IN THE

SOME OF

SOVIET

]PROBLEMS

NATIONAL

IN THE

EMBANKMENT-TYPE

COMMITTEE

DESIGN

AND

ON

LARGE

DAMS

CONSTRUCTION

DAMS*

A. A. B o r o v o i and V. I. Vutsel'

Of the many problems associated with the design, construction, and investigation o f e m b a n k m e n t - t y p e dams, the Congress paid special attention to the construction of i m p e r m e a b l e elements in darns, their location within the d a m body, the construction materials (soil, concrete, asphaltic concrete, etc.), the technology of carrying out the work, and the required equipment. Attention wa~ also paid to f u l l - s c a l e observations and investigations and protection of the d a m slopes (materials and methods of construction). These questions were covered in 53 papers submitted by 27 countries. IMPERMEABLE

ELEMENTS

OF D A M S

Several types of i m p e r m e a b l e elements o f dams are examined in 39 papers (see T a b l e 1). As is evident, attention was focussed mainly on dams with clay cores, no doubt due to this type of antiseepage provision having the widest application. Position

of Impermeable

Elements

in Dams

As is well known, the position and form of the core significantly affect the stressed state within the core and the dam as a whole, and also the stability of the slopes and deformations in the body of the d a m . E. Reinius (Sweden, D - 2 * ) d e s c r i b e d laboratory investigations on sand models with cores having different slope angles, loaded by hydrostatic pressure. The tests established that for a vertical core the pressure distribution in the dam foundation has a significantly greater nonuniformity than is the case for a sloping core. It was noted that the foundation stress distribution approaches uniformity as the core is inclined further from the vertical. The dam body deformations parallel to the core also reach their highest values for a vertical core, and the least values for a core slope of 2 : 1 (the flattest slope tested). The author presented a stability analysis of theupstream slope of a d a m under drawdown conditions, and proposed rabies for determining the allowable core slope according to the internal friction angles of the core and the supporting prisms. The design method recommended presumes that the slip surface passes along the upstream boundary of the core, which can hardly be accepted as being correct; the most probable failure surface will obviously pass through the core mass, where the effective stresses are usually smaller than at its boundary, owing to the pore pressures retained in the clay m a t e r i a l of the core when the upstream water level is lowered. A n a l y z i n g the distribution of seepage gradients in cores, the author focussed attention on the significant i n crease in their magnitude in their downstream segments. He suggested that in order to even out the gradients, the downstream core boundary should be curved so as to thicken the core in the zone of its contact with the foundation. A. D. Penman and D. A. Charles (U.K.,D-18) presented the results o f full-scale observations of the d e f o r m a tions and stresses in the Scammonden (70 m high) and Bryan (90 m high) dams with sloping and v e r t i c a l cores, respectively. The authors noted that in the Bryan dam, wtfich has a clay core, the maximum stresses at its contact with the downstream supporting prism were observed at the end of the construction period, and that they did not increase after reservoir filling. This can be explained by the fact that the core moisture content at the t i m e of p l a c e m e n t exceeded the optimum value, and that the pore and l a t e r a l pressures were high. The v e r t i c a l deformations in the two dams were very similar in nature, as their profiles were g e o m e t r i c a l l y very similar. The disparity in the a b * Based on the Transactions of the l l t h International Congress on Large Dams, Vol. III, Madrid (1973). THere and henceforth this indicates the paper number. Translated from Gidrotekhnicheskoe Stroitel'stvo, No. 3~ pp. 51-,55, March,1974. 265

266

A. A, BOROVOI AND V. I. VUTSEL' TABLE I

Paper No.

