brief history of testing and collapse....bulletin of the seismological society of america. vol. 59,...

Bulletin of the Seismological Society of America. Vol. 59, No. 6, pp. 2231-2251. Decembe5 1969

SUBSIDENCE RELATED TO UNDERGROUND NUCLEAR

NEVADA TEST SITE

BY F. N. ttOUSER*

EXPLOSIONS,

ABSTRACT

The U. S. Geological Survey is investigating the mechanism of collapse of nuclear explosion cavities for its value in containment, with the hope of eventually determin. ing those geologic environments most susceptible to early collapse or most adaptable to the incitation of collapse. This is a progress report on a part of these studies.

The investigations to date have been confined largely to the subsidence of the ground surface in the desert alluvium of Yucca Flat by collapse of volcanic rocks and (or) alluvium into the large cavities formed by nuclear explosions.

The sinks thus formed are as much as 60 m (meters) deep and 250 m in radius; they have forms that are commonly variations of inverted cones or sectors of spheres. The outer half of each sink is an inward slide of Slightly disoriented but relatively undisturbed alluvium. The major physical disturbance within the slide mass is by subsidiary spoon-shaped cracks and slide surfaces along which individual blocks have adjusted within the glide mass. Each individual block moves a net distance that is small relative to the total movement of the main slide. The cracks or glide surfaces are assumed to flatten with depth within the slide and to join the major surface of failure at the base of the main slide mass. The spoon-shaped surfaces, where they intersect the top surface of the slide, dip steeply to vertically. In plan they make a pattern of curving, intersecting cracks.

The depth of burial of the explosion cavity in alluvium relative to its size affects the profile of the sinks, the height of chimneys for collapses not extending to the surface, and, to an extent, the likelihood that later surface subsidence will occur.

A working model of collapse includes: (a) maintenance of the explosion cavity until main collapse occurs; (b) a two-phase collapse taking place in several seconds or several tens of seconds, culminating in surface subsidence if the cavity is sufficiently close to the surface. The first phase of collapse in alluvium, at least, consumes 70 - 90 per cent of the collapse period; collapse propagates upward at rates of 50-80 ft/sec. The second phase shows faster rates of upward propagation, a lesser degree of particulation of the collapse material, and ends with the drop of a central mass of alluvial material that initiates formation of the sink. The shape, size, and distance of vertical drop of this alluvial plug control the size and form the resulting sink. Acceleration of this mass ranges from 0.35 to 0.8 g.

INTRODUCTIOK

Brief history of testing and collapse. Underground nuclear testing was suggested as recently as 1956 (Griggs and Teller, 1956), and the first test was conducted in 1957. Subsequent underground testing during the succeeding 11 years has demonstrated the advantages of less dependence on weather conditions, the elimination of radio- active fallout, and improvements in economy, instrumentation, public safety, and scientific experimentation.

* Publication authorized by the Director, U. S. Geological Survey.

2231

2232 B U L L E T I N OF T H E S E I S M O L O G I C A L S O C I E T Y OF A M E R I C A

The first contained underground uuclear test was the RAINIER event on September 19, 1957. The explosion had a yield of 1.7 kt (kilotons) equivalent TNT and was detonated in relatively soft nonwelded tuff at a vertical depth of 274 m in Rainier ~Iesa. The cavity formed by the explosion was presumably nearly spherical and had a 20-m radius; this cavity collapsed to form a more or less cylindrical chimney 20-28 m in radius and 118 m in height (Thompson and Misz, 1959).

The first surface subsidence above a nuclear explosion site resulted from collapse of the Blanca cavity on October 30, 1958. The entire collapse occurred in tuff and within 26 seconds of the explosion. The irregularly elongated surface depression is estimated to be 30 m wide, 120 m long, and no more than 5 m deep (Wilmarth and ~'icKeown, 1960). The first sink in alluvium formed 27.5 minutes after the FISHER event on December 3, 1961. This explosion had a yield of 13.5 kt centered at a depth of 364 m. The sink measured 15 m in depth and 87 m in average radius.

Through January 1969, more than 265 underground tests had been announced. ~,Iost of the tests in Yucca and Frenchman Flats are known to have collapsed within a few minutes to a few hours. Subsidence of the surface has most commonly aceom- panied this main collapse; less commonly, subsidence has occurred months to years later, or not at all.

The U. S. Geological Survey is investigating the mechanism of collapse for its value in containment, on behalf of the U. S. Atomic Energy Commission and the Defense Atomic Support Agency. The objective of this work is to determine those geologic environments most susceptible to early collapse, or most adaptable to the incitation of collapse. This is a progress report dealing with some of the current results in the study of the form, structure, and rate of near-surface movement and their relationship to cavity depth. A possible model of collapse based on this information is also proposed. Before discussing subsidence, it is well to review briefly emplacement methods and explosion effects, especially those that may influence the collapse.

General information on emplacement methods and effects of an explosion. Scores of nuclear devises, ranging in yields from < 1 to about 1,000 kt (about 1 megaton), have been detonated at the Nevada Test Site and elsewhere in tunnels and in drill holes as much as 3.7 m i~ diameter and 1,400 m in depth. The test media used have included alluvium, tuff, basalt, granite, carbonate rocks, rhyolite, and salt.

Each underground test, whether a cratering test or a contained test, is described in terms of scaled depth of burial. For most tests conducted at the Nevada Test Site in alluvium or tuff, where the mean bulk density of the medium is between 1.8 and 2.3 g/cc (grams per cubic centimeter) the scaled depth of burial can be approximated by considering the depth of burial in terms of the cube root of the yield.

Scaled depth of burial = Depth of burial Wll3

where W is yield in kilotons of equivalent TNT. Figure 1 shows diagrammatically a drill-hole emplacement for a test in Yucca Flat,

Nevada Test Site. Holes as much as 1.8 m in diameter are drilled to the desired depths, the final casing string--commonly about 1.2 m in diameter--is cemented in place throughout the length of the hole, and a canister containing the nuclear device and certain diagnostic instruments is lowered to the desired depth. The casing is then stemmed with gravel and cement plugs; the stemming simulates the average density of the surrounding media. Some recent tests in Yucca Flat have been conducted in

SUBSIDENCE RELATED TO UNDERGROUND NUCLEAR EXPLOSIONS 2233

uneased, but stemmed holes thus saving considerable expense and time in site development. For many tests, one to three (rarely more) smaller diameter holes nearby have contained additional instruments.

Some sites have been developed by means of tunneling nearly horizontally beneath mesas of tuff in areas just north and west of Yucca Flat. Most commonly, several side drifts are constructed from one main adit for reasons of economy both in money and land use.

