Some parts of the Wall were not very well built. Sections of the curtain wall collapsed west of the fort at Wallsend and west of the bridge over the North Tyne at Chesters, whilst at Housesteads the north wall of the fort fell down more than once. Similarly, one of the stone turrets of the turf wall (T54A) fell into the ditch and had to be replaced by another, positioned slightly further south of it, which was then incorporated into the stone wall when it was built. There is evidence that the north face of the gateway at Milecastle 37 (Housesteads) collapsed northwards too.
There may be several reasons for this beyond simple shoddy workmanship (which cannot be completely discounted). The bonding material used for the core of the wall – a clay or earth matrix holding rubble – may have been vulnerable to faulty pointing leading to penetration and erosion by the elements. Even now, bits of the consolidated curtain wall can break off after frost (just one of many reasons why not to walk on it). Water may have played a part too – a stream undermined the curtain wall west of Wallsend.
All of this may have lain behind the reconstruction work that was undertaken in the reign of Severus, leading to the notion that this emperor was responsible for building the Wall in the first place.
Further reading: Breeze 2006