1. Geotechnics and Slope Stability - R 0
Short Description
Slope stability of a ground...
Description
LUBILIA HYDROPOWER PROJECT
SLOPE STABILITY ANALYSIS REPORT FORM INTAKE UP TO FOREBAY TANK VS HYDRO (UGANDA) LTD VS CONSULTING ( PVT) LTD NO.18B , JOSHPH FRAZER RD, COLOMBO -06 REPORT NO. VSC/LUBILIYA /GEO/001 REV.N0.00 – MAR 2016
CONTENTS 1.0
Geo-techniques and slope stability analysis of ground from intake up to forebay tank 1.1 The Contract requirements
1
1.2
3
Slope stability of weir, Intake and along the Head race canal from intake
1
up to forebay tank 2.0
3.0
Selection of Soil parameters for stability analysis
8
2.1
Available soil parameters for stability analysis
8
2.2
Logging of test pit data with site observations
10
2.3
Estimation of C and Phi values of the soil layers at the test pit
20
Slope Stability Analysis
22
3.1
The Slope stability analysis software (Slope/W)
25
3.2
Soil Parameters for the slope stability analysis in the Slope/W
26
3.3
Slope stability analysis in the Slope/W
27
4.0
Conclusion and recommendations
43
5.0
Annexure
44
Annex. 01 - Ground slope variation plan along the HRC trace Annex. 02 - Details of the test pit plotted on the test pits cross sections Annex. 03 - Cut slope section drawings Annex. 04 - Geotechdata.info data sheets.
ii
List of Tables
01
Average slope and the excavation depths of the ground along LPP line
3
02
Test pit location and C, Phi values
8
03
C and phi values at Ch 0+147 m
20
04
Selected sections for Slope Stability analysis and C and ‘Phi’ values
23
05
Selected test pits’ Shear strength parameters for the slope stability
26
analysis 06
Summary of the Slope stability analysis
43
iii
List of Figures 1
Soil log- CH1, CH2, CH3
10
2
Soil log CH4, CH5, CH6
11
3
Soil log CH7, CH8, CH9
12
4
Soil log CH10, CH11, CH12
13
5
Soil log CH13, CH14, CH15
14
6
Soil log CH16, CH17, CH18
15
7
Soil log CH19,CH20,CH21
16
8
Soil log CH22, CH23, CH24
17
9
Long term stability of original ground at Ch 0+124.83 m
27
10
Short term stability of excavated ground (During Construction) with
27
5kN/m2 surcharge load for the road at Ch 0+124.83m 11
Long term stability of excavated ground (After Construction) with
28
40kN/m2 surcharge load for the canal at Ch 0+124.83m 12
Short term stability of excavated ground (After Construction) with
28
40kN/m2 surcharge load for the canal at Ch 0+124.83m with seismic load 13
Long term stability of original ground at Ch 0+220.13 m
29
14
Short term stability of excavated ground (During Construction) with
29
5kN/m2 surcharge load for the road at Ch 0+220.13m 15
Long term stability of excavated ground (After Construction) with
30
40kN/m2 surcharge load for the canal at Ch 0+220.13m 16
Short term stability of excavated ground (After Construction) with
30
40kN/m2 surcharge load for the canal at Ch 0+220.13m with seismic load 17
Long term stability of original ground at Ch 0+574.18 m
31
18
Short term stability of excavated ground (During Construction) with
31
5kN/m2 surcharge load for the road at Ch 0+574.18 m 19
Long term stability of excavated ground (After Construction) with
32
40kN/m2 surcharge load for the canal at Ch 0+574.18m
iv
20
Short term stability of backfilled ground (After Construction) 40kN/m2
32
surcharge load for the canal at Ch 0+574.18 m with seismic load 21
Long term stability of original ground at Ch 0+774.14 m
33
22
Short term stability of excavated ground (During Construction) with
33
5kN/m2 surcharge load for the road at Ch 0+774.14 m 23
Long term stability of excavated ground (After Construction) with
34
40kN/m2 surcharge load for the canal at Ch 0+774.14 m 24
Short term stability of Backfilled ground (After Construction) 40kN/m2
34
surcharge load for the canal at Ch 0+774.14 m with seismic load 25
Long term stability of original ground at Ch 1+002.77 m
35
26
Short term stability of excavated ground (During Construction) with
35
5kN/m2 surcharge load for the road at Ch 1+002. 77 m 27
Long term stability of excavated ground (After Construction) with
36
40kN/m2 surcharge load for the canal at Ch 1+002.77 m 28
Short term stability of Backfilled ground (After Construction) 40kN/m2
36
surcharge load for the canal at Ch 1+002.77 m with seismic load 29
Long term stability of original ground at Ch 1+331.84 m
37
30
Short term stability of excavated ground (During Construction) with
37
5kN/m2 surcharge load for the road at Ch 1+331.84m 31
Long term stability of excavated ground (After Construction) with
38
40kN/m2 surcharge load for the canal at Ch 1+331.84m 32