When a nuclear device is detonated--in alluvium of Yucca Flat, for example--the energy of the explosion is released in about a tenth of a microsecond, and the tempera-

Emplacement hole, stemming,and nuclear device canister

f

FIG. 1. Example of drill-hole emplacement of a nuclear tes t in Yucca Flat , Nevada Test Site.

ture in the immediate vicinity of the device is raised to several million degrees Kelvin and the pressure to many kilobars (Germain and Kahn, 1969). At these temperatures and pressures, the emplacement hardware and surrounding alluvium are vaporized and melted and the initial cavity around the device expands spherically udthin tens of milliseconds to a radius that is dependent largely on the yield, lithostatie pressure, and water content. The cavity is lined with melted rock, and the alluvium is fractured out to distances of several times the initial cavity radius. For a short distance from the explosion, for a brief time (tens of milliseconds) after detonation, the alluvium reacts hydrodynamically. The shock energy propagates in all directions and creates seismic waves. A slight bulge occurs at the ground surface and high-angle surface tensional fractures are formed. Then from rarefaction of the seismic wave, flat-lying tensional

2234 B U L L E T I N OF T H E S E I S M O L O G I C A L S O C I E T Y OF AMERICzk

fractures are formed in the near-surface. According to Germain and Kahn (1969), the cavity will continue to grow by what is termed "gas acceleration" only if the rarefaction wave returns from the surface in sufficient strength before initial cavity growth has stopped. This timing is largely a function of distance (depth) and the sonic velocity of the medium. As soon as the cavity has formed, the molten roek shell begins to drain downward to form a puddle in the bottom of the cavity; the ground surface returns to a position near or somewhat above its preshot level, minus any compaction that may have taken place.

At a later time, ranging from several seconds to several hours, the explosion cavity collapses to form a chimney. If the test has a large-sealed depth, the top of the chimney will end short of the ground surface. If, on the other hand, the test is at moderate depth in alluvium or volcanic rock, the collapse will continue to the surface and form a sink.

SINKS

The term "sink" was first used by Houser and Eekel (1962) to describe topographic depressions formed by collapse above nuclear explosion sites in Yucca Flat. Sink is preferred to the term "crater" to emphasize the essential factor of subsidenee--a major genetic difference from explosion craters. Neither shape nor size is implied.

When observed in a cursory way, the sinks at Nevada Test Site appear as a random agglomeration of holes and depressions. A variety of profiles is shown in Figure 2, along with site designations for typical examples. In plan view, few sinks are perfeet circles; most are slightly oval or elliptieal and most have some scalloping of their rims. Aerial views of intensively used areas show a slight resemblance to a well-eratered lunar surface. Individual sinks range from small to large, shallow to deep; slopes are smooth, rounded, or jagged, and fiat, eoneave, or convex. They resemble plates, saucers, bowls, funnels, spoons, and immense drill holes. A few exhibit extreme asymmetry in profile and outline, and some extreme eccentricity with respect to ground zero or the low point. The extreme dimensions of sinks in Yucca Flat are 3-250 m in radius and 1 to nearly 60 m in depth. The larger sinks represent volumes approaching 2.8 X 106 cubic meters.

Far from being random, the size, shape, and structure of the sinks all appear at this stage of our knowledge to be results of: (1) the size and shape of the cavity, and (2) the type and thickness of the overlying rock material--and perhaps quite s!gnifi- eantly, its conditioning by the explosion. When comparisons are made in these terms, the differences among the sinks begin to blend and a systemmatie variation emerges.

Morphology. A classification of the morphology of the more common forms of sinks - - the saucer- and funnel-type sinks--has been derived from detailed field study and photogrammetric mapping of several sinks in northern and central Yucca Flat. Within most sinks and surrounding areas, five zones are defined and are numbered consecu- tively outward (Figures 3 and 4). Different proportions of the morphologic zones are recognizable in at least 90 per cent of the sinks formed to date; the remainder are those in which zonal definition has been impossible owing to coverage by surfieial landslides formed in the final stage of subsidence.

Zone 1 comprises the eentral crudely circular part of the sink with maximum downward displacement. Zone 1 may be flat or may have the shape of a saucer, bowl, or funnel. The interior of zone 1 is cut by anastomotic and irregular, generally radial fractures. These commonly are represented on the vertically displaced surface by low pressure ridges of loose material; net vertical offset on these fractures ranges from


aImost nothing to about a meter, but most show less than 2 decimeters. Par t or all of % " (Varnes, 1958), zone 1 may be eovered with landslides, commonly as ~ana runs"

that are formed by oversteepening of slopes higher in the sink. The outer limit of

~ V - - r - - e o m = - 7 - ~

A

2rn. 6Z ~ r n . C

GZ I - - . ~ I I r n . - ' ~

5m. 60m.

~zq ~ - 0 --400m. . - - , E ~ °'~ . / - " - - - ~ ,

E (U3 on)

~ z F

~-~___._. f IZSm: i . > - - ~

G

- - 1 8 0 r n : S J

H (U3 bj)

1 2 O r e : - - ~ f " " - - " ' - ' ~

58m

~ 1 Note :Variable scale (in meters)

FIG. 2. A variety of sink profiles.

zone 1 is marked by either curved faults or monoelines--abruptly steepened slopes that are approximately concentric to the central, low part of the zone.

Sink depth is measured to the lowest point in zone 1. Where this point is on the preshot surface as evidenced by eorrelatable preshot pads, roads, or vegetation, as it is for sinks of moderate to small relief, the depth is a true indication of amount of vertieaI drop below the sink edge. In sinks of greater relief, where the bot tom is covered

2236 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AI~'IERIC&

extensively with landslide debris, the true structural depth can be estimated by projecting the slopes of the sink that are clearly part of the preshot Yucca Valley surface.

Zone 2 is characterized by tilted fault blocks of remarkably consistent inward dip toward the low point of the sink. The zone may be circular or somewhat irregular in outline and highly variable in width. The blocks are outlined by intersecting and sub- parallel concentric faults and fractures that dip steeply outward, at least in the near surface. Two types of structures are present as the basis of the predominant relationship of these blocks (Figure 4, example A). In one type, the inner block along these faults most commonly shows downward movement with respect to the adjacent outer

EXPLANATION

FauH scarp or series of closely spaced scarps showing down- dropped side

\ \ I I

/ I

J I

I /

Zone [imils

i Monofiline

Fault showing down- d r o p p e d b l o c k .

