changes occur in the under-lying material that, in time, can seriously affect the performance of struc-tures founded upon it. Weathering and decomposition of the underlying materials may be accelerated. The organic materials may shrink on dry-ing, thus causing settlements of the ground surface. Serious structural damage to buildings can result unless proper design and construction practice is followed. This Note describes some problems which develop-ed in a residential area locatdevelop-ed on marshland in Ottawa.
In the fall of 1969, at the request of the Regional Municipality of
ｏｴｴ｡ｷ｡Ｍｃ｡ｲｬ･ｴｯｮｾ a survey was made of an area on which domestic
dwell-ings had been constructed fifteen to twenty years previously. The in-vestigation was prompted by complaints to the city that the ground was settling causing severe damage to the property. A tour of several city blocks within the area showed that streets, sidewalks, and driveways re-quired considerable and frequent repair; garages and fences were greatly distorted. In general, the houses and fire hydrants were undamaged, al-though the ground around them appeared to have subsided 12 to 18 in. The lawns and gardens had marked depressions with soft centers. Most of these soft spots had cavities below the sod cover. The "springy"
character of the ground surface suggested that the subsoil was highly or-ganic.
2
-At the request of the authors, the city excavated two test pits in the centre of the area. Although heavy rains prece ded the investigations, the test pits were remarkably dry. The soil profile consisted of 6 iri. of turf mixture of decomposed peat and shells, which was probably the spoil excavated from the house basements. One foot of decomposed organic peat was found below this. It contained little or no mineral soil although
some small tree roots were found. This layer rested upon 1 ft of brown-ish grey marl, consisting almost entirely of small shells (Figure 1).
Competent grey limestone which underlies the area was exposed at the bot-tom of the test pit. A photograph of the soil profile is shown in Figure 2.
The marl was heavily fissured with vertical wedge shaped channels up to 5 in. wide; some contained organic matter which had fallen in from above. Some of the voids extended vertically through the peat to the sur-face where the "sink-holesll appeared. A chemical analysis of the marl
is given in Table I.
Before the marshland was converted into a residential area, the ground water table was at or near the surface. In this saturated con-dition, the peat and marl were stable. When the area was drained and sewer and water services were installed, the ground water level dropped, permitting the peat to decompose more rapidly. Water from rains, snow melt and frequent watering by the residents, percolated through the organic layer. The humic acids formed by the decomposing peats reacted with the calcium of the marL Gradually both the soluble organic material and the calcium went into solution and drained away, slowly at first, then more rapidly when the fissures in the marl enlarged into channels. This de-terioration of the marl and peat has resulted in a general subsidence in the area over a period of 15 to 20 years. As the houses were founded on shallow bedrock, they were undarn.aged. Structures which rested on the organic or marl layers, however, were affected. Figures 3 to 8 illustrate the effects of the subsidence.
TABLE I
CHEMICAL ANALYSIS OF MARL
Carbonates: 74. 7% by weight (Treatment with HC..e) Organic Content: 2.60/0 (Treatment with H
202) Soil Particles: 22. 6%
Figure 1
Brownish grey marl, made up almost entirely of small shells
Figure 2
Figure 3
Subsidence of soil around a house basement founded upon bedrock. The exposed foundation
indicates that about 17 in. of subsidence occurred since the house was constructed.
Figure 4
Subsidence around fire hydrant. The water line and the fire hydrant must be supported upon the shallow bedrock at this location. As the surround-ing soil subsided, it exposed the hydrant, caussurround-ing it to appear to emerge from the ground. The pro-tective collar usually placed level with the ground indicates a subsidence of about 2 ft.
Figure 5
Subsidence around pier supporting a verandah. The old concrete cap on top of a pier founded upon bed-rock is now about 8 in. above a newly paved drive-way. The concrete cap could have been placed at or below original ground level, indicating that the subsidence was at least 1 ft at this location.
Figure 6
Earth mound around a tree. If the tree was planted originally in level ground, the bump must be caused by the root system holding the organic soil in place, whereas the soil was able to subside at some distance away from the tree.
"
Figure 7
Garage distorted by subsidence. As the garage was founded upon the ground surface, non-uniforITl subsidence caused it to tilt and warp so that the door could not close. Behind and to the side of the garage, sink-holes exist in the ground where the person is standing.
Figure 8
Cavity under sidewalk. Non-uniforITl subsidence of the soil reITloved support of the sidewalk creat-ing a 6-in. deep cavity illustrated in the photo. At this location, the paved street required con-siderable repairs.
