STRUCTURAL ANALYSlS OF HlSTORICAL CONSTRUCTIONS P. Roca, J.L. González, A.R. Mari and E. Oftale (Eds.)

© CIMNE, Barcelona 1996

THE ANGKOR'S TEMPLE IN CAMBODIA: PRELIMINARY STRUCTURAL STUDIES ANO STRATEGY OF INTERVENTION

G.Croci Ulliversi/y of Rome "La SapiCllza"

Viu Eudu::.::.ill/!fI 18

I~00184 Roma, Italy

SUMMARY Angkor represenls a complex of temples built belwecn lhe II Ih and 13th centuries in lhe aneienl capitais of lhe Khmer Empire Angkor's Temples, very important examples Df brickwork masonry and dty stone bloek Slructure, was built in lhe middle of the Carnbodian rain forest and they are neglected till 19th century Luxuriant vegetation and seasonal rnonsoon flooding are lhe mos! important causes 01' structures decay and partial or total eollapse. In order to avoid the use of reinforced concrele, and to propose new strategies for safeguard inlerven lion , which are more sllitable and revcrsiblc, if possible, some studies are carried out In Ihi s pape r the preliminary strllctu ral bchaviour studies are presenled and related possiblc stnltegies orinlervenlion are proposed .

I. HI STORICAL SURVEY Angkor represents a complcx of lemples mainly bu ilt between lhe IX and XI II

centuries; Angkor Thom and Angkor VaI, the aneien! capitais, are a witness of lhe splendour ofthc Khmer Empire (fig. I) . A peculiar charactcrislic of lhe main Icmples is lhe pyramidal shape that the complex of lowers, galleries and Icrraccs assumcd as a whole, so thal usually they are called "lllo11ntain temples" (fig 2): lhe Bakong, IX century, is Orle 01' lhe first cxamples of this conceplion. In lhe X ccnlllly lhe "tllollnlain tem pie" 01' Pre Rup was built and is one o f lhe lasl irnpor1anl exatllples oI' brickwork Illasonry belore il was substitutcd by dry stone block struclures. The ternples were progrcssivcly surrounded by a system of barray and tlloals. Following lhe Iradition of lhe Khrner ruler vcry frequently each king abandoned the previo us capital lo build a new one always in lhe some area; lhe sp lendid "Baphon lemple" was buil! in thc XI centu lY; lhe houses and Royal Palaces, differenlJy from the temples, were built on timber so Ihal no trace oflhe buildings remains today In lhe X II cenlllly "Angkor VaI", undoubtcdly lhe masterpiece of Khmer al1 was buil!; shor11y aHerwards lhe eapila lmoved lO Angkor Thorn, a large arca in the centre ofwhich ri ses lhe IlluulLlailL lcrnplc "Bayon" . The cnd 01' lhe XII ccntury represem lhe end of lhe construct ion of these exceplional lemples, although the splendour of Angkor continued for anolher centUly. Angkor was lina1Jy abandoned in lhe XV cenlury and il has been only in lhe halfofthe XIX century lha! Angkor began lo be rediscovered to show to lhe world lhe exeeptional, the rigorous geometrical organisat ion, of ils architecture and lhe incredib le unily in a struclural conccpti on Ibat has developed Ihrough four centuries.

2. THE MATERIALS AND THE DECA Y

2. I Soil and t'oundation The soil of lhe area \Vere Angkor rises is substanlially madc of sand and clay; its

bchaviour is conditioned by lhe prcsence of waler bOlh as deep and superficial leveis relaled to lhe seasonal rains The templcs generally are based 011 a layer or compaeled sand afier having removed the superficial parto The foundations, that anyway remain superficial, are realised usually

142 STRUCTURAL ANALYSIS OF HISTORICAL CONSTRUCTIONS

Fig I : AII$!,kuI" Vaf

i'ig 2: Tlle AIIKkor's "mO/llllail1 temple.\""

G" eROO! The Angkor's Tcmpk in C:lInbodia 143

Fig 3: Dolt/ages dl/e 10

lhe !)"oi! .\"eUlemellls

Fig..J: Sleel slirrlljJs 10

co1llraSl sfone flake oi!

