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Exploratory shaft design for a high-level waste repository in salt
Exploratory Shaft Design for a High-Level Waste Repository in Salt S. A. G. Poppen, A. I. Cooley and M. B. Mirza Abstract--The authors describe the exploratory sha]t design ]or a high-level nuclear waste repository in salt. The exploratory sha]t ]acility in salt is one o[ three ]acilities--one each in basalt, tu[[ and salt--to be developed [or the O[[ice of Civilian Radioactive Waste Management Program o] the U.S. Department o[ Energy. The authors discuss design criteria ]or the sha[t; the influence o[ the geotechnical conditions on sha[t lining; the sha]t construction process; the two types o[ sha]t lining to be used; and determination o] the loads on the sha]t lining.
Introduction he U.S. Department of Energy (DOE) is responsible for siting and constructing a licensed geologic repository for commercially generated high-level nuclear waste. W i t h i n DOE, the Office of Civilian Radioactive Waste Management (OCRWM) Program has this responsibility. T o accomplish this mission, the O C R W M Program is engaged in a m u h i d i s c i p l i n a r y effort to identify, qualify, and develop repository sites for high-level nuclear waste. T h e current strategy of the O C R W M Program is to identify candidate repository sites in basalt, tuff and salt and to develop exploratory shafts at those sites. Within OCRWM, the Salt Repository Project Office (SRPO) has identified the salt site for the development of exploratory shafts. T h e proposed Exploratory Shaft Facility (ESF) in salt will have several primary purposes. T h e shafts will provide access to the reference repository horizon to permit in-situ testing of the salt horizons in order to: (1) Verify the salt repository design parameters and validate performance assessment models. (2) Demonstrate shaft constructibility. (3) Demonstrate the capability of adequately sealing penetrations into the salt.
T
Present address: M. B. Mirza, PB-KBB, 11767 Katy Freeway, Suite 700, Houston, TX 77079, U.S.A. This paper first appeared in the 1987 R E T C Proceedings (© 1987, Society of Mining Engineers, Inc.). We are grateful to the Society of Mining Engineers, Inc. for granting permission to reprint the article.
An exploratory shaft is required (by 10 C F R Part 60) for conducting in-situ testing at depth prior to submittal of a license a p p l i c a t i o n for construction authorization of a repository. Therefore, construction of an ESF will also facilitate the licensing process. T h i s paper presents the preliminary design of the shaft excavation through the H i g h Plains aquifer, which is the p r i n c i p a l source of fresh water in the Texas Panhandle area. T w o 12-ft(3.6-m-) diameter circular shafts will be sunk to a depth of about 2600 ft (780 m), using ground freezing to stabilize the unconsolidated and saturated rocks. T h e paper discusses the basic design criteria used in the design, the significant geotechnical conditions influencing the shaft lining design, and the design methodology of the shaft l i n i n g and seal systems to be used for protecting the Ogallala and Dockum aquifers. Figure 1 is an artist's view of the ESF.
Design Criteria T o mitigate adverse impacts on the environment and the important aquifers, the ESF design must satisfy some extremely stringent criteria. T h e site clearing will be kept to a m i n i m u m and the areas disturbed by grading must be graveled or seeded with natural grasses. T h e salt storage piles are designed to minimize adverse impacts to the environment, and groundwater monitor wells will be installed around the salt piles to m o n i t o r the quality and levels of groundwater. T h e saline runoff p o n d (retention pond) will minimize seepage of saline water into the ground. T h e shaft liners will be watertight to prevent vertical and horizontal move-
Tunnelling and Underground Space Technology, Vol. 3, No. 2, pp. 183-192, 1 9 8 8 . Printed in Great Britain.
R~sum~--Les auteurs dbcrivent une premibre conception d'un puit pour un dkp6t de dkchets hautement radioacti]s dams du sel. Ce premier puit dams du sel est un des trois dep6ts (dams du sel, du basalt et du tu][) ~ Otre dkveloppks pour le bureau de dechet radioacti] civil du dkpartement de l'knergie ambricain. Les auteurs discutent des critbres de la conception pour le puit, de l'in]luence des conditions gbotechniques sur le rev~tement du puit, du prockdk de construction du puit, des deux types de rev~tement utilisks ainsi que de la dbtermination des contraintes sur le rev~tement du puit.
0886-7798/88 13.00 + .00 Pergamon Press plc
merit of water into the shaft or along the lining. T h e shaft lining and operational seals are designed for a 100-year life with m i n i m a l maintenance procedures. T h e ESF shafts will be excavated and structurally lined using proven methods and materials. T h e design minimizes any adverse impacts that the facility may cause to the environment and any damage to the site, should it be found suitable for a repository.
Influence of Geotechnical Conditions on Shaft Lining
Geology T h e Palo Duro Basin in the Texas Panhandle is a gently sloping depression between the Amarillo Uplift and the Matador Arch that is composed of relatively uniform sedimentary formations. T h e formations to be encountered in shaft construction consist of variably cemented sediments overlying generally flat-lying, i.e. less than 5°dip, sandstones, sihstones, shales, limestones, dolomites and evaporite sequences r a n g i n g in geologic age from U p p e r Permian to Pliocene. A stratigraphic column, shown in Fig. 2, has been extrapolated from the DOE p r o g r a m wells, Detten No. 1 and G. Friemel No. 1, drilled in Deaf Smith County. T h e depths and thicknesses of each formation assume uniform thinning of the beds between the wells. A brief, generalized description of the anticipated formational lithologies is as follows: • T h e Ogallala Formation, extending from the surface to a depth of about 360ft (108m), consists of variably cemented fine sand, silts, clays, and thin caliche beds grading to coarse sands and occasional
18 3
Figure 1. Configuration for the Exploratory Shaft Facility in salt.
gravels near the base of the formation. The sediments are very weakly cemented in some areas, but are mostly unconsolidated. The Ogallala is considered a major freshwater aquifer. l The Dockum Group consists of about 600 ft (180 m) of variably indurated siltstones with localized beds of sandstones and shales in the mid- to lower portion of the formation. The sandstone is a member of the Dockum Group, which yields large quantities of fresh water in localized basin areas. l Underlying the Dockum Group is the Dewey Lake Formation, consisting primarily of siltstones, and possibly including some shale beds. 0 The Alibates Formation consists primarily of anhydrite beds, but may have some interbedded shales and siltstone. l The Salado and Yates Formations consist of siltstones interbedded with shales. l The Upper Seven Rivers Formation contains the first significant salt beds; it is characterized by interbedded siltstones, shales and some anhydrite. The Lower Seven River Formation contains interbedded siltstones and shales, and may have some thin beds of halite and anhydrite. The Upper and Lower Seven Rivers Formations are approximately 140 and 185ft (42 184
T UNNELLINC
AND
and 56 m) thick, respectively. The Queen/Graysburg Formation, which is approximately 245 ft (74 m) thick, generally consists of interbedded shales and siltstones, but may also have some thin halite sequences. l The Upper San Andres Formation is typified by cyclic evaporitic sequences of limestone, dolomites, anhydrites and halite. Siltstones and shales are interbedded throughout the sequence. The Upper San Andres is anticipated to be about 520 ft (156 m) thick. 0 The top of the Lower San Andres Unit 5 Salt is anticipated to be at a depth of about 2285 ft (686 m). The Unit 5 Salt is typified by about 100 ft (30 m) of halite interbedded with thin mudstone or shale seams. The seams are typically less than 1 in. (2.5 cm) thick, a n d a r e generally spaced 5-10 ft (1.5-3 m) apart. The Unit 5 Salt is separated from the underlying Unit 4 Salt by anhydrites, dolomites, and shales. l The top of the Lower San Andres Unit 4 Salt is expected to be at a depth of about 2475 ft (743 m). The salt sequence is anticipated to be in excess of 150 ft (45 m) thick. The salt is interbedded with thin mudstones and/or shale seams. These seams are typically much less than 1 in. (2.5 cm) thick, and are generally spaced 5-10 ft (1.5-3 m) apart.