Name o f dam

Country

Type o f i m p e r m e a b l e e l e ITlent

3,4, 9 6 8 12 16 18 19, 53 18, 26 30 31 32 33 34 35 38 39 41 42 46 48 49 51 52

Cethana Mauthaus Graha mstown Blenheim-Gilboa Lench Sea m mo nd en High Aswan Bryan Mattmark Huinco Nurek Queen Elizabeth II Finstertal Nurek Gardiner Churchill Fails Ramganga Biis Letschau Poso de Los Ramos Tirayana Ghrib and Bou-Hanifia Ogliastro

Australia West Germany Australia USA West Germany UK Egypt UK Switzerland Peru USSR UK Austria Canada Canada Canada India India Austria Spain Spain Algeria Italy

Concrete m e m b r a n e (deck) Clay core Clay suspension Clay core Asphaltic c o n c r e t e membrane Clay m e m b r a n e Clay core The s a m e Clay m e m b r a n e Concrete m e m b r a n e (deck) Clay core The same Asphaltic c o n c r e t e core CIay core The same The same The same The same Asphaltic concrete core Concrete m e m b r a n e (deck) Spain Asphaltic concrete membrane The same

solute values of the v e r t i c a l deformations can be attributed to the difference in the d a m heights, However, the hotizontal deformations differed substantially, as they depend on the position of the core in the dam body, The observed Scammonden deformations did not exceed the values one could e x p e c t in a homogeneous dam without a core. Some increase in the deformations was observed onIy in the zone i m m e d i a t e l y adjoining the core. At the same time, in the Bryan d a m the effect of the vertical core was reflected more n o t i c e a b l y in the d e f o r m a t_tons of the dam body. After reservoir filling, both dams showed small supplementary deformations which, e.g., in the Bryan dam, amounted to only 60~ of the total. M. Takahashi and K. Nikayama (Japan, D-29) examined, with the Miboro, Oshirakawa, and other dams as examples, the question of the effect of local construction conditions on the design and methods o f construction of e a r t h - a n d - r o c k f i l l dams. Thus in the construction of the Yanase (115 m) and Mizwkubo (105 m) dams, the core m a t e r i a l was weathered rock which was removed from the slopes, then placed in thin layers, and finally broken up by bulldozer traffic. The resulting material thus contained fine-grained particles (minus 5 mm fraction) amounting to 50 or 60070, which was sufficient to secure stability against seepage when combined with the corresponding consolidation during the c o n s t m c g o n period. T h e paper paid great attention to the results of comprehensive computations o f the stressed state and slope stability of the dam, carried out for different core positions, and taking into account the properties o f the core m a t e rial. With their a n a l y t i c a l investigations (carried out by the nonlinear f i n i t e - e l e m e n t method) as the base, the authors recommend that in seismic regions the core should be positioned close to the dam axis; elsewhere, provided the core m a t e r i a l is sufficiently deformable, it should be inclined slightly away from the v e r t i c a l . Of considerable interest are the results of stability investigations o f a dam 98 m high, located in a seismic region and founded on alluvia, carried out for various possible positions o f the core, which were presented by Y. M. Merti, L N. Srivastava. and S. K. Bhatia (India, D-40). The dam is l o c a t e d in a region of high seismic activity. A horizontal seismic a c c e l e r a t i o n of 0.15g was adopted in the design, assumed acting on both the dam and the part o f the foundation above the slip surface. The upstream and downstream slopes of the darn are 1 : 2 . 5 and 1: 2.0,

PROBLEMS IN DESIGN AND CONSTRUCTION

267

~IA0,07

--I

5

--6

--

7

Fig. 1. Ogliastro dam, with an asphaltic concrete membrane (deck). 1) Concrete wall; 2) layer of rock 300 m m m a x i m u m size, 2 m thick; 8) rockflll body of limestone with c a l c i t e ; 4) large-sized rock (riprap); 8) asphaltic c o n crete membrane (deck); 6) layer of homogeneous clay; q) limestone.

6

"-'r-~'~'~-?n:~=q '"~ )t

4

\

4

\

" 5

k

6

---

r

~ 1

\, ~'