144 STRUCTURAL ANALYS IS DF H1STOR1 CAL CONSTR UCTlONS

Fig 5: lJefl'noral iol/ dl/e lo lhe r o()!s

G. CROCJ I Th!.; AngJ..:.or's Tl.:mpJc in Cambodia 145

with lat c rit e blocks and oOen !lave sullered relevant selllelllenls producing inc1inations and dalllages to lhe strllClures (lig 3), lhe cause nf lhe seltlements is likcly due lo lhe cyelical variatioll of lhe w,Her lable rclated lo lhe seasonallllonsoon rainfall , il is possible however Ihal the varialioll of lhe hydraulic condilions especially dllring the cen luries when Angkor has bcen abandoned have playcd a rcleva nt role as wel!

22 StnJclural materiais Three are lhe main kind oflllilleliab Ihal h,l VC becn used

- brickwork slructllrcs made by sun cooked brick5, connecled with a monar Iha l in rea lity is a kind ofvegclable g lue (pre Rup) . - lalerile it is a reddish very porous material used as blocks for lhe foundaliol15: il1 some cases il is uscd for wa ll s and lhe lower pari 01' slruc lures The slrenglh is good especially 10 the surti.cc where an hard cruSI (aboul 20 em) is prodll ced, benealh Ihis CnJst lhe material is yclJower and so l h.:r~ lIslIally lhe deeay is limited - sandstollc (grcs) it is a more compact material uSlIally grcyi5h used as blocks 101' lhe conslruclion oflhe bllildings Tbe strcnglb is good blll it sufTers a decay subslanlially due to the wa ler (rising waler fór capi llarily. e lc ) espccially visible aI the basis of Ine piers. so mc picrs bcsidcs Ilake ofl lo ngi tll dinally as in the qllarries Ihey were cut horizontally, lo llowing the natural st ratiti calion of lhe sandstone which represent lines of discontinuity. vely freq uent sleel sti rrups to con lrast Ihis phenomenon arc visible (fig 4) . A peculiar kind of coarsed-gained sandslone, Is represented by Ine arkose Ihat Is also frcquently used ror lhe linlcls and the small COIUlllllS in lhe brick buildings

2 3 Thc decay Thc causes 01' dccay, besides lhe natural processes, can be related lo

physioehemical-biogeOl:helllical aspccIs. 10 lhe abnormolls devclopmenl o f lhe vegclalion especially in tne cenluries when Allgkor was aballdoned, so Ihat even Irecs over lhe ICllIples (fig 5) ha ve dcvclop alld Ihc roots could enlargc thcjoinls and displace lhe block s 11 i5 lhe \Va lcr ho\Vcvcr Ihal ha5 playcd lhe main role penetrating inlú lhe slones dlle lo Ihc rain. Ihc percolation and lhe risillg walers, thesc wa lers, rich of minerais brought fi'o lll lhe 50il, caused dissolulion and oxidation oflhe slone il self. Little by little lhe cohesion was brokcl1 dO\VIl, and di si nlegralion progressed, phenomena of e ro sion and l1aking oflhc sandslone are frcqucll[ (fig 4)

3 TlI E STRUCTU RAL BEIIA VIOUR

3. The sl ru ctura l typologies We can consider thrcc main Iypc 01' slruclures

- lhe galleries and Iheir cOllncclions (Iigs 5.6) - lhe lowers (fig 7) • lhe slructure5 lllassive5 which provides lhe Icrraces of the "Illounlain lemples" (fig 8); these st ruc lures are partieularly subjcclcd 10 the cOcei 01' lhe rainwatcr wl1ich penetrales through lhe joinls crea ling slIpplcmcll lary prcssures 0/1 the retaining waUs and possible selllemenls on the foundalions. lhe now oul of lhe \Vater, besides, creales possible seepage and loosing of lhe sandfill which represcnls a cause of funher deformalions. The slructures reali sed wil h dry blocks presel1t vertical joints little e flccli ve as lhe overlapping is insuflicienl OI' IOlally absenl 50 Ihal lhe longitudinal slnJclura l con linuil y i5 mi ssed o r very \Veak, Ihis sl ruc tural imperfcclion could bc juslified by thc need to leI the 5tructllres p1ll11y lO lollow lhe settlements oflhe soil. probably we1l known to the Khmer builders, due 10 lhe seasonal varialion of lhe \Valer, during the dry season in fact the Ircnd of lhe soil is lO contracl, whilst in the 1ll0llS00n seasol1lhe soi l expands .