l
U NDERGROUND S PACE T E C H N O L O G Y
Impact of Geotechnical Conditions Rocks and Soils. The data base for the design indicates unconsolidated soils in the Ogallala and weakly cemented rocks in the Dockum Formation. Below the Dockum, down to the shaft bottom, low-strength rocks and salt are indicated. The soils of the Ogallala Formation, in particular, and the weakly cemented rocks of the Dockum are too weak to allow open-hole shaft excavations under saturated conditions. Therefore, stabilization by freezing will be required for both these formations. Excavation below the Dockum will require that some of the rocks be restricted in their exposure to moisture. The design provides for rockbolts, wire mesh, shotcrete and concrete as the preliminary support. Some localized conditions may require steel ring beams. No major geologic discontinuities that would contribute additional weakness in the ground have been identified. To compensate for salt creep, the design provides for overexcavation both in the lined and unlined shaft sections in salt. Some shales also may creep. The planned shaft construction sequence allows the creep pressures to dissipate before the concrete is placed. Groundwater. The hydrogeological data base identifies the Ogallala and Dockum Formations as aquifers of potable water. The formations below Volume 3, Number 2, 1988
FORMATIONS
i
GENERALIZED LITHOLOGY
0"
COARSE TO FiNE SAI4)S, SILTS & CLAYS WITH A ZONE OF CALICNE NEAR SURFACE.
OGALLALA WATER TABLE
IF.
-,4r
20~)" iFIIl|linllllll]l
i IIIII 1il III 360'
LOCALIZED BEDS OF GRAVEL NEAR THE BASAL CONTACT AND BF.J.OW UNCONSOLIDATED t RUNI4NG )SANDS
laJ I-O O..
SILTSTONE AND SHALES WITH LOCALIZED BEDS OF SANDSTONES
DOCKUM
DEWEY LAKE
~6o"
SALADO YATES UPPER
SHALE (WATER SENSITIVE ) ANHYDfllTE/DOLOMITE
~e,
ALIBATES
SHALE & ~LTSTONE, ANHYDI~TE BED AT BASE
m, .95, SEVEN
LOWER SEVEN
SHALE & ~LTSTONE HALITE WITH BEDS OF SHALE & ANHYI~TE
RIVERS 13~"
RIVERS mz0,--
INTERBEDOED SLTSTONE. & SHALE WiTH SANDSTONE & A BASE OF HALITE & ANHYORITE
INTF.~ SHALE & SiL TSTONE
QUEEN /
GRAYBURG JTSS" " ; : ; ' ; : ; ; ; ; ; ; I ............
UPPER SAN ANDRES "I-H-I-H'H+I-H~
AN~TE & SHALE AT BASE OF THE FORMATION
2285" HALITE I N T ~ MTH SHALE & AI4rr01tiTE.
LOWER SAN ANDRES Unll" S SALT
247B"
NDmzoN 2sss"
EXPLORATORY Unl+ 4 SALT
2635"
SAME AS UNIT 5
21'ZO
U n l t 3 SALT
Figure 2. Generalized stratigraphic column/or the site o[ the repository in salt.
Volume 3, Number 2, 1988
TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY 185
the Dockum are untested, except for an interval below Unit 4 Salt. For design purposes it is necessary to assume that water may flow from any formation penetrated by the shaft. Unit 4 Salt is assumed to be dry. Therefore, the watertight shaft lining will extend from the surface to the top of Unit 4 Salt. T h e water in the Ogallala and Dockum Formations will be frozen for soil stabilization in this shaft section. T h e water flowing into the shaft during excavation and lining operations from below the Dockum Formation will be controlled by pressure grouting or by collection in water rings.
Shaft Construction Ground Stabilization Prior to Excavation G r o u n d stabilization prior to shaft excavation will be needed to provide safe working conditions in the shaft. Conditions adverse to a safe working environment d u r i n g shaft construction will include excessive waterflows, unstable g r o u n d conditions and a c o m b i n a t i o n of flowing water with unstable ground. Based on the available data, it appears that the soils of the Ogallala Formation and the weakly cemented rocks of the Dockum are too weak to allow openhole excavation under saturated conditions (Roesner et al. 1983). Below the Dockum, down to the shaft bottom in Unit 4 Salt, relatively low- to moderate-strength rocks and salt are dominant. Anhydrite is the only highstrength rock identified below the Dockum. T h e hydrostratigraphy below the Dockum indicates a total water inflow rate into the shaft excavation of less than 4 0 - 5 0 g p m u p o n initial exposure, decreasing to a total inflow of less than 10gpm after one year if allowed to drain. T h e soils of the Ogallala do not lend themselves to stabilization by grouting (Mohr 1964), and dewatering by p u m p i n g is at best uncertain in terms of time and expense (Roesner and Poppen 1984). Freezing of the Ogallala and Dockum would stabilize the soils and rocks and control the waterflow to create a safe working environment for shaft excavation. This method has proven to be cost-effective and timecertain. Freezing of the Ogallala and Dockum formations will involve the following:
• Installing and operating a freeze plant of sufficient capacity. • Circulating a coolant through the freeze pipes. • Conducting an ultrasonic profile of the frozen g r o u n d between the freeze holes. • Measuring and evaluating the fluid flow from the shaft center relief hole. • Measuring, monitoringandevaluating the ice wall creep in the shaft excavation relative to freeze pipe deformation. • Measuring, monitoringand evaluating the ice wall creep relative to support pressure applied and freestanding shaft wall height. Dimensioning of the ice wall thickness is based on instantaneous and timedependent strengths of the frozen soils and rocks and a safety factor of more than 2.0. Determination of the ice wall thickness is based on the modified Domke equation (Domke 1915) to incorporate preliminary lining support pressure. =0.5 [ - - d )+ d_...._~] LI" (1+ r, (ri+d)
P-Poo
K
= pressure on the ice wall (psi); Po0 = allowable stress of the preliminary lining support or support pressure (psi); K = time-dependent strength of the frozen soil (psi); d = required ice wall thickness(in in.); ri = inside radius of the ice wall (in in.).