D a s h e d w h e r e

i~defini~e

Joints

Limit of major ~ " . ZONE q / . . . . ZONE 5 concentric sink " -~ - - -

f r a c t u r e s

0 30 60 90 120 150 METERS I _ _ I I 2 _ _ t _ _ J

FIe. 3. Morphology of a sink in east-central Yucca Flat , Nevada Test Site.

block or, in other words, reverse fault movement (Figure 4, example B). The other type has the inverse relationship giving rise to normal fault movement between blocks (Figure 4, example C). Both types are the result of rotation of the blocks as they moved down and toward the center of the sink; their difference probably reflects a basic difference in the curvature of the underlying zone of failure. The major faults in zone 2 generally range from a few centimeters to 3 meters in vertical offset.

Zone 3 includes the blocks of alluvium that have subsided but have not rotated, or that have rotated through only a few degrees of are toward the center of the sink. Individual blocks are bounded by intersecting concentric faults and fractures of varied curvature (Figure 5). The tops of the blocks are virtually level or dip slightly, earn- manly inward, rarely outward. As a result of differential downward movement of the blocks, grabens are common and horsts are few.


Zone 4 is immediately outside the area of major subsidence and contains the most remote evidence of the sink. The zone can be as wide as 15 m on the larger sinks-- being widest in sinks of little relief and narrowest in sinks of greatest relief. The normal structural features of zone 4 are concentric fractures like those in zone 3. As expect- able, these are more numerous near the inner boundary and become more sparse outward. Vertical offset is generally absent along most fractures throughout the zone, but minor differences in elevation are present between the outer and inner limits.

Zone 5 consists of the original Yucca Flat terrain around the sink. At some sinks in Yucca Flat, this zone--out to a few hundreds of meters and rarely to thousands of meters--includes fractures related directly to the detonation. Other fractures are re-

Ground ze(o

__•_•oFie 1 5 -~--ZoneC~*-Zone5 ,i "~ Zone 2 I . ~ _] _ _ _ _ I Oriqinal voll_~ surface

A,

" Zone I t"

ondslide debris /

. Emplacement rest. hole /

Two basic structural varieties occur in zone 2 A

I J

B. C. FIG. 4. Generalized morphologic zones of a typical sink in Area 3, Yucca Flat , Nevada Test Site.

lated indirectly to movement induced in the underlying tuff bedrock, or result indirectly from the subsidence that forms the sink.

Displacements in and around sinks. Vertical and horizontal displacement of the ground surface in the sinks and in the surfaces outside the sinks results from both the explosion and the subsidence. Examples of explosion-released movement are numerous along major natural faults, such as the Area 3 and Yucca faults in Yucca Flat, and such movement has been experienced as far as 500 W 1/3 and 1,000 W 1/3 feet from many tests (Dickey, 1968). Structural study of the amount, rate, and sequence of movement in or close to the sinks provides important clues as to the deeper subsurface movements tha t may be taking place during collapse.

Vertical displacement in zones 4 and 5 that is probably related to the explosion effects. Precision topographic maps show also that much of zones 4 and 5 around most sinks

2238 B U L L E T I N O F T H E SEISI~IOLOGICAL S O C I E T Y O F A-~IERICzk

is lower than the same area was before the test. The amount of change decreases outward; isopaehs of the change are grossly concentric to the sink, but show the valley surface to be more undulatory than it was preshot. The maximum vertical depression measured to date is 6 m at the edge of a large sink in central Yucca Flat. Depressed areas are volumetrically large for intermediate (200-1,000 kt) yield tests and progressively smaller for low-intermediate (20 200 kt) and low (<2 0 kt) yield tests. For intermediate yield events in tuff with alluvium cover, the volume approaches 50 per cent of the volume of the sink. The information analyzed to date suggests

zonos "',

/" // Zone3 ( / " " -- ~ , ~-~ .%. \ \ t/Zone i f / . " \'xXN~ $.\ \

I ' | 1 / / Zone 2 ~\ \ \ ' ~ \ \ , j l \ ~ .

/ / ~ ~ \ " I f i r , zonal \ ] . , / ,, I I I I I , I p \ I \ ~ I ~

,y,: /l )j I ( ~ G Z / I I

.L ',,i( t \ ',,\ _ . ) / / / j / ,;

\ \ l g /

. , * * \ ,.,////?

0 30 60 METERS I I I

Approximate scale

FIG. 5. Typical distribution of concentric fractures of zones 2, 3, and 4 in a sink in northern Yucca Flat, Nevada Test Site.

tha t these depressions are related to the explosion rather than to subsidence. Compac- tion of alluvium by the explosion seems to be the prime candidate, inasmuch as a propo,tionate amount of depression is not observed for tests in more competent rock such as tha t in Pahute l~esa.

Permanent displacements in sink sw]ace. The amount of vertical movement in a given sink is well generalized by the topographic configuration in most sinks. Excep- tions are (1~ where late slides and falls of material from oversteepened slopes now rest upon the former ground surface; and (2) those parts of the sink where there has been a relatively large horizontal component of movement.


Surfieial landslides, block falls, and sand flows are confined to the sinks of moderate to high relief (ratio of radius to depth, R~/D,, < 4.0). Block falls are common in moderate-relief sinks that for other reasons have faults of large offset in zone 3 or at the zones 3 and 4 boundary. Sand and debris flows are found on low slopes and in zone 1 of sinks with R~/D~ ratios less than 3.0. As the ratio to depth decreases, more block falls and debris flows will cover more and more of the slopes. Below a R~/D, ratio of 2.5, the entire sink slope will be covered by surfieial landslides. Quite naturally, accurate determination of the depths, radii, or structural zones for definition of the subsidence in these sinks is impossible.

Detailed preshot and postshot photogrammetrie mapping of a sink in east-central Yucca Flat (R~ = 137 m, D, = 27 m) has shown significant horizontal components of inward movement in the various morphologie zones (Figure 6). The amount of horizontal movement, as shown by preliminary analysis of more than 200 control points in this sink, is greatest in zone 2; it decreases in both radial directions to nearly a meter near the lowest part of zone 1 and to a few eentimeters in zone 4. Similar orders

Zone

I =ooo ! =ooe, I =V I zo°e ,=one, zooe - - 2 Preshot surface 3 ~ 4~ 5 - -

( Projection from [

Depth, Irneters) 0

50

60

,90

0 30 60 90 METERS I ~ ~ L _ _ I (Arrows indicating movement

Gre exaggerated 4X)

FIG. 6. Surface movement in a typical sink of moderate relief, east-central Yucca Flat.

of magnitude of horizontal inward movement have been confirmed at several other sinks in Yucca Flat.

