ministry of memorandum transportation and infrastructure · 2018-01-17 · from literature, manual...
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Ministry of Transportation and Infrastructure M E M O R A N D U M
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John SC Lau, M.Eng., P.Eng.
May 8, 2017
Bridge Engineer, Design Consultant Liaison
Southern Interior Region Technical Memo: Project 37195‐0001 – Nass Bridge, Rock Foundation Design 1.0 Background
This memo is limited to bridge foundation design on rock. All other factual data and design recommendations can be found in the two preceeding reports: Nass Bridge, Preliminary Geotechnical Report (September 21, 2016) and Nass Bridge, Geotechnical Design Report, Roadway and Approaches (April 12, 2017). Design of piers and abutments is based on 50% Detailed Design drawings provided by Stantec on April 18, 2017. The piers are envisaged as spread footings founded on bedrock, as is the west abutment (depending on depth to bedrock). The east abutment will be founded on a MSE wall, details of which were provided in the April 12, 2017 Report. The arrangement is shown in Figure 1 below, and in greater detail in the Appendix:
Figure 1 ‐ General Arrangement of Foundations From 50% Detailed Design Drawings
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2.0 Structural Loads Factored structural loads at the east pier were provided on April 5, 2017, load cases at the west pier and west abutment were provided April 20, 2017. Table 1 – Loading Used for Foundation Analyses
* See the Appendix for sketches showing details of load cases Additionally Stantec has provided that the west abutment bearing pressure envelope (ULS 1) varies linearly from the approach side at 60 KPa to 300 kPa on the bridge side. 3.0 Summary of Rock Parameters The following parameters were provided in the Geotechnical Design Memo dated April 12, 2017:
Consequence Factor 1.0
Shallow Foundation Resistance Factor 0.45
Rock Slope Factor of Safety (Static) 1.5
Em, Rock 12 GPa
Range of GSI 54 ‐ 66
Friction angle under 0.5 MPa distributed load 35o
Factored Bearing 1.6 MPa Details of drilling and lab testing are presented under the same cover. Generally the rock is very strong and prone to failure along healed discontinuities. The lowest UCS test occurring along a healed discontinuity is 32.1 MPa and is used as an estimate of strength for design. Rock mass geometry on each side of the river is shown in Table 2 below:
B1 J1 J2 J OTHER
Location Slope Angle Dip Dip Direction Dip Dip Direction Dip Dip Direction Dip Dip Direction
West Slope 25 46 194 86 83 69 36 2 43
East Slope 55o 44 205 62 302
Table 2 ‐ Slope and Discontinuity Geometry on West and East slopes
Structure Loading Scenario Vertical Load Shear Load Wind Load
West Abutment
SLS 6150 kN
ULS 1 8340 kN
West Pier
SLS 19600 kN (+/‐) 190 kN
ULS 1 26000 kN (+/‐) 250 kN (+/‐) 190 kN
ULS 1 + 3 18300 kN (+/‐) 700 kN
East Pier
SLS 11400 kN (+/‐) 330 kN
ULS 1 + 2 15400 kN (+/‐) 460 kN
ULS 3 + 4 15400 kN (+/‐) 520 kN
ULS 4 12960 kN (+/‐) 460 kN (+/‐) 520 kN
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4.0 Foundations at East Pier The pier on the east side was analysed using loading provided by Stantec. All of the loading scenarios were considered, and ULS 1 + 2 was found to be the governing load case. For this project, the slopes are seismic performance category 1, assuming typical understanding and a consequence factor of 1.0, as per Table 6.2b in the BC Supplement to the Canadian Highway Bridge Design Code, the minimum required factor of safety (FOS) for global stability of the slope is 1.54. The pier on the east side of the river is at the crest of a rock slope with a potential for wedge failures resulting from the intersection of bedding (B1) and the prevalent joint set (J1). Due to very strong rock, bearing capacity is straightforward and would be the same as given for the west pier and abutment (Section 5.0). However, at the east pier, global stability of the slope is the design challenge. Although the existing slope is generally 55o or less, slope stability and wedge stability modelling is sensitive to the slope angle, thus a local maximum (59o, as provided by Stantec) was used in all analyses. Based on drilling data and observed slope condition, dry conditions were modelled. Presumptive mi values for siltstone (5‐7) and D values ranging from 0.7 to 1.0 were input into RocScience RocLab V 1.0 (in addition to calculated GSI, Em and friction angle). Output cohesion, tensile strength and Hoek‐Brown parameters (m and s) were then used for design. A summary of output is presented in Table 3 below. Table 3 ‐ RocLab Output Parameters, East Pier