186
h
t = 1.3 P0 ~
(Sanger and Sayles)
The calculations of the free-standing ice wall heights for the various frozen soil and rock are iterated until the deflection of the freeze pipes falls within safe limits. T h e ice wall thickness will vary from approx. 16 to 20 ft (4.8-6 m), requiring 28 freeze holes drilled to an a p p r o x i m a t e depth of 1000 ft (300m) (see Fig. 3). Additional holes will be drilled for temperature and deformation surveys. A center relief hole drilled in the shaft center will provide escape for fluids that otherwise might be trapped inside the ice wall, compromising stability. Each shaft will require a freeze plant with a capacity of approximately 400 tons of refrigeration.
Excavation Method and Sequence T o minimize the disturbance to the strata surrounding the shaft, mechanical excavation techniques will predominate at the shaft excavation perimeter. Conventional drill-and-blast methods will supplement mechanical excavation techniques where appropriate.
where P
This modified equation is derived from the Domke formula (Domke 1915): d = ri [0.29
P
×g+ 2.3
P
The free-standing unsupported height of the shaft wall is calculated by substituting the ice wall thickness, determined by the Domke equation for the thickness in the equations proposed by Vialov (1976), and Sanger and Sayles (1979). t = X/~ P0 -h K
• Drilling of freeze holes, temperature control holes and a shaft center relief hole. • Installation and testing of freeze pipes in the ground. • Conducting an ultrasonic survey profile of the unfrozen ground between the freeze holes. • Installing and operating a temperature-control system
or
where
t
Po
K h
(Vialov)
= ice wall thickness (in in.); = pressure on the ice wall (psi); = time-dependent strength of the frozen soil (in psi); = free-standing excavation wall height (in in.).
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Shaft Lining T h e shaft lining materials for the SRP shafts have been selected on the basis of their proven performance of watertighmess and stability. T w o types of lining will be installed in the shaft: a preliminary l i n i n g and a final lining. T h e preliminary shaft lining will consist of one or more of the following elements: • Rockbohs and wire mesh. • Rockbohs, wire mesh and shotcrete. • Cast-in-place concrete. • Precast concrete blocks. • Steel ribs and lagging/liner plate. T h e preliminary lining will provide a safe working environment for personnel in the shaft. Liner plate will be used in the construction of the shaft collar to a depth of approx. 90 ft (27 m). Precast concrete blocks will be used in the frozen shaft section, where immediate support without heat generation against the ice wall will be required. Rockbolts and shotcrete will be used as a means of minimizing disturbance and deterioration of the strata surrounding the shaft. T h e final shaft lining will consist of a circular steel membrane with loadbearing concrete on the inside to a depth of 1710 ft (513 m). T h e lining from 1710 to 2475ft (513-743m) will have an additional steel membrane on the inside of the shaft to confine the concrete. The Volume 3, Number 2, 1988
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• ~ l t ~
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Figure 3. Ground .freezing arrangement/or the salt repository.
outer steel membrane will be impermeable, watertight shaft liner. T h e inner steel membrane is not watertight. T h e inner and outer steel plate thicknesses will be balanced to reduce the concrete thickness for efficiency of installation and a uniform cross-section. T h e inner steel membrane will be anchored to the concrete by anchors a n d / o r steel hoops (Fig. 4). T h e final l i n i n g will be installed in two phases so that the duration of the freeze process can be reduced. In the first phase, the final l i n i n g will be installed Volume 3, Number 2, 1988
immediately after the excavation through the frozen section is complete. T h e foundation for this lining will be constructed in the Alibates at a depth of 1036 ft (311 m) (see Fig. 5). T h e freeze p l a n t will be demobilized once the final l i n i n g in the frozen section is complete. T h e foundation for the lower section of the final lining will be constructed immediately above Unit 4 Salt (Fig. 6). A concrete support ring will supplement the final liner foundation below the foundation. T h i s ring will confine the rock bearing the weight of the entire
shaft liner column and, thus, permit a controlled distribution of stresses into the strata. T h e design of the shaft lining uses accepted structural analysis for circular mine shafts (Link 1976).
Loads on the Shaft Lining A shaft l i n i n g is subject to a complex set of superimposed external pressures or loads. T h i s is especially true of watertight, composite shaft liners in non-competent strata. T o account for
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 187
M
I.II~G SECTI~ NO. SIIIAFT ¢OU.~tR DEPTH O"
-s"
v
M
OF Og~LLALA
Q
OF POTAmLIE AQUFER TOP OF N , a A T D M OF N.llAT[S
t I
II
I
Q 180"
® 1470"
® too,
®
®
TOP OF SAI.T UNiT
4
24"NV
I~'~AVATION 14"-0"0M,
SHAFT t - i
Figure 4. Sha[t lining details.
188
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
V o l u m e 3, N u m b e r 2, 1988
T /
TOP a r N.liATES
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ASPHALT~
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ASPI~.T SO
S~U~EO ASm~.T
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OIMUT
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TOP OF FOUNDATION 102S'
~
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a IO"
CHI[MCAL
s w ~ . ~ T
I
~ S E D CemXT ml0UT
TOPOIr
S 0a0~ I~ NOTE 2
e 60" m-ll'*)
Figure 5. Sha[t lining pressure envelope.
Volume 3, Number 2, 1988
TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY 189
FIMSl4ED SHAFT OIAMETER
these loads, a complex, closed-h)rm mathematical model has been developed to determine the total load on both the preliminary and final ESF shaft linings. The total load (PT) is expressed as follows:
~,~
6 0ROUT PIPES II 80" SANDED CEMENT gROUT
j
,
PT = Po + Po + P,,,
TOiSl0N ANCHORS (TYP)
where P,, = uniform horizontal pressure (psi); Po = n o n u n i f o r m horizontal pressure (psi); P,n = other loads (psi).
S a t ~ r l r pprdi e a 0 ,
[LIJ
I__ OlElar.m. SF.AL
The uniform horizontal pressure is the result of hydrostatic pressure and ground pressure. Therefore,
IQ::
P,, = Pw + Pc;
I F-
I" I
¢ImENT ~IOUT
where Pw = hydrostatic pressure (psi); Pc, = uniform ground pressure (psi).
L/) I mmlJ~ IIEIfJL III ~ ~ IIAIK Iq. ~'
moEmoTh.on pL ~.
CBENT ml0UT
• WI~tT PIPES • SO" m
FOUIOATION CON~qETE
4. uam Si.AII
smxcra~PLAT.__tt m
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4
Z41'8"
EXCAVATICN GIA. IN SALT
q
I I
I Figure 6. L o w e r shaft lining ]oundation. Table 1. Allowable material stresses.