I t seems significant that at a late phase of collapse the surface was responding to a void which was much closer to the surface than the shot cavity and which was of smaller lateral dimension than the sink. The resultant movement direction displayed in Figure 6 is not directed toward the shot cavity, which was at a depth of 434 m (projections are shown) except within zone 1 where the dominant movement is, of course, downward. Rather, the projections in zones 2 and 3 are toward a region about 100 to 170 m beneath zone 1.

Subsurface structure. Little has been observed of the subsurface structure of sinks. Exposures are rare at depths greater than 2 m and they are unknown at depths greater than 6 m; most are the result of erosion. At this stage, much is to be inferred from the displacements and from the following description of near-surf nee mapping and its interpretation on the basis of landslide experience. A number of struetural features of importance to subsurface interpretation have been mapped at one sink in east-central Yucca Flat, U3co, and confirmed by earing at another in the same area, U3cn.

2240 B U L L E T I N O F T H E S E I S ~ I O L O G I C A L S O C I E T Y O F A M E R I C A

The basic structure at U3eo shows that the outer part of zone 2 is underlain by an inward-dipping failure surface, that the outer part of zone 2 is a slide block (Figure 7), that zone 3 consists of subsidiary slump blocks, and that zone 4 has cracked tensionally and moved slightly in response to lateral release of pressure. The surface of failure beneath zone 2 is a shear that intersects the ground surface at the zones 2 and 3 boundary at relatively low angles (20 ° to 25°), but shows the increased inward dip (~ 45 °) where last observed about one-third the radial distance inward from zone 3. The alluvium in the slump block above the shear dips inward at angles tha t parallel the sink slope defined in zone 2. The alluvium below the shear is undisoriented and fiat-lying. Zone 3 alluvium is fractured and faulted at high angles, and its movement downward and inward suggests a zone of failure at shallow depth.

--'l~" --Zone I GZ /

]

I

,I

/ /

Assumed chimney [ Projection from side of boundGry shot cavify to sink edge

I I

/DEPTH

I - °

/ / Surface of failure [ 20

/ /

/ /

30

40

!

5O

0 iO 20 METERS I i I

FIG. 7. Profile of typical sink of moderate relief showing relation of shear underlying sink with projections from shot cavity.

The hypothesis that this shear marked the subsurface boundary of the collapsed material was again tested by coring vertically through the U3en sink about half of the radius from GZ so as to be outside the shot cavity radius. From study of the attitudes of the alluvium in core, the shear at the base of zone 2 is estimated to be about 45 m deep. From this information an att i tude and position of the shear has been inferred that is similar to that at U3co.

Analogies of the structure in the outer part of the sink to normal landslides would seem appropriate and justifiable, and if such analogies are correct in principle, then many of the methods and interpretations used in landslide analysis can be applied to provide inferences (Baker and Yoder, 1958; Ritchie, 1958; Varnes, 1958) of deeper subsurface structures in the more central parts of the sinks. Although study is in progress on the geometries, we may be justified, on the above basis, in projecting the failure surface under zone 2 nearly to a circular zone beneath the zones 1 and 2 boundary.

Movement during subsidence. The sequence of movement during the subsidence of

S U B S I D E N C E R E L A T E D TO U N D E R G R O U N D N U C L E A R E X P L O S I O N S 2241

sinks has provided an additional basis for subsurface structural interpretation. A number of aerial oblique motion-picture films have been studied for general sequence of activity. Enlarged positive prints of individual frames from one film were used to measure the beginning, ending, and amount of movement and fracturing in different parts of the sink. The relative amount of fracturing in each zone was estimated and is portrayed in Figure 8 by the width of the symbol. The times of opening of fractures are roughly indicated by the appearance of shadows in the film; their closing is rea- sonably well indicated by dust fountains along the traces of the fractures in zones 2 and 3. Key points of activity presented in Figure 8 are summarized below in relation to time after first subsidence of the surface was detected near ground zero.

Zone l

Zone 2

Zone 3 ~

Zone 4 -~

"Movement Downward

F?c~dlol Fractures

Forming

Closing' b

Movement

Inward

Downward

Concentric Fractures

Forming

. Closing

Movement

Inward and downward

Concentric Fractures Forming

Closing

Movement and fractures

Seconds after f irst detected movement in sfte area

0.0 1.0 2.0 5.0

- 4

.Ill.

imm||

D

Not determined

mmmlm

mmmn

FIG. 8. Chronology of movement and fracturing during subsidence of a sink in Area 3, Yucca Flat, Nevada Test Site.

(1) First movement of the ground surface at the site was in zone 1. Average acceleration was 8 m/see/see or 0.8 g, and average velocity was slightly more than 7 m/see.

(2) Zone 2 movement started about 0.25 see after zone 1--horizontally inward first, then downward. Both components of movement ended at the same time. Ve- locities ranged from 4.9 m/see near zone 1 to 2.1 m/see near zone 3.

(3) Zone 3 movement appears to have started about the time the inner part of zone 2 stopped.

(4) Radial fractures of zone 1 formed and elosed mainly during the major part of zone 1 movement- -probably in response to increasing lateral pressure from inward zone 2 movement.

(5) Concentric fractures in zone 2 likewise formed and closed during the major par t

22Zi2 BULLETIN OF TI-IE SEISMOLOGICAL SOCIETY OF AMERIC3_

of that zone's movement, starting on the inner side and proceeding to the outer boundary.

(6) Concentric fractures of zone 3 formed and closed in a comparatively brief time interval and, as those in zone 2, did so progressively outward.

(7) Concentric fractures of zone 4 were not detected separately; they are assumed to have opened after zone 3 movement. Surface mapping shows that most do not close.

VERTICAL EXTENT AND ]~OR~[ OF COLLAPSE AS RELATED TO EXPLOSION CAVITY

Chimneys and sinks. The cause of collapse--the void of the explosion cavity--quite naturally exerts a strong influence on the vertical extent and shape of the collapsed

/ / / i ' ncavty Frustum model Cylinder model

FIG. 9. Two chimney models for collapse of underground nuclear explosion cavities.

material. As might be expected, the size of the cavity largely determines the volume of the collapsed material and the volume of the sink. Other things being equal, the larger the cavity the larger will be the chimney and the sink. There is some suggestion, too, that the amount or degree of collapse may increase disproportionately with larger cavities. Then, too, tile shape of tile cavity has a marked effect on the type of subsidence.

