Soil Mechanics and Foundation Engineering, VoL 32, No. 1, 1995

EMERGENCIES AND CA TASTROPHES

RESTORATION OF THE FAILED SECTION OF THE KREMLIN

WALL IN NOVGOROD

P. A. Konovalov and V. T. Makos" UDC 69.059.22:624.15

The emergency situation in the Novgorod Kremlin, which was caused by stability loss of an artificially placed embankment is analyzed. It is established that the artificial embankment is reinforced with wooden elements. Design proposals for restoration of the failures in the Kremlin wall are discussed.

In April 1991, a section of the Kremlin wall collapsed in Novgorod between the Spasskii and Knyazha towers (Fig.

1). A commission of the State Committee on Construction, Council of Ministers of the USSR recorded facts concerning the emergency, made a number of suggestions and recommended to continue investigation of the properties of the embank- ment soils, the loads acting on the bed, and the development of wail settlements over time.

The Kremlin wall is a three-layer stone structure (rubble masonry on limestone, which is protected on both sides

by brick) with a width of 5 m along the bottom, a height of approximately 10 m, and a width of 4.2 m along the top at the level of the military passageway (the load at the level of the lower surface of the wall was 1250 kN). The wall was built in the fifteenth century on an artificially placed earth embankment with a slope of approximately 1:2, which was bounded on one side by a water-fiIled moat, and on the other by the IeveI area of the Kremlin courtyard. The masonry of the walt had

a number of vertical cracks. After the embankment had been cleared of debris, a distinct pattern of deformations was revealed in plan in the

collapsing wall. As had been established by inspection, the wall has no foundations in the universally adopted sense of this word. They are replaced by two rows of cobblestones established over the height; in that case, the lower row is com-

pressed halfway into the upper clayey contact layer of the bed. It is apparent from the position of the external edge of the cobblestones that the wall has shifted downward along

the slope in an arch; in that case, its edge points, which are situated near the boundaries of the wail collapse, have zero displacement, while in the middle of the arch, they amount to approximately 100 cm. This suggests the existence of a

downward shift in the soil mass along the slope, which also confirms the sliding failures of soil along the edges of the collapse zone.

Several alternate schemes were proposed for strengthening the beds and foundations in the failed section. One Italian firm proposed to cut through the wall from the top with a type of rootlike piles, pass them through the fill stratum, push them into the dense underlying soils, and, in turn, stabilize the situation. The "State Institute for the Projection of Special Hydraulic Construction" recommended a set of technical engineering measures to rehabilitate the wall and founda- tions and to strengthen the beds, which were in no way inferior to the Italian solution. Chemical grouting of the soils in the artificial bed, which was called for in that case, however, was rejected, since the fill stratum, which is extremely valuable to archeologists, would irrevocably lost.

The cooperative "Sindikat" proposed measures providing for the development of zones of guaranteed stability in the form of several embedded buttresses cutting through the slope in the transverse direction.

The engineering solutions presented by the "Scientific-Research Institute for the Design and Exploration of Special Construction" and "Sindikat" were recognized as the most proven expertise.

In connection with the fact that the wall foundations were under a rock burst, it is possible to judge the dynamics of the emergency process completely definitively: displacement of the soil mass (loss of stability of the bed soils), which encompassed part of the wall foundations, occurred initially, and the wall that had deformed collapsed onto it soon thereafter.

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Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 21-25, January-February, 1995.

0038-0741/95/3201-0024512.50 �9 Plenum Publishing Corporation

Fig. 1. General appearance of emergency section from moat side.

A trench (archeological dig) with planform dimensions of 36 x 4 m was located at a distance of 11 m from the

northwestern section of the segment that had been retained under the initiative of G. M. Shtender to investigate the

construction and technical condition of the embankment in the breach. It was established by the diggers that the embankment is a complex engineering structure, which was to us,

primitively unusual for earth structures of the fifteenth century. According to a reconstruction built by V. A. Popov and N.