Two cases were developed for the range of potential disturbance conditions. On the east side of the bridge the rock quality varies, and so the lower bound of GSI was assumed. From literature, manual excavation of rock is typically modeled as D = 0.7 and blasting is modelled as D = 1.0. Although we expect to manually excavate the rock, we wanted to consider the event where disturbance due to construction is higher than typical. As a preliminary check slope modelling was conducted with the above parameters using RocScience Slide V 5.0. The factor of safety (FOS) for global stability of the slope if assuming typical disturbance meets minimum design critera, and is 1.74. However, in the case of greater disturbance, the FOS is 1.38, indicating there is a potential for slope failure. A detailed analysis of the slope was then conducted using RocScience Swedge V 6.0. The slope was modelled as two potential scenarios: Scenario 1: Upon inspection of the excavated rock surface, there no discrete wedge or apparent discontinuities that might result in wedge failure. In this scenario we modelled the rock as having the minimum strength properties Table 3, assuming both high disturbance, and tensile strength contribution rather than cohesion. The lowest FOS for this wedge occurs under ULS 1 loading and is 1.84. As this is the limiting load case, the slope will be stable under all other potential loading cases.
Case GSI mi D φ
C [kPa]
Tensile [kPa] m s
1 Lower GSI, High Disturbance 54 5 1 33 180 80 0.187 0.0005 2 Lower GSI, Typical Disturbance 54 5 0.7 40 250 100 0.399 0.0013
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Scenario 2: There is a discrete wedge and/or fractures visible within the footprint or near to the footing. In this event we assume failure occurs as sliding along two planes, and so the resistance is best modelled as friction angle without contribution from tensile or cohesive strength. The friction angle of the rock for this analysis was assumed to be 35o (as per from the Barton‐Bandis analysis, April 12, 2017 report). Under this condition, the FOS is 1.10, indicating that the slope would not be stable, and will require mitigation. Because this scenario is not inevitable nor necessarily considered likely, it is recommended that rock bolts be provided as a provisional item.
4.1 Rock Bolt Design, For Provisional Item Using the computer program Swedge and assuming the rock mass geometry noted in Table 2, the calculated resistance required to stabilize the design wedge maximum loading (ULS 1) is 3.1 MN. If rock bolts are required it is estimated that up to seven rock anchors may be required with the following properties: Drill Hole Diameter 96 mm, minimum Minimum Bond Length 3.5 meters Free Stressing Length ~4.4 meters (8 meter thread bar minus stickup and bonded zone) Threadbar: Steel Grade 830/1035 Mpa Diameter 32 mm Tensile capacity 500 kN A schematic of the rock bolt is provided in the Appendix. The allowable rock/grout bond was modelled as 450 kPa. This was calculated from a presumptive value typical of siltstone (0.9 MPa) and the required minimum factor of safety of 2.0 found in the PTI manual. All anchors shall be proof tested in conformance with PTI excepted as noted below. The working load of each anchor is 442 kN. The calculated test load for the anchor would typically be 589 kN (1.33 X DL), however, given that the threadbar is made from high strength steel, brittle yield is a concern. As such, it is recommended that anchor testing stay under 80% of bar yield load (0.8 x 673 kN), and be set at 1.20 x DL (531 kN). The specified bolt design is based on available information, and will have to be verified if observed conditions in construction do not match those assumed. 5.0 Foundations at West Pier and West Abutment Table 1 below summarizes rock parameters on the west side of the bridge, where bedding favorably dips into the slope, and rock quality is observed to be less variable and of better quality. Table 1 ‐ RocLab Output Parameters, West Pier
Case GSI mi D φ
C [kPa]
Tensile [kPa] m s
3 Average GSI, Typical Disturbance 66 7 0.7 47 490 214 1.081 0.0072