■Restoration Project of The N1 Tower Prasat Suor Prat

One of the purpose of the conservation and restoration of Prasat Suor Prat towers is to set a restoration model for laterite made towers and material restoration methods for laterite blocks. Until now, we see some restoration project of platform and wall but there are very few examples of restoration on laterite made towers. So we need to research and develop, masonry method, repairing methods of laterite block and materials for restoration. Full-scale survey and analysis will be done for the foundation and platform which are the factor for inclination of the towers. We also can grasp precious data for research works for the future conservation and restoration. The results of research will be reflected to the actual restoration works.

Moreover this restoration works includes many themes to carry on. The relations between the towers and terrace, influence of north-pond. A total survey and research that considers the environment around tower are needed for conservation and restoration of N1 tower. We expect that these results and experiences here will turn out as the basic data or reference materials for the consideration of the remaining eleven towers.

■The significance of the whole Dismantling and Reassembling Works

For the preservation of the cultural heritage, it should not be, needless to say to preserved in the present state. In order to preserve the Tower N1 safely without dismantling, we have to condition the soil in vast area and reinforce the construction, after straightening the lean with concrete or steel frame support or jacking up on a large scale. We had to give up these methods considering huge cost involved, preservation of the scenery of the Plaza, environmental protection from ecological viewpoint, and effect of technical transfer to Cambodian people.

The Tower N3 toward the central section of the Northern Group was dismantled of the upper part from the platform and foundation in 1960ﾕs and it was reassembled, by reinforcing its stone material and the tower itself with cement, mortar and so forth. But, probably, immediately after the completion of the reassembling the tower presumably began to lean toward the north pond. As is known in this case, we presume collapse of many Angkor monuments is caused by the structure of the platform and foundation. Upon studying carefully on several methods of restoration, we have decided, for the first time in Angkor, to dismantle the whole structure of the building including its platform and foundation, investigate the causes of collapse, and then to reconstruct the tower.

■The Situation of Deterioration and Collapse

As the investigation, the N1 Tower itself inclined to northwest direction, it made some stones fall down from the upper part, and they were scattered in the pond. Platform elements moved toward the northern pond, gap openings between stone joint in the north part was connected with the embankment elements, and compared with the southeast corner of the foundation, it subsided about 35 cm at the foundation part and 50 cm at the first layer of the lowest wall stone. Furthermore, serious gap openings between main chamber and antechamber caused roof elements to collapse and drop inside, and pushed the walls out so the upper part inclined outside. The southern wall happened to collapse in previous time of the movement. The restoration of N1 Tower started from research and analysis of the situation of deterioration and collapse.

**【trench】A digging slot dug during an excavation in order to confirm the internal situation of the monumental area.

■Solving the Reasons of Deformation from Dismantling

Deformation of the Towers：Among 12 towers except the southern most one which has been in failure, there is a common characteristic of deformation of structures, especially for the towers near North and South ponds. Horizontal distance of the opening at the bottom is wider than that at the top of the towers. Horizontal widening is noticed at the laterite blocks of the foundations.

The Structure of the platform and Foundation：Photo in the bottom right shows the NS vertical section viewed from west side. The upper portion of right hand side is manmade compacted layer. The laterite blocks of eight layers were laid upon the compacted fill as to protect the foundation fill. The encircled number in larger size shows the block at the present position. Block lines in white show the estimated original position. The number in smaller size shows the estimated original position of the same element block. To clarify the structure of the platform, excavation of the North Pond is also conducted. (Photo in the bottom left)

How to trace the original position：There are two points to be discussed in the bottom right picture. One is a layer line of block(6), the right side block of the same line is found at the higher level that have been at the same level when constructed. Another is the deformation of manmade compacted layer near the block(6). These compacted layers were also horizontal in the original situation. These two characteristics of the displacements of laterite blocks and filled layer lead to a conclusion that laterite blocks were slipped down along the boundary between blocks and fill. The horizontal displacements were associated with the vertical downwards movements.

Conclusion：The deformation of the laterite blocks is continuous and not concentrated at some point. It is very hard to imagine the inside movement of the manmade fill and the blocks. Without dismantling process, the mechanism of spreading out of laterite blocks due to sliding was never realized. Geotech unit reports one of the interesting facts found by dismantling process.

■Laterite*** Block Repair

Restoration works for cultural heritage is aimed at reusing the original blocks as much as possible. But when reassembling, a new stabilization is required, so we must give sufficient strongness and endurance by restoration. Through out destruction and mortar tests, we have established a suitable manual for the damaged laterite blocks such as fixing the detached parts, bonding he broken parts, filling the lost portion, supplement of new parts and replacing new block. In the case, laterite blocks that required structural reinforcement will be reinforced using stainless steel plate or bar. Polymer cement mortar mixed with a laterite powder is used to connect stainless steel reinforcement bar or fixing stainless steel bolt.

More than 2300 laterite blocks were dismantled from the N1 Tower. Among then, 600 needed repairing or supplement of new laterite. In October 2003, the laterite repairing works achieved 60%.

***【 laterite 】Red soil or rock often seen in tropical and subtropical regions, rich in aluminum and iron components. Formed by leaching of rocks in the weathering process. What we call 'laterite blocks' are the blocks which are cut and hardened into a shape suitable for use in buildings.

■Sandstone Repair

The N1 tower of Prasat Suor Prat is mainly built of laterite, sandstone is also used partially in important parts. The restoration method and materials applied here are followed from the experiences and the manuals which we made through out the restoration of the BAYON Northern Library.