潛盾隧道之施工常遭遇的問題包括(1)隧道開挖面之穩定性檢討。(2)施工引致之地表沉陷分布及對建物之影響評估。(3)作用於環片之土壓力大小評估。因此隧道設計者必須考慮隧道開挖時，開挖面之穩定性，並藉以決定施工方式及開挖引致之周圍地盤沉陷問題。盾尾空隙閉合後地盤應力釋放作用在環片上之土壓力大小，用以決定隧道環片之強度及架設時機。 單隧道開挖面穩定試驗結果顯示，在相同覆土深度時，隨著未襯砌長度之增大，單隧道崩潰時之超載係數降低、最大地表沉陷量(dmax)增大。當P/D 時， C/D越大時dmax越大；但當P/D 時， C/D越大時dmax反而越小。另外，在相同C/D之下，平面應變試驗(P/D=∞)之dmax遠大於三維試驗(P/D=0、0.5、1及1.5)之dmax。在相同未襯砌長度時，深徑比越大時其地表沉陷槽寬度也越大。三維破壞機制約在P/D=1.2過渡至平面應變狀態。 襯砌周圍土壓力分布試驗結果顯示，隨著C/D的增加盾尾孔隙閉合後的正規化土壓力分布將趨近於定值且側壁之土壓力最大，將試驗結果與現場資料比較發現有極相似之分布，並由孔隙水壓變化可知當C/D越大其拱化效應越顯著。新挖隧道達大變形前將造成既存隧道近新挖隧道側周圍土壓力有較大影響，且其最終正規化土壓力將隨C/D增加而趨於定值。 The problems a designer often faces on the construction of the shield tunnel in soft ground are：(1) the face stability of a tunnel; (2) influence of the magnitude and extent of surface settlement trough caused by tunneling on the safety of building nearby a tunnel ;(3) estimate of the earth pressure on tunnel segments. A series of 3-D centrifuge tunnel models with cover-to-diameter ratios (C/D) from 1 to 3 and unlined length-to-diameter ratios (P/D) from 0 to 1.5 are used to investigate the failure mechanism of excavated tunnel face and to measure the earth pressure acting on the liner after tail voids closure and on the liner of a existed tunnel due to nearby new tunneling. The measured overload factor at collapse decreases with an increase of P/D for the tunnel having the same C/D. The maximum surface settlement (dmax) increases with an increase of P/D and it reaches the largest value in the plane strain model (P/D reaches infinite) for the tunnel having the same C/D. dmax increases with increasing C/D in the case of P/D less than or equal to 0.5,however, it decreases with increasing C/D in the case of P/D greater than or equal to 1. In addition, the larger the cover-to-diameter ratio is, the wider the width of settlement trough. Three dimensional failure mechanism will transfer to two dimensional failure mechanism if P/D is greater than 1.2 from the observed profile of displacement vecter before and failure. The magnitude normalized of earth pressure acting on the segment may become constant distribution state for a deeper tunnel and the maximum normalized earth pressure appear at side walls of a tunnel. The extent of the arching effect in tunnel surrounding becomes obvious for a deeper tunnel from the observation of the change of pore water pressure in the ground. The new tunnel nearby an existed tunnel will increase the earth pressures at the side near to the new tunnel. The magnitude of the normalized earth pressure acting on the existed tunnel will become a constant distribution state for a deeper tunnel.
