Changes in Weak Soils Over Time

Temporal changes in weak
natural and artificial soils –
influence on geotechnical
characteristics
Peeter Talviste

Discussed medium
• Varved clays in western Estonia
• Oil shale retorting waste

Common aspects
• Very porous medium –
consisting 2/3 of voids and 1/3
solids in volume in the worst
case
• Have maintained that porous state under
overburden due to bonding
• Tend to lose geochemical bonding if
environmental conditions change

Common aspects
• Very porous medium –
consisting 2/3 of voids and 1/3
solids in volume in the worst
case
• Have maintained that porous
state under overburden due to
bonding
• Tend to lose geochemical bonding if environmental
conditions change

Common aspects
• Very porous medium – consisting
2/3 of voids and 1/3 solids in
volume in the worst case
• Have maintained that porous
state under overburden due to
bonding
• Tend to lose geochemical
bonding if environmental
conditions change

Related hazards
• By increasing cohesional
component bonding also
determines shear strength
of the medium
• Degradation of bonds means
decrease of the shear strength
• That may lead to the landslide or
yield of the dams of the waste
depositories

Related hazards
• By increasing cohesional
component bonding also
determines shear strength
of the medium
depositories
shear strength =  x tan() + c,
where :
 is vertical stress;
 is friction angle;
c is cohesion, often bonding.

Related hazards
• By increasing cohesional component
bonding also determines shear
strength of the medium
• Degradation of bonds
means decrease of the
shear strength
depositories
where :

Related hazards
• By increasing cohesional component
bonding also determines shear
strength of the medium
• That may lead to the
landslide or yield of the
dams of the waste
depositories
where :

Objectives of the thesis
• Landslides related objectives
• “Stress-history” related objectives
• Oil retorting waste management related
objectives

Landslide related objectives:
– To compare varved clays with clays in other landslide
areas and to understand the nature of the
geomechanical properties of varved clays
– To compare the Estonian landslides with the landslides related to the similar clays
– To explain the mechanism of strength decrease in varved clays

– To compare varved clays with clays in other landslide areas and to understand the
nature of the geomechanical properties of varved clays
– To compare the Estonian landslides with the
landslides related to the similar clays
– To explain the mechanism of strength decrease in varved clays

– To compare varved clays with clays in other landslide areas and to understand the
nature of the geomechanical properties of varved clays
– To compare the Estonian landslides with the landslides related to the similar clays
– To explain the mechanism of strength decrease in
varved clays

“Stress-history” related objectives:
– To establish the relationship between varve
thickness and water content of the varved clay
– To construct a spatial distribution model of the varved clay in Pärnu and to
explain some aspects of the post-glacial paleogeographical conditions in the
area

“Stress-history” related objectives:
– To establish the relationship between varve thickness and water content of
the varved clay
– To construct a spatial distribution model of the
varved clay in Pärnu and to explain some aspects
of the post-glacial paleogeographical conditions in
the area

Oil retorting waste management related
objectives:
– To propose the possible methods for cementation
control by describing the geochemical and
geomechanical deterioration of cementation
– By describing the geochemical and geotechnical aspects of black ash
cementation envisage the direction of future investigations in order to obtain
low permeability and high strength depositories

Oil retorting waste management related
objectives:
– To propose the possible methods for cementation control by describing the
geochemical and geomechanical deterioration of cementation
– By describing the geochemical and geotechnical
aspects of black ash cementation envisage the
direction of future investigations in order to obtain
low permeability and high strength depositories

Subject is discussed in two major panels:
• Late glacial varved clays: long term stability of the
erosional river valleys in western Estonia
• Shale oil processing wastes: composition and
cementation with implication to the long term
stability

Late glacial varved clays
• Geotechnical properties of the varved clay in Pärnu
• Comparison of the varved clay complex in Pärnu with
common landslide related clay deposits
• Analysis of landslides in Pärnu county
• Analysis of temporal stregth changes in terms of stress
release at erosional river valleys
• Correlation between geotechnical properties of the varved
clay and post-glacial conditions

