# A Case Study on Distresses of Concrete Pavements Supported on a Retaining Wall
## 1 Introduction  
According to the ASCE 2021 Infrastructure Report Card [1], the evaluation of bridge conditions across the United States has yielded an overall grade of C. This grading underscores the imdling state of the nation's bridges, necessitating targeted efforts to rectify a range of concerns. Notably, a pressing issue arises from the identification of 7.5% of the country's bridges--equivalent to 46,154 structures--as being subpar or exhibiting structural deficiencies. Within the wide-ranging bridge network of the state of Texas, a substantial portion, namely 18% of Texas' bridges, has been classified as being in unsatisfactory condition [2]. Embankments and retaining walls are integral parts of the bridge system and provide a smooth transition from roadways to bridge decks [3]. A bridge approach slab is another essential component of the bridge system that acts as an intermediate bridge to span the end portion of the pavement on the roadway embankment/retaining wall directly behind the bridge abutment. In Texas, drilled shafts are commonly used to support bridge abutments, while steeper slabs are utilized to support the bridge approach slabs on the pavement side [4]. Such a difference in support system typically induces differential settlement between the pavement side and the abutment side, creating an uneven transition onto the bridge. There are other factors that can cause unsatisfactory performances of pavement structures supported on embankments/retaining walls, including poor conditions of the subgrade soil, inadequate construction practices, compression of the embankment/retaining wall fill materials, and drainage issues [5]. Furthermore, many researchers reported that a considerable number of current bridge approach slabs contain voids underneath them resulting from water infiltration and consequent material erosion [6; 7; 8]. A pavement structure is a multi-layered system, and the subgrade serves as a foundation of the pavement structure. Evaluations of pavement performance can be broadlydivided into two tasks: (a) the assessment of the conditions of pavement structures and (b) investigation of the quality of the subgrade. Assessing conditions of pavement structure often involves the identification of distresses through visual surveys, determination of slab performance through deflection testing [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20], and checking the quality of concrete via coring [21; 22; 23; 24; 25]. [PERSON] et al. [26] conducted a comprehensive investigation into the utilization of falling weight deflectometer (FWD) data for mechanistic-empirical pavement design and rehabilitation procedures, establishing the efficacy of FWD tests in pavement evaluation and rehabilitation. Investigating the quality of the subgrade requires direct access to the subgrade, and the dynamic cone penetrometer (DCP) test has been widely used to determine the California bearing ratio (CBR) or resilient modulus (\(M_{r}\)) of the subgrade due to its rapid setup and simplicity [27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Results from the DCP tests can be used not only for the quality assessment of newly prepared subgrade but also for in-service roadways when adopting maintenance strategies. For example, if the results of DCP tests performed for in-service roadways indicate the high quality of the subgrade, maintenance measures can include simply treating the surface pavement layer. However, if the DCP investigations show the poor quality of the subgrade, then extensive stabilization measures or the replacement of the subgrade may be needed [37]. Evaluating pavement structures built on roadway embankments/retaining walls poses additional challenges compared to typical pavement structures. Maximum investigation depths of DCP tests are typically limited to 9 to 10 ft (2.7 to 3 m) due to the bending of the driving shaft, and such investigation depths may be sufficiently deep to assess the subgrade quality for typical pavement structures.