Material Concrete:
4 0 0 0 psi 6 0 0 0 psi 8 0 0 0 psi
Steel:
ASTM A - 6 3 3 Gr.C ASTM A - 6 3 3 Gr.E
Buckling (psi)
Non-uniform and uniform (psi)
Uniform (psi)
3800 5100 6100
2600* 3900* 5200*
2100 2500 3050
49,000 59,000
44,500t 53,000t
29,000 35,000
* Based on ACI permissible stresses of 0.65 x fc as well as on guidelines for calculating shaft linings (7) for uniform plus non-uniform pressures. t Based on ratio of allowable stresses/yield stress for uniform plus nonuniform pressures as recommended by Link (7).
190
TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY
G r o u n d pressures are calculated on a stratigraphic unit-by-unit basis. Classic soil mechanics formulae are used for non-cohesive soils. For competent rock strata, ground pressures are negligible. Where the rock is predicted to fail and a relaxed zone develops, an interactive equation predicts the extent of this relaxed zone for a given support pressure. Additional loads resulting from freezing and thawing are considered in zones where the ground is frozen. Salt presents special problems due to its viscous behavior under pressure, known as "creep". Theoretically, full lithostatic pressures will eventually come to bear on a liner in salt zones. However, in reality this p h e n o m e n o n can be avoided by backing the liner with a yielding foam where possible. Except for the rare case that requires designing for full lithostatic pressure, hydrostatic pressures typically are higher than ground pressures. However, hydrostatic pressures will not bear on non-watertight linings such as the preliminary liner. Conversely, ground pressures will not bear on the final liner in areas where the asphalt seal column is used behind the liner; rather, the liner will be loaded only by the hydrostatic head of the asphalt column. The n o n - u n i f o r m load is calculated by varying a portion of the uniform pressure around the circumference of the lining, in accordance with the procedure established by Link et ai. (1976). The design also considers loads resulting from: (1) Time-delayed rock displacements. (2) Internal residue stresses. (3) Thermal effects. (4) Seismic effects. (5) Effects of construction methods. (6) Shaft e q u i p p i n g loads including those created by shaft furnish-
Volume 3, Number 2, 1988
LINING PRESSURE (PSI) 200 400 600 800 I000 1200 1400 1600 1800 2000 I
O
I
I
I
I
I
I
I
I
WATER TABLE
200 -
ASPHALT PRESSURE ON LINING(PA)
I000
TOTAL PRESSURE ENVELOPE
GI
FLUID PRESSURE ( PW )
t/3
TOTAL PRESSURE(PT)
2000
2475
NOTE: I. TOTAL PRESSURE (PT)= ROCK/SOIL PRESSURE + FLUID PRESSURE. 2. THE TOTAL PRESSUREENVELOPE=BITUMENPRESSURE ON LINING + ALLOWABLE PRESSURE ON PRELIMINARYLINING.
Figure 7. Summary o[ pressures acting against the shaft lining pressure envelope.
applies to the unlined shaft section in Unit 4 Salt. A creep allowance is designed into the shaft diameter, resulting in a 14-ft-(4-m-) diameter shaft. T h e allowable lining material stresses for different loading conditions are given in T a b l e 1. T h e final lining dimensions are given in Table 2.
ings, hoisting equipment, and instrumentation. (7) Shaft station area. (8) Shaft bottom plug. These loads and their applicability to specific sections of lining are evaluated on a case-by-case basis. For example, shaft-equipping loads are not a p p l i e d to the preliminary liner because no furnishings are attached to it. Pressures acting against the shaft l i n i n g are summarized in Fig. 7. Under atmospheric conditions the salt will creep unrestrained. T h i s condition
Seals Sealing of the watertight shaft lining sections will employ asphalt and "chemical seal".
Table 2. Final lining dimensions. Steel* Outer
Concrete
inner
Depth
t
t
Fs'
(ft)
t
fc'
(in.)
(in.)
(ksi)
(in.)
(ksi)
0.5 0.625 0.625 0.625 0.625 0.625 0.625
---1.00 1.25 1.375 1.625
50 50 50 60 60 60 60
25.5 25.5 25.5 24.75 24.5 24.25 24
6 6 8 8 8 8 8
0-1150 1150-1470 1470-1710 1710-2135
2135-2260 2260-2390 2390-2475
* To compensate for corrosion, the steel thickness is increased by 1 / 8 in. and is included in the steel dimensions shown above, except for the interval 0 - 1 1 5 0 ft which is protected by the bitumen envelope.
Volume 3, Number 2, 1988
Asphalt will be placed in the annular space between the outer watertight steel membrane and the shaft wall or primary shaft lining. T h e asphalt will have a penetration rate of 2 5 - 3 2 m m / 5 s at 25°C. It will be weighted with limestone flour of a fineness of 100% passing 200 mesh. T h e specific gravity of this asphalt mixture (approx. l . a g / c m 3) will exceed the specific gravity of the formation fluids. Therefore, the hydrostatic pressure of the asphalt c o l u m n will exceed the hydrostatic head of the fluid behind the shaft lining and will displace it. T h e asphalt will also occupy near-field pore space and fill small formation cracks. T h e bottom portion of the asphalt column will consist of absorbing cushions of sand and densely sanded asphalt. Where asphalt is not used, the watertight shaft lining will be terminated by sealing the annular space with a chemical seal. Chemical seals use a polymeric sealing c o m p o u n d called "Chemical Seal R i n g " (CSR) (Dellinger and Bough:on). CSR will be mixed with gravel and placed as a slurry that will be pliable enough to penetrate into nearfield cracks. After emplacement, the slurry will harden to form an elastic solid. T h e polymer material will expand when it comes into contact with water and develops swelling pressure. For this seal to be effective, it needs to be confined. Confinement of the seal material will be achieved by pressure g r o u t i n g with a cement slurry in the a d j o i n i n g shaft sections. Thus, the seal material itself will be pressurized t h r o u g h c o n f i n e m e n t for g r e a t e r effectiveness. Chemical seals are p l a n n e d for the lower shaft l i n i n g sections at approx. 2460 and 1045 ft (738 and 314 m) below the surface (Fig. 5). [] Acknowledgement
T h e preliminary shaft design presented in this paper has been completed under contract with the U.S. Department of Energy, Chicago Operations, Salt Repository Project Office. T h e authors wish to thank the S R P O for g r a n t i n g permission to present this paper. References
Dellinger, T. B. and Boughton, L. D. Unique materials mix used to seal. E. 6*M. ]. 166 (6): 114-118. Domke, O. 1915. Uber die Beanspruchung der Frostmaner beim Schachtabteufen nach Gefrierverfahren. Glueckau] 51 (47): 1129-1135.
Link, H., Lutgendorff, H. O. and Stoss, K. 1976. Guidelines for calculating shaft linings in incompetent strata. Glueckaul, 2nd edn. Essen, West Germany: Coal Mining Association.