\. o

\. o

T

Fig. 2. Details of asphaltic concrete membrane and its connection with the concrete wall. 1) Bituminous seal; 2) asphaltic concrete membrane; a) strengthening mesh; 4) bonding layer; g) porous asphaltic concrete; 6) prepared bed; 7) bituminous emulsion; 8) concrete wall; 9) drain pipe. respectively. With the dam slopes ftxed at these constant values, the effect of the core position on their stability was studied for different water leveIs upstream and downstream, with and without allowance for seismic forces. The stability analyses were carried out by the modified Swedish method, using an electronic computer. As a resutt, it was established that, for the m i n i m u m safety factor against sliding of the upstream d a m face, flattening of the core slope leads to a lowering of the slip surface into the foundation. As the upstream boundary of the core is steepened to 1 : 0.4, the c r i t i c a l slip surface and the stability coefEcient are both raised. The effect of any additional steepening of the upstream core boundary on the dam slope stability was not examined as the core thickness could not be reduced further. Analyses carried out with seismic forces taken into account confirmed the expediency o f also steepening the downstream core boundary. The optimum steepness was established as being 1 : 0.2. The authors noted that for other positions of the core boundaries, the dam was found to be unstable. These conclusions agree with those o f Paper No. D-29, which is also concerned with the stability of dams in seismic regions. M. Maksimovic (Yugoslavia. D-44) described the results of an analysis aimed at finding the optimum position of a tbXn clay core. such as would secure a favorable stress distribution in the core. An investigation was m a d e of a problem similar to that examined by Reinius (D-2). Particular attention was paid to the d e v e l o p m e n t of an "arching effect" in the core, w h i c h - as is well k n o w n - is manifested by the interaction between the core and the supporting prisms, as the d a m deforms. The stressed state was computed by the f i n i t e - e l e m e n t method; the results established that the o p t i m u m inclination angle of the upstream core boundary lies within the range 1 : 0 . 6 to 1: 0.5.

268

A. A. BOROVOI AND V. I. VUTSEL' Materials Dam

for Impermeable

Elements

The papers presented at the Congress examined examples of the application in modern construction practice of different soil types and some other mateo oo 0 ~ " rials for dam cores and membranes. It should be noted that the use of natural and artificial mixtures o o 1 of clay soils and various forms of large-sized rock materials, which enable their strength characteristics to be raised and their deformability to be reduced, has found wider application in recent times;* Also, Fig. 3. Core detail of the dam of the Upper Taranten as a component of the mixture, there is often a hydroelectric scheme. 1) Rockfill; 2) prisms of finerock; crashed rubble of such weak rocks as shale and argil3) watertight core; 4) trench; 5) steel forms. lite. In the paper of V. Laurenz (West Germany, D-6) it is stated that, after examining several possible alternatives for the core of a dam 61 m high, a vertical core was adopted, composed of shale and graywacke. The narrow central part was composed of the same mixture, but with the addition of 1% bentonite and 5% sand to impart plasticity and impermeability to the material. Of great practical interest was the experience gained in the construction of the Biis dam (100 m), described in the paper presented by B. R. Ralta, I. C. Malkhotran. and M. L. Agarval (India. D-42). For the core of this dam, use was made of the sandstones and argitlites from the excavation for the spillway channel, tunnels, and other structures. During construction of the roads and driving of the tunnels, proposals were made regarding the possibility of obtaining materials suitable for placing in the core directly from the damsite area. It had been observed that the blasted quasi-rock materials broke down easily on being passed over by bulldozers. After the placement and compaction of trial banks of argillites and sandstones mixed in different proportions corresponding to those occurring in nature, confirmation was obtained of this possibility of constructing the dam core from such materials, provided the appropriate technology was used; this varied only slightly from the procedures usually adopted. The layers were placed and spread to a thickness of 20 cm and compacted by sheepsfoot rollers to a finished depth of 15 cm (foot pressure of 35 kg/cm2). Three rollers were operated simultaneously. Tests on samples removed from the fill gave the following characteristics: average volumetric density, 1.96 tons/m3; permeability coefficient, 10 -7 to 10 "8 cm/sec; average plasticity index. 9.5%; angle of internal friction. 29.6*; cohesion, 1.15 kgf/cm 2. The paper by V. I. Vutsel', P. Listrovoi, M. P. Malyshev, and V. I. Shcherbina (USSR, D-32) was devoted to the whole complex of laboratory and experimental-production investigations of the material placed in the core of the Nurek dam, the largest in the world. This material consists of a natural mixture of sandy loam and largesized rubble.l" The paper also describes the investigations of crack formation in the Nurek dam core, in particular the definition of locations where cracks could possibly occur and the depth of their propagation, and also examines measures for preventing their formation. The papers concerned with the provision of impermeable asphaltic-concrete elements in dams can be divided into two groups. The first examines asphaltic-concrete membranes (seven papers), and the second investigates such cores for dams (three papers). This ratio of papers is not accidental, as asphaltic concrete is used in compacted fill construction mainly as an impermeable decking over the upstream slopes, concurrently strengthening them. Asphaltic concrete has been used extensively in recent years for decking the upstream faces of dikes, and for facing the ponds of pumped-storage stations and artificial water-supply reservoirs. Of this group, the paper o f G. Baldovina and A. Girardini (Italy, D-52) deserves special mention; this describes the construction of a dam with an asphaltic-concrete membrane 17 cm thick. The 22-m high dam stores water. The foundation consists of a layer of clays (8 m) underlain by limestones. The main body of the dam was placed in limestone rockfill; the layer under *The use of soil mixtures for cores is recommended in various standards for dams, published in the USA in 1971 (EM. 1110-2-2300). ~Detailed information is presented in Trudy Gidroproekta, No. 32 (1973).

