At a deeper level, above the tunnel axis, a narrower settlement trough develops, giving a maximum settlement . This Note considers available field measurements and centrifuge model test data, and addresses the question of how the widths of the settlement profiles and the magnitudes of settlement vary with depth above tunnels constructed in clays.粘土中隧道開挖引起的地中沉降
Mair, R. J., Taylor, R. N.& Bracegirdle,
A. (1993). Gkotechnique
43, No. 2, 315-320
Subsurface settlement profiles above tunnels in clays R. J. MAIR,* R. N. TAYLORt and A. BRACEGIRDLE*
KEYWORDS: case history; clays; settlement; tunnels. INTRODUCTION For tunnelling schemes in urban areas, constraints of existing tunnels and deep foundations often lead to new tunnels being constructed close beneath such structures, as shown in Fig. 1. Designers assessing the likely effect of tunnelling on structures relatively close to the tunnel crown need to know how subsurface settlement profiles develop, and how these relate to surface settlement profiles. The effect on structures such as those shown in Fig. 1 depends on the width of the subsurface settlement profile and on the magnitude of the settlement. Fig. 2 shows a tunnel at depth z,, below ground level; a settlement trough develops at the ground surface giving a maximum settlement 6, over the tunnel centre line. At a deeper level, at a distance (z. - z) above the settlement trough tunnel axis, a narrower develops, giving a maximum settlement 6, over the tunnel centre line. This Note considers available field measurements and centrifuge model test data, and addresses the question of how the widths of the settlement profiles and the magnitudes of settlement 6, and S, vary with depth above tunnels constructed in clays.
Smax= 6,. The width of the settlement profile is defined by the important parameter i, which is the distance from the tunnel centre line to the point of inflexion of the trough (shown in Fig. 2); the total half-width of the settlement trough in practical terms is given by about 2.5i. Figure 3 shows data obtained by O’Reilly& New (1982) of the parameter i plotted against depth of tunnel axis below ground level z0 from field measurements of surface settlements above UK tunnels in clays. O’Reilly& New proposed the linear relationship (shown in Fig. 3) i= 0.432,+ 1.1
(2) it is often reasonable to (3)
For practical assume that i= Kz,
SURFACE SETTLEMENT PROFILES A considerable amount of data is available from field measurements of surface settlement profiles above tunnels in clays (e.g. Schmidt, 1969; Peck, 1969; Clough& Schmidt, 1980; O’Reilly& New, 1982; Rankin, 1988). It has been found that the shape of the surface settlement troughs developing during tunnel construction is reasonably well represented by a Gaussian distribution, as shown in Fig. 2. The settlement S is defined as 5= Snl,, exp ( -x2/2?) (1)
The data are reasonably consistent with K= 0.5, as shown in Fig. 3. Rankin (1988) plotted i against z. for field measurements above tunnels both in the UK and worldwide, and concluded that K= 0.5 was a reasonable fit to most of the data. The settlements caused by tunnelling are often characterized by the term‘volume loss’ (sometimes referred to as‘ground loss’), expressed as a percentage of the notional excavated volume of the tunn
el. The volume of the settlement trough (per metre length of tunnel) is obtained from integration of equation (l), and is given by v,= .\l(27+S,,, where D is the excavated diameter (4) of the tunnel.
v,=$$ where D is the excavated diameter of the tunnel. For tunnels in clays, settlements during tunnel construction usually occur under undrained (constant volume) conditions, in which case the volume V, represents the additional quantity of clay excavated, over and above the theoretical volume of the tunnel of excavated diameter D. Combining equations (3)-(5) gives S mBx= 0.313VLDZ/i 315 (6a)
is the maximum settlement, which where S,, occurs above the tunnel centre line; in Fig. 2 Discussion on this Technical Note closes 1 October 1993; for further details see p. ii. * Geotechnical Consulting Group. t City University, London.