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 191
Mohr, F. 1964. Schachtbautechnik (Mine Shaft Engineering). Germany: H. Hubener K. G. Goslar. Oellers, T. 1983. Asphalt as a sealant in shaft construction. Glueckauf 119 (12): 567-570. Roesner, E. K. and Poppen, S. A. G. 1984. Shaft sinking and tunnelling in the oil sands of Alberta. Paper presented at AOSTRA Seminar on Underground
192
Excavating in Oil Sands, May 19, 1978. Roesner, E. K., Poppen, S. A. G. and Konopka, J. C. 1983. Stability during shaft sinking (a design guide for ground support of circular shafts). In Proc. First Int. Conf. on Stability in Underground Mining (August 1982). New York: AIME. Sanger, F. J. and Sayles, F. H. 1979. Thermal and rheological computations
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
for artificially frozen ground consttm+ tion. In Ground Freezing/Development.~ in Geotechnical Engineering, Vol. 26. Amsterdam: Elsevier Scientific Publishing. Vialov, S. S. et al. 1976. Strength and creep of frozen soils and calculations in ice-soil retaining structures, (F. J. Sanger, te~h. ed.).
V o l u m e 3, N u m b e r 2, 1988
Introduction he U.S. Department of Energy (DOE) is responsible for siting and constructing a licensed geologic repository for commercially generated high-level nuclear waste. W i t h i n DOE, the Office of Civilian Radioactive Waste Management (OCRWM) Program has this responsibility. T o accomplish this mission, the O C R W M Program is engaged in a m u h i d i s c i p l i n a r y effort to identify, qualify, and develop repository sites for high-level nuclear waste. T h e current strategy of the O C R W M Program is to identify candidate repository sites in basalt, tuff and salt and to develop exploratory shafts at those sites. Within OCRWM, the Salt Repository Project Office (SRPO) has identified the salt site for the development of exploratory shafts. T h e proposed Exploratory Shaft Facility (ESF) in salt will have several primary purposes. T h e shafts will provide access to the reference repository horizon to permit in-situ testing of the salt horizons in order to: (1) Verify the salt repository design parameters and validate performance assessment models. (2) Demonstrate shaft constructibility. (3) Demonstrate the capability of adequately sealing penetrations into the salt.
T
Present address: M. B. Mirza, PB-KBB, 11767 Katy Freeway, Suite 700, Houston, TX 77079, U.S.A. This paper first appeared in the 1987 R E T C Proceedings (© 1987, Society of Mining Engineers, Inc.). We are grateful to the Society of Mining Engineers, Inc. for granting permission to reprint the article.
An exploratory shaft is required (by 10 C F R Part 60) for conducting in-situ testing at depth prior to submittal of a license a p p l i c a t i o n for construction authorization of a repository. Therefore, construction of an ESF will also facilitate the licensing process. T h i s paper presents the preliminary design of the shaft excavation through the H i g h Plains aquifer, which is the p r i n c i p a l source of fresh water in the Texas Panhandle area. T w o 12-ft(3.6-m-) diameter circular shafts will be sunk to a depth of about 2600 ft (780 m), using ground freezing to stabilize the unconsolidated and saturated rocks. T h e paper discusses the basic design criteria used in the design, the significant geotechnical conditions influencing the shaft lining design, and the design methodology of the shaft l i n i n g and seal systems to be used for protecting the Ogallala and Dockum aquifers. Figure 1 is an artist's view of the ESF.
Design Criteria T o mitigate adverse impacts on the environment and the important aquifers, the ESF design must satisfy some extremely stringent criteria. T h e site clearing will be kept to a m i n i m u m and the areas disturbed by grading must be graveled or seeded with natural grasses. T h e salt storage piles are designed to minimize adverse impacts to the environment, and groundwater monitor wells will be installed around the salt piles to m o n i t o r the quality and levels of groundwater. T h e saline runoff p o n d (retention pond) will minimize seepage of saline water into the ground. T h e shaft liners will be watertight to prevent vertical and horizontal move-
Tunnelling and Underground Space Technology, Vol. 3, No. 2, pp. 183-192, 1 9 8 8 . Printed in Great Britain.
R~sum~--Les auteurs dbcrivent une premibre conception d'un puit pour un dkp6t de dkchets hautement radioacti]s dams du sel. Ce premier puit dams du sel est un des trois dep6ts (dams du sel, du basalt et du tu][) ~ Otre dkveloppks pour le bureau de dechet radioacti] civil du dkpartement de l'knergie ambricain. Les auteurs discutent des critbres de la conception pour le puit, de l'in]luence des conditions gbotechniques sur le rev~tement du puit, du prockdk de construction du puit, des deux types de rev~tement utilisks ainsi que de la dbtermination des contraintes sur le rev~tement du puit.
0886-7798/88 13.00 + .00 Pergamon Press plc
merit of water into the shaft or along the lining. T h e shaft lining and operational seals are designed for a 100-year life with m i n i m a l maintenance procedures. T h e ESF shafts will be excavated and structurally lined using proven methods and materials. T h e design minimizes any adverse impacts that the facility may cause to the environment and any damage to the site, should it be found suitable for a repository.
Influence of Geotechnical Conditions on Shaft Lining
Geology T h e Palo Duro Basin in the Texas Panhandle is a gently sloping depression between the Amarillo Uplift and the Matador Arch that is composed of relatively uniform sedimentary formations. T h e formations to be encountered in shaft construction consist of variably cemented sediments overlying generally flat-lying, i.e. less than 5°dip, sandstones, sihstones, shales, limestones, dolomites and evaporite sequences r a n g i n g in geologic age from U p p e r Permian to Pliocene. A stratigraphic column, shown in Fig. 2, has been extrapolated from the DOE p r o g r a m wells, Detten No. 1 and G. Friemel No. 1, drilled in Deaf Smith County. T h e depths and thicknesses of each formation assume uniform thinning of the beds between the wells. A brief, generalized description of the anticipated formational lithologies is as follows: • T h e Ogallala Formation, extending from the surface to a depth of about 360ft (108m), consists of variably cemented fine sand, silts, clays, and thin caliche beds grading to coarse sands and occasional
18 3
Figure 1. Configuration for the Exploratory Shaft Facility in salt.