146 STRUCTURAL ANALYSIS OF I-IISTOR ICAL CONSTRUCTIONS

Fig 8: lhe ma:;sil'f! Il!rrCICe slrllc ture

Fig 7: A IVlVer ~f AIIgkor VaI

G. CROCI I Thc Angkor's Tcrnplc 111 C:unbod ia 147

3.2 The vaults The main slruclural characleristic of the Khmer's architecture is lhe conception of

lhe vaulls Ihal are realised wilh corbelled blocks wilh horizontal joinls instead than with lhe struclurally more efficienl wedge shaped blocks with radial joints as currently used in lhe European architecture; lhe former in fact presents a lower bearing capacity and a larger sensitivily to any rotalion of lhe foundalions (fig. 9). A malhematical analysis of Ihis kind of vaults shows anyway Ihal in relation with lhe limiled spans, lhe Slresses are cornpatible wilh Iht;: rnalt.!rials (fig . 14) and in particular lhe horizontal shear stresses in the joints do not exceed the friction strenglh; in other words lhe inclination of lhe resultant force remains inside lhe friclion cone (figs. 10, 11). The vauhs Ihen, in lhe o riginal situation, were stab le. The choice of the Khmer buildcrs does nol appear Iherefore related neither to lack of knowledge nor to "ritual belives" as somelime il has been said ("lhe wedged arches does nol rest , only lhe corbelled arches repose") ; it is likely instead thal Ihis Solulion came from a precise evaluation of lhe following advanlages: - simpler cutting being parallel the two main Opposile faces; - simpler and quicker construction as il was possible lhe firsl blocks lO be placed without centering; lhe upper ones bcsides cou ld take advanlage of a certain longitudinal co­operalion finding support on lhe blocks of lhe arches already buill , 50 lha! ali lhe vaull cou ld have been realised without centering; - efficient water proof; in facI whilst in the radial joints lhe waler can vel)' easi ly penetrate, in the horizontal joints il becames more difficult. In some vaults lhe walerproofing is further improved realising lhe joints with a slight outward inc\ination although this however worsens lhe friction slrenglh; this lechnique is probably relaled with the large tradition of lhe Khmer builders in the conslruction of wood roofs, where the tiles are just superposed wilh an outward incJination. The slide between lhe blocks (fig . 12), Ihat easierly occurs ai lhe upper leveis were the load is lower, can be related lO a reduction of lhe friction due to lhe wealhering of lhe surfaces (vegetation, lichens, bacleria, elc .). The main more dangerous cause however is due lo lhe increase of the horizontal forces produced by settlements and rOlations of lhe foundations; when Ihere is an oulward rotalion more Ihan lhe sliding it is a geometrically instability oflhe blocks to be easily reached (figs 13 , 14). In gallcries just one side stands on a continuous wall and the olher on disconlinuous pillars; in Ihese cases lhe gallery on this side is completed by half vaults associated with some horizontal struts, 10 ensure lhe contrast; oRen these slruts have been broken (and subsequenlly rebuilt) due to rei ative vertical settlemenls so lha! a slide belween lhe blocks has been produced .

