PERMFILE57051

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Date: 6/18/1999
• Doc Name: OBJECTORS EXHIBITS
• Media Type: D
• Archive: No

8rineTield Subsidence ar Windsor, Onlario •
Only four of the reference points in tltc south-
tvesterly area have experienced a change of cleaa•
lion in excess of 6.5 inches in the period of
observation (1954 to 1968). Of these, hvo arc lo-
cated within the sinkhole area, mid the other hvo
are within 100 Feet of it. The data suggest a north-
westward or westward tilt involving a subsidence of
66 inches at the westerly end of the tilted area, and
an elevation of 12.5 inches of dte easterly end. The
tilting movement took place at a rapidly decreasing
rate until about 1965, when the rate of movement
of the three subsiding points seems to have as-
sumed afairly constant rate of about one inch per
year. The reference point which went up had
apparently become stationary by the beginning of
1967.
Inasmuch as the notable subsidence in the years
following sinkhole formation took place at a rap-
idly decreasing rate, it is probably attributable to
settlement and readjustments in the rock debris
underlying the broken rock surface at depth. The
apparently constant rate of settlement of about
one inch per year presently experienced by three
of the reference points in or near the sinkhole azea
is of the same order as that observed in many areas
v,•here salt extraction has been practiced. It appears
to result from the slow creep of salt pillars,
whether left by design in salt mining or fortui-
tously during brine extraction. It differs qualita-
tively as well as quanti[atiyely from the accelerat-
ing subsidence associated v,dth progressive roof
failure.
MECHANISM OF SUBSIDENCE
After the formation of the sinkhole in February,
1951, it was clear that the gradual subsidence of an
area with a diameter of abort[ 2000 Eeet as well as
the rapid subsidrncc of its central portion were the
result of the collapse of the roof of one or more
cavities in underlying salt bed;. heverthcless, the
data then available left a number of unanswered
questions- In his report of October 27, 1954, Dr.
liarl Terzaghi suggested two alrernaticr hypotheti-
cal models of the events which led to sinkhole for-
mation, One of these involved subsidence of only
that part of the rock surface which tmderlies the
sinkhole, i. e., localized rock subsidence. The sec-
ond involved the more conventional hypothesis of
the subsidence of the rock surface underlying the
entire subsiding area, i.e., general subsidence.
Concerning the hypothesis of localized settle-
ment, he ,,Tote, in part, as follows:
305
"lF the bedrock s~zcc surrounding the sinkhole
area remained practically stationary throughout
the years, the observed subsidence can only be ex-
plained b}• assuming [liar tl,c bedrock surface in
the sinkhole area... descended through a space
with a columc equal to that through which the
surrounding ground surface mo,rd down. It must
further be assumed that the clay advanced in radial
directions towards the sinkhole area, from dis-
tances of more than one thousand feet .. .
... 1'et a Clovv of clay over such dis-
tances ...beneath an almost horizontal surface is a
process without knov,•n precedent. Therefore the
writer cannot accept it unless check borings show
[hat the quantity oC cla}• located above the sub-
sided portion of the rock surface has reall}• in-
creased ..."
The recommended check borings were never
made, bttt tvvo other items of information suggest
that the settlement of the surface of the ground
outside the sinkhole area vvas not the result of flow
of clac toward the center of the bou•I of subsid-
ence. The first of these concerns the character of
the clay. In the opinion of man}' who are familiar
v,ith i[, this clay is a stiff and highly immobile
material. The second item is the great redaction in
the rate of settlement of points located in periph-
eral parts of the subsiding area following sinkhole
formation. If [low of cla}' had taken place prior to
the major subsidence, such flow should have con-
tinued therefore, because the average gradient
toward the center of the bowl v,•as at least as high
after the subsidence and subsequent filling opera-
tion as it vvas prior to sinkhole formation. It thus
appear; that the h} pothesis of localized subsidence
is no longer tenable.
If, as suggested by the second hypothesis, the
subsidrncc of the ground surface vvas everywhere
essentially identical with that oC the bedrock, the
rock surface underlying a large area outside of [he
sinkhole subsided at rates which were locally as
great a; 7 inches per }'ear. Such a rate of subsid-
ence could be explained onl} iE this area ,sere
underlain by one or more cavities with inadequate
roof support.
if surface settlements did indeed accurately re-
flect the deformation of the bedrock surface, suc-
cessive surface profiles provide a valuable clue to
the nature of the deformation of the bedrock sur-
face. Figure 5 shows a series of such profiles
through the westerly part of the bowl of subsid-
ence, from a point within the sinkhole area to the