gravels near the base of the formation. The sediments are very weakly cemented in some areas, but are mostly unconsolidated. The Ogallala is considered a major freshwater aquifer. l The Dockum Group consists of about 600 ft (180 m) of variably indurated siltstones with localized beds of sandstones and shales in the mid- to lower portion of the formation. The sandstone is a member of the Dockum Group, which yields large quantities of fresh water in localized basin areas. l Underlying the Dockum Group is the Dewey Lake Formation, consisting primarily of siltstones, and possibly including some shale beds. 0 The Alibates Formation consists primarily of anhydrite beds, but may have some interbedded shales and siltstone. l The Salado and Yates Formations consist of siltstones interbedded with shales. l The Upper Seven Rivers Formation contains the first significant salt beds; it is characterized by interbedded siltstones, shales and some anhydrite. The Lower Seven River Formation contains interbedded siltstones and shales, and may have some thin beds of halite and anhydrite. The Upper and Lower Seven Rivers Formations are approximately 140 and 185ft (42 184
T UNNELLINC
AND
and 56 m) thick, respectively. The Queen/Graysburg Formation, which is approximately 245 ft (74 m) thick, generally consists of interbedded shales and siltstones, but may also have some thin halite sequences. l The Upper San Andres Formation is typified by cyclic evaporitic sequences of limestone, dolomites, anhydrites and halite. Siltstones and shales are interbedded throughout the sequence. The Upper San Andres is anticipated to be about 520 ft (156 m) thick. 0 The top of the Lower San Andres Unit 5 Salt is anticipated to be at a depth of about 2285 ft (686 m). The Unit 5 Salt is typified by about 100 ft (30 m) of halite interbedded with thin mudstone or shale seams. The seams are typically less than 1 in. (2.5 cm) thick, a n d a r e generally spaced 5-10 ft (1.5-3 m) apart. The Unit 5 Salt is separated from the underlying Unit 4 Salt by anhydrites, dolomites, and shales. l The top of the Lower San Andres Unit 4 Salt is expected to be at a depth of about 2475 ft (743 m). The salt sequence is anticipated to be in excess of 150 ft (45 m) thick. The salt is interbedded with thin mudstones and/or shale seams. These seams are typically much less than 1 in. (2.5 cm) thick, and are generally spaced 5-10 ft (1.5-3 m) apart.
l
U NDERGROUND S PACE T E C H N O L O G Y
Impact of Geotechnical Conditions Rocks and Soils. The data base for the design indicates unconsolidated soils in the Ogallala and weakly cemented rocks in the Dockum Formation. Below the Dockum, down to the shaft bottom, low-strength rocks and salt are indicated. The soils of the Ogallala Formation, in particular, and the weakly cemented rocks of the Dockum are too weak to allow open-hole shaft excavations under saturated conditions. Therefore, stabilization by freezing will be required for both these formations. Excavation below the Dockum will require that some of the rocks be restricted in their exposure to moisture. The design provides for rockbolts, wire mesh, shotcrete and concrete as the preliminary support. Some localized conditions may require steel ring beams. No major geologic discontinuities that would contribute additional weakness in the ground have been identified. To compensate for salt creep, the design provides for overexcavation both in the lined and unlined shaft sections in salt. Some shales also may creep. The planned shaft construction sequence allows the creep pressures to dissipate before the concrete is placed. Groundwater. The hydrogeological data base identifies the Ogallala and Dockum Formations as aquifers of potable water. The formations below Volume 3, Number 2, 1988
FORMATIONS
i
GENERALIZED LITHOLOGY
0"
COARSE TO FiNE SAI4)S, SILTS & CLAYS WITH A ZONE OF CALICNE NEAR SURFACE.
OGALLALA WATER TABLE
IF.
-,4r
20~)" iFIIl|linllllll]l
i IIIII 1il III 360'
LOCALIZED BEDS OF GRAVEL NEAR THE BASAL CONTACT AND BF.J.OW UNCONSOLIDATED t RUNI4NG )SANDS
laJ I-O O..
SILTSTONE AND SHALES WITH LOCALIZED BEDS OF SANDSTONES
DOCKUM
DEWEY LAKE
~6o"
SALADO YATES UPPER
SHALE (WATER SENSITIVE ) ANHYDfllTE/DOLOMITE
~e,
ALIBATES
SHALE & ~LTSTONE, ANHYDI~TE BED AT BASE
m, .95, SEVEN
LOWER SEVEN
SHALE & ~LTSTONE HALITE WITH BEDS OF SHALE & ANHYI~TE
RIVERS 13~"
RIVERS mz0,--
INTERBEDOED SLTSTONE. & SHALE WiTH SANDSTONE & A BASE OF HALITE & ANHYORITE
INTF.~ SHALE & SiL TSTONE
QUEEN /
GRAYBURG JTSS" " ; : ; ' ; : ; ; ; ; ; ; I ............
UPPER SAN ANDRES "I-H-I-H'H+I-H~
AN~TE & SHALE AT BASE OF THE FORMATION
2285" HALITE I N T ~ MTH SHALE & AI4rr01tiTE.
LOWER SAN ANDRES Unll" S SALT
247B"
NDmzoN 2sss"
EXPLORATORY Unl+ 4 SALT
2635"
SAME AS UNIT 5
21'ZO
U n l t 3 SALT
Figure 2. Generalized stratigraphic column/or the site o[ the repository in salt.
Volume 3, Number 2, 1988
TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY 185
the Dockum are untested, except for an interval below Unit 4 Salt. For design purposes it is necessary to assume that water may flow from any formation penetrated by the shaft. Unit 4 Salt is assumed to be dry. Therefore, the watertight shaft lining will extend from the surface to the top of Unit 4 Salt. T h e water in the Ogallala and Dockum Formations will be frozen for soil stabilization in this shaft section. T h e water flowing into the shaft during excavation and lining operations from below the Dockum Formation will be controlled by pressure grouting or by collection in water rings.
Shaft Construction Ground Stabilization Prior to Excavation G r o u n d stabilization prior to shaft excavation will be needed to provide safe working conditions in the shaft. Conditions adverse to a safe working environment d u r i n g shaft construction will include excessive waterflows, unstable g r o u n d conditions and a c o m b i n a t i o n of flowing water with unstable ground. Based on the available data, it appears that the soils of the Ogallala Formation and the weakly cemented rocks of the Dockum are too weak to allow openhole excavation under saturated conditions (Roesner et al. 1983). Below the Dockum, down to the shaft bottom in Unit 4 Salt, relatively low- to moderate-strength rocks and salt are dominant. Anhydrite is the only highstrength rock identified below the Dockum. T h e hydrostratigraphy below the Dockum indicates a total water inflow rate into the shaft excavation of less than 4 0 - 5 0 g p m u p o n initial exposure, decreasing to a total inflow of less than 10gpm after one year if allowed to drain. T h e soils of the Ogallala do not lend themselves to stabilization by grouting (Mohr 1964), and dewatering by p u m p i n g is at best uncertain in terms of time and expense (Roesner and Poppen 1984). Freezing of the Ogallala and Dockum would stabilize the soils and rocks and control the waterflow to create a safe working environment for shaft excavation. This method has proven to be cost-effective and timecertain. Freezing of the Ogallala and Dockum formations will involve the following:
• Installing and operating a freeze plant of sufficient capacity. • Circulating a coolant through the freeze pipes. • Conducting an ultrasonic profile of the frozen g r o u n d between the freeze holes. • Measuring and evaluating the fluid flow from the shaft center relief hole. • Measuring, monitoringandevaluating the ice wall creep in the shaft excavation relative to freeze pipe deformation. • Measuring, monitoringand evaluating the ice wall creep relative to support pressure applied and freestanding shaft wall height. Dimensioning of the ice wall thickness is based on instantaneous and timedependent strengths of the frozen soils and rocks and a safety factor of more than 2.0. Determination of the ice wall thickness is based on the modified Domke equation (Domke 1915) to incorporate preliminary lining support pressure. =0.5 [ - - d )+ d_...._~] LI" (1+ r, (ri+d)
P-Poo
K
= pressure on the ice wall (psi); Po0 = allowable stress of the preliminary lining support or support pressure (psi); K = time-dependent strength of the frozen soil (psi); d = required ice wall thickness(in in.); ri = inside radius of the ice wall (in in.).
