3.3 Evaluation of Subsurface Drainage Conditions below Pavement
From the settlement data from ground improvement, the overlapping of sand blanket and secondary canals shown in Figure 24 and the discontinuity of the sand blanket caused by the box culverts, the airfield pavement in the WEA and WESA zones can be subdivided into forty-six (46) individual blocks as shown in Figure 25. For each block, the subsurface drainage condition of the subgrade sand beneath the pavement is evaluated based on the connections to the nearby canals. The results indicated that at least twenty-eight (28) blocks do not have direct link to the nearby canals. The remaining blocks have sand blanket connected to the secondary canals with drainage through weep holes on the side panels, and some areas have connected to unlined temporary canals which would be blocked by new box culverts in future airport expansion. At present, over 50% of the airfield pavement has no apparent subsurface drainage connection to the canals with summary tabulated in Table 3. If the water is allowed to enter the blockage subgrade sand, then there is no possibility for the water to drain out as observed in Taxilane T11 (Figure 26). The measured water level in Taxilane T11 during February 2007 was maintained at elevation of +0.0 m MSL, and this was the same level as the top edge of the secondary canal. It can be concluded that the stagnant water level in the taxilane represents a major defect in the subsurface drainage system if the pavement structures are designed with some freeboard clearance