CIVL3811 Lecture Slides - Week 5 - 2022 PDF

Tunnel Engineering Rock Mechanics CIVL3811 Engineering Design and Construction School of Civil Engineering | Faculty of Engineering THE UNIVERSITY OF SYDNEY The University of Sydney Page 1 Basic Data Types Nine basic data types for a tunnel design: 1.Intact Rock Strength: This parame...

Tunnel Engineering Rock Mechanics CIVL3811 Engineering Design and Construction School of Civil Engineering | Faculty of Engineering THE UNIVERSITY OF SYDNEY The University of Sydney Page 1 Basic Data Types Nine basic data types for a tunnel design: 1.Intact Rock Strength: This parameter is measured laboratory 2.Field Stresses 3.Ground Water 4.Drill Core Quality (Fracture Density) 5.Joint Spacing 6.Joint Persistence (length) 7.Joint Orientation 8.Joint Contour (Shape) 9.Joint Aperture and Surface Condition The University of Sydney Page 2 Main characteristics The University of Sydney Page 3  Jointed Rock The University of Sydney Page 4 The University of Sydney http://terracoregeo.com/home/ Page 5 https://www.researchgate.net/publication/326129056_A_Preliminary_Investigation_for_Character ization_and_Modeling_of_Structurally_Controlled_Underground_Limestone_Mines_by_Integratin g_Laser_Scanning_with_Discrete_Element_Modeling/figures?lo=1 The University of Sydney Page 6 The University of Sydney https://www.researchgate.net/publication/=1312577500_Modeling_a_Shallo Page 7 https://www.sciencedirect.com/science/article/pii/S0013795216301314#f0015 w_Rock_Tunnel_Using_Terrestrial_Laser_Scanning_and_Discrete_Fracture _Networks/figures?lo  https://www.geocomp.com/GeoTesting/Laboratory/Rocks The University of Sydney Page 8 http://www.dr-roland- braun.com/EN/evaluation/3d_in_situ_str ess/index_3d_in_situ_stress.html The University of Sydney https://www.researchgate.net/publication/267459830_Total_Sand_Manage Page 9 ment_Solution_For_Guaranteed_Flow_Assurance_In_Subsea_Developme nt/figures?lo=1 The University of Sydney Page 10 Rock mass classification – Rock mass classification schemes can/maybe – beneficial at the feasibility and preliminary design stages of an underground structure; – used as a check-list to ensure that all relevant information has been considered (the simplest level); – used to build up a picture of the composition and characteristics of a rock mass to provide initial estimates of support requirements (empirical design methods for rock support); – and/or estimates of the strength and deformation properties (a more advanced level). – Rock mass classification schemes cannot – replace engineering analysis and design procedures The University of Sydney Page 11  Q-System The University of Sydney Page 12 The University of Sydney Page 13  RQD The University of Sydney Page 14  joint set number, Jn The University of Sydney Page 15  joint roughness number, Jr (critical joint walls) The University of Sydney Page 16  joint roughness number, Jr The University of Sydney Page 17  joint alteration number, Ja The University of Sydney Page 18  Stress Reduction Factor (SRF) The University of Sydney Page 19  Joint water reduction factor Jw The University of Sydney Page 20  (Jw/SRF) – The third quotient (Jw/SRF) consists of two stress parameters. SRF is a measure of: 1) loosening load in the case of an excavation through shear zones and clay bearing rock, 2) rock stress in competent rock, and 3) squeezing loads in plastic incompetent rocks. It can be regarded as a total stress parameter. The parameter Jw is a measure of water pressure, which has an adverse effect on the shear strength of joints due to a reduction in effective normal stress. Water may, in addition, cause softening and possible out-wash in the case of clay-filled joints. It has proved impossible to combine these two parameters in terms of inter-block effective stress, because paradoxically a high value of effective normal stress may sometimes signify less stable conditions than a low value, despite the higher shear strength (Bieniawski , 1989; Grimstad and Barton , 1993) The