Short term stability of excavated ground (After Construction) with
38
40kN/m2 surcharge load for the canal at Ch 1+331.84m with seismic load 33
Long term stability of original ground at Ch 1+635.75m
39
34
Short term stability of excavated ground (During Construction) with
39
5kN/m2 surcharge load for the road at Ch 1+635.75m 35
Long term stability of excavated ground (After Construction) with
40
40kN/m2 surcharge load for the canal at Ch 1+635.75m 36
Short term stability of excavated ground (After Construction) with
40
40kN/m2 surcharge load for the canal at Ch 1+635.75m with seismic load
v
37
Long term stability of original ground at Ch 1+904.61 m
41
38
Short term stability of excavated ground (During Construction) with
41
5kN/m2 surcharge load for the road at Ch 1+904.61 m 39
Long term stability of excavated ground (After Construction) with
42
40kN/m2 surcharge load for the canal at Ch 1+904.61 m 40
Short term stability of excavated ground (After Construction) with
42
40kN/m2 surcharge load for the canal at Ch 1+904.61 m with seismic load
vi
GEO-TECHNIQUES AND SLOPE STABILITY ANALYSIS OF GROUND FROM INTAKE UP TO FOREBAY TANK 1.1
The contract requirements
The stability of the permanent cut slopes in both overburden / residual soils and rock shall be designed to address potential circular sliding failure and structurally controlled failure surfaces respectively for the following loading conditions:
Self weight.
Fully saturated under steady state seepage.
DBE (Design Basis Earthquake).
The minimum FOS for the ‘local’ stability of permanent slopes of cuttings, embankments and other parts of the works formed from natural ground materials shall be adopted in the design of the Works. The ‘global’ stability of slopes ( i.e. an assessment outside of the immediate zone of works on a slope) shall be assessed taking account of the proposed works, both temporary and during long term operations.
Loading Condition
‘Local’ Slope Stability FOS
Long term steady state - In areas of identified risk to properties (houses, schools, roads, etc) Long term steady state - After construction stage (All other locations)
1.4
1.3
Short term - Construction stage (temporary works)
1.2
Short term - During the Design Basis Earthquake
1.05
Slope reinforcing elements shall be designed to meet the service life defined in Section 3.1.4 These requirements shall apply to slopes associated with the
access roads
weir and head works
penstock
headrace channel and other excavated slopes within the project footprint.
1
Temporary slopes excavated as part of the construction process may adopt lower factors of safety, as agreed with the Employer’s Technical Consultant at the final design stage, subject to the final reinstatement having equivalent permanent FOS.
Seismic design The Contractor shall use the following design peak ground acceleration (PGA) for the Design Basis Earthquake (DBE), which shall be adopted in the analysis and design of the Works:
PGA Seismic Loads (Horizontal) DBE
All Structures 0.16
g
1.2 Slope stability of weir, Intake and along the Head race canal from intake up to Fore bay tank
General The weir of the Lubiliya project has been planned on Lubiliya river and the intake of the project has been proposed on the left bank. The head race canal of Lubilia project has been planned to construct on the left bank of the Lubilia River to deliver water from intake up to fore bay tank. The length of the Head race canal is 2030 m. The existing ground along the HRC has to be disturbed to form a stable flat bed to found the structures. The terrain where the structures are going to be built is a sloping terrain from LHS to RHS towards the river with an average slope varies from 10 deg. to 45 deg. to the horizontal. The table - 01 summarises the average slope along the HRC trace. The stability of the disturbed ground and the overall stability of project trace are needed to be identified and the cut slopes shall be identified not to have long term ground slips due to excavation.
The table - 01 includes the average excavation depth at the centre line of the HRC trace. At certain sections, the excavation depth is about 3 m or more. As the canal is having about 2030 m length, the critical sections of the ground along the head race canal were selected after careful study of the ground and soil parameters to analyse for the slope stability. The selected sections can represent the ground stability after cut very reasonably.
