THE RESTORATION OF THE PISÈ HERITAGE: EXPERIENCED RESULTS FROM SAFEGUARDING OPERATIONS FOR THE MARRAKECH WALLING

Marrakech was founded in 1062 during the Almoiravide kingdom (1060-1147) by the Sultan Youssef Ben Tachfine who had protected it with a defensive wall, which had never stopped to be extended according to the city’s enlargements, in particular during the Almohade (1145-1248) et Saadien (1554-1659) kingdoms. Presently it is a fabric of 19 km in length, of 6/8 meters of height and a relatively constant thickness of 0,6 meters. It is provided with nine doors which give access to the various agricultural areas of the region, with around 200 guard little towers, located approximately at any 35 metres.
The walling is built with pisé, a construction technique which is made up by compacting, into a formwork, various following layers of rammed earth in association with at least some whitewash. The cohesion of this natural concrete is ensured by the clay, so that the other components (river sediment, sand and gravel ) can create the frame which can resist to compression and cracking. This material’s use offers a lot of advantages regarding its cost, easy and speed execution and its optimum characteristics of thermal and acoustic insulation. On the other hand, its solidity and durability are strongly dependant on the ensured protection from water action.
Actually, the main source of the Pisé vulnerability is due to its high sensitivity to the water action (streaming, infiltration, splashing up, rain). The material earth, multiphase, porous and permeable, is made up with a solid matrix and some empty spaces which can hold a lot of water under any shape (solid, liquid or vapour), so allowing its circulation. The latest can occur, under the gravity effect of the water at the liquid state, with the major effect of phenomena of climbing-capillarity, which affects the ground floors and creates hazard for the stability of the structure; or even under the shape of vapour, it can be moved by the action of temperature and/ or pressure gradient.
The material chosen for the restoration should then have imperatively the same properties of the water circulation. Conversely, a barrier will be established which will trouble and hinder the circulations, can hold the water accumulation in the interface between the two materials, getting, as consequence, the rapid showing of reject phenomena.