The Ruins of Jiaohe is a national major preservation unit of cultural relic. As an ancient earthen city site

it is more intact and biggest in the world currently. The Observation Platform is a major and typical building in the Ruins of Jiaohe. The primary damages of the Observation Platform include surface weathering

unstable roof of cave and many crannies. Based on analysis of the damages forming

some key techniques of protection and reinforcement of earthen site have been studied. It is suggested that when the depth of base recess is less than 30 % the width of the block and is very smaller than thickness of cantilever block

it should not be repaired and only used the method for preventing surface weathering. With bamboo-wood anchor and PS-C grouting

the anchoring unit power of wood can be adopted 5 kN/ m by test. NATW

the technique for reinforcing the roof of cave

can acquire effective result for preserving its appearance. In the arid region of western China

when applying PS material to reinforce ancient earth-structure sites and prevent the surface weathering

we should adopt low density (smaller 5 %) of the PS

and the method of many times penetrating. The number of permeating should not be less than 3 times in windward side

and should amount to 4-5 times in leeward side. The reasonable working-time needs choice. By the examination of engineering practices

ideal result has been obtained after applying these protective techniques.