地铁与隧道施工力学2

Rock mass classification
Engineering rock mass classification
General factors
intact rock strength Rock mass classification fracturing intensity shear strength of fractures geometrical relationship between fracture patterns and the excavation groundwater
Tunnels in weak rock
Assembly of a friction joint in a top hat section steel set
Top hat section steel sets Installation of sliding joint top hat section steel sets immediately behind the face of a tunnel being advanced through very poor quality rock.
Tunnels in weak rock
Practical example
Estimate of rock mass properties For the altered porphyry and fault material, GSI = 15, mi = 12 and ⌠ci = 10 Mpa, ⌠cirm = 0.4 MPa. Rock mass strength =0.1 In situ stress if r0=2 m, then rp=9.3 m and U=0.4 m, substantial support is required. Support pressure =0.35 Experience suggests: U/r0<= 0.02 In situ stress Support pressure=1.4Mpa
Rock mass classification
Engineering rock mass classification
Rock quality designation index (RQD)
RQD is defined as the percentage of intact core pieces longer than 100 mm (4 inches) in the total length of core (Deere et al 1967).
Tunnels in weak rock
Spiling in very poor quality clay-rich fault zone material.
Engineering rock mass classification
Introduction Most of the multi-parameter classification schemes (Wickham et al (1972) Bieniawski (1973, 1989) and Barton et al (1974)) were developed from civil engineering case histories in which all of the components of the engineering geological character of the rock mass were included. In underground hard rock mining, however, especially at deep levels, rock mass weathering and the influence of water usually are not significant and may be ignored. Different classification systems place different emphases on the various parameters, and it is recommended that at least two methods be used at any site during the early stages of a project.
Practical example
Estimate of rock mass properties For the granodiorite, GSI = 55, mi = 30 and ⌠ci = 100 Mpa ⌠cirm = 23 MPa. Rock mass strength =5 In situ stress This shows that the size of the plastic zone and also the induced deformations will be negligibly small. Furthermore, no permanent support should be required for the tunnel.
Rock mass classification
Engineering rock mass classification
Classifications involving stand-up time
Lauffer (1958) proposed that the stand-up time for an unsupported span is related to the quality of the rock mass in which the span is excavated. Lauffer's original classification has since been modified by a number of authors, notably Pacher et al (1974), and now forms part of the general tunneling approach known as the New Austrian Tunneling Method. The significance of the stand-up time concept is that an increase in the span of the tunnel leads to a significant reduction in the time available for the installation of support. For example, a small pilot tunnel may be successfully constructed with minimal support, while a larger span tunnel in the same rock mass may not be stable without the immediate installation of substantial support.
Practical example
Estimate of rock mass properties
1 Forepoles – typically 75 or 114 mm diameter pipes, 12 m long installed every 8 m to create a 4 m overlap between successive forepole umbrellas. 2 Shotcrete – applied immediately behind the face and to the face, in cases where face stability is a problem. Typically, this initial coat is 25 to 50 mm thick. 3 Grouted fiberglass dowels – Installed midway between forepole umbrella installation steps to reinforce the rock immediately ahead of the face. These dowels are usually 6 to 12 m long and are spaced on a 1 m x 1 m grid. 4 Steel sets – installed as close to the face as possible and designed to support the forepole umbrella and the stresses acting on the tunnel. 5 Invert struts – installed to control floor heave and to provide a footing for the steel sets. 6 Shotcrete – typically steel fiber reinforced shotcrete applied as soon as possible to embed the steel sets to improve their lateral stability and also to create a structural lining. 7 Rockbolts as required. In very poor quality ground it may be necessary to use self-drilling rockbolts in which a disposable bit is used and is grouted into place with the bolt. 8 Invert lining – either shotcrete or concrete can be used, depending upon the end use of the tunnel.
Tunnels in weak rock
Full face 10 m span tunnel excavation through weak rock under the protection of a forepole umbrella.
Practical example
Estimate of rock mass properties Installation of 12 m long 75 mm diameter pipe forepoles in an 11 m span tunnel top heading in a fault zone.
Rock mass classification
Engineering rock mass classification
Terzaghi's rock mass classification Intact rock Stratified rock Moderately jointed Blocky and seamy rock Crushed but chemically intact rock Squeezing rock Swelling rock by Terzaghi (1946)
Shotcrete Concrete lining Closely spaced steel sets
Tunnels in weak rock
Sliding joint top hat section sets.
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Estimate of rock mass properties