# A Case Study on Distresses of Concrete Pavements Supported on a Retaining Wall
## 3 Evaluation of the Performance of the Subject Section
### Evaluation of Soil Conditions in MSE Wall  
#### 3.3.1 Soil Modulus from DCP Test and CPT  
Performances of pavement structures are often directly related to the conditions of embankments or retaining walls on which the pavement structures are supported. To evaluate soil conditions in the MSE wall of the subject section, DCP tests were conducted at 17 locations. DCP tests are commonly used to assess the subgrade conditions beneath the pavement structure, but the maximum investigation depth of a DCP test is typically limited to less than 10 ft (120 inches or 3 m). Since the maximum height of the MSE wall of the subject section was about 25 ft, CPTs were performed at five additional locations to characterize soil conditions at deeper depths. Two DCP tests (DCP4 and DCP8) and one CPT test (CPT8) were performed as controls at locations where relatively less distresses were observed, and the rest of the DCP tests and CPTs were performed at locations where major distresses were observed. The locations of DCP tests and CPTs conducted in the subject section are shown in Figure 12.  
To have direct access to the backfill soils, coring or drilling was performed through the pavement structures and DCP tests were conducted through the cored or drilled holes with investigation depths varying from 81.4 in (2.1 m) to 102.7 in (2.6 m) below the pavement surface. Out of 17 tests, 3 DCP tests (DCP1 ML, DCP1 IL, and DCP9 OS) had early refusals, perhaps due to the presence of the cement-treated backfills, and, therefore, the results from the three DCP tests are not presented in the subsequent sections. The results of DCP tests are usually interpreted from the cumulative DCP blow counts vs. depth curves to determine the resilient moduli of soils. The slope of the cumulative DCP blow counts vs. depth curve represents a penetration rate (i.e., penetration depth per blow), and the steeper slope indicates a greater penetration rate or lower modulus of soil. [PERSON] et al. [48] proposed the following equation to estimate the resilient modulus (\(M_{t}\)) of soil from the penetration rate of DCP:  
\[M_{t}[ksi]=2.55\bigg{(}\frac{292}{PR^{1.12}}\bigg{)}^{0.64} \tag{1}\]  
Figure 11: Photos showing a reversed slope of riprap at the bottom of the MSE wall of the subject section compared to the riprap on the east side of the bridge. where \(PR\) = penetration rate in mm/blow. Figure 13 shows this process, taking results from DCP3 OL LWP as an example. Resilient moduli of soils in the MSE wall, determined via the aforementioned procedure, at the locations of all 15 non-refusal DCP tests are presented in Figure 14.  
Weighted average values of resilient moduli were then computed at each DCP test location using the thicknesses of sublayers of soils as weights, and they are presented in Table 1. At control sections, weighted average values of resilient moduli were 15.8 ksi (108.9 MPa) at DCP4 ML and 17.8 ksi (122.7 MPa) at DCP8 OL LWP. On the other hand, results from DCP2 OL LWP, DCP2 ML, DCP2 IL, DCP3 ML, DCP3 IL, DCP6 OL LWP, and DCP7 OL LWP tests show  
Figure 12: Locations of DCP test and CPTs performed at the subject section (OS = outside shoulder, OL = outside lane, ML = middle lane, IL = inside lane, IS = inside shoulder, RVP = right wheel path, and LWP = left wheel path). Figure 13: Determinations of resilient moduli and thicknesses of soil layers in the MSE wall, using results from DCP3 OL LWP as an example (blue, orange, and gray colors represent the subdivided layers based on the penetration rates; the vertical red lines on the right-side figure represent the resilient modulus determined for each sublayer). that the weighted average moduli at these non-control locations are less than 10 ksi (69 MPa). The lowest value of the weighted average of resilient modulus was observed at DCP2 ML, with a value of 4.4 ksi (30.3 MPa), roughly one-fourth of the value observed at the control location, namely DCP8 OL LWP.