1

Closure of the Teutschenthal backfill mine –challenge for a geomechanical safety concept

Till Popp, Wolfgang MinkleyInstitute for Geomechanics GmbH (IfG)

Karsten Maenz, Erik FillingerGTS Teutschenthal

Washington, DC

September 7-9, 2016Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a

wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear

Security Administration under contract DE-AC04-94AL85000. SAND2016-XXXX.

2

Motivation - Study of industrial analogues

Hazardeous waste is used for stabilization underground openings

„Long-term-containment“ has to be guaranted,

but because the mines were not designed as repositories

Sophisticated geotechnical „Safety concept“ are required

Backfilling of underground openings in German salt mines (UTV)

Learning from conventional closure concepts may have an important benefit for geological storage of radioactive waste, e.g. for(1) validating the geomechanical approaches used for integrity

analysis of the salt barrier

(2) shaft and drift seal realisation

3

Underground disposal or backfilling measures in German salt mines

4 underground repositories (UTD) 10 mines with backfill measures (UTV)

Regulations and Requirements in Germany: TA Abfall (Technical Instructions on Waste) Technische Regeln Versatz (technical rules backfill) Versatzverordnung 2002 (backfill regulation)

• Proof of long-term safety :- Assessment of the geological barriers- Geomechanical integrity analysis- Long term szenario analysis

• A geotechnical safety concept, based- on the geological, hydro-geological site

situation and waste data- Geotechnical closure concept

4

General safety concept according to the German Regulations

The unique host rock properties of salt enable a fast and total inclusion / encapsulation of the waste and its hazardous constituents without any further barriers needed (in the best case).

Concept of complete inclusion

If the used salt rock formation shows any deficits (e.g. homogeneity, thickness) properties of the host rock might become offset by means of a so-called multi-barrier system.

Multi-barrier system

After Brasser (GRS)

5

History / Facts – GTS Teutschenthal

MF Salzmünde

MF Teutschenthal

Begin of mining 1906 Potash and rocksalt mining at 600 m – 900m depth 3 connected mining fields

- MF Teutschenthal- MF Angersdorf- MF Salzmünde

4 shafts ca. 15 mio. m3 mined underground

openings 3 rock bursts (RB): 1916, 1940, 1996 1982 shut down of the mine Since 1992 GTS: backfilling of the

mining area Teutschenthal 2026 End of backfilling

RB 1940

RB 1996

MF Angersdorf

RB 1916 5 km

6

Backfilling measures in the Teutschenthal mine

Direct backfillingas bulk material or contained within big bags.

Conditioned backfillingPreparation of suitable backfill by a special mixing.

Slurry backfillingThis is hydraulically transported directly to the excavations to be backfilled, where it hardens.

ca. 150,000m3 / y

ca. 70,000m3 / y

7

Overview Teutschenthal – Where are the challenges?

Partly flooded potash seamRock burst risk?

Main Anhydrite = Inflow of water! - Migration path?

Direct backfillStorage for fluids?

Hydraulic backfill – significant water amount!

600,000 m3 NaCl-Brine

3 fluid filled Caverns1 Mio. m3 NaCl-brine

Rock burst area 1996Integrity of the salt barrier?

MF Salzmünde- Shaft sealing- 100,000 m3 Fluids

Mining in the rocksalt barrierBackfilling

Schneesalz –drift- Sealing of the drift

Salzmünde

Teutschenthal

Saale

Halle

MF Teutschenthal

MF Angersdorf

8

The geological barriers – prerequisite for safe containment

Rockburst area 1996

Schneesalz-drift

Rock salt-mining

Z4 Aller-Rock salt(ca. 20m)

T4 Red salt clay(ca. 15m)

Z3 Leine-Rock salt(ca. 100m)

A3 Main anhydrite(50m)

Na2 Staßfurt-Rocksalt(ca. 500m)