There are two current schools of thought regarding the shape of the final collapse zone--or chimney--beneath the sink (Figure 9). One holds the shape to be an inverted cone frustum (W. W. Hakala, unpub, rept., 1968; R. It. Berry and W. W. Hakala, unpub, rept., 1968; D. E. Rawson and R. F. Rohrer, unpub, rept., 1968). The other holds that the collapse zone is fundamentally a right cylinder with a radius equal to, or slightly greater than, the cavity radius (Boardman et al, 1964; Piper and Stead, 1965; and Witherspoon, 1966). If collapse does not extend to the surface, the chimney terminates upward in some form of dome. Intuitively, one would expect this to b e accomplished most easily by the cylindrical model (requiring less of a span) and


awkwardly by the frustum--depending on how far its slopes had gone astray from the vertical. For purposes of this report and for reasons beyond the scope of this report, the cylindrical model of chimney collapse is assumed.

The void represented by the original explosion cavity is transmitted upward during collapse, but it does not appear to be distributed evenly as additive porosity throughout the vertical extent of the chimney. This is shown by: the general lack of voids and the compactness of the matrix in the region of the cavity, the abundance of voids in the region about two cavity radii above the zero point, and an upward decrease in structural relief. In competent rock such as densely welded tuff and granite the additive porosity would be more evenly distributed by bulking of the rock, but over a shorter vertical range of chimney. At two sites, the chimneys collapsed from pumiceous vitric tuff into overlying densely welded tuff and were considerably foreshortened as a result. Preliminary calculations show that other things being equal (such as cavity

Sink rodius (metere) Sink depth ( m e t e r s )

60

120

1

180 l

240

3.2 5.8 9.5

SINK D E P T H VERTICALLY ,

I

© I

16 24

EXAGGERATED 4

0 eO METERS I I I

> 50

TIMES

FIG. I0. Graphic comparison of ratio of sink radius to sink depth resulting from a 15 meter radius explosion cavity at various depths in Area 3 alluvium, Yucca Flat, Nevada Test Site.

size and depth), the height of chimney growth will be 3.5 times greater in vitric tuff than in welded tuff, and 1.5 times greater than in zeolitized tuff.

The typical sinks developed from the spherical cavity are the saucer or funnel types (Figure 2, C, D, F, and G) ; the systemmatie variation among these sinks as a function of depth of burial is shown in Figure i0.

As a result of any one or several emplacement or geologic situations, nonsphericity of the explosion cavities at Nevada Test Site has taken two general forms: (i) elonga- tion along the vertical axis, forming voids with a shape approaching some variation of a cylinder with hemispherical ends, and (2) enlargement in a horizontal plane, creating a form of ellipsoid. These departures from sphericity probably account for many of the variations observed in surface subsidence. Sinks with similar configurations can originate in other ways, such as through greater bulking attendant with competent rock, or late partial collapse of apical voids. The results described, therefore, are based on assumptions of homogeneous alluvium and scaled depths of burial between 120 and 170 m (situations that generally lead to surface collapse during the main collapse of the cavity).

2244 B U L L E T I N OF T H E S E I S M O L O G I C A L SOCIETY OF A M E R I C A

In general, sinks resulting from collapse of cavities that had been elongated in a vertical direction are commonly steep sided and deep relative to radius. Examples to date (similar to those illustrated in Figure 2, A and B) are those sinks of relatively small radius, generally less than 25 m. The ratios of radius to depth are less than 3.0, and generally less than 2.5, discounting later collapse of the sides. The shape of the steep-sided sinks over vertically elongated cavities are analogous to the "pit-holes" in alluvium over shallow coal mines in Indiana (Young and Stoek, 1916, p. 46-47), pit craters along the southeast rift zone of Kilauea, Hawaii, and the high narrow collapse at the Athens mine, Negaunee, Michigan, described by Allen (1934).

Sinks resulting from collapse of cavities enlarged in horizontal dimension are generally broad. The broad configuration of the Bilby sink (Figure 2, E) is considered to have resulted in part from a flattened cavity, although part is also the result of bulking of the large proportion (60 percent) of tuff in the collapse medium. Chimneys formed over broad cavities may particulate less and aetually drop more as a mass, somewhat analogous to some broad subsidences over subhorizontal coal mines throughout parts of the United States and Europe (Young and Stoek, 1916; Briggs, 1929).

Effect of explosion cavity depth. Two features of collapse show an influenee of depth of the cavity: (1) the sink profile, expressed by the ratio of sink radius/sink depth (R,/D,), and (2) the height of chimney collapse of deeply buried tests where no initial surface subsidence occurs. To date, only tests in alluvium that presumably have given rise to spherical eavities have been studied and the following discussion pertains solely to results under such conditions unIess otherwise noted.

Sink profile. As expressed by the ratio of R,/D,, sink profile increases directly with increasing depth (Houser, unpub, rept., 1969) of explosion cavities of a given size. The sink radius (outer limit of zone 4 is used currently by this writer) shows a good correlation with cavity size (and yield, since there is a dependence between the two) regardless of depth throughout the range of detectible sinks in homogeneous alluvium. The close cavity size-sink radius relationship, therefore, implies that sink depth varies inversely with increasing depth of burial. In effect, as the depth to the cavity increases, the depth of the sink imparted by a cavity of a given size decreases. Let's take an example.

If one visualizes a spherical cavity of 15 m in radius buffed in unsaturated alluvium of east-eentral Yucca Fiat at a depth of 120 m, the sink formed above the cavity would have a radius of about 38 m and a depth of about 12 m (Figure 10). If the cavity were at a depth of 150 m, the sink depth would be about 4= m; at 185 m, 1.5 m deep; and at 215 m, about 0.5 m. If the cavity is at a depth greater than 215 m, the sink really lacks any structure by which to identify it, and the subtle topographic depression typical of zones 4 and 5 (detectable only by surveying) may obscure its existence.

As the sink radius remains virtually eonstant with the increased depth of burial, and the sink depth decreases, it is obvious that the sink volume also varies inversely with increasing depth of burial. For a depth of burial of 105 m in our example, the sink volume would be at its maximum and equal to about the cavity volume.

