In the poem, "The City Planners," several tract houses collapse because they've been built on ground that is not stable. The collapse is due to poor planning caused by political corruption. The houses have been built on a clay foundation—this is notoriously unstable and prone to erosion.
The grass that might have held the ground together is shown in the first stanza to have been “discouraged.” Political conspirators are mentioned in the fifth stanza. The clay foundations are rendered as “clay seas,” in the fourth stanza.
The collapse of the houses, further, is tied to a larger failure in planning connected with our overly complacent attention to the environment, as evidenced by the mention of glaciers and oil in stanzas two and four.
Please see:http://www.enotes.com/topics/margaret-atwood for more about Margaret Atwood.