K2 Potash seam Staßfurt

Shaft HalleShaft SaaleShaft Teutschenthal

T3 Grey salt clay

Main Geologicalbarrier

Shaft Salzmünde

Dry cap rock

Cav. 3

Cav. 1 Cav. 2

Lower Bunter

Mined potash seam

MF TeutschenthalMF Salzmünde MF Angersdorf

ca. 1 Mio. m3 NaCl-brine

Hydraulic backfill ca. 0,6 Mio. m3 NaCl-brine

ca. 0,1 Mio. m3 MgCl2-brine

Water bearing cap rock

9

Rock burst Teutschenthal September 11, 1996

2,5 m2

After the rock burst no water inflow into the mine were observed

10

Geomechanical analysis of the rock burst

MP 1434Modell

rock burst 1996due to pillarcreep failure

subs

iden

ce (

m )

0

0,6

1,2

time ( years )10 20

collapsed mining area 1996

measuring point 1434

N S

bunter sandstone

rock salt

1600 m

rock saltanhydritecarnallitite

900 m

model

• Numerical modelling:Precast of the - Event – it will happen soon!- Order of magnitude of the rock burst

Reliability of the models has been demonstrated

11

New drift between Teutschenthal and Angersdorf

• Above the rock burst field 1996• Ca. 3 km long• Cross section: 18,2 m2

Geological cross section

Rock burst area

New drift

Shaft Teutschenthal

Shaft Saale

Shaft Halle

Unique access to the dynamically loaded salt barrier above the rock burst area

Rock burst area

12

Proof of salt barrier integrity of rock salt after the rock burst 1996

Stress state in the barrierMinimal stress measurements

WesternUnaffected area

Easternunaffected

area

Rock burst area

Lithostatic stress: 16 MPa

• Significant unloading of the rock burst area (ca. 30%),

• but outside of this area only small interactions are visible

• Stress recovery takes place

13

Proof of salt barrier integrity of rock salt after the rock burst 1996

Hydraulic integrity of the unloaded salt barrierFluid-pressure decay in sealed boreholes

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100 110

Zeit [Tage]

Dru

ck im

Prü

frau

m [b

ar]

0,0

5,0

10,0

15,0

20,0

25,0

30,0

radi

ale

Aus

brei

tung

[m

m] d

er

Sätti

gung

sfro

nt s

at=1

00%

Messung Bohrung TTL_8Berechnung, Fall a TTL_83,00E-221,00E-22

Fall a TTL_81. Prüfabschnitt (Steifigkeit 150 MPa):0,0 - 2,0 mm: k = 1,0E-20 m2, n = 2,0‰2,0 - 6,5 mm: k = 2,0E-21 m2, n = 1,2‰6,5 - 8,4 mm: k = 1,0E-22 m2, n = 0,7‰

Fall a TTL_82. Prüfabschnitt (Steifigkeit 500 MPa):0,0 - 4,5 mm: k = 5,0E-20 m2, n = 2,5‰4,5 - 6,7 mm: k = 1,0E-20 m2, n = 2,0‰6,7 - 13,8 mm: k = 4,0E-21 m2, n = 1,4‰13,8 - 17,5 mm: k = 2,0E-21 m2, n = 1,2‰17,5 - 22,2 mm: k = 5,0E-22 m2, n = 0,8‰ 22,2 - ... mm: k = 5,0E-23 m2, n = 0,5‰

5E-23

1E-22

Flui

d pr

essu

re (b

ar)

Time (days)

Rad

ial s

prea

ding

(mm

) of

the

fluid

sat

urat

ion

fron

t

• Recalculation of the fluid flow into the salt results in 10-22 m2

The integrity of the salt barrier is not affected, i.e.