3.3 The towers The towers follow lhe same general concept ion of lhe vaults with two main

adva nlages: - the const ruclion is easier as il is possible to grow with horizontal rings withoul any support ; - the shear forces are lower so that lhe ri sk of sl iding between lhe blocks is usually excJuded. On lhe other hand lhe highness of lhe towers associated with lhe lack of efficient connections on lhe vertical planes makes these SlruClures very weak to even little differential soil settlements which create supplementary horizontal Slresses and wide lhe joints; till unstable silualions and collapses of some blocks can be crealed. The progressive developmenl of detachmenls is clearly highlighted by a non linear analysis

148 STRUCTURAL ANALYSIS OF HISTORICAL CONSTRUCT10NS

Fi).:.Y: (·ro.\,\' ,\'1.:(."//(111 \ '1('11' 'l li ('(whdkd 1'(1/(1,

hg lO: JJ{ffercl/I kmd,\' 'lore:/,,:.\' he!J(l\'/(Jffr

G. CROOI The Angkor's Temple in C~mbodia

limi t friction cone

true orch

Flg. !! : !·"'c llo!Jal he/UII'IUIII" (/11(/ III/II! ("(Jlle

Fi;..:. 12: "lhe -'"lide ht'llt 'l'L' /I lhe IIppel" h!ock.,

corbel orch

149

150 STRUCTURAL ANALYSIS OF HISTORI CAL CONSTRUCTIONS

Sai I settlements: inward rotation

thrust

increcsl'! the thrust

- possible sliding of the bJocks @ - new equilibrium with deformed shope (J)

Ng 13

Sai l settlements: outward rotation

the thfust does not Bubstontiolly chonge

- rocking af the blocks

- possibJe instobility of the shope

Ihear .tflngt~

A B C O

- reduction of the sheor strength as the contoct between the blocks is reduced

Fig/4

N

G. CROCI I Thc Angkor's Ternplc in Carnbodi.:l 15 1

which shows how lensile slresses are produced in the corners and in correspondence of the lintels (figs. 15, 16). In lhe dry block slructures lhe crack and possible fai lure af lhe lintel can bring to a progressive phenomenon ofloosing and then collapse oflhe overhanging blocks.

4. STRATEGY OF INTERVENTION The Angkor temples presenl a so large variety of situations, decay and damages

Ihal each one requires a differenl solution; besides lhe studies are nOI presently compleled 50 that JUSI some general criteria will be presented taking in mind thal the solution muSI be found balancing lhe minimum alteration with the stability requirements . We can distinguish substantially two situations (fig . 17): a) Interventions on lhe foundations. these interventians are necessary in lhe case of non slabilised soil settlements; it must be realised anyway that some cyelical deformations due to the cyelical variation of the waler table cannot be eliminated. When lhe deformalions have became incompatible with lhe structure stabilily it would be necessary to recover part af lhe settlemenls and inclinations with special equipments Uacks, etc.). b) Interventions on lhe structures; these interventians musl mainly achieve one or bolh lhe tasks: • to improve lhe friction in the horizontal joints; this can be obtained, insertillg some pieces ofthe same original stone (laterite or sandstone), or of other suitable material , in some holes drilled in lhe horizontal plans of the joints, alternatively incJined in two different directions, these holes can be realised fram lhe interior wilhout reaching the externai surfaces; - 10 improve or to create global horizontal conneclions; solutions of this kind can be obtained inserting transversal tie rods, (treated against lhe corrosion), slightly preslressed .

152 STRUCTURAL ANALYSlS OF HI$TORICAL CONSTRUCTIONS

Fi)!.. 15: Maflll:J/Il1liclIllllodel (!! I 'I"!' /(111'\ !OIl'CI": dcat! /o(/(I m}(l~I 'sis

J'ig lfí: (ú/Ck's /11 lhe 10m'/' Imlel

G. CROCII The Angkor's Tcmple in Cambodia

Interven tion criterio

al Carrection af lhe ratations a t the foundol ion levei (big d isplocements)

- In lerver'llionB on lhe foundolionB .. a t- oetive forc .. (jock, •... )

- monitorln; to contrai th. d.lormotion

b) Improvement of the structurol behoviour (little displocements)

- intervenHon on the joinlS lo inereese lhe st-eer resistonce - reduetion of lhe thrusl wilh tie bor~

thrust

H· loH

~ BC D N

Fig / 7

153