The Kyrgyz Republic:
Disaster Hazard Mitigation Project (DHMP), Component A 2
Relocation of TP3 and TP18 material to TP6
Preliminary Quality Assurance/Quality Control Program
Appendix 7:
Geotechnical Stability Calculations for Evaluation
of Proper Relocation Technologies
1
Table of Content
1
2
Introduction ............................................................................................................................ 3
Input parameters for the stability calculations ......................................................................... 4
2.1
Soil properties: ................................................................................................................ 4
2.2
Excavation Technologies and Loading Scenarios ........................................................... 4
2.2.1
A) Standard technology ............................................................................................ 4
2.2.2
B) Alternative Technology ........................................................................................ 5
3 Geotechnical Stability Calculations ......................................................................................... 6
3 Conclusions.......................................................................................................................... 10
2
1
Introduction
This report compiles the results of geotechnical stability calculations prepared for evaluating the
suitability and for dimensioning of excavation technologies (hydraulic excavator), temporary cover
placement technology (dozer) and with regard to trafficability of temporary cover surfaces by
trucks.
The results of the recent shearvane test program are of critical importance regarding developing
the excavation technology. The results show that only a certain part of the tailings volume is characterized by undrained shear strength values below 25 kPa. These tailings can also be mixed with
the overlying sandy materials in order to allow a conventional transport in standard lorries without
encapsulation. In addition such mixed material would also be easier to place and compact in the
depository on TP 6. The lower part of the tailings body shows shear strength values above 25
kPa. Based on these results the standard excavation technology has been developed. It consists
of a layerwise excavation of an upper 1m thick layer of soil dozed onto the air-exposed tailings
together with an underlying tailings layer.
Based on this volume balance this option would start with the local removal of the overlying layer
only by dozing within the pond area. The first layer of the tailings will be excavated by a hydraulic
excavator standing on a cover layer of ca. 1 m thickness. Tailings and cover material shall be
excavated together and mixed by dumping on the truck. Afterwards a dozer places a new cover
layer on the free-lying tailings surface. Then the next layer of cover soils and underlying tailings
shall be excavated and so on. Air-drying can also be used to the maximum extent possible. Under
dry weather conditions the cover layer shall be placed on top of the tailings shortly before excavation. In case of insufficient trafficability of the cover layer the thickness of the cover layer is to be
increased.
If the consistency of the respective tailings layer do not allow for dozing of soils to create a trafficable working platform such tailings will have to be excavated by excavators standing more distant from the excavation place. Such excavators can be hydraulic excavators with long boom or
cable excavators. This alternative excavation technology may be needed for relocation of a minor
amount of tailings. Such pulpy tailings will then have to be transported using specific lorries with
watertight lockable containers. The alternative excavation technology and transport technology is
presented with the Working Design.
Due to the excavation of TP 3 material the natural ground profile will be restored. Contaminated
subsoil will be removed during ongoing excavation works. It is to be expected that decomposed
rock underlying the tailings will be radioactively contaminated by seepage of tailings pore water or
processing water. Therefore such weak soil and decomposed rock materials will be removed
during ongoing excavation works.
3
2
Input parameters for the stability calculations
2.1
Soil properties:
Soil properties of the interim cover:
Soil group according to DIN 18196:
Specific weight, earth-moist:
Friction angle:
Cohesion:
SU; GU; GU*; SU*
21.0 kN/m³
30.0 °
0 kPa (calculation value: 1 kPa)
Soil properties of the tailings:
water saturated
specific weight, water saturated: 17.5 kN/m³
2.2
Excavation Technologies and Loading Scenarios
2.2.1 A) Standard technology
1. loading case: The temporary interim cover will be placed
Placement of a layer with a thickness of 1.0 m in one layer by a dozer from type Komatsu D37/PX21 on air-exposed tailings surface
Technical data of Komatsu D37/PX-21:
Weight: 7770 kg; width of the tracks: 600 mm;
Length of the tracks between the axes: 2240 mm
Footprint of the tracks: 2.688 m2; soil pressure: 28.4 kPa
2. loading case: Excavation of tailings and overlying temporary cover soil by a crawler excavator
A 1.0 m thick interim layer was placed before by the dozer.