Chimney height. None of a selected group of tests studied that were scaled deeper than 200 W 1In meters in alluvium of northern Yucca Flat is known to have collapsed to the surface; no test at less than 160 W */a meters is known not to have eventually collapsed ta the surface. For chimneys (collapse not reaching ground surface), Board- man et al, (1964) have found chimney height to be 4.3.5 and 5.3 times the cavity radius, respeetively, for collapses in granite and tuff. Reeent studies I made of collapses in alluvium and tuff show that the deeper a cavity of a given size, the shorter

SUBSIDENCE RELATED TO UNDERGROUND NUCLEAR EXPLOSIONS 2 2 4 5

will be the chimney. On the basis of meager in format ion for tests in a l luv ium of nor th-

ern Yucca Fla t , (a) m a x i m u m ch imney deve lopment could be accomplished with

bur ia l at abou t 150 W 1/3 meters, and (b) no ch imney would develop above cavities

TABLE 1

SUMMARY OF A GRADATION IN CRATERING AND COLLAPSE EFFECTS RESULTING FROM INCREASING

SCALED DEPTH

[See Figure 11 for graphic presentation of the data described here]

Approximate Scaled Characteristic Effects Depth

(meters)

Effects quoted and slightly modified from Stead (1969, Table 1)

0

0-3

3-60

60-90

"Detonation at land surface . . . Small shallow crater forms through compaction of soil and rock, and scour by the extremely turbulent gases of the fireball." " . . . Size of crater increases s l i g h t l y . . . " over that produced from detonation at surface.

"Throwout crater forms. As scaled depth increases, air blast and the fraction of radioactivity and hot gases released to the atmosphere gradually diminish while the seismic forces increase." At scaled depth of 45 meters " . . . Throwout crater of maximum volume, its depth being about half that of the point of burst . . . . As scaled depth iucreases further, diameter add volume of crater diminish."

No throwout crater formed by ejection of rock fragments along ballistic trajectories. " . . . Additionally, over a narrow range of Scaled depth from about 200 [60] to possibly as high as 250 feet [75 meters], a mounding crater (also ~ermed a retarc, from crater spelled backward) may be formed from bulking of rock fragments during fallback which causes the true crater to overfill above the original ground surface."

Effects abstracted from study by Houser in 1969 (unpub. data) of low-yield tests forming relatively spherical cavities in northern Yucca Flat alluvium and given as examples of a continuum that will telescope markedly depending on rock type and, perhaps, yield.

90-140

140-175

175-200

200-400

> 400

Sinks form with average ratio of radius/depth (R~/Ds) = 4.6. [Three of four examples vented.]

Sinks will probably form over a majority (~60 percent) of tests within 24 hours of detonation with average R~/Ds = 12.

Chimney collapse over remaining tests will range in height from 7.5 to 12 times radius of cavity (Re) within 24 hours after detonation, and will extend to surface within several years or when shocked by tests within 185 W ~ meters slant range.

Sinks probably will not form within 24 hours of detonation; chimneys will collapse to heights ranging frbm 5 to 7.5 times cavity radius. About half of the chimneys will eventually collapse to surface and form steepsided sinks of Rs < 15 meters and D~ < 7.5 W -~ meters at least as long as 6½ years later.

Chimney collapse, but no sinks formed during 24-hour period after detonation. Chimneys will range in height from > 1 to 5 times Rc; no examples have subsequently

collapsed to surface. Collapse of small to moderate-sized cavities may not occur until a few weeks after

detonation. Late chimney collapse probably occurs, especially if cavity shocked by nearby tests or eroded by ground water.

exceeding abou t 400 W 1/3 meters. The reduct ion of chimney height relat ive to cavi ty size with increased scaled depth of bur ia l is propor t ional ly much greater in a l luv ium t h a n in tuff.

Similar relat ions p robab ly exist for other rock media, except t h a t surface react ion is more sensit ive to increases in thickness of competen t rock (sonic velocities in excess

2246 B U L L E T I N O F T H E S E I S M O L O G I C A L S O C I E T Y O F A M E R I C A

of 3,000 m/see). Where competent rock comprises a considerable proportion of the total thickness of the collapse medium, no sink will form at sealed depths greater than about 120 m for cavities as large as about 100 m in radius.

A gradation in cratering and collapse effects with depth of burial. We have considered the effect of cavity depth on both the vertical extent of chimney growth and sink profile in a relatively homogeneous medium, such as alluvium. The variations described can be seen to be a rather continuous extension of the variation observable in surface craters ranging from a burst at the surface to one at greater than optimum crater depths (Nordyke, 1961), and an expansion of the efteets described by others (Piper and Stead, 1965; Teller et al, 1968). This gradation is summarized in Table 1, and

I (o)

i

/ \

i/~ J , ,

i-",+ z ioo) , , I

I I I I ' I \ I i . I

"~ ~ { t 7/(i70 )

Appfroximafe scaled depth of burial (WTmeters) in parentheses.

Figures A through D are modified from Nordyke 0961, p.3452); E on basis of Stead (1969)~ F through L are determined on basis of recent study of unpubtished data.

B C D E

{>~0) , ~.// \\,~it i ! O H (40) " ~"~ ̂ , \ [ / '

, I t * °u~ L ~ I ( 6 0 )

(t'~, I | I I 1 I

t J t, '~ t ~ )~(2oo)

l Ik _A I l I. -A', I l : I ' t 2 . . (240)

z f - \ i . . ' " . ",

I

/ ' b ) ( 3 7 o ) %....

FIG. 11. Generalized gradatioll in cratering and initial collapse effects resulting from explosions of the same yield at different depths of burial in alluvium, at the Nevada Test Site.

generalized examples are given in Figure 11. Such an elaboration is possible because study has been confined to one medium- alluvium. Current studies by the U.S. Geological Survey are aimed at developing a better understanding of a segment (sealed depth > 90 m) of this gradation in terms of the various types of tufts and other media, with the hope that eventually these generalizations can be made more precise for other rock types as well.

A POSSIBLE MODEL OF COLLAPSE

The cause of the formation of chimneys and sinks over underground nuclear tests is, of course, collapse of the explosion cavity. For purposes of discussion, the collapse period begins at the time when first major collapse of the cavity begins. This is taken to be at the initiation of the final seismic signal tha t identifies activity that leads to either surface collapse or to cessation of significant close-in seismic activity for those

SUBSIDENCE RELATED TO UNDERGROUND NUCLEAR EXPLOSIONS 2 2 4 7

collapses not extending to the surface. It is assumed that collapse occurs when pressure in the cavity has decreased to a point where the explosion-modified strength of the rock medium can no longer sustain the upper dome of the cavity.

The time that collapse occurs after detonation has been investigated in relation to yield, cavity size, depth of burial, general type of medium, and other factors. The best correlation to date seems to be between time of surface collapse and depth of burial (D. L. Orphal, unpub, rept., 1968), and correlations improve when the tests in alluvium and tuff are separately considered.