Dynamic events such earth-quakes are not a risk in the long-term (at sufficient barrier thickness)

14

Long-term szenario of fluid migration - Safety concept

Squeezing out of fluids

Inhibited fluid flow

Migration of fluids through the EDZ and fractures

Drift and shaft seals

Disturbed /crushed zone

fractures

Rockburst area 1996

Schneesalz-drift

Rock salt-mining

Z4 Aller-Rock salt

T4 Red salt clay

Z3 Leine-Rock salt

A3 Main anhydrite

Na2 Staßfurt-Rocksalt

K2 Potash seam Staßfurt

Shaft HalleShaft Saale

T3 Grey salt clay

Main Geologicalbarrier

Dry cap rock

Cav. 3

Cav. 1 Cav. 2

Lower Bunter

Containment of fluids:• Main anhydrite• Pore space in crushed salt

Shaft Teutschenthal

Shaft Salzmünde

15

Long term storage capacity of fluids inside the salt formation

Main Anhydrite : Salt mine Roßleben Salt rocks : Werra-deposit (bedded salt)

Super-critical CO2(tertiary volcanism)

Residual salt liquids(salt formation processes)

Residual salt solutions in fractured reservoirs

5 brine inflow events with ca. 5 mio. m3 saturated brine:

1909: 0,5 Mio. m3

1921: 0,5 Mio. m3

1939: 2,3 Mio. m3

1982: 1,1 Mio. m3

1986: 0,6 Mio. m3

Fossil salt solutions out of the main anhydrite:

Tritium and C14 -tests

Salt solutions and gases in salt formations

(µl - ml bis > 1000 m3)

Hauptanhydrit

CO2 – Gletscher nach Ausbruch

CO2-reservoirs up to several 100.000 m3:• Cracks / fractures• Cavernous openings

Kristallgrotte Merkers

Natural and technical analogues

Salt mine Rossleben

16

Demands for sealing measures - Prototype concept shaft Saale

Technical Workabilit

ye.g. site

concrete, shot concrete,

pre-consolidation

Material behavio

ure.g. heat

development during setting,

shrinking

Long term

stabilityGeochemical compatibility with the host

rockTightnesse.g. Sealing

element

Strengthe.g. abutment

Overburden

MgO-concrete: Long-term stable in contact with MgCl2

Crushed salt / clay

MgO-concrete

17

Shaft seal - Proof of hydromechanical integrity

Development of a shaft seal concept Prevention of water inflow to the mine Prevention of outflow of contaminated fluids

from the waste

746,2

712,1

Lösungsspiegel

MgO-Beton

Asphaltmastix

BinäresCalcigelgemisch

WL

DE 1A

Füllsäule aus Salzgrus(bis 380 m)- Option Haldensalz

(Füllsäule - ortsstab iler Sorelbausto ff)

700,8

698,5

682,3

681,8

DE 2

Salzgrus(verdichtet)

Hilfs-W L

Bitumen

Pegmatitanhydrit

Leinesteinsalz

RoterSalzton

Aller Steinsalz

Grenzanhydrit

DE 1B

MgO-Beton

DE 2

DE 1A

DE 1B

WL

Pbottom ca.18 MPa

t

Ptop ca.8,4 MPa

Crushed salt / clayelement

Considered processes• Convergence-

induced ompaction of the sealing elements

• Water saturation of the bentonite sealing element DE2 Development of swelling pressure

• In the crushed salt / clay element decrease of the permeability due to associated compaction and healing processes

Change of the load support from the lower abutment to the upper sealing (abutment) elements The integrity of the innovative shaft seal

concept with MgO- and crushed salt elements was demonstrated by numerical modelling.

18

Conclusions

Shaft condition 20.05.2004 © Lars Baumgarten

2013/14: Pre-studies (i.e. safety proof) finished

in 2016/17: Re-opening of the shaft2018 – 2020 Installing of the improved

shaft seal

Industrial analogues give valuable input for assessment of radioactive waste repositories:• Concept of “complete inclusion”• Geotechnical multi-barrier systems

Lessons learned at the Teutschenthal site

• The geological barrier “salt” with sufficient thickness is not at risk at dynamic events (e.g. earth quakes).

• Re-calculation of failure events allows qualification and proof of the numerical tools used for geotechnical integrity analysis.

• Significant amounts of fluids may be stored in salt.• MgO-concrete was identified as powerful building

material for sealing systems in salt formations where potash rocks exist.

• Sealing concepts (for shafts or drifts) are developed, which will be realized within the next 10 – 15 years.