K. Stetsenko, the embankment is constructed in the following manner [1]. In the bed of the embankment 14-15 m wide, wooden poles 12-15 cm in diameter with pointed ends = 1 m in

length were driven against one another vertically into the layer of native clayey peaty soil. Piles were driven very closely

in several rows along the edges of the fill, and less densely over the remainder of the area. A beam placed longitudinally was used to control the width of the bed of the embankment. The compacted layer of clayey soil 40-60 cm thick, which had been excavated from the moat along the perimeter of the embankment was placed over the heads of the piles. In the

second horizon, a virtually continuous decking consisting of several transverse rows of timbers or logs that had not been

debarked, was placed along the embankment. The length of the remaining shafts did not exceed 3-4 m. The thickness of

the timbers was 0.1-0.2 m, and the distance between rows =0.5 m. Several timbers have notches and other marks suggesting their reuse. Eleven levels of foundations beams were installed over the height of the fill (Fig. 2). The length of

the tier along the transverse foundation beams was "composed" of individual timbers placed "in an overlapping fashion."

Beginning with the fourth level (counting upward), longitudinal foundation beams up to 30 cm thick, which were

used to hold the "butt joints" together, were introduced as rei~orcement.

A meter-thick layer of slightly plastic clay, which played the role of scarf joint, served as the roof of this earthen structure. The overall height of the reinforced portion of the embankment was 5.5 m (Fig. 3). Flagstones were placed

along a platform made of one-two rows of large cobblestones.

The embankment and moat assumed the same slope, since their was no horizontal bench-type area in the bed of the outer slope of the embankment prior to the moat.

The wooden foundation beams, which play the role of reinforcement in the bed, are deformed in the vertical

direction under the load applied by the walls, and have a general incline toward the moat, as well as breaches up to 10 cm wide. The faces of the breaches are covered with mold and traces of decay; this suggests the considerable age of the breaches that have occurred. Discontinuities in the soil layers with a differential of from 0.1 to 0.4 m in their vertical positions are visible in the section.

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Fig. 2. Section of embankment with archeological dig and exposure of transverse rein- forcement formed from wooden foundation beams (photograph taken from Kremlin courtyard).

~ 28 MPa

Fig. 3. Schematic diagram showing reinforcement of em- bankment by wooden elements: 1) clay scarf joint; 2) wooden reinforcing foundation beams; 3) short wooden poles.

Ground water was detected within the structure of the artificially reinforced bed at two levels: in seven-eight rows of foundation beams and in the lower row in the form of water pockets. In that case, both levels of ground water did not come in contact with one another. It is therefore possible to consider the fill part of the Kremlin's hydraulic system. The water regime was not disturbed on opening the dig.

Rotation of the reinforcing foundation beams with their failure at the center of the wall is explained by continuous displacements of the slope along the slip surface in the direction of the moat over time. Ih that case, the line of intersection between the slip surface and the lower surface of the wall foundation was directed toward its center as a result of nonuniformity of the stratifications of soil layers and anisotropy. Under these conditions, the front part of the foundation beams, sinking with the soil mass along the slip plane, acquired a reverse slope with separation from the rear portion of

the foundation beams. The rear portions of the foundation beams, which were carried along by the shifting soil mass, inclined in the direction of the moat, forming a characteristic flow over the internal portion of the wall.

In our opinion, the change in the wall's stiffness as a result of the appearance and subsequent development of a longitudinal rupture crack in the wall could have exerted a definite influence on the development of shear strains and displacements of the bed of the wall in the direction of the moat. Our attention is turned to the intimate mutual relation between the bed deformations and crack formation in the wall: the development of nonuniform bed deformations lead to loss of the wall's integrity; this, in turn, contributed to an increase in bed deformations, including shear displacements.

