# Experimental Study on Dynamic Response Characteristics of RPC and RC Micro Piles in SAJBs
## 1 Introduction  
Jointless bridge is the one with continuous superstructure and without moveable joint (MJ) or movable deck joint (MDJ) between the superstructure and abutment. It has a link slab between the outer ends and girders. It includes the integral abutment jointless bridge (IAJB), the semi-integral abutment jointless bridge (SAJB), and the deck-extension abutment jointless bridge (DAJB) [1; 2]. IAJB and SAJB are jointless bridges with integral or semi-integral abutment, and they have been widely used in the developed countries for its integrity and economy, such as North America, Europe, and Australia [3; 4; 5]. At present, although semi-integral abutment jointless bridges (SAJB) are widely used abroad for these properties composed of good integrity, low-cost, and a good comfortable driving environment to vehicles. However, SAJB has relatively poor seismic performance compared with IAJB [6; 7]. Meanwhile, the longitudinal structures of bridge and the constraint structures of transverse anti-falling beams in SAJB are easier to damage under seismic load [8; 9; 10; 11; 12; 13]. Therefore, [PERSON] [14] and [PERSON] [15] proposed a new type of micro piles made by concrete to improve the seismic behavior of SAJB. They were constructed in the backfill filling soil behind abutment below approach slabs, as shown in Figure 1. The micro-piles can be taken as a vertical brace structure for the approach slab to reduce the settlement and subsidence of the approach slab. In addition, the micro-piles can work as a longitudinalelastic constraint to enhance the resistance of girder and approach slab under temperature loads and other effects. As a result, the expansion and contraction between the approach slab and road can be reduced. During the earthquake action, the micro-pile and soil interaction can dissipate the seismic energy to play a role as damping equipment and also increase the horizontal constraint to prevent the girder from failing. However, the design of the approach slab in SAJB is relatively complicated due to the elimination of the expansion joint. The pile foundations below the approach slab require high flexibility performance to accommodate the cyclic horizontal deformation of the approach slab generated by the girder's thermal expansion and contraction as well as earthquake action. Micro-piles usually refer to those small-diameter piles within a diameter of 100 to 300 mm and a slenderness ratio of more than 30 [16]. [PERSON] et al. [17] proposed the application of micro-pile in detail, which lays the foundation for the research and development of the micro-pile in the future. At present, the studies on micro-pile are more extensive, but it mainly focuses on the testing research studies under static load or finite element (FEM) analysis, and lack the related experimental investigation under dynamic load. The most completed book on micro-pile have been published by the United States Federal Transportation Administration (DOT), and it aims to provide engineers with comprehensive technical guidance and application design of the micro-pile in practical engineering [18]. [PERSON] [19] and [PERSON] [20] have done some work on the dynamic responses of structure-soil interaction based on the shaking table tests. It is found that the seismic responses of structure and soil were more sensitive to input motions with richer low-frequency components. Meanwhile, the frequency spectrum characteristics of input ground motions clearly affected the lateral displacement of the structure. Furthermore, the influences of mismatches on the simulated dynamic response of a simple structural model were further investigated. [PERSON] et al. [21] conducted the mechanical properties of single micro pile in sand under axial cyclic loading, focusing on the characteristics of side friction between the micro pile and the cast-in-situ pile. However, it is well known that accidental loads (strong earthquakes) will generate greater damage to the pile foundation of the bridge.