###### Abstract  
Tunnels may suffer severe damage when passing through an active fault in high-intensity earthquake zones. The present study aims to investigate the performance of an isolation layer composed of a rubber-sand mixture, an emerging trend in low-cost seismic mitigation studies. Based on the Ngong tunnel in the Nairobi-Malaba Railroad in Kenya, Africa, the effect of the rubber-sand isolation layer on the acceleration and strain of the tunnel lining was investigated through a shaking table test under small normal fault creep-slip and subsequent seismic shaking. The influences of the length of the isolation layer and the rubber content in the mixture were analyzed by numerical simulation. The results indicate that the isolation layer slightly reduces the acceleration response of the tunnel lining within the fault and obviously reduces the permanent strain of the invert and crown within the fault under small normal fault creep-slip and subsequent seismic excitation. The mitigation effect of the isolation layer is related to the length of the isolation layer and the rubber content in the mixture. In the case of this study, the length of the isolation layer is triple the fault width (influence range of the fault) and the appropriate enhancement of the rubber content of the isolation layer offers favorable conditions for mitigation effect, respectively. t  
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fault zones [6]. The tunnels of the Nairobi-Malaba Railroad in Kenya are vulnerable to damages from active fault creep-slip with a rate of 0.2-2 mm/year and potential strong earthquakes in the East African Rift Valley [7,8]. The influence of combined fault creep-slip and subsequent earthquakes on tunnels has attracted much attention. [PERSON] et al. [9] first showed that tunnels should have a relatively small segment length under combined normal fault dislocation with a magnitude of 2 m and subsequent seismic shaking. [PERSON] et al. [10] found that initial normal fault creep-slip reduces the overall stiffness of the tunnel in subsequent seismic excitation. [PERSON] et al. [11] elucidated the damage mechanism of normal fault-crossing tunnels under fault rupture and subsequent seismic excitation. Flexible joints and segmented tunnels were investigated for the adaptation of mountain tunnels to fault dislocation with amplitude ranges from 1.5 to 5 m and subsequent seismic shaking, as well as fault dislocation alone, by [PERSON] et al. [12] and [PERSON] et al. [13, 14, 15, 16, 17, 18]. The fault dislocations in the above studies were meter-level dislocations. The literature [19, 20, 21] showed centimeter-level dislocations in faults moving with creep-slip rates of 0.1-1 mm/year, which resulted in a 1-10 cm displacement within a century. Fault-crossing tunnels may be subjected to centimeter-level dislocation and strong earthquakes. Therefore, besides flexible joints and segmented tunnels, isolation layers may be a theoretical method for the adaption of tunnels to small normal fault creep-slip and subsequent seismic shaking. Isolation layers have been extensively studied as an aseismic or shock absorption method for the tunnel [22, 23, 24]. Flexible materials with a low modulus and high damping ratio of seismic isolation layers, such as foamed concrete, geofoam, rubber-sand, and rubber, can be adapted to the larger deformation aroused by low-frequency seismic excitation [25, 26, 27, 28]. Rubber-sand mixtures are considered as an environmentally friendly and low-cost geotechnical seismic isolation (GSI) technology [29, 30]. GSI can be achieved by introducing a low-modulus layer for decoupling the structure from the ground shaking [27]. [PERSON] et al. and [PERSON] et al. [31, 32, 33] revealed through shaking table tests that the addition of a rubber-sand mixture beneath a structure can activate some effective detuning mechanisms, thus protecting the structure from earthquakes.