Repeated cyclic variations in the water regime in the artificial bed of the wall led to decay and failure of the foundation beams, decomposition of the organic material in the soils, and to a change in the seepage heads and forces; this ultimately contributed to a reduction in the stability of the beds of the walls and towers of the Novgorod Kremlin.

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9~

Fig. 4. S c h e m a t i c d iagram of embankmen t s t ra t i f i ca t ion (I - a l te rnate s c h e m e of soi l cond i t i ons 1-5, 7; II - a l ternate scheme 1-4, 6, 8). I) c lay cu to f f : ~ -- 20 ~ C = 40 kPa, e = 0 .65 ; 21 f i | |ed sandy l oam and s~nd;

= 9 ~ , e = 0 . 9 3 ; 3) f i l led c layey loams and c lays: ~f = 10 ~ , ~] = 6 ~ , ~ = 8 ~ , C = 8 kPa, C I = O, C H = 1 kPa, e = 0 . 9 5 ; 4} pea ty f i l l soi ls:

1 3 ~ ~1 = 8 ~ ~ll 10~ Cf = 10 kPa, CII = 3 kPa; 5) sand of m e d i u m f ineness and m e d i u m densi ty : ~f = 35 ~ ~1 = 3 2 ~ ~11 = 3 5 ~ C f = 1 kPa, C! = 0 .7 kPa, Cj! = 1 kPe; 6) semi -hard re f rac to ry clay: ~f = 1 6 ~ ~t = 1 3 ~ ~11 = 1 4 ~ Cf = 4 .7 kPa, C I = 36 kPa, CI! f 40 kPa, e -- 0 .8 ; 7) grave l ly s a n d : ~ f -- 4 0 ~ ~1 = 3 6 ~ ~11 = 4 0 ~ C = 1 kPa, C! = 0 .7 kPa, Cii = 1 kPa, e = 0 .66 ; 8) h igh ly p last ic c layey loam: ~ f = 16 ~ ~ = 12 ~ t#ll = 13 ~ C f = 13 kPa, C I = 1 kPa, CIj = 6 kPa, e = 0 . 7 7 ; 9) long i tud ina l rup ture crack in wa l l ; 10) c o m p u t e d sl ip plane.

Calculation of slope stability, which was performed in accordance with the method of circular-cylindrical slip surfaces by the Kiev "Giprograzhdanpromstroi" and, independently by the joint-stock company "Yupiter" from Saint Peterburg, indicated that the safety factor for stability, which is the ratio of the sum of the moments of all confining forces about the center of rotation of the sliding masses to the sum of the moments of all shearing forces about the same center, was 0.8-1.1. This factor characterizes the "slope/Kremlin-wall" system as unstable. The calculations were performed even before exposure of the reinforcement in the bed and prior to a detailed investigation of the soil properties of the bed by the

Leningrad. Designs developed by the cooperative "Sindikat" called for a system of buttresses, which could be installed 12 m

on centers along the slope of the embankment on its external side and which would not connected to the wall foundations. The body of buttresses is formed by concrete piles built by the method of embedment through a grillage. A gravelly-rubbly

layer at a depth of 12 m from the upper edge of the slope serves as the base of the piles. The piles are placed at an incline of 60 ~ to the horizon to eliminate forces that result in their bending and shearing. On embedding piles in three rows, the soil mass between them, which, together with the piles and grillage, forms a buttress that retains the slope and structures on it from sliding, is compacted. At the present time, this development of "Sindikat" has been completely implemented in the field (between the Spasskii and Knyaza towers).

Codification and analysis of data on the geological-engineering studies of the bed soils in the Kremlin walls (primarily data derived by the Leningrad ) made it possible to construct schematic diagrams of the stratification of the embankment from the Voskresensk arch to the Dvortsova tower, one of which is shown in Fig. 4.

The upper part of the embankment is composed of layers of old fill soils underlain by original soils. Where the order of occurrence of the fill soils and the thickness of their layers are virtually the same in all cross sections, their underlying layers were found to be different; this forced us to introduce two alternate schemes of soil conditions to the calculation.