Mode of chimney collapse. Measurements of the rate of chimney collapse in alluvium, coupled with the chronology of subsidence of the sink at the surface, suggest that collapse may occur in two main structural phases. For present purposes, the earlier phase is termed the "particulate" phase and the later phase is termed the "massive" phase. The massive phase may occur only with surface collapse but this is not known. There are indications from some analyses now in progress that the collapse zone affected by the particulation phase may relate closely to Chimney height in those collapses not extending to the surface.

During collapses to the surface in Yucca Flat, both phases are accomplished within a time interval consisting of several seconds to more than 10 see, depending mainly upon the depth of the cavity and to some extent the rock type. Our only experience on rate of propagation is for collapses in alluvium (D. E. Rawson and R. F. Rohrer, unpub, rept., 1968; Sisemore, 1968; W. R. Perret, written commun., 1968). The par- tieulate phase comprises the first 70-90 percent of the total collapse period in material of an incompetent nature such as alluvium or vitric tuff. The rate of particulate collapse in dry alluvium at one site in northern Yucca Flat, U2bd, was about 15 m/see, and in the saturated alluvium of the Faultless site, 25 m/see--two remarkably similar rates despite vast differences in chimney radius and water content of the rocks.

After the particulate phase is completed, either the collapse medium will dome and collapse will cease, or the massive phase will propagate to the surface. The distinction as to whether and where massive collapse takes over is due probably in part to the level of seismic energy at the time and in part to the thickness and competence of the uncollapsed alluvium or other rock material relative to the diameter and domal curvature of the chimney. The term "massive" is not meant to imply that the collapsing material necessarily falls completely intact, but is used tentatively to characterize the dominant differences between the two phases.

2~leager information would suggest that the two phases differ in rate of propagation -- the rate for the massive phase being at least several times faster than the rate during the particulate phase. Propagation rate is a function of the size of the fragments and the frequency with which they break away. The average fragment size (one dimension) during the massive phase approaches twice that of the particulate phase but may be much larger.

The topmost 30 m or so of the alluvium is thought to subside more or less as a unit, termed here the "central plug" of the chimney. The top of the central plug is the bottom of the lower portion of the sink after surface subsidence, and is approximately equal to zone 1. The massive phase, therefore, might well be divided into two subphases. The first subphase may or may not be present and, if it is present, it does not drop as a discrete mass. The second subphase is always present for initial collapses extending to the surface and does drop as a discrete mass. Whether a central plug is typical of chimney collapse that does not reach the surface is not known. Careful study of posteollapse drilling records may help in this regard.

Subsidence of the central plug (zone 1), then, is the last part of the chimney col-

2248 BULLETIN OF THE SEIS~IOLOGICAL SOCIETY OF AMERICA

lapse and it initiates the sink subsidence. The size and shape of the material involved in the massive phase, especially the central plug, and the vertical distance of drop are considered the critical dimensions that determine the size and configuration of the sink.

At this point in our interpretation of the subsurface structure, the shape and size of the central plug can only be inferred. The shape would range from cylindrical to an upward-converging cone frustum. The cylindricai shape results in steep-sided sinks. The frustum shape is believed basic to the more common sinks with sloping sides--the "saucer" or "funnel" types. For sinks of great relief, the convergence of the sides of the cone frustum (central plug) is probably relatively stuN1 and the vertical drop of the plug relatively great; in sinks of moderate to little relief, the convergence of the sides increases progressively and the vertical drop decreases progressively.

SUMMARY

For purposes of summary, the activities of subsurface collapse and surface subsidence are integrated chronologically, beginning with the explosion, into four periods: (1) detonation, (2) precollapse, (3) collapse, and (4) postcollapse. Individual activities for these periods are noted in Figure 12.

(1) Detonation period (as much as severn hundred milliseconds in dura t ion) - vaporization, melting, compaction, and massive radial displacement create cavity; rarefaction waves reflected from ground surface may impinge upon cavity during initial growth and may promote additional preferential cavity growth. Size of cavity is determined by explosion energy level, rock medium including water content, and overburden pressure; shape of cavity is controlled or influenced in large part by geologic inhomogeneity, rarefaction, and emplacement methods.

At the end of the detonation period the medium is fractured, especially above the cavity, and compacted. In thick alluvium the ground surface around GZ may have been depressed as the surface expression of this compaction; for intermediate yields (200 to 1,000 kt) the maximum vertical depression at GZ can be in excess of 6 m, and the volume can be as much as 50 per cent of the final sink volume.

1. Activities noted in Figure 12. Detonation period: (1) Initial cavity growth. (2) Initial radial fractures. (3) Ground rise and movement on nearby natural fractures, if any. (4) Surface fractures. (5) Rarefaction fracturing. (6) Surface depression probably due to compaction. (7) Rarefaction enlargement of cavity.

Precollapse period: (8) Strain release during P (pressure) and T (temperature) decrease; molten rock

drains to cavity bottom. (9) Possible late strain release high in site area.

(10) Cavity sides collapse, speeding P and T decrease and rendering upper dome less stable.

Collapse period: (11) Main eoUapse--"particulate" phase. (12) "Particulate" phase ends ~ distance to surface with apical void. (If cavity

had been somewhat deeper, collapse period would have ended here.) (13) Continuation of main collapse--early "massive" phase flaring by inward

radial shearing into apical void.


(14) Late "massive" phase--central plug becomes unstable from radial under- cutting and drops, beginning zoned surface subsidence.

A.

f 1

I >'l /

B.

\

o r

\ ¢

/ / / \ ",,

C ,

FIo . 12. C h rono l og i ca l s u m m a r y of co l l apse - r e l a t ed ac t iv i t i e s .

(15) Inward and downward movement progresses outward through zone 2, then zone 3.

(16) Inward movement of zone 4 blocks, culminating sink movement. (2) Precollapse period (several minutes to several hours)--pressure and tempera-

rare decrease steadily; much seismic activity related probably to (a) strain release in

2250 BULLETIN OF THE SEISSIIOLOGICAL SOCIETY OF ASIERICA

vicinity of cavity, (b) movement along nearby faults and fractures, and (e) minor spalling of cavity walls. Just before collapse, seismic activity may spread to other levels higher in site; strain release along faults and fractures may continue, and the cavity pressure and temperature may drop rapidly. Horizontal enlargement of cavity by inward collapse of sides may occur now; this subordinate collapse probably ac- counts for the increase in chimney radius over cavity radius, commonly 5-15 per cent.