PROBLEMS IN DESIGN AND CONSTRUCTION

269

the membrane consisted of a graded mass with a m a x i m u m stone size of 300 m m (Fig. 1). Watertighmess of the dam is secured by a single-layer asphaltic-concrete membrane (Fig. 2), connected to the upstream concrete wall, which is embedded in the clay foundation. At the junction of the membrane and the wail, the asphaltic concrete is reinforced with a mesh of synthetic fiber. Before placing the asphaltic concrete, the submembrane layer was sprayed with a special solution for d e stroying vegetation. The external surface was sprayed with a l i m e - v i n y l solution to prevent softening of the asphaltic concrete. According to the authors' estimates a s i n g l e - l a y e r asphaltic-concrete membrane was e c o n o m i c a l l y justified in this case. Complete watertighmess of the joints was achieved by heating the asphaltic concrete by infrared rays and sinking copper strips into them by vibrators. The analysis of the performance of asphaltic-concrete i m p e r m e a b l e membranes presented in a paper by K. Belbakhar, B. Montel, and L. Chervier (Algiers, D-51) is of interest because it gives a true appreciation of the r e l i a b i l i t y of this type of decking. These dams, namely Ghrib, 72 m high (19'26-a8); Bou-Hanifia 55 m high (19a041); and Sarno, 28 m high (1947-84), are o p e r a t i n g i n a hot c l i m a t e and the remits of an e x a m i n a t i o n of the condition o f the asphaltic concrete on these dams therefore a t t r a c t the attention of specialists. The highest of these, the Grib dam, was constructed of roekfill and partially of placed stone, with a fairly steep upstream slope o f 1 : 1 in the lower part and 1 : 0.7 in the upper. The asphaltic concrete deck consists of two layers bedded on porous concrete. The deck surface is protected with a thermal insulating layer 1{) c m thick, consisting o f reinforced porous concrete strengthened by reinforcement which is fixed at the dam crest. After a service period of 25 years, local bulges appeared in the deck. An investigation into their causes showed that the fault lay in an unsatisfactory design and not in the selected type of i m p e r m e a b l e membrane, because the construction d e t a i l did not provide the necessary bond of the asphaltic concrete to the concrete layer, as was d e manded by the great steepness. French specialists (D-7), on the basis of analyses of service experience with similar dams in France and Algeria, recommend that asphaltic-concrete membranes (decks) should not be placed on slopes steeper than 1 : 1.5. This condition is considered easy to meet since modern practice virtually excludes crane p l a c e m e n t on steep slopes. This paper also focusses attention on ensuring a reIiable bedding, and recommendations based on dam construction experience are made, which can be briefly formulated as follows: p l a c e m e n t of the submembrane layer 2 to 4 m thick, comprising rockfill containing no stones of greater size than 150 to 180 mm; leveling o f this layer, with compaction longitudinally along the slope, and with the addition of fine gravel o r l S - t o 3 0 - m m rubble; and stabilization of the surface by spraying with bituminous emulsion. It was noted that the thickness of the asphaltic-concrete membrane is not amenable to computation; it should be looked upon as a flexible decking which should follow the slope deformations without losing its ffandamental property of watertighmess. Therefore, membrane thickness is less important than its abitity to deform plasticly. Another paper (D-27) presented by a working group of the French National C o m m i t t e e on Large Dams was devoted to new materials for dam membranes. Some difficulties in their arrangement and their considerable cost when using traditional materials aroused interest in such innovations as membranes of vinyl resin or butyl rubber, and also of reinforced earth. Membranes employing films of vinyl resin can be applied in a way similar to asphaltic concrete. At the same t i m e they have several advantages owing to their greater flexibility and resistance to shear and, hence, a higher service r e l i a b i l i t y can be anticipated. The paper presents the remits of investigations into the mechanical properties of vinyl film and compares it with asphaltic concrete and asbestos-bituminous coverings. Butyl rubber is widely used in France in various gelds, including hydraulic engineering construction; it possesses watertighmess, high m e c h a n i c a l strength, and deformability. In the opinion of the authors, this m a t e r i a l is very convenient in industrial work and doubtiess will find a wide application in dam construction in the future. Several examples are p r e sented, e.g., the Miel dam, which was constructed of very p e r m e a b l e material, with a butyl rubber membrane 1 mm thick protected with layers of sand (20 cm) and rockfill (90 cm). The paper describes the prospects of using reinforced earth, based on an invention of A. Vidal, a French e n g i neer. Reinforced dams and cofferdams have been and are being constructed in several countries, designed to pass flood discharges, thus reducing or completely e l i m i n a t i n g the cost o f flood diversion works. The first dam of this type, which was reinforced with steel rods, was built in Mexico in 1939. In recent years, many reinforced rockfill dams have been built in Australia, Turkey, Egypt, and other countries." "See Transactions of 10th International Congress on Large Dams, Inform~nergo, 1972.