2
Table 01: Average slope and the excavation depths of the ground along LPP line Section No
Chainage (m)
C.L. Excavation depth (m)
Existing Ground Slope (deg)
1 2 3 4 `5 6 7 7-1 7-2 7-3 7-4 7-5 7-6 7-7 8 9 9-1 9-2 9-3 9-4 10 11 12 13 14 14-1 15 15-1 16 17 17-1 17-2 17-3 17-4 18 19 20 21 22
0+022.81 0+034.11 0+064.54 0+091.4 0+122.63 0+156.08 0+190.48 0+220.13 0+254.34 0+300.52 0+335.4 0+362.75 0+433.46 0+460.06 0+490.02 0+524.61 0+574.18 0+614.71 0+659.67 0+690.26 0+733.13 0+774.14 0+832.04 0+871.84 0+904.8 0+933.51 0+961.85 1+002.77 1+029.89 1+058.42 1+094.95 1+131.84 1+184.50 1+219.4 1+249.22 1+307.27 1+331.84 1+358.97 1+382.69
5.55 2.87 0.82 2.66 0.00 4.87 3.12 2.07 4.42 3.20 4.25 0.00 2.91 2.25 0.50 2.26 3.04 1.20 0.76 2.12 3.69 2.46 2.05 2.61 2.06 3.41 1.76 1.32 1.36 2.71 4.07 3.53 1.91 3.67 3.42 2.91 0.65 3.49 4.43
40 36 35 40 30 37 48 36 36 27 24 39 45 38 15 20 34 32 44 31 31 39 21 30 26 15 31 47 32 27 25 40 41 39 38 30 31 30 31
3
22-1 22-2 23 24 25 26 26-1 26-2 27 27-1 28 29 30 31 32 32-1 33 34 35 35-1 35-2 35-3 36
1+416.93 1+437.15 1+483.43 1+507.62 1+538.00 1+568.00 1+585.24 1+614.26 1+635.75 1+659.28 1+690.81 1+741.55 1+776.44 1+819.47 1+848.78 1+882.4 1+904.61 1+941.05 1+978.11 1+994.08 2+006.29 2+019.04 2+029.73
3.48 3.52 4.39 4.23 0.68 1.53 1.75 0.39 2.83 1.37 0.22 1.50 1.46 0.31 1.17 1.73 1.76 1.32 0.08 0.41 1.43 2.05 2.66
38 36 37 37 30 45 45 10 41 45 35 32 34 40 24 28 22 26 29 26 27 30 26
The slope angles of the HRC trace from intake up to the Forebay tank were divided into three categories of slopes as given below. a. Category 01 - 10˚ ≤ β ≤ 20˚ b. Category 02 - 20˚≤ β ≤ 30˚ c. Category 03- 30˚ ≤ β ≤ 45˚ The ground sections were selected from these three categories to identify the most suitable cut slopes to have long term and short term stability of the ground as given in the design criteria. These categories were further marked on the HRC layout for easy reference. The exhibit- 1, 2, 3, 4 & 5 provide the slope variation and slope directions along the HRC trace. The same drawing is given as Annex- 01 to the report to be printed on A3 papers.
4
Exhibit 1
Exhibit 2
5
Exhibit 3
Exhibit 4
6
Exhibit 5
Exhibit 6 7
2.0 Selection of Soil parameters for Stability analysis 2.1
Available Soil parameters for stability analysis
In view of studying the ground stability of the existing ground and the constructed ground, the soil parameters necessary to study the stability of the slopes were determined by carrying out laboratory investigation on the soil samples collected along the from intake up to the forebay tank and power house area. Twenty four numbers of test pits have been dug in the canal trace to ascertain the soil type and the necessary soil parameters for the stability analysis. The test pit locations along the Head race canal line and the summary of test results of the soil samples are given in table -02. The same data were used in modelling and analysing the stability of the slopes. The details of the test pits and evaluation of C & Phi at the laboratory are given in the Geology report for Lubilia. Table 2: Test pit location and C, Phi values TP No
Depth/(m)
CH1 CH2 CH3 CH4 CH5 CH6 CH7 CH8 CH9 CH10 CH11 CH12 CH13 CH14 CH15 CH16 CH17 CH18 CH19 CH20 CH21 CH22 CH23 CH24
1.9 2.1 1.5 3.7 1.5 1.9 1.5 1.5 1.5 2.4 1.8 1.8 2.1 2.9 2.7 2.3 2.3 1.6 1.5 1.4 1.6 2.7 2.7 2.1
Drained parameters C' (deg) (kN/m^2) 6 38 9 37 18 36 5 40 22 35 14 34 3 39 13 30 6 34 8 35 19 29 9 40 11 40 5 36 8 32 7 38 18 36 18 35 4 42 3 39 7 38 5 42 22 36 16 39
ᶲ'
Un-drained Parameters C (deg) (kN/m^2) 9 32 2 37 6 31 7 39 8 38 11 32 5 31 14 22 6 33 7 32 19 24 9 34 7 35 1 34 6 31 4 37 3 33 8 30 4 39 4 33 3 37 2 41 10 35 5 37
ᶲ
Bulk Density (kN/m^3) 17.4 15.8 15.8 18 18.5 16.9 15.3 17.7 15.4 15.8 16.1 17.7 15.8 15.2 15.8 18.1 16.6 16 16 15.1 15.8 0.7 17.5 16.5
Ground water Table No No No No No No No No No No No No No No No No No No No No No No No No
8
The water table has not been encountered in all the test pits discussed above along the Head race canal line and in the bore holes. Hence, there is no a permanent water table present along the trace.