Shale oil retorting waste
• Semi-coke
– Cementation of semi-coke deposits
– Stability of semi-coke cementation
– Semi-coke cementation controls
• Black ash
– Hydration, cementation and stability of black-ash

Late glacial varved clays: long term
stability of the erosional river
valleys in western Estonia

Geotechnical properties of the
varved clay complex in Pärnu

Geotechnical properties of the varved clay complex in Pärnu

0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
0.0 20.0 40.0 60.0 80.0 100.0 120.0
IPc,%
wLc, %
B
C
D
E
B
E
C
D

0
5
10
15
20
25
30
35
40
45
N
Clay content, %
B
C
D
E

Comparison of varved clay complex
in Pärnu with common landslide
related clay deposits – stiff fissured
clays and quick clays

Comparison of varved clay complex in Pärnu with common landslide related
clay deposits – stiff fissured clays and quick clays
• Comparison is made using the normalization of clay units via
void index – Iv – proposed by Burland (1990)
IV = (e-e*100) / (e*100-e*1000) = (e-e*100)/CC*)
• Porosity of the reconstituted clay is determined by ICL – intrinsic compression line – unique
relation between the applied stress and normalized void ratio
• Burland (1990) also proposes that sedimentation compression line (SCL) differs from ICL due
to clusters of particles forming during sedimentation

• Comparison is made using the normalization of clay units via void index – Iv – proposed by
Burland (1990)
• Porosity of the reconstituted clay is determined by ICL –
intrinsic compression line – unique relation between the
applied stress and normalized void ratio
• Burland (1990) also proposes that sedimentation compression line (SCL) differs from ICL due
to clusters of particles forming during sedimentation
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
10 100 1000 10000
VoidindexIvo
σ'vo, kPa
ICL, Burland 1990

• Comparison is made using the normalization of clay units via void index – Iv – proposed by
Burland (1990)
• Porosity of the reconstituted clay is determined by ICL – intrinsic compression line – unique
relation between the applied stress and normalized void ratio
• Burland (1990) also proposes that sedimentation compression
line (SCL) differs from ICL due to clusters of particles forming
during sedimentation
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
10 100 1000 10000
VoidindexIvo
σ'vo, kPa
SCL, Burland 1990
ICL, Burland 1990

clay deposits – stiff fissured clays and quick clays (Table 3 and Figure 5)

• Possible quick clay indicators – non–swelling clay minerals (illite,
chlorite), with activity less than ac <0.5 (Mitchell, 1976) and
wN/wL > 1.1 (Larsson and Ahnberg, 2003).

678 vane tests

• Varved clays in Pärnu are soft low sensitivity clays
• Clay is slightly overconsolidated OCR = 1.3...1.6 (Kalm et al., 2002)
• At river valleys OCR is bigger due to stress release after erosion
• Varved clay has higher wN/wL ratio compared to the analogues
• There is no evidence of “quick clay phenomenon “ , however at some locations (Sauga, Audru
landslides) some clay sequences have remarkably higher water content compared to the
average
• I propose that overconsolidation at high void ratio is due to cemented bonds, possibly
carbonates

• Clay is slightly overconsolidated OCR = 1.3...1.6 (Kalm et al.,
2002)
average
carbonates

• At river valleys OCR is bigger due to stress release after
erosion
average
carbonates

• Varved clay has higher wN/wL ratio compared to the
analogues
average
carbonates

• There is no evidence of “quick clay phenomenon “ , however
at some locations (Sauga, Audru landslides) some clay
sequences have remarkably higher water content compared
to the average
carbonates

average
• I propose that overconsolidation at high void ratio is due to
cemented bonds, possibly carbonates

Analysis of landslides in Pärnu county

Three different types of landslides:
A – large slides in varved clays
B – slides in marine sands
C – small (4...15 m) slides in varved clays in the bank of
the flow channel
A
B

Three different types of landslides:
A – large slides in varved clays
B – slides in marine sands
C – small (4...15 m) slides in varved
clays in the bank of the flow channel
A
B
C

Small, type C landslides:
• Are “build up” by erosion and are often related to the rapid
fall of water level – similar to the slides in intact clays. Due to
higher OCR not always happening at critical state fully
softened strength.
• Occur in course of strength softening process – similar to the coastal cliff ruptures in stiff
fissured clays. Scale difference due to differences in OCR (OCR > 10...20 v OCR = 1.6...2)
• Trigger larger landslides similar to “quick clay” slopes. Flake-type slides are not observed.