270

A. A. BOROVOI AND V. I. VUTSEL'

TABLE 2

Of great practical interest also was another paper 03-28) presented by a working group of the French Maximum M e a n s t o n e Massofstone Thickness National C o m m i t t e e on Large Dams, concerning the wave height m [dimension of maximum of rockfill [dimension. (rip-rap)laye~ m provision o f dam cores in areas where it is difficult ~ ', [Ds~ [kg or impossible to construct these elements out of clay 0,31 0--0,31 O, 20 45 soils. The paper examines three types of cores: an 0.38 0,31--0,61 0,25 90 asphaltic mix with l a r g e - s i z e d stone inclusions; con0.46 0,61--I ,22 0,31 226 0.61 I, 22--1,83 O, 38 680 crete diaphragms; and i n j e c t e d cores. A core of an 0,76 1,83--2,44 0.46 1 150 asphaltic mix (55%) and stones (45%) was built in 0,91 2.44--3.05 0,61 1 180 1969 in the dam of the Upper Taranten hydroelectric scheme. The asphaltic mix had a temperature of 185 to 195"C. The stones were dropped into the mix without preheating. Recommendations were made regarding the optimal dimensions of the stones. It was established that the m i n i m u m width o f such cores should be 0.5 m. For a dam 60 m high, a width o f 1 m is sufficient. The cores can be vertical or sloping, with an obligatory e m b e d m e n t of 2 to 3 m into the rock foundation. A core of asphaltic mix imparts flexibility and watertighmess. Cost comparisons showed that such a membrane is equivalent to one of asphaltic concrete, and is 45% cheaper than one of reinforced concrete. A significant part of the paper is taken up by a description of the technology (Fig. 3). Antiseepage walls o f concrete placed in situ permit interesting decisions to be m a d e regarding the provision o f antiseepage elements of foundations and dams. The paper included a detailed description of the construction technology for thick and thin walls developed in France. The diaphragms are formed by injecting a b e n t o n i t e - c e m e n t mix below the shoe of a steel sheet pile, which is then withdrawn. The construction of injected cores is provided for at several dams currently in the design stage. It is proposed first to place the rockfi11 to full height, followed by injection o f a "foam" of cement mix. A description of the method of preparing the "foam" is presented, together with its properties in the fresh and hardened states.* Of the papers concerned with dams with reinforced-concrete membranes (decks), those presented by three groups of Australian specialists (]3-3, D-4, D-9) are of interest. They contain detailed descriptions of the design, technology, and full-scale observations of the behavior of the reinforced-concrete m e m b r a n e (deck) for the Cethana dam (110 m), also a very efficient method of concreting the membrane of this and other dams, by the e m p l o y m e n t of a sliding form. It was noted that the Cethana dam could have been constructed with a clay core, as suitable materials were a v a i l a b l e . But the difficulty of such a material under winter conditions, which would have caused interruptions in the work, and the higher cost of a c l a y - c o r e dam, necessitated construction of a dam with a thin reinforced-concrete membrane over a slope of 1 : 1 . 3 (built in 1971). After the d a m a g e suffered by dams with reinforced-concrete decks, owing to disruption of their joints, the l i m i t i n g height of such dams as established by practice was fixed at 100 m. Therefore, the design solution of an external membrane and the d a t a from full-scale observations of the Cethana dam are of undoubted practical interest. The reinforced-concrete membrane o f the Poso de Los Ramos dam (I. A. Jerrechas, Spain, D-48) is distinguished by a somewhat unusual feature. T h e dam is 97 m high (it is later proposed to raise it to 134 m) and in addition to the reinforced-concrete deck has a pelymer membrane to prevent water loss, as the dam impounds a water supply for the city o f Madrid. SLOPE