186
h
t = 1.3 P0 ~
(Sanger and Sayles)
The calculations of the free-standing ice wall heights for the various frozen soil and rock are iterated until the deflection of the freeze pipes falls within safe limits. T h e ice wall thickness will vary from approx. 16 to 20 ft (4.8-6 m), requiring 28 freeze holes drilled to an a p p r o x i m a t e depth of 1000 ft (300m) (see Fig. 3). Additional holes will be drilled for temperature and deformation surveys. A center relief hole drilled in the shaft center will provide escape for fluids that otherwise might be trapped inside the ice wall, compromising stability. Each shaft will require a freeze plant with a capacity of approximately 400 tons of refrigeration.
Excavation Method and Sequence T o minimize the disturbance to the strata surrounding the shaft, mechanical excavation techniques will predominate at the shaft excavation perimeter. Conventional drill-and-blast methods will supplement mechanical excavation techniques where appropriate.
where P
This modified equation is derived from the Domke formula (Domke 1915): d = ri [0.29
P
×g+ 2.3
P
The free-standing unsupported height of the shaft wall is calculated by substituting the ice wall thickness, determined by the Domke equation for the thickness in the equations proposed by Vialov (1976), and Sanger and Sayles (1979). t = X/~ P0 -h K
• Drilling of freeze holes, temperature control holes and a shaft center relief hole. • Installation and testing of freeze pipes in the ground. • Conducting an ultrasonic survey profile of the unfrozen ground between the freeze holes. • Installing and operating a temperature-control system
or
where
t
Po
K h
(Vialov)
= ice wall thickness (in in.); = pressure on the ice wall (psi); = time-dependent strength of the frozen soil (in psi); = free-standing excavation wall height (in in.).
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Shaft Lining T h e shaft lining materials for the SRP shafts have been selected on the basis of their proven performance of watertighmess and stability. T w o types of lining will be installed in the shaft: a preliminary l i n i n g and a final lining. T h e preliminary shaft lining will consist of one or more of the following elements: • Rockbohs and wire mesh. • Rockbohs, wire mesh and shotcrete. • Cast-in-place concrete. • Precast concrete blocks. • Steel ribs and lagging/liner plate. T h e preliminary lining will provide a safe working environment for personnel in the shaft. Liner plate will be used in the construction of the shaft collar to a depth of approx. 90 ft (27 m). Precast concrete blocks will be used in the frozen shaft section, where immediate support without heat generation against the ice wall will be required. Rockbolts and shotcrete will be used as a means of minimizing disturbance and deterioration of the strata surrounding the shaft. T h e final shaft lining will consist of a circular steel membrane with loadbearing concrete on the inside to a depth of 1710 ft (513 m). T h e lining from 1710 to 2475ft (513-743m) will have an additional steel membrane on the inside of the shaft to confine the concrete. The Volume 3, Number 2, 1988
mlJm NIl i
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• ~ l t ~
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Figure 3. Ground .freezing arrangement/or the salt repository.
outer steel membrane will be impermeable, watertight shaft liner. T h e inner steel membrane is not watertight. T h e inner and outer steel plate thicknesses will be balanced to reduce the concrete thickness for efficiency of installation and a uniform cross-section. T h e inner steel membrane will be anchored to the concrete by anchors a n d / o r steel hoops (Fig. 4). T h e final l i n i n g will be installed in two phases so that the duration of the freeze process can be reduced. In the first phase, the final l i n i n g will be installed Volume 3, Number 2, 1988
immediately after the excavation through the frozen section is complete. T h e foundation for this lining will be constructed in the Alibates at a depth of 1036 ft (311 m) (see Fig. 5). T h e freeze p l a n t will be demobilized once the final l i n i n g in the frozen section is complete. T h e foundation for the lower section of the final lining will be constructed immediately above Unit 4 Salt (Fig. 6). A concrete support ring will supplement the final liner foundation below the foundation. T h i s ring will confine the rock bearing the weight of the entire
shaft liner column and, thus, permit a controlled distribution of stresses into the strata. T h e design of the shaft lining uses accepted structural analysis for circular mine shafts (Link 1976).
Loads on the Shaft Lining A shaft l i n i n g is subject to a complex set of superimposed external pressures or loads. T h i s is especially true of watertight, composite shaft liners in non-competent strata. T o account for
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 187
M
I.II~G SECTI~ NO. SIIIAFT ¢OU.~tR DEPTH O"
-s"
v
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OF Og~LLALA
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OF POTAmLIE AQUFER TOP OF N , a A T D M OF N.llAT[S
t I
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Q 180"
® 1470"
® too,
®
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TOP OF SAI.T UNiT
4
24"NV
I~'~AVATION 14"-0"0M,
SHAFT t - i
Figure 4. Sha[t lining details.
188
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
V o l u m e 3, N u m b e r 2, 1988
T /
TOP a r N.liATES
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a IO"
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s w ~ . ~ T
I
~ S E D CemXT ml0UT
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e 60" m-ll'*)
Figure 5. Sha[t lining pressure envelope.
Volume 3, Number 2, 1988
TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY 189
FIMSl4ED SHAFT OIAMETER
these loads, a complex, closed-h)rm mathematical model has been developed to determine the total load on both the preliminary and final ESF shaft linings. The total load (PT) is expressed as follows:
~,~
6 0ROUT PIPES II 80" SANDED CEMENT gROUT
j
,
PT = Po + Po + P,,,
TOiSl0N ANCHORS (TYP)
where P,, = uniform horizontal pressure (psi); Po = n o n u n i f o r m horizontal pressure (psi); P,n = other loads (psi).
S a t ~ r l r pprdi e a 0 ,
[LIJ
I__ OlElar.m. SF.AL
The uniform horizontal pressure is the result of hydrostatic pressure and ground pressure. Therefore,
IQ::
P,, = Pw + Pc;
I F-
I" I
¢ImENT ~IOUT
where Pw = hydrostatic pressure (psi); Pc, = uniform ground pressure (psi).
L/) I mmlJ~ IIEIfJL III ~ ~ IIAIK Iq. ~'
moEmoTh.on pL ~.
