Regarding the influence of seepage on face support pressure and ground collapse mode in shallow shield tunnel excavation

an experimental system composed of a shallow shield tunnel excavation-seepage model and a setup of measurement and data acquisition is designed and established

with consideration of pore water pressure in the excavation chamber and face plate opening ratio. The saturated soil pressures and pore water pressures at the face and settlements of surrounding strata are measured under various steady-state seepage conditions

as the soil input into the excavation chamber is gradually increased. Correspondingly

numerical simulations and limit equilibrium calculations are carried out. The major findings are: the effective earth pressure at tunnel face decreases as excavation volume loss increases

but plateaus after reaching a limit value; the necessary support pressure for face stability is related to the allowable collapse range of the stratum

the limit effective earth pressure

i.e. the minimum necessary support pressure at the tunnel face corresponds to the stratum limit collapse range. Seepage increases the limit effective earth pressure of excavation face

which shows roughly linear relationship with the hydraulic head difference between excavation face and ground surface. Seepage may increase the limit collapse range of the soil ahead of the tunnel face

but has little effect on the limit collapse range of the soil behind tunnel face; if the relative hydraulic head difference between tunnel face and ground surface is small (lessthan or equal to 0.33)

seepage will mainly increase soil settlements

while the stratum limit collapse range only increases slightly; if the relative hydraulic head difference is large (greater than 0.33 and less than 1.00)

seepage will enlarge the stratum limit collapse range

whereas the maximum settlement of stratum decreases slightly; if the relative hydraulic head difference is very large (greater than or equal to 1.00)

the influences of seepage on both the settlement and limit collapse range of stratum have essentially reached their limits. The stratum limit collapse range may be divided into three zones stacking vertically: an inverted pyramid at the bottom within the height of tunnel face

an inverted prismoid at the top within a certain depth range below the ground surface and an inverted prismoid of a certain height in between; out of these three zones

the tapered angles of the inverted pyramid increase in longitudinal and transverse directions when the relative hydraulic head difference increases

and they exert the most significant influence on the limit collapse range of stratum.