(3) Coliapse period (several seconds to several tens of seeonds)--collapse of upper- cavity dome initiates the first or "particulate" phase of chimney propagation toward the surface at rates, in alluvium, on the order of 15-25 m/see. Top of zone that is defined by the particulate phase is probably top of the chimney for collapses not extending to the surface. Sides of chimney are probably cylindrical and radius of chimney probably exceeds cavity radius by about an average factor of 0.1. For tests that are greatly overburied, the height of chimney is probably foreshortened; in extreme eases (for example: depths > 400 W 1/a meters in some alluvium) collapse may not occur or may be so minor as to preclude precise seismic determination. For tests at less than 180 W 1/a meters in alluvium (about 120 W lta meters in volcanic rocks) the second or "massive" phase of chimney collapse begins during the last quarter of the collapse period, with a propagation rate at least 3 times faster than during the particulate phase.

Chimney may be cylindrical and, if so, early massive phase includes a flaring of the domal top of the chimney by inward radial subsidence (displacement) of material into the existing apical void, which enlarges upper part of chimney into the form of an inverted 445 ° cone frustum.

Dropping of the "central plug"--the final action of massive phase and the initial action of sink subsidence--begins when flared dome has enlarged beyond the point of stability. Plug commonly drops at less than acceleration of gravity (three known determinations range from 0.35 to 0.8 g), perhaps because inward radial displacement during flaring is still in progress. Size and shape of plug and amount of vertical drop are primary factors in determining size and shape of sink. Landslide movement begins in sink zones 2, 3, and 4 and fracturing progresses outward from central plug as inner stress is removed. Fractures are formed and closed progressively outward as dictated by amount of movement and the strength of the material.

(4) Postcollapse period (infinite period of time)--additional surface subsidence of sink or parts of sink occurs as result of collapse of small voids remaining under bridged spaces in chimney. This is particularly common where later water inflow into chimney occurs from either surface or from saturated rocks in subsurface. For subsurface chimneys, periodic collapse occurs especially when tests are conducted within a distance of 185 W I/~ meters. Many subsurface chimneys collapse to the surface (as much as 7 years later), mainly as steep-sided sinks (pot-holes) that may continue to show minor enlargement in the future as new stress is applied to bridges in chimney. For tests in northern Yucca Flat alluvium, no test scaled deeper than 200 W ~/a meters has yet been known to collapse to the surface; no test at less than 160 W 1/a meters is known not to have eventually collapsed to the surface.

REFERENCES

Allen, C. W. (1934). Subsidence resulting from the Athens system of mining at Negaunee, Michi- gan, Trans. Am. Inst. Mining Metall. Eng. 109, 195-202.

Baker, R. F. and E. J. Yoder (1958). Stability analyses and design of control methods, Chapter 9, of Landslides and engineering practice, Ed. by E. B. Eckel, Natl. Acad. Set., Natl. Res. Coun- cil, Highway Res. Board Spec. Rept. 29, 189-216.

Boardman, C. g . , D. D. gabb and g . D. iV[cArthur (1964). Responses of four rock mediums to contained nuclear explosions, J. Geophys. Res. 69, 3457-3469.


Briggs, H. (1929). Mining subsidence, Edward Arnold and Co., London, 215 pp. Dickey, D. D. (1968). Faul t displacement as a result of underground nuclear explosions, in Ne-

vada Test Site, Ed. by Eckel, E. B., Geol. Soc. Am. Memoir 110, 219-232. Germain, L. S. and J. S. Kahn (1969). Phenomenology and containment of underground nuclear

explosions, Chapter 4, in U.S. Atomic Energy Comm. Rept. NVO-40. Griggs, D. and E. Teller (1956). Deep underground test shots, California Univ., Livermore, Lau-

rence Radiation Lab. Rept. UCRL-4659, 9 pp. Houser, F. N. (1968). Application of geology to underground nuclear testing, Nevada Test Sit%

in Nevada Test Site, Ed. by E. B. Eckel, Geol. Soc. Am. Memoir 110, 21-33. Houser, F. N. and E. B. Eckel (1962). Possible engineering uses of subsidence induced by contained

underground nuclear explosions, in Geological Survey research I962, U.S. Geol. Survey Prof. Paper 450-C, C17-C18.

Nordyke, M. D. (1961). Nuclear craters and preliminary theory of the mechanics of explosive crater formation, J. Geophys. Res. 66, 3439-3459.

Piper, A. M. and F. W. Stead (1965). Potential applications of nuclear explosives in development and management of water resources--Principles, U.S. Geol. Survey TEI-857, issued by U.S. Atomic Energy Comm. Tech. Inf. Service, 128 pp.

Ritchie, A. M. (1958). Recognition and identification of landslides, Chapter 4, of Landslides and engineering practice, Ed. by E. B. Eckel, Natl. Aead. Sci., Natl. Res. Council, Highway Res. Board Spec. Rept. 29, 48-68.

Sisemore, C. J. (1968). Written communication recorded in p. 13-14 of Plowshare Program, Feb- ruary 1, 1968-April 30, 1968, II , compiled by D. D. Rabb, California Univ., Livermore, Law- rence Radiation Lab. Rept. UCRL-50008-68-2.

Stead, F. W. (1969). Potential uses of nuclear explosions for water management project, in Aqua- rius Study Report, by Arizona Atomic Energy Comm., U.S. Atomic Energy Comm. and U.S. Dept. of Interior.

Teller, E , W. K. Talley, G. tI . Higgins and G. W. Johnson (1968). The constructive uses of nuclear explosives, McGraw-Hill Book Co., New York, 320 pp.

Thompson, T. L. and J. B. Misz (1959). Geologic studies of underground nuclear explosions Rainier and Neptune [Nev.]--Final Report, California Univ., Livermore, Lawrence Radiation Lab. Rept. UCRL-5757, 58 pp.

Varnes, D. J. (1958). Landslide types and processes, Chapter 3, of Landslides and engineering practice, Ed. by E. B. Eckel, Natl. Acad. Sci., Natl. Res. Council, Highway Res. Board Spec. Rept. 29, 20-47.

Wilmarth, V. R. and F. A. McXeown (1960). Structural effects of Rainier, Logan, and Blanca underground nuclear explosions, Nevada Test Site, Nye County, Nevada, in Short papers in the geological sciences, U.S. Geol. Survey Prof. Paper 400-B, B418-B423.

Witherspoon, P. A. (1966). Economics of nuclear explosions in developing underground gas stor- age, California Univ., Livermore, Lawrence Radiation Lab. Rept. UCRL-14877, 29 pp.

Young, L. E. and H. H. Stoek (1916). Subsidence resulting from mining, Illinois Univ. Bull. 13, 205 pp.

U. S. GEOLOGICAL SURVEY DENVER, COLORADO

Manuscript received July 2, 1969.

brief history of testing and collapse....bulletin of the seismological society of america. vol. 59,...

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