CBENT ml0UT
• WI~tT PIPES • SO" m
FOUIOATION CON~qETE
4. uam Si.AII
smxcra~PLAT.__tt m
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4
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EXCAVATICN GIA. IN SALT
q
I I
I Figure 6. L o w e r shaft lining ]oundation. Table 1. Allowable material stresses.
Material Concrete:
4 0 0 0 psi 6 0 0 0 psi 8 0 0 0 psi
Steel:
ASTM A - 6 3 3 Gr.C ASTM A - 6 3 3 Gr.E
Buckling (psi)
Non-uniform and uniform (psi)
Uniform (psi)
3800 5100 6100
2600* 3900* 5200*
2100 2500 3050
49,000 59,000
44,500t 53,000t
29,000 35,000
* Based on ACI permissible stresses of 0.65 x fc as well as on guidelines for calculating shaft linings (7) for uniform plus non-uniform pressures. t Based on ratio of allowable stresses/yield stress for uniform plus nonuniform pressures as recommended by Link (7).
190
TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY
G r o u n d pressures are calculated on a stratigraphic unit-by-unit basis. Classic soil mechanics formulae are used for non-cohesive soils. For competent rock strata, ground pressures are negligible. Where the rock is predicted to fail and a relaxed zone develops, an interactive equation predicts the extent of this relaxed zone for a given support pressure. Additional loads resulting from freezing and thawing are considered in zones where the ground is frozen. Salt presents special problems due to its viscous behavior under pressure, known as "creep". Theoretically, full lithostatic pressures will eventually come to bear on a liner in salt zones. However, in reality this p h e n o m e n o n can be avoided by backing the liner with a yielding foam where possible. Except for the rare case that requires designing for full lithostatic pressure, hydrostatic pressures typically are higher than ground pressures. However, hydrostatic pressures will not bear on non-watertight linings such as the preliminary liner. Conversely, ground pressures will not bear on the final liner in areas where the asphalt seal column is used behind the liner; rather, the liner will be loaded only by the hydrostatic head of the asphalt column. The n o n - u n i f o r m load is calculated by varying a portion of the uniform pressure around the circumference of the lining, in accordance with the procedure established by Link et ai. (1976). The design also considers loads resulting from: (1) Time-delayed rock displacements. (2) Internal residue stresses. (3) Thermal effects. (4) Seismic effects. (5) Effects of construction methods. (6) Shaft e q u i p p i n g loads including those created by shaft furnish-
Volume 3, Number 2, 1988
LINING PRESSURE (PSI) 200 400 600 800 I000 1200 1400 1600 1800 2000 I
O
I
I
I
I
I
I
I
I
WATER TABLE
200 -
ASPHALT PRESSURE ON LINING(PA)
I000
TOTAL PRESSURE ENVELOPE
GI
FLUID PRESSURE ( PW )
t/3
TOTAL PRESSURE(PT)
2000
2475
NOTE: I. TOTAL PRESSURE (PT)= ROCK/SOIL PRESSURE + FLUID PRESSURE. 2. THE TOTAL PRESSUREENVELOPE=BITUMENPRESSURE ON LINING + ALLOWABLE PRESSURE ON PRELIMINARYLINING.
Figure 7. Summary o[ pressures acting against the shaft lining pressure envelope.
applies to the unlined shaft section in Unit 4 Salt. A creep allowance is designed into the shaft diameter, resulting in a 14-ft-(4-m-) diameter shaft. T h e allowable lining material stresses for different loading conditions are given in T a b l e 1. T h e final lining dimensions are given in Table 2.
ings, hoisting equipment, and instrumentation. (7) Shaft station area. (8) Shaft bottom plug. These loads and their applicability to specific sections of lining are evaluated on a case-by-case basis. For example, shaft-equipping loads are not a p p l i e d to the preliminary liner because no furnishings are attached to it. Pressures acting against the shaft l i n i n g are summarized in Fig. 7. Under atmospheric conditions the salt will creep unrestrained. T h i s condition
Seals Sealing of the watertight shaft lining sections will employ asphalt and "chemical seal".
Table 2. Final lining dimensions. Steel* Outer
Concrete
inner
Depth
t
t
Fs'
(ft)
t
fc'
(in.)
(in.)
(ksi)
(in.)
(ksi)
0.5 0.625 0.625 0.625 0.625 0.625 0.625
---1.00 1.25 1.375 1.625
50 50 50 60 60 60 60
25.5 25.5 25.5 24.75 24.5 24.25 24
6 6 8 8 8 8 8
0-1150 1150-1470 1470-1710 1710-2135
2135-2260 2260-2390 2390-2475
* To compensate for corrosion, the steel thickness is increased by 1 / 8 in. and is included in the steel dimensions shown above, except for the interval 0 - 1 1 5 0 ft which is protected by the bitumen envelope.
Volume 3, Number 2, 1988
Asphalt will be placed in the annular space between the outer watertight steel membrane and the shaft wall or primary shaft lining. T h e asphalt will have a penetration rate of 2 5 - 3 2 m m / 5 s at 25°C. It will be weighted with limestone flour of a fineness of 100% passing 200 mesh. T h e specific gravity of this asphalt mixture (approx. l . a g / c m 3) will exceed the specific gravity of the formation fluids. Therefore, the hydrostatic pressure of the asphalt c o l u m n will exceed the hydrostatic head of the fluid behind the shaft lining and will displace it. T h e asphalt will also occupy near-field pore space and fill small formation cracks. T h e bottom portion of the asphalt column will consist of absorbing cushions of sand and densely sanded asphalt. Where asphalt is not used, the watertight shaft lining will be terminated by sealing the annular space with a chemical seal. Chemical seals use a polymeric sealing c o m p o u n d called "Chemical Seal R i n g " (CSR) (Dellinger and Bough:on). CSR will be mixed with gravel and placed as a slurry that will be pliable enough to penetrate into nearfield cracks. After emplacement, the slurry will harden to form an elastic solid. T h e polymer material will expand when it comes into contact with water and develops swelling pressure. For this seal to be effective, it needs to be confined. Confinement of the seal material will be achieved by pressure g r o u t i n g with a cement slurry in the a d j o i n i n g shaft sections. Thus, the seal material itself will be pressurized t h r o u g h c o n f i n e m e n t for g r e a t e r effectiveness. Chemical seals are p l a n n e d for the lower shaft l i n i n g sections at approx. 2460 and 1045 ft (738 and 314 m) below the surface (Fig. 5). [] Acknowledgement
T h e preliminary shaft design presented in this paper has been completed under contract with the U.S. Department of Energy, Chicago Operations, Salt Repository Project Office. T h e authors wish to thank the S R P O for g r a n t i n g permission to present this paper. References
Dellinger, T. B. and Boughton, L. D. Unique materials mix used to seal. E. 6*M. ]. 166 (6): 114-118. Domke, O. 1915. Uber die Beanspruchung der Frostmaner beim Schachtabteufen nach Gefrierverfahren. Glueckau] 51 (47): 1129-1135.
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