University of Sydney Page 21  What can Q-System tell us ? The University of Sydney Page 22 Q-System - Example 1 The rock mass is a strong competent jointed sequence of rocks (RQD=70%) with an average uniaxial compressive strength (UCS) of approximately 200 MPa determined with standard laboratory testing procedures using drill core samples. There are 3 major joint sets with little water present. The critical joint walls are smooth and wavy. The Q-system for rock mass classification RQD J R JW Q= × × JN J A SRF Rock quality designation (RQD). Number of joint sets (JN). Roughness of the most unfavourable joint or discontinuity (JR). Degree of alteration or filling along the weakest joint (JA). Water inflow (JW). Stress condition given as the stress reduction factor (SRF) The University of Sydney Page 23  Solution – Fair Rock Mass Quality RQD=70% – Three joints sets Jn = 9 – Unaltered Joint walls Ja = 1 – Jr = 2 The critical joint walls are smooth and wavy – Dry excavations or minor inflow Jw = 1 – SRF Moderate slabbing after > 1 hour in massive rock – 𝜎𝜎UcS/ 𝜎𝜎1= 200/50=4 SRF= 27.5 – Q= RQD/Jn x Jr/Ja x Jw/SRF – = 70/9 x 2/1 x 1/27.5 – = 0.57 The University of Sydney Page 24 Q-System -Application - DesignSupport and Reinforcement Tabulated version by Barton A summary graph of recommendations by Grimstad based on Barton’s Table. It is for different combinations or rock quality, Q, and on Equivalent Span. The proposed graph was developed for permanent support in civil tunnels, shafts and caverns. The University of Sydney Page 25 The University of Sydney Page 26 The University of Sydney Page 27 Excavation Support Ratio ESR The value of ESR is related to the intended use of the excavation and to the degree of security which is demanded of the support system installed to maintain the stability of the excavation. The University of Sydney Page 28  Equivalent Dimension - De In relating the value of the Q - system to the stability and support requirements of underground excavations, Barton et al (1974) defined an additional parameter which they called the Equivalent Dimension, De, of the excavation. The University of Sydney Page 29  De=6.25 RQD = 40 ⇒ Q = 0.45 The University of Sydney Page 30  Bolt Length The length L of rock bolts can be estimated from the excavation span and the Excavation Support Ratio ESR: Span 2 + 0.15 × L= ESP The University of Sydney Page 31 Support System – Reinforcement Rock Bolts (typically less than 3 meters long) – Cable Bolts (typically greater than 5 meters long) – Surface Support Plates – Straps – Mesh – Concrete (Shotcrete) The University of Sydney Page 32  Reinforcement Rock Bolts The University of Sydney Page 33  Cable Bolts The University of Sydney Page 34 The University of Sydney Page 35 The University of Sydney Page 36  Cable Bolts The University of Sydney Page 37  Plates The University of Sydney Page 38  Straps The University of Sydney Page 39  Mesh Reinforcement The University of Sydney Page 40 Shotcrete Shotcrete is a key for rapid tunneling excavation and fast cycle times in tunnels around the globe. Rapid early strength development of the shotcrete is critical in order to enable short cycle times and ensure efficient rates of progress in tunneling. https://bestsupportunderground.com/shotcrete-sprayed-concrete-lining-scl/?lang=en The University of Sydney Page 41  Thin shotcrete design. Shotcrete Failure Mechanisms -Adhesion If adhesion fails, then it can either fail in flexure or the bolt heads could punch through. Once you’ve got the mechanism, you can assess the strength. The University of Sydney Page 42 Thin shotcrete design. Failure Mechanisms –Direct Shear Again, once you’ve got the mechanism, you can assess the strength. The University of Sydney Page 43 Thin shotcrete design. Failure Mechanisms –Compression / Tension. Uncommon for thin shotcrete The University of Sydney Page 44  Shotcrete with Fiber‐reinforced https://bestsupportunderground.com/fibre-reinforced-shotcrete/?lang=en The University of Sydney Page 45