The physicomechanical properties of the soils were borrowed from accounts of the Leningrad. The strength characteristics are given for the case of the shear testing of soils in the consolidated state.

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The bearing capacity of the bed of the Kremlin wall was calculated analytically from the SLIDE program for the

cases with and without loading of the wall mass. The external slopes of the embankment assumed the following inclines to the horizon in various sections of the

segment investigated: 25, 30, 35, and 46 ~ . Only the extreme values of the embankment slope (25 and 46 ~ ) were intro-

duced to the calculation, i.e., just two alternate schemes were computed. Calculation of the slope stability of the embankment with no load (i.e., in the absence of the wail) indicated that

the stability factor or safety factor was less than unity over the expanse of all sections from the Voskresensk arch to the

Dvortsova tower, and more precisely, amounted to 0.48-0.74 as a function of soil conditions. This gives us the full right to assume that the embankment was strengthened (most likely by reinforcement) even

during its construction, otherwise it would have lost stability early on during its placement. Calculation of the slope stability of the embankment under load (i.e., under the vertical action of the mass of the

Kremlin wall on the surface of the embankment) indicated that the stability factor is diminished, and amounts to 0.41-0.61

as a function of soil conditions, i.e., is two or three times lower than that required. With this stability factor, naturally, none of the segments of the Kremlin embankment in the area that we investi-

gated between towers could have functioned for approximately five hundred years, but would have failed even at the time of construction. Reinforcement of the fill with the wooden foundation beams most likely enhanced the strength of the soil mass. Situated within the bounds of the zone of tensile deformations, the reinforcement disrupted the uniform pattern of deformations and prevented the formation of collapse surfaces in the soils, as a result of which the latter acquired increased stiffness and shear strength. As the wooden foundation reinforcing the soil mass of the embankment decayed, however, its

shear strength diminished; this may have resulted in loss of its stability over time. In the second stage, the "Sindikat MVT" and Kiev "Giprogrezhdanpromstroi" developed plans for the restoration

of the crumpled section in the segment between the Spasskii and Knyazha towers. The design called for restoration of the archeological dig by filling it layer-by-layer with clayey soil compacted every 50 cm and the placement of a layer of geotextile prior to restoration of the crumpled section. The clay cover should be restored along the top of the embankment. A reinforced-concrete slab 500-600 mm thick with openings in the central section was proposed as a wall foundation to take up the pressure of 0.08 MPa at the level of the lower surface. The computed settlements of this slab (for the worst soil properties) amounted to 5.1-6.4 cm. A sand cushion 30-60 cm thick is located beneath the monolithic slab.

To reduce the mass of the wall and ensure its high rigidity, the design specifies that the wall section be built with an internal frame and brick facing. A similar solution was implemented in 1863 during restoration of river-facing Kremlin

sections that had collapsed unexpectedly. At that time, four months passed from the moment of collapse of the sections to their complete restorations, while at the present time, only debates on the matter of the method used to restore the crum-

pled section have continued over four years.

CONCLUSIONS

1. It is demonstrated by investigations and calculations that the principal cause of the collapse of the Kremlin wall

in Novgorod is loss of bearing capacity of its bed soils. 2. For an extended time, reinforcement of the fill embankment by wooden elements has ensured its elevated

bearing capacity, which has steadily diminished as the organic materials have undergone gradual decay and will lead to loss of stability and failure of the structures resting on the embankment.

3. It is expedient to restore the crumpled section of the wall in the open alternate scheme to lower the pressure on the bed, and also to implement a set of measures proposed in the design under consideration to ensure the stability of the

embankment slopes.

LITERATURE CITED

. Digs in the Novgorod Kremlin. Novgorod and Novgorod earth~ Conference Materials [in Russian], No. 7,

Novgorod (1993), pp. 18-33.

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