PROTECTION

OF DAMS

The correct selection of the type and details of the slope protection o f a dam, particularly on the upstream side, usually presents great difficulties if one takes into account not only the e c o n o m i c factors but also the service conditions, with due regard to their combined effects on the protective work (wave action, ice pressure, and several other phenomena generally characterized asweathering). From this group the most interesting was the paper o f * Similar designs prepared by Gidroproekt jointly with the State Trust for Design o f S p e c i a l Hydraulic Structures (Gidrospetsproekt) showed that the injection of dam cores after placing the rockfill to the full height o f the dam (nearly 100 m) is e c o n o m i c a l l y not justified. Better results can be obtained with sequential injection by stages, as dam construction proceeds.

PROBLEMS IN DESIGN AND CONSTRUCTION

271

K. V. Taylor (U.S.A., D-13), which contains a detailed analysis of all types of slope protection for the upstream and downstream faces of dams, including s o i l - c e m e n t facings, which have found fairly wide a p p l i c a t i o n in the U.S.A. A large part of the paper consists of a section dealing with methods of calculating wave forces on the protective facings. In the U.S.A., the materials which have found the greatest application for protecting the upstream slopes are rockfill (riprap) and reinforced concrete. T a b l e 2 shows the requirement with regard to rockfill (riprap) protection according m wave height. These data were determined on the basis of service e x p e r i e n c e with dams having slopes ranging from 1 : 2 to 1 : 4. The granulometric composition (mechanical analysis) should m e e t the following criteria: the ratio of m a x i mum stone size to the average (effective) dimension should not be less than 1.5; the lower boundary of the protection should be submerged not less than 2.5 m below the lowestwater level: and the specific gravity should exceed 2.6. The paper includes a d e t a i l e d description o f slope protection with s o i i - c e m e n t , concrete, reinforced concrete, asphaltic concrete, and other materials. It is noted that the d a m a g e which has occurred m a d e it possible to establish that the fundamental cause of destruction of such protective layers has been an inadequate standard of the work in placing the filter (bedding) layers or an incorrect fixing of the thickness and granulometric composition of these Iayers. In the economic comparison of possible alternatives - a matter which the paper draws attention to - the effect of the type of protection on the dam crest elevation should be taken into account, because t h e designed height of wave ran-up is less for rockfill (riprap) and h i g h e r for concrete and s o i l - c e m e n t slope facings. The downstream dam slopes which are constructed of materials which are subjected to erosion are usually sown with grasses whose types depