It is well-known that the performance of Portland cement concrete (PCC) pavement depends, to a great extent, on the support condition under the slab. The Texas Department of Transportation (TXDOT) recognized the importance of the support condition and requires the use of a stabilized base, either a 10.16-cm hot mix asphalt layer or a minimum 2.54-cm asphalt concrete over a 15.24-cm cement-stabilized layer. For pavement design purposes 81.4 MPa/m has been used as the default value for the modulus of the subgrade reaction (k value) for the support provided by these bases. The use of a single value of 81.4 MPa/m for the k value in rigid pavement design was partly because of the insensitivity of the required slab thickness to the modulus of the subgrade reaction in the current TXDOT rigid pavement design procedure. To improve rigid pavement design, TXDOT developed a new design procedure for continuously reinforced concrete pavement (CRCP) on the basis of mechanistic-empirical principles and plans to implement the procedure. In this design procedure, the slab support has rather more substantial effects on the required slab thickness than the current TXDOT design procedure. To increase the reliability of the new pavement design procedure, accurate evaluations of the k value are important. A test section was constructed in which various base types were used, and a number of field tests were conducted to estimate the range of k values for the support systems that are currently used or expected to be utilized, including the placement of nonwoven geotextile in lieu of the asphalt layer. The current practice of using one default value of 81.4 MPa /m needs to be revised, and the use of nonwoven geotextile is not recommended until further evaluations prove its viability.