Characteristic data of the excavator CAT 319 D L:
Weight:
19550 kg
Soil pressure (tracks):
45 kPa
Width of the tracks:
600 mm
Supporting track length:
3650 mm
Footprint of the two tracks:
4.38 m2
Width of the outside edge of the tracks: 2800 mm
Minimum width of the wooden planks below the excavator for reducing soil pressure (without side
overlap) 2.8 m * 3.65 m = 10.22 m2, this means reduction of the soil pressure to a maximum of ca.
19 kPa
Geometric boundary conditions for the location of the excavator:
Minimum distance of the excavator from the excavation border: 2.0 m
Slope ratio of the edge of the temporary cover v : h = 1 : 1
Slope height: 2.0 m (1 m tailings + 1 m interim cover)
4
Characteristic data of the truck
Truck KAMAZ 55111
Transportation truck for bulk material, 3 axes
total weight:
22400 kg
tyres:
10.00 R20
Axle load, front:
5550 kg
Soil pressure, front:
ca. 125 KN/m²
Axle load, rear:
2 x 8430 kg
Soil pressure, rear:
ca. 95 KN/m²
total length:
6.70 m
2.2.2 B) Alternative Technology
Characteristic data of the equipment:
Hydraulic excavator type Liebherr R 934 C Litronic with long arm (11.5 m slope mono arm)
maximum range:
weight:
track width:
track running length:
footprint of 2 tracks:
Soil pressure (tracks)
21.05 m
34.16 to
750 mm
3848 mm (axle distance)
5.772 m2 (relating to the axes distance)
(this means 58.1 kPa calculated soil pressure)
55 kPa (statement of producer)
Width of the undercarriage (chassis) HD-SL: 3395 mm (see documentation)
Length of the track running gear:
3848 mm (axle distance)
Footprint of wooden planks for reducing soil pressure:
at least 13.06 m2
Calculated soil pressure:
25.7 kPa
The use of the wooden planks below the excavator will lead to a similar soil pressure as using the
standard technology.
5
3
Geotechnical Stability Calculations
FLAC3D 2.1 was used to calculate the stability for different technologies and scenarios.
Only static loading cases were taken into account. The wooden planks below the excavators as
stated above shall serve to reduce the soil pressure and shall be used always and all the time.
Development of the model for excavator and truck:
- Slope ratio v : h = 1 : 1
- Slope height: 2.0 m (1 m tailings + 1 m interim cover) for the excavators
- For calculations with the truck the model was finer discretized because of nearly point load
of the tyres
Figure 1:
Model for excavator use
Model for using the dozer:
- Dozer for a 1.0 m interim cover, tailings underneath
- Interim cover at the border v : h = 1 : 1 sloped
Consideration of soil buoyancy:
There were calculations done for selected examples under consideration of the buoyancy. It has
been assumed that there will be a permanent water table at the level of the slope foot of the tailings. A seepage line within the 1 m tailings slope was not applied, because the water table within
the tailings is to be lowered by drainage measures, if needed.
The seepage line can vary in practice during the excavation work (QA/QC programme). By assumption of a specific weight and buoyancy the withstanding forces in the stability calculations will
be lower and the safety factor will reduce. The calculation results under consideration of the
buoyancy
are
shown
in
6
Table 1 in brackets and marked red.
For the static calculations only the undrained strength of the tailings (cohesion) was varied to
determine the safety factor. The following table shows the calculation results.
7
Table 1:
Summary of standard safety calculations
Cohesion of
tailings
[kPa]
10
Standard technology
Excavator
CAT 319 D L
2 m high
slope
S = 0.95
(Sw=0.92)
15
S = 1.41
20
S = 1.89
truck
KAMAZ
55111
S = 0.71
S = 1.06
(Sw=1.02)
S = 1.39
Alternative technology
Dozer Komatsu D37/PX-21
1 m high slope
Distance to the slope border
0m
1m
S = 1.25
S = 1.14
(Sw=1.23)
S = 1.35
S = 1.72
Long arm excavator
Liebherr R 934 C
2 m high slope
S = 0.92
S = 1.38 (Sw=1.34)
Results – Excavation without implementation of buoyancy
Use of the dozers:
In this calculation case with the dozer directly at the slope border and cohesion of the tailings of
10 kPa a slope collapse will be stated with S=1.14. The tailings under the interim cover are not
affected in that case (Figure 2). Experiences with the feasibility of the barely manageable work
with those dozers at tailings material with ca. 10 kPa cohesion are confirmed here. To demonstrate that the construction equipment and the existing load are shown as orange rectangle in the
figures.