9
2.2 Logging of test pit data with site observations CH1, CH2 & CH3 Test pits data According to test pita data given in geology report of test pits No. CH1, CH2 & CH3 has been dug close to Ch0+124.8m where the de-sand tank is located. The soil type of three test pits are described as sandy clay up to depth of 1.4 m from the ground level. The soil layers as identified in CH1 to CH 3 are given below, a. Layer 01 - From ground level up to 0.3 m - Sandy clay b. Layer 02 - From 0.3 up to 1.4 m - sandy clay with cobbles and boulders c. Layer 03 - From 1.4m - completely weathered rock and the depth is not given
The photos no. 01, 02 & 03 taken at the site excavated test pits (CH1, CH2 & CH3) are given below.
Photo No 01
Photo No 02
Photo No 03
The test pits CH1, CH2 & CH3 soil layer description was refined with the site observations as given in the soil log. (Soil log - CH1, CH2 & CH3)
Figure 1 : Soil log- CH1, CH2, CH3
10
CH4, CH5 & CH6 Test pits data According to test pits data given in geology report of test pits No. CH4, CH5 & CH6 has been dug close to Ch0+220.13m. The soil type of three test pits are described as cohesive clay up to depth of about 1.0 m from the ground level. The soil layers as identified in CH4 to CH 6 are given below, a. Layer 01 - From ground level up to 0.6 - 1.2m - Cohesive clay b. Layer 02 - From about 1.0m up to 2.0m - Sandy clay c. Layer 03 - From about 2.0m - completely weathered rock and the depth is not given
The photos no. 04, 05 & 06 taken at the site excavated test pits (CH4, CH5 & CH6) are given below.
Photo No 04
Photo No 05
Photo No 06
The test pits CH4, CH5 & CH6 soil layer description was refined with the site observations as given in the soil log. (Soil log - CH4, CH5 & CH6)
Figure 2 : Soil log CH4, CH5, CH6
11
CH7, CH8 & CH9 Test pits data According to test pits data given in geology report of test pits No. CH7, CH8 & CH9 has been dug close to Ch0+574.18m. The soil type of three test pits are described as cohesive clay up to depth of about 0.3m from the ground level. The soil layers as identified in CH7 to CH 9 are given below, a. Layer 01 - From ground level up to 0.3m - Cohesive clay b. Layer 02 - From about 0.3m up to 0.8m - Sandy clay c. Layer 03 - From about 0.8m - Highly weathered rock and the depth is not given
The photos no. 07, 08 & 09 taken at the site excavated test pits (CH7, CH8 & CH9) are given below.
Photo No 07
Photo No 08
Photo No 09
The test pits CH7, CH8 & CH9 soil layer description was refined with the site observations as given in the soil log. (Soil log – CH7, CH8 & CH9)
Figure 3: Soil log CH7, CH8, CH9
12
CH10, CH11 & CH12 Test pits data According to test pits data given in geology report of test pits No. CH10, CH11 & CH12 has been dug close to Ch0+774.14m. The soil type of three test pits are described as cohesive clay up to depth of about 0.4 m from the ground level. The soil layers as identified in CH4 to CH 6 are given below, a. Layer 01 - From ground level up to 0.4m - Cohesive clay b. Layer 02 - From about 0.4m up to 0.8 m - Cohesive clay with cobbles c. Layer 03 - From about 0.8 m - completely weathered rock and the depth is not given
The photos no. 10, 11 & 12 taken at the site excavated test pits (CH10, CH111 & CH12) are given below.
Photo No 10
Photo No 11
Photo No 12
The test pits CH10, CH11 & CH12 soil layer description was refined with the site observations as given in the soil log. (Soil log – CH10, CH11 & CH12)
Figure 4 : Soil log CH10, CH11, CH12
13
CH13, CH14 & CH15 Test pits data According to test pits data given in geology report of test pits No. CH13, CH14 & CH15 has been dug close to Ch 1+002.77m. The soil type of three test pits are described as cohesive clay up to depth of about 1.5m from the ground level. The soil layers as identified in CH4 to CH 6 are given below, a. Layer 01 - From ground level up to 0.5m- Cohesive clay b. Layer 02 - From about 0.5m up to 1.5m - cohesive clay with cobbles c. Layer 03 - From about 2.0m - completely weathered rock and the depth is not given
The photos no. 10, 11 & 12 taken at the site excavated test pits (CH13, CH14 & CH15) are given below.
