Page 54 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 5 Terzaghi’s Rock Load Theory 37

TABLE 5.3 Terzaghi’s Rock Load Concept as Modified by Deere

Rock class and RQD (%) Rock load Remarks
condition 95–100 (Hp) Same as Table 5.2
90–99 Zero Same as Table 5.2
I. Hard and intact
85–95 0–0.5 B
II. Hard stratified
or schistose 75–85 0–0.25 B Same as Table 5.2

III. Massive moderately 0.25 B–0.35 Types IV, V, and VI reduced
jointed (B þ Ht) by about 50% from
Terzaghi values because
IV. Moderately blocky (0.2–0.6) water table has little effect
and seamy (B þ Ht) on rock load (Terzaghi,
(0.6–1.10) 1946; Brekke, 1968)
V. Very blocky and 30–75 (B þ Ht) Same as above
seamy 3–30 (1.1–1.4)
(B þ Ht) Same as above
VI. Completely crushed (1.10–2.10)
(B þ Ht) Same as above
VIa. Sand and gravel 0–3 (2.10–4.50)
(B þ Ht) Same as Table 5.2
VII. Squeezing rock at NA Up to 80 m
moderate depth NA irrespective of the Same as Table 5.2
NA value of (B þ Ht)
VIII. Squeezing rock at Same as Table 5.2
great depth

IX. Swelling rock

B ¼ tunnel span; Ht ¼ height of the opening; and Hp ¼ height of the loosened rock mass above the tunnel
crown developing load (Figure. 5.1).

Source: Deere et al., 1970.

replaced by pneumatically filled lean concrete. These improvements in tunneling tech-
nology preserve the pre-excavation strength of the rock mass and use it as a load-carrying
structure to minimize roof convergence and restrict the height of the loosening zone
above the tunnel crown.

Consequently, support pressure does not increase directly with the opening width.
Based on this argument, Barton, Lien, and Lunde (1974) advocated that the support
pressure is independent of opening width in rock tunnels. Rock mass-tunnel-support-
interaction analysis of Verman (1993) also suggested that the support pressure is
practically independent of the tunnel width, provided support stiffness is not lowered.
Goel, Jethwa, and Dhar (1996) also studied the effect of tunnel size on support pressure
and found a negligible effect of tunnel size on support pressure in non-squeezing ground