The calculations with the dozer in 1 m distance to the slope edge gives higher safeties. It is therefore required to use the long arm shields and to grant a minimum distance of 1 m from the slope
shoulder line to the front of the dozer. The presented figures of soil displacements show the collapse state only qualitatively. The dimension of the displacements is not considered.
Dozer
Figure 2:
Displacements in the calculation case with dozer directly at the slope border and
cohesion of the tailings = 10 kPa
Use of the excavators:
For the calculations of the use of excavators (ordinary and with long arm) on a 2 m high embankment another collapse mechanism is stated. The 1 m thick tailings layer in the lower part of the
slope is pressed out affecting a are large area of the embankment. Despite the lower soil pressure
of the excavator (standard technology) in comparison to the dozer, lower safety factors are stated
at the respective undrained shear strengths of the tailings material. For the calculations it was
assumed that also the long arm excavator despite of bigger reach is standing above the tailings
material.
8
Excavator
Figure 3:
Displacements in the calculation case with a excavator, in 2 m distance from the
slope border, cohesion of the tailings = 15 kPa
Use of the trucks:
Similarly to the use of excavators the stability calculations for truck use show a large-area pressing out the tailings material towards the slope. The effects are shown with the displacement figure
Figure 4. As the truck tyres will nearly act as point loads, in practice the interim cover must guarantee a minimum load bearing capacity, that will be reached by compacting.
Figure 4:
Displacements in the calculation case with truck on the interim cover, cohesion of
the tailings = 15 kPa
Results of the considerations of soil buoyancy
The calculation results show minimum reductions (< 5 %) of the safety factor in comparison to the
calculations without buoyancy. For example the following figures 5 and 6 present the grid densities and calculation results for the case “dozer in 1 m distance to the slope border”.
9
Figure 5:
Presentation of the density distribution by consideration of the soil buoyancy
sities in kg/m³)
(den-
Dozer
Figure 6:
3
Displacements in calculation case with dozer in 1 m distance from the slope border
and cohesion of the tailings = 10 kPa, with consideration of soil buoyancy
Conclusions
For performing the excavation works a minimum safety factor of 1.3 is to be met. On the basis of
the stability calculations and the developed diagrams below the following required minimum values were determined for the undrained shear strength (cohesion) of the tailings:
-
ordinary excavator - minimum cohesion cmin= 14 kPa
dozer (in 1m distance from the slope border) - cmin= 11 kPa
truck - cmin= 19 kPa
long arm excavator - cmin= 15 kPa
Please note that the above stated geometric assumptions and load affecting points will apply. The
largest undrained strength of 19 kPa of the tailings material will be required by truck use. Because
of the relatively small tyre footprint soil pressure caused by trucks is large. The excavators and
dozers assumed for these stability calculations can be used if the tailings cohesion would be larger than 15 kPa.
10
Diagram 2:
Geotechnical Safety vs. undrained cohesion: Excavator; Standard technology
Normal excavator,
2 m distance to the slope
2
1.9
1.8
Factor of Safety
1.7
1.6
1.5
1.4
1.3
w ith buoyancy
1.2
Linear (no buoyancy)
1.1
1
0.9
0.8
9
10
11
12
13
14
15
16
17
18
19
20
21
Tailings cohesion [kPa]
Diagram 2:
Geotechnical Safety vs. undrained cohesion: Dozer; Standard technology
Dozer
1 m distance to the slope
1.8
Factor of Safety
1.7
1.6
1.5
1.4
w ith buoyancy
1.3
Linear (no buoyancy)
1.2
1.1
9
10
11
12
13
14
15
16
Tailings cohesion [kPa]
11
Diagram 3:
Geotechnical Safety vs. undrained cohesion: Truck;
Standard Technology & Alternative technology
Truck KAMAZ 55111
1.5
1.4
Factor of Safety
1.3
1.2
1.1
1
w ith buoyancy
0.9
Linear (no buoyancy)
0.8
0.7
0.6
9
10
11
12
13
14
15
16
17
18
19
20
21
Tailings cohesion [kPa]
Diagram 4:
Geotechnical Safety vs. undrained cohesion: Long-arm Excavator;
Alternative technology
Excavator, longarm
10 m distance to the slope
1.5
Factor of safety
1.4
1.3
1.2
1.1
w ith buoyancy
1
Linear (no buoyancy)
0.9
0.8
9
10
11
12
13
14
15
16
Tailings cohesion [kPa]
12