Photo No 13
Photo No 14
Photo No 15
The test pits CH13, CH14 & CH15 soil layer description was refined with the site observations as given in the soil log. (Soil log – CH13, CH14 & CH15)
Figure 5 : Soil log CH13, CH14, CH15
14
CH16, CH17 & CH18 Test pits data According to test pits data given in geology report of test pits No. CH16, CH17 & CH18 has been dug close to Ch1+331.84m. The soil type of three test pits are described as cohesive clay up to depth of about 1.6 m from the ground level. The soil layers as identified in CH16 to CH 18 are given below, a. Layer 01 - From ground level up to 1.2m - Cohesive clay b. Layer 02 - From about 1.2m up to 1.6m - Cohesive clay with cobbles c. Layer 03 - From about 1.5m - completely weathered rock and the depth is not given
The photos no. 16, 17 & 18 taken at the site excavated test pits (CH16, CH17 & CH18) are given below.
Photo No 16
Photo No 17
Photo No 18
The test pits CH 16, CH17 & CH18 soil layer description was refined with the site observations as given in the soil log. (Soil log – CH16, CH17 & CH18)
Figure 6 : Soil log CH16, CH17, CH18
15
CH19, CH20 & CH21 Test pits data According to test pits data given in geology report of test pits No. CH19, CH120 & CH21 has been dug close to Ch1+635.75m. The soil type of three test pits are described as cohesive clay up to depth of about 0.8 m from the ground level. The soil layers as identified in CH19 to CH 21 are given below, a) Layer 01 - From ground level up to 0.8m - Cohesive clay b) Layer 02 - From about 0.8m up to 1.5m - completely weathered rock c) Layer 03 - From about 1.5m - Highly weathered rock and the depth is not given
The photos no. 19, 20 & 21 taken at the site excavated test pits (CH19, CH20 & CH21) are given below.
Photo No 19
Photo No 20
Photo No 21
The test pits CH19, CH20 & CH21 soil layer description was refined with the site observations as given in the soil log. (Soil log – C19, CH20 & CH21)
Figure 7 : Soil log CH19,CH20,CH21
16
CH22, CH23 & CH24 Test pits data According to test pits data given in geology report of test pits No. CH22, C23 & CH24 has been dug close to Ch1+904.61m. The soil type of three test pits are described as cohesive clay up to depth of about 2.5 m from the ground level. The soil layers as identified in CH22 to CH 24 are given below, d. Layer 01 - From ground level up to 0.4m - Grayish cohesive clay e. Layer 02 - From about 0.4m up to 2.5m - Brown cohesive clay f. Layer 03 - From about 2.5m - completely weathered rock and the depth is not given
The photos no. 22, 23 &24 taken at the site excavated test pits (CH22, CH23 & CH24) are given below.
Photo No 22
Photo No 23
Photo No 24
The test pits CH22, CH23 & CH24 soil layer description was refined with the site observations as given in the soil log. (Soil log – C22, CH23 & CH24)
Figure 8 : Soil log CH22, CH23, CH24
17
The test pit data plot on the ground cross sections
The Test pit data were further taken in to ground cross section drawings and they are given as Exhibit 7 and 8. The same cross section are also given in Annex- 02, to be printed on A3 papers. These sections were used in analysing the ground cross sections for slope stability.
Exhibit 7
18
Exhibit 8
19
2.3 Estimation of C and Phi values of the soil layers at the test pit
Analysis of the ground sections for the slope stability requires soil parameters of all the soil types in the cross section. The test pits no. CH1, CH2 & CH3 are having three layers of soil according to the test pits data and site observations. The soil parameters of the first layer are not available in the Geology report for Lubiliya. Hence, it (i.e. Sandy clay) had to be determined using “Handbook of geotechnical investigation and design tables” prepared by Consulting Geotechnical Engineer, Burt G. Look, assigned for sandy clay as C=10 kPa & phi=28°.
The soil properties of the second layer (i.e. Sandy clay with cobbles and boulders) have been determined for the three test pits which were located approximately 6 m from each other by laboratory tests. The values of C and Phi are having different values for each test pit as given in table 03. Table 03 : C and phi values at Ch 0+147 m Test pit No
Depth ( m)
C(kPa)
Phi (deg)
CH-01
1.9
6
38
CH-02
2.1
9
37
CH-03
1.5
18
36
So, the most appropriate C and Phi value necessary for the analysis had to be selected considering the nearest test pit to the Canal trace centre line. The 3rd soil layer of CH 1 to CH 3 is completely weathered rock. The depth of the layer and soil parameters are not available. Completely weathered rock describe in Geo-tech classification as “all rock material is decomposed and/or disintegrated to soil. The original mass structure is largely intact”.
Completely weathered rock layer can be reasonable assume as sand, silt clay by its
definition given above. Accordingly, the C & Phi values can be taken from the Geotechdata.info, assigned for compact soil packing material with N (SPT) value nearly 10 as 35° to 40°. Hence, the phi value of the weathered rock was selected the low value of the range, that is 35 deg.
20
As the completely weathered rock is generally below 1.5m, it is reasonable to assume that the layer packing as dense soil and due to other possible factors, C can be selected as
50kPa.
The
Geotechdata.info sheets used in this report is given in Annex-03.
The selection methodology of C and Phi values were same for the other test pits as well.
21
3.0 Slope stability analysis
The trace of the head race canal passes through cultivated lands. The houses close to the canal trace has been acquired and settlement has been completed. The weir location and in the vicinity of the weir there are no houses. The power house location also does not have houses in and around. Hence, the roads, weir, head race canal, fore bay tank, penstock and power houses are constructed with a fairly good distance to houses in the whole project area.
The depth of excavation in most of the cut sections are less than 3m. There are few sections having a cut depth of 4 to 6 m. As the cut slopes are high, the cut slope stability has to be checked during construction and after construction considering the importance of stable cut slope for the function of the project. In analysing the ground for slope stability, the challenge a geotechnical engineer encountered is lack of uniformity in mechanical properties of soil such as soil gradation properties, fine content, liquidity parameters and shear strength. These mechanical properties are varying significantly with the soil deposit history. So, selection of representative section to evaluate the stability has to be done with careful evaluation of the ground. Same time it was consider the safe and conservative soil parameters in order to minimize the risk.
In the analysis, it was considered that the water table is at surface level to obtain saturated condition which may be critical for long term as well as short term analysis.
Considering the average slopes, test pits locations and depth of excavation for forming the canal base, eight sections were selected to analyse the slope stability of the ground in HRC trace. The C and ‘phi’ values of selected sections and the other details are given in Table -03.
22
Table 04: Selected sections for Slope Stability analysis and C and ‘Phi’ values Chainage /(m)
0+124.82
0+220.13
0+574.18
0+774.14
1+002.77
1+331.84
1+635.75
1+904.61
TP No CH1 CH2 CH3 CH4 CH5 CH6 CH7 CH8 CH9 CH10 CH11 CH12 CH13 CH14 CH15 CH16 CH17 CH18 CH19 CH20 CH21 CH22 CH23 CH24
Depth/(m) 1.9 2.1 1.5 3.7 1.5 1.9 1.5 1.5 1.5 2.4 1.8 1.8 2.1 2.9 2.7 2.3 2.3 1.6 1.5 1.4 1.6 2.7 2.7 2.1
Drained parameters
Un-drained Parameters
C' (kPa)
ᶲ'(deg)
C (kPa)
6 9 18 5 22 14 3 13 6 8 19 9 11 5 8 7 18 18 4 3 7 5 22 16
38 37 36 40 35 34 39 30 34 35 29 40 40 36 32 38 36 35 42 39 38 42 36 39
9 2 6 7 8 11 5 14 6 7 19 9 7 1 6 4 3 8 4 4 3 2 10 5
ᶲ
Bulk Density (deg) (kN/m^3) 32 37 31 39 38 32 31 22 33 32 24 34 35 34 31 37 33 30 39 33 37 41 35 37
17 16 16 18 19 17 15 18 15 16 16 18 16 15 16 18 17 16 16 15 16 17 18 17
Selected Test pits for analysis _ _
_ _ _ _ _ _ _ _ _ _ _ _ _ _
The Selected ground sections as given in table-04 were analysed using Slope/W software for the a) Existing slopes before excavation ( Long term stability analysis) b) Excavated sections with 5 kN/m2 surcharge load for the road at construction stage.( Short term) c) The ground with constructed canal load and water load ( Long term). The canal load of 40 kN/m2 was used as surcharge load after construction. d) The ground with constructed canal load and water load with seismic conditions ( Short term)
23
As there is no a water table below ground level, fully saturated under steady state seepage condition was not considered in the design/ checked.
The FOS As there are no houses, schools and roads close proximity to the canal and other structures, a FOS of 1.3 will be kept for the long term stability of the cut slopes, Short term (Construction Stage) 1.2 and FOS of 1.05 will be kept under seismic conditions.
Selection of excavation slope angle
The minimum excavation slope was tried with the ground cross sections selected for analysis. The staring excavation slope was selected considering the depth of the excavation. For the residual soil layer and weathered rock layer, a 15 deg. slope was sufficient up to 6 m. If the cut depth is more than 6 m, excavations are proposed to carry out with a burm at 3 m height from the excavation bottom The 4 m wide burm is going to be used as the construction road of the forebay and head race canal. . Accordingly, the slope of the excavation in weathered rock layer is proposed as 10deg and after the burm is 15 deg. Also, the construction road of 4 m wide which is proposed to construct on LHS of canal and at about 3 m vertical height also considered in the analysis. The analysis of the slope stability with the canal operation load and 5 kN/m2 load on the construction road suggest to have a 15 0 to satisfy the required factors of safety in all the cross sections.
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3.1 The Slope stability analysis software (Slope/W)
SLOPE/ W commercial software released by Geo- Slopes International (http://www.geo-slope.com, Student Version of Geo Studio 2007) was used to analyse the ground sections for slope stability. SLOPE/W is a leading slope stability CAD software product for computing the factor of safety of earth and rock slopes. SLOPE/W can effectively analyze both simple and complex problems for a variety of slip surface shapes, pore-water pressure conditions, soil properties, analysis methods and loading conditions.
Using limit equilibrium, SLOPE/W can model heterogeneous soil types, complex stratigraphic and slip surface geometry, and variable pore-water pressure conditions using a large selection of soil models. Slope stability analyses can be performed using deterministic or probabilistic input parameters. Stresses computed by a finite element stress analysis may be used in addition to the limit equilibrium computations, for the most complete slope stability analysis available. The analysis were done using Morgenstern – Price analysis type and Entry and exit slip surface option provided in the SLOPW / W were used where the critical failure mode considered.
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3.2 Soil Parameters for the slope stability analysis in the Slope/W
Eight ground sections selected for analysis as given in the Table 05, were analysed for the long term stability, short term stability and short term stability with earth quake conditions at flood conditions. The C, phi and density of completely weathered rock was selected as 50KN/m2, 35deg and 20kN/m3 respectively and C, phi and density of sandy clay was selected as 10KN/m2, 28deg and 15kN/m3 respectively. The cut slope angle was selected as 10 degree at the completely weathered rock and 15 degree was selected at the cut slope at the residual soil. The analysis results are presented in below analysis graphics. Table 05 : Selected test pits’ Shear strength parameters for the slope stability analysis
Chainage /(m)
TP No
Depth/(m)
0+124.83 0+220.13 0+574.18 0+774.14 1+002.77 1+331.84 1+635.75 1+904.61
CH3 CH4 CH8 CH11 CH15 CH18 CH21 CH23
1.5 3.7 1.5 1.8 2.7 1.6 1.6 2.7
Drained parameters C' (kPa) 18 5 13 19 8 18 7 22
Un-drained Parameters
ᶲ'(deg)
C (kPa)
ᶲ(deg)
Bulk Density (kN/m^3)
36 40 30 29 32 35 38 36
6 7 14 19 6 8 3 10
31 39 22 24 31 30 37 35
16 18 18 16 16 16 16 18
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3.3 Slope stability analysis results - the Slope/W a. Analysis for Chainage 0+124.83 m (Test Pits – CH1, CH2, CH3)
Figure 09: Long
Figure 10:
term stability of original ground at Ch 0+124.83 m
Short term stability of excavated ground (During Construction) with 5kN/m2
surcharge load for the road at Ch 0+124.83m
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Figure 11:
Long term stability of excavated ground (After Construction) with 60kN/m2
surcharge load for the De-sand tank at Ch 0+124.83m
Figure 12:
Short term stability of excavated ground (After Construction) with 60kN/m2
surcharge load for the de-sand tank at Ch 0+124.83m with seismic load
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b. Analysis for Chainage 0+220.13 m (Test Pits – CH4, CH5, CH6)
Figure 1 : Long
term stability of original ground at Ch 0+220.13 m
Short term stability of excavated ground (During Construction) with 5kN/m2 surcharge load for the road at Ch 0+220.13m Figure 14:
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Figure 15:
Long term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 0+220.13m
Figure 2
: Short term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 0+220.13 m with seismic load 30
c. Analysis for Chainage 0+574.18 m (Test Pits – CH7, CH8, CH9)
Figure 3 :
Long term stability of original ground at Ch 0+574.18 m
Short term stability of excavated ground (During Construction) with 5kN/m2 surcharge load for the road at Ch 0+574.18 m Figure 4:
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Figure 19:
Long term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 0+574.18m
: Short term stability of backfilled ground (After Construction) 40kN/m2 surcharge load for the canal at Ch 0+574.18 m with seismic load Figure 5
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d. Analysis for Chainage 0+774.14 m (Test Pits – CH10, CH11, CH12)
Figure 6:
Long term stability of original ground at Ch 0+774.14 m
term stability of excavated ground (During Construction) with 5kN/m2 surcharge load for the road at Ch 0+774.14 m Figure 7: Short
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Figure 23:
Long term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 0+774.14 m
Short term stability of Backfilled ground (After Construction) 40kN/m2 surcharge load for the canal at Ch 0+774.14 m with seismic load Figure 8:
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e. Analysis for Chainage 1+002.77 m (Test Pits – CH13, CH14, CH15)
Figure 9 :
Long term stability of original ground at Ch 1+002.77 m
Short term stability of excavated ground (During Construction) with 5kN/m2 surcharge load for the road at Ch 1+002. 77 m Figure 10:
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Figure 11:Long
term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 1+002.77 m
Short term stability of Backfilled ground (After Construction) 40kN/m2 surcharge load for the canal at Ch 1+002.77 m with seismic load Figure 12:
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f.
Analysis for Chainage 1+002.77 m (Test Pits – CH16, CH17, CH18)
Figure 13 : Long
Figure 30:
term stability of original ground at Ch 1+331.84 m
Short term stability of excavated ground (During Construction) with 5kN/m2
surcharge load for the road at Ch 1+331.84m
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Figure 31 :
Long term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 1+331.84m
Figure 14 :
Short term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 1+331.84m with seismic load 38
g. analysis for Chainage 1+635.75 m (Test Pit – CH19, CH20, CH21)
Figure 15 : Long
Figure 34 :
term stability of original ground at Ch 1+635.75m
Short term stability of excavated ground (During Construction) with 5kN/m2
surcharge load for the road at Ch 1+635.75m
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Figure 35:
Long term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 1+635.75m
Figure 36:
Short term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 1+635.75m with seismic load
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h. Analysis for Chainage 1+904.61 m (Test Pits – CH1, CH2, CH3)
Figure 37: Long
Figure 38:
term stability of original ground at Ch 1+904.61 m
Short term stability of excavated ground (During Construction) with 5kN/m2
surcharge load for the road at Ch 1+904.61 m 41
Figure 39:
Long term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 1+904.61 m
Figure 40:
Short term stability of excavated ground (After Construction) with 40kN/m2
surcharge load for the canal at Ch 1+904.61 m with seismic load 42
Table 6 : Summary of the Slope stability analysis - FOS
Chainage / (m) 0+124.82 (Desand Tank) 0+220.13 0+574.18 0+774.14 1+002.77 1+331.84 1+635.75 1+904.61
Long termExisting
Short termDuring construction
Long term After Construction
Short term - After construction at earthquake
1.455
1.948
2.231
2.382
1.418 1476 1.537 1.633 1.634 1.793 1.690
1.401 1.829 1.630 1.935 2.362 2.390 1.859
2.557 2.239 2.456 2.099 2.216 2.884 2.970
1.942 1.642 1.775 1.522 1.601 2.119 1.996
4.0 Conclusion and recommendations The Slope stability analysis of the ground from intake up to the forebay tank of Lubiliya SHP project was carried out to determine the safety of slope after construction of the project. In the analysis of the slopes, the following parameters were taken in to account, a) The ground conditions of the natural surfaces, b) Slope of the ground, c) Depths of excavations, d) Construction of structures & road, e) Groundwater level and alignment of the waterways Soil parameters and the other necessary information for the analysis were taken from the Lubilia geology report done by CaCl Consulting limited. The geology report include, a) Laboratory tests results b) Test pits c) Borehole details d) Site observation results Accordingly, the slope stability analysis was carried out for eight representative sections along the trace shows that long term ground stability is sufficient for the existing ground and the proposed excavation slopes for HRC trace and roads. When the short term stability and stability at design based earth quake conditions were considered, the required FOS was achieved with a cut slope of, a) 15 deg. to the vertical in residual soils b) 10 deg to the vertical in completely weathered rock. 43
c) Further, the depth of excavation in most of the cut sections are less than 5m. There are few sections having depth of the cut more than 5 m in residual soil. If the depth of cut more than 5 m, cut slope should be increase to 35 deg. to the vertical in residual soils
The cut slope stability was checked during construction and after construction considering the importance of stable cut slope for the function of the project. In the analysis, it was considered that the water table is at surface level to obtain saturated condition which may be critical for long term as well as short term analysis. A sample drawing ( Annexure - 03) is attached to the report with the proposed cut slopes and other dimensions which are necessary for the construction to maintain the same slopes in cut as recommended by the analysis.
The De-sand tank ground area at Ch 0+124.82m was considered and analysed by using test pit CH3 soil parameters and site observations and other seven test pits also were analysed along the HRC trace to find stability of the slope. Totally eight sections were analysed and found out that the sections are stable for all four cases and factor of safety values also satisfied, which are in more than the minimum FOS values. However, it is recommended to implement slope stability vegetation, divert rain water paths cross the cut slopes using built up drains to prevent erosion and percolation into the ground to prevent any slope instability issues.
The excavation will encounter rock boulders within the excavation. They shall not allowed to remain in the cut slopes and shall be removed carefully to prevent them drop down accidentally.
If locations which are having different soil properties than the properties given in the analysed sections are encountered, a geologist shall be consulted for establishing safe cut slope angles.
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5.0 Annexure The section stability analysis report for, Annex. 01 - Ground slope variation plan along the HRC trace Annex. 02 - Details of the test pit plotted on the test pits cross sections Annex. 03 - Cut slope section drawings Annex. 04 - Geotechdata.info data sheets.
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