Apron Slab between my house front fence wall and the large open drain

The 20 year old apron slab was cracking and making hollow sound when I walked on it. I hammered the slab and found 100mm void beneath.

This hollowness was caused by:
(a)  consolidation settlement by the un-compacted soil behind the drain,
(b)  loss of soil by seepage, flowing rain water through cracks, joints and the drain weep holes.

As the solid fence wall sits on footing just next to the drain, such loss of soil will weaken the foundation and in future, the wall may tilt and fail. Therefore, I hacked off the slab, fill in with stones and sand and re-cast the slab. 
Maintenance is essential to ensure lasting performance of any product.  
I am sure after I finish this work (I am still doing, only 3m per day), I need not worry of the wall for another 15 years!