• Are “build up” by erosion and are often related to the rapid fall of water level – similar to the
slides in intact clays. Due to higher OCR not always happening at critical state fully softened
strength.
• Occur in course of strength softening process – similar to the
coastal cliff ruptures in stiff fissured clays. Scale difference due
to differences in OCR (OCR > 10...20 v OCR = 1.6...2)
• Trigger larger landslides similar to “quick clay” slopes. Flake-type slides are not observed.

• Are “build up” by erosion and are often related to the rapid fall of water level – similar to the
slides in intact clays. Due to higher OCR not always happening at critical state fully softened
strength.
• Occur in course of strength softening process – similar to the coastal cliff ruptures in stiff
fissured clays. Scale difference due to differences in OCR (OCR > 10...20 v OCR = 1.6...2)
• Trigger larger landslides similar to “quick clay” slopes. Flake-
type slides are not observed.

Large, type A landslides:
• Occur at fully softened critical state strength similar landslides
at gentle slopes in stif fissured clay and intact clay.
• Are retrogressive, similar to “quick clay” slides happen in many stages. Though varved clays
are soft highly plastic clay with low sensitivity.
• Follow the old rupture zones similar to the slides in stiff fissured clays. Strength in these
zones corresponds to undrained weakened (but on residual) strength rather than drained
residual strength.

• Occur at fully softened critical state strength similar landslides at gentle slopes in stif fissured
clay and intact clay.
• Are retrogressive, similar to “quick clay” slides happen in
many stages. Though varved clays are soft highly plastic clay
with low sensitivity.
• Follow the old rupture zones similar to the slides in stiff fissured clays. Strength in these
zones corresponds to undrained weakened (but on residual) strength rather than drained
residual strength.

• Occur at fully softened critical state strength similar landslides at gentle slopes in stif fissured
clay and intact clay.
• Are retrogressive, similar to “quick clay” slides happen in many stages. Though varved clays
are soft highly plastic clay with low sensitivity.
• Follow the old rupture zones similar to the slides in stiff
fissured clays. Strength in these zones corresponds to
undrained weakened (but on residual) strength rather than
drained residual strength.

Analysis of the history and future
of the strength in terms of stress
release at erosional river valleys in
western Estonia

Analysis of the history and future of the strength in terms of stress release at
Soil softening or strength decrease of overconsolidated
fissured clays due to stress release:
• In case of pure, erosion related overconsolidation, the modern
soil mechanics predicts that the total softening is due to stress
release and corresponding swelling.
• The process takes time due to low permeability of clay but ultimately leads to lower strength.
• Critical state drained strength is the lowest possible strength due to soil softening.

• In case of pure, erosion related overconsolidation, the modern soil mechanics predicts that the
total softening is due to stress release and corresponding swelling.
• The process takes time due to low permeability of clay but
ultimately leads to lower strength.
• Critical state drained strength is the lowest possible strength due to soil softening.

• In case of pure, erosion related overconsolidation, the modern soil mechanics predicts that the
total softening is due to stress release and corresponding swelling.
• The process takes time due to low permeability of clay but
ultimately leads to lower strength.
• Critical state drained strength is the lowest possible strength
due to soil softening.

Long term strength decrease due to stress release of
Cambrian clay, Estonia
1. Strength decrease is
evident
2. Approx. 10 000 years
since the last glaciation
receeded

Strength decrease due to shear strain or strain
softening, London clay:
• At the slope area shear stresses occur in addition to the normal
stress release due to erosion.
• According to Skempton (1970) developing shear strain may lead to opening of fissures due to
dilatancy and somewhat faster water inflow. Skempton followed a higher water content in the
narrow zone along the slip surface in stiff fissured London clay.
• Full softening in slope area developed within 30...40 years (Skempton, 1964, 1970), a short
period compared to approx. 10 000 years in Sillamäe case.

• At the slope area shear stresses occur in addition to the normal stress release due to erosion.
• According to Skempton (1970) developing shear strain may lead
to opening of fissures due to dilatancy and somewhat faster
water inflow. Skempton followed a higher water content in the
• Full softening in slope area developed within 30...40 years (Skempton, 1964, 1970), a short
period compared to approx. 10 000 years in Sillamäe case.

• At the slope area shear stresses occur in addition to the normal stress release due to erosion.
• According to Skempton (1970) developing shear strain may lead to opening of fissures due to
dilatancy and somewhat faster water inflow. Skempton followed a higher water content in the
• Full softening in slope area developed within 30...40 years
(Skempton, 1964, 1970), a short period compared to approx.
10 000 years in Sillamäe case.

Soil softening of the bonded soft clays due to erosion
• Some varved clay sequences in western Estonian river valleys
have additional overconsolidation due to bonding effect
• That has lead to the much looser compaction and higher water content compared to SCL of
Burland (1990)
• Bonding effect may hold back swelling (Bjerrum, 1973) and soil softening due to stress release

• Some varved clay sequences in western Estonian river valleys have additional overconsolidation
due to bonding effect
• That has lead to the much looser compaction and higher water
content compared to SCL of Burland (1990)
• Bonding effect may hold back swelling (Bjerrum, 1973) and soil softening due to stress release

• Some varved clay sequences in western Estonian river valleys have additional overconsolidation
due to bonding effect
• That has lead to the much looser compaction and higher water content compared to SCL of
Burland (1990)
• Bonding effect may hold back swelling (Bjerrum, 1973) and soil
softening due to stress release

Soil softening of the bonded soft clays due to shear strain
• However at slope areas shear stain developing due to shear
stresses may break down the bonds (Quinn, 2009)
• The overconsolidation caused by bonding disappears and due to already loose and wet saturated
state the strength decrease may develop immediately
• That kind of process tends to expand beyond initially softened zone and trigger a landslide (Quinn,
2009)

• However at slope areas shear stain developing due to shear stresses may break down the bonds
(Quinn, 2009)
• The overconsolidation caused by bonding disappears and due to
already loose and wet saturated state the strength decrease may
develop immediately
• That kind of process tends to expand beyond initially softened zone and trigger a landslide (Quinn,
2009)

• However at slope areas shear stain developing due to shear stresses may break down the bonds
(Quinn, 2009)
• The overconsolidation caused by bonding disappears and due to already loose and wet saturated
state the strength decrease may develop immediately
• That kind of process tends to expand beyond initially softened
zone and trigger a landslide (Quinn, 2009)

Timeframe for soil softening:
• Stiff fissured clay due to erosion – thousands of years
(Cambrian clay in Sillamäe, Estonia)
• Stiff fissured clays due to erosion and shear strain – 30...40 years (London clay)
• Soft clay, slightly overconsolidated due to bonding – years (Audru landslide) or minutes
(Sauga landslide)

• Stiff fissured clay due to erosion – thousands of years (Cambrian clay in Sillamäe, Estonia)
• Stiff fissured clays due to erosion and shear strain – 30...40
years (London clay)
• Soft clay, slightly overconsolidated due to bonding – years (Audru landslide) or minutes
(Sauga landslide)

• Stiff fissured clay due to erosion – thousands of years (Cambrian clay in Sillamäe, Estonia)
• Stiff fissured clays due to erosion and shear strain – 30...40 years (London clay)
• Soft clay, slightly overconsolidated due to bonding – years
(Audru landslide) or minutes (Sauga landslide)

Correlation
between the
geotechnical
properties of the
varved clay and
post-glacial
conditions

Gentle erosional scarp of the varved clay surface in Pärnu
Correlation between the geotechnical properties of the varved clay and post-
glacial conditions

Selective erosion supported by clay thickness v lower
clay surface height analysis
glacial conditions
~4 m

Some paleo-geographic conclusions:
• Lowest water level in the Pärnu Bay after the Billingen
drainage event at 0 to -2 m
• Selective erosion at least of about upper 4 m of clay occur at the area between the erosional
scarf and recent shore line in Pärnu
• Area north from the erosional scarf was temporarily emerged and due to periodic freezing-
melting cycles upper clay interval has lost his varve structure (unit A)
glacial conditions

• Lowest water level in the Pärnu Bay after the Billingen drainage event at 0 to -2 m
• Selective erosion at least of about upper 4 m of clay occur at
the area between the erosional scarf and recent shore line in
Pärnu
• Area north from the erosional scarf was temporarily emerged and due to periodic freezing-
melting cycles upper clay interval has lost his varve structure (unit A)
glacial conditions

• Lowest water level in the Pärnu Bay after the Billingen drainage event at 0 to -2 m
• Selective erosion at least of about upper 4 m of clay occur at the area between the erosional
scarf and recent shore line in Pärnu
• Area north from the erosional scarf was temporarily emerged
and due to periodic freezing-melting cycles upper clay interval
has lost his varve structure (unit A)
glacial conditions

Shale oil processing wastes:
composition and cementation with
implications to long term stability

Objectives for shale oil retorting waste
management
• Ensure the low permeability of landfill body to
minimize the infiltration of leachate
• Confirm high mechanical stability of the deposits to reduce the possibility
of a failure and consequent pollution of the environment

Objectives for shale oil retorting waste
management
• Ensure the low permeability of landfill body to minimize the infiltration of
leachate
• Confirm high mechanical stability of the
deposits to reduce the possibility of a failure
and consequent pollution of the environment

Semi-coke - processing
• Blackish granular material which contains mineral and organic
part, residue that forms in absence of oxygen
• Coking occurs at temperatures 350-400°C, later residue is heated to 900-1000°C to burn out
as much of the organic as possible
• Combustion stage is too short and organic residue remains 6-10%.
Semi-coke
Grain size distribution of semi-coke

• Blackish granular material which contains mineral and organic part, residue that forms in
absence of oxygen
• Coking occurs at temperatures 350-400°C, later residue is
heated to 900-1000°C to burn out as much of the organic as
possible
• Combustion stage is too short and organic residue remains 6-10%.
Semi-coke

• Blackish granular material which contains mineral and organic part, residue that forms in
absence of oxygen
• Coking occurs at temperatures 350-400°C, later residue is heated to 900-1000°C to burn out
as much of the organic as possible
• Combustion stage is too short and organic residue remains 6-
10%.
Semi-coke
Semi-coke in microscopy

Semi-coke - geotechnical properties
Semi-coke

Semi-coke – high liquid limit
Semi-coke

Cementation of semi-coke deposits
• The dominant hydration product of
semi-coke is secondary ettringite –
Ca6Al2(SO4)3(OH)12*26H2O
• The precipitation kinetic of ettringite in the lab is rapid, in
the depository much slower and takes 2-3 weeks
• Ettringite is responsible for strength development in
deposits
• Precipitation of ettringite causes significant reduction of
hydraulic conductivity (less tha 10-8 m/s)
Semi-coke

• The dominant hydration product of semi-coke is
secondary ettringite – Ca6Al2(SO4)3(OH)12*26H2O
• The precipitation kinetic of ettringite
in the lab is rapid, in the depository
much slower and takes 2-3 weeks
deposits
Semi-coke
Dynamics of ettringite content,
field test

• Ettringite is responsible for strength
development in deposits
Semi-coke

deposits
• Precipitation of ettringite causes
significant reduction of hydraulic
conductivity (less tha 10-8 m/s)
Semi-coke

Stability of semi-coke cementation
• Geochemical stability of the ettringite is sensitive towards
pH and is stable at pH>10.7
Semi-coke

Semi-coke
towards temperature and is unstable at temperature above 70°C

Semi-coke
>1000°C

Semi-coke
>1000°C
<70°C
<70°C
<70°C
<70°C

towards temperature and is unstable at temperature above 70°C
Semi-coke

• Geomechanical crushing of the
ettringite “needles”
Semi-coke

• Geomechanical crushing of the
ettringite “needles”
Semi-coke
Confined compression curve on granular material
Confined compression line of semi-coke

Semi-coke

Semi-coke
Total shear strength

Semi-coke
Decreasing cohesion
Total shear strength

Semi-coke
Total shear strength =
Decreasing cohesion
Frictional strength +

Semi-coke cementation controls
• Reduction of the
porosity
– by mechanical
compaction
– by co-deposition
with ashes (TTP ash,
black-ash) by filling
the large voids in
semi-coke
Semi-coke

porosity
– by mechanical
compaction
– by co-deposition with ashes
(TTP ash, black-ash)
Semi-coke

porosity
– by mechanical
compaction
– by co-deposition with ashes
(TTP ash, black-ash)
Semi-coke
Lanfill capacity increase
By 25...30% (in tons) by
maintaining the designed
shape and volume

porosity
– by mechanical compaction
black-ash)
Semi-coke
Ash semi-coke ratio 1:4

porosity
– by mechanical compaction
black-ash)
Semi-coke
Ash semi-coke ratio >1:4

Black-ash – processing
• Black-ash is fine grained TTP fly ash like
material
• Coking occurs in presence of oxygen
• Coking temperatures are higher
• Contains only few percent of organics
• Mineral composition of black-ash differs from composition of
semi-coke
Black-ash
Black ash grain size
distribution

• Black-ash is fine grained TTP fly ash like
material
• Mineral composition of black-ash differs from composition of
semi-coke
Black-ash

• Black-ash is fine grained TTP fly ash like material
• Mineral composition of black-ash differs
from composition of semi-coke
Black-ash

Black-ash – geotechnical properties
Black-ash

Cementation of black-ash
• No evidence of massive
ettringite or gypsum
precipitation
• Binding phases are ferrite and
belite hydration outcome and
later hydrocalumite
• Samples did not expand nor crack (common for
TTP ash with ettringite cementation)
• Cementation leads to a small but steady
decrease of voids volume. It is proposed that is
due to sedimentation of minerals in the pore
space rather than rapid crystallization.
Black-ash

• No evidence of massive ettringite or gypsum
precipitation
• Binding phases are ferrite and belite hydration
outcome and later hydrocalumite
• Samples did not expand nor
crack (common for TTP ash with
ettringite cementation)
• Cementation leads to a small
but steady decrease of voids
volume. It is proposed that is
due to sedimentation of
minerals in the pore space
rather than rapid crystallization.
Black-ash

Black-ash

Stability of the black-ash cementation
• In long term the cementation bonds in black-ash are more
stable (compared to TPP ash and semi-coke) due to hydration
of belite and only very low content of unstable ettringite.
• The possibilities of strength control via initial void ratio, use of different black-ash fractions,
cementation bonds composition and location in the voids needs further study as it may
influence the re-use of black ash as a material.
Black-ash

Stability of the black-ash cementation
• In long term the cementation bonds in black-ash are more stable (compared to TPP ash and
semi-coke) due to hydration of belite and only very low content of unstable ettringite.
• The possibilities of strength control via initial void ratio, use of
different black-ash fractions, cementation bonds composition
and location in the voids needs further study as it may
influence the re-use of black ash as a material.
Black-ash

Conclusions – geotechnical properties of varved clays:
1. Varved clays in Pärnu are mainly soft low sensitivity clays with
high water content close to the liquid limit, often
overconsolidated.
2. Although activity chart suggest that some samples are similar to “quick clay” formation, the
wN/wL ratio and measured sensitivity does not indicate the phenomenon of “quick clays” in
Pärnu area.
3. The overconsolidation of varved clays in Pärnu is ue to cemented bonds and that has led to the
higher water content than normal. Nature of bonding is not investigated but carbonate
mineralization could be considered as possible cementing agent.
4. The recent landslides in Pärnu area have several yielding mechanisms and in some aspects are
similar to the landslides in “quick clays” and in other aspects to the landslides in
overconsolidated clays. Geotechnical properties of the clay units, namely the water content and
overconsolidation ratio, are determining the rupture mechanism.
Conclusions

1. Varved clays in Pärnu are mainly soft low sensitivity clays with high water content close to the
liquid limit, often overconsolidated.
2. Although activity chart suggest that some samples are similar
to “quick clay” formation, the wN/wL ratio and measured
sensitivity does not indicate the phenomenon of “quick clays” in
Pärnu area.
Conclusions

Pärnu area.
3. The overconsolidation of varved clays in Pärnu is ue to
cemented bonds and that has led to the higher water content
than normal. Nature of bonding is not investigated but
carbonate mineralization could be considered as possible
cementing agent.
Conclusions

Pärnu area.
4. The recent landslides in Pärnu area have several yielding
mechanisms and in some aspects are similar to the landslides in
“quick clays” and in other aspects to the landslides in
overconsolidated clays. Geotechnical properties of the clay units,
namely the water content and overconsolidation ratio, are
determining the rupture mechanism.
Conclusions

Conclusions – landslides and temporal strenght reduction
of varved clays
5. The nature of soil softening to the fully softened critical state
strength can be discussed in context of erosion related stress and
creep, which follow shear stresses in slope area. Timeframes for
the soil softening are from minutes (in Sauga landslide case) to the
years (in Audru landslide case) and decades (in the stiff fissured
clay slopes) up to the thousands of years (stiff fissured clay in
Sillamäe without significant shear stresses in massive clay beds).
6. The soil softening is inevitable and the question is not if the slopes steeper then critical would fail,
but when they fail. The critical slope inclination of ½ from the critical state friction angle can be
suggested for practical use.
Conclusions

Conclusions – landslides and temporal strenght
reduction of varved clays
5. The nature of soil softening to the fully softened critical state strength can be
discussed in context of erosion related stress and creep, which follow shear stresses in
slope area. Timeframes for the soil softening are from minutes (in Sauga landslide case)
to the years (in Audru landslide case) and decades (in the stiff fissured clay slopes) up to
the thousands of years (stiff fissured clay in Sillamäe without significant shear stresses in
massive clay beds).
6. The soil softening is inevitable and the question is not if
the slopes steeper then critical would fail, but when they
fail. The critical slope inclination of ½ from the critical state
friction angle can be suggested for practical use.
Conclusions

Conclusions – geotechnical properties of varved clays
and paleogeographical reconstructions
7. Low-stand of Yoldia Sea in the Pärnu Bay can be placed to an
altitude of 0 to -2 m, which is rather similar to the earlier
proposed minimum level in the Pärnu area between 3 and 0 m
a.s.l.
8. The formation and spatial distribution of desiccated clay unit A can be discussed and explained
as emerged from the sea during the water level low-stand after the Billingen event.
Conclusions

Conclusions – geotechnical properties of varved clays
and paleogeographical reconstructions
7. Low-stand of Yoldia Sea in the Pärnu Bay can be placed to an altitude of 0 to -2 m, which is
rather similar to the earlier proposed minimum level in the Pärnu area between 3 and 0 m a.s.l.
8. The formation and spatial distribution of desiccated clay unit
A can be discussed and explained as emerged from the sea
during the water level low-stand after the Billingen event.
Conclusions

Conclusions – semi-coke
9. Semi-coke is a high porosity material due to high organic
content and geochemical reactions starting immediately if
mixed with water. The high void ratio is held up by ettringite
bonding. However, the ettringite bonding is unstable due to
geochemical degradation and geomechanical crushing.
10. Initial porosity of semi-coke can be controlled by compaction. Smaller initial void ratio
leads to lower permeability, hence lower geochemical degradation rates of ettringite and
lower compressibility, hence lower geomechanical crushing of ettringite.
11. Attempts to control the initial porosity of semi-coke by adding fine TTP ash in order to
fill the large scale voids of semi-coke did lead to expansion of the mixtures if the TTP ash
content exceeded 20-25% in the mixture.
Conclusions

9. Semi-coke is a high porosity material due to high organic content and geochemical reactions starting
immediately if mixed with water. The high void ratio is held up by ettringite bonding. However, the
ettringite bonding is unstable due to geochemical degradation and geomechanical crushing.
10. Initial porosity of semi-coke can be controlled by compaction.
Smaller initial void ratio leads to lower permeability, hence lower
geochemical degradation rates of ettringite and lower
compressibility, hence lower geomechanical crushing of ettringite.
11. Attempts to control the initial porosity of semi-coke by adding fine TTP ash in order to fill the large
scale voids of semi-coke did lead to expansion of the mixtures if the TTP ash content exceeded 20-25%
in the mixture.
Conclusions

9. Semi-coke is a high porosity material due to high organic content and geochemical
reactions starting immediately if mixed with water. The high void ratio is held up by
ettringite bonding. However, the ettringite bonding is unstable due to geochemical
degradation and geomechanical crushing.
10. Initial porosity of semi-coke can be controlled by compaction. Smaller initial void ratio
leads to lower permeability, hence lower geochemical degradation rates of ettringite and
lower compressibility, hence lower geomechanical crushing of ettringite.
11. Attempts to control the initial porosity of semi-coke by
adding fine TTP ash in order to fill the large scale voids of
semi-coke did lead to expansion of the mixtures if the TTP
ash content exceeded 20-25% in the mixture.
Conclusions

Conclusions – black-ash
12. Black ash exhibits self-cementing properties when
mixed with water reaching compressive strength values >6
MPa. Most of the strength was gained during the first 30
days, and futher increase was only minor.
13. The self-cementing properties of black ash are controlled mainly bu the hydration of
belite, precipitation of secondary calcite and, to the lesser extent, by hydrocalumite. This
type of cementation can be considered chemically stable.
14. The possibility that the strength of black ash could be controlled via initial void ratio
or by use of different fractions of black ash needs further assessment. The composition of
cementation bonds and their location in the voids needs further study as it is important
for possible re-use of the black-ash.
Conclusions

12. Black ash exhibits self-cementing properties when mixed with water reaching
compressive strength values >6 MPa. Most of the strength was gained during the first 30
13. The self-cementing properties of black ash are
controlled mainly bu the hydration of belite, precipitation of
secondary calcite and, to the lesser extent, by
hydrocalumite. This type of cementation can be considered
chemically stable.
14. The possibility that the strength of black ash could be controlled via initial void ratio
or by use of different fractions of black ash needs further assessment. The composition of
cementation bonds and their location in the voids needs further study as it is important
for possible re-use of the black-ash.
Conclusions

12. Black ash exhibits self-cementing properties when mixed with water reaching
compressive strength values >6 MPa. Most of the strength was gained during the first 30
13. The self-cementing properties of black ash are controlled mainly bu the hydration of
belite, precipitation of secondary calcite and, to the lesser extent, by hydrocalumite. This
type of cementation can be considered chemically stable.
14. The possibility that the strength of black ash could be
controlled via initial void ratio or by use of different
fractions of black ash needs further assessment. The
composition of cementation bonds and their location in the
voids needs further study as it is important for possible re-
use of the black-ash.
Conclusions

A. Einstein:
Things should be made as
simple as possible, ...
Conclusions

A. Einstein:
Things should be made as
simple as possible, but not
one bit simpler.
Conclusions

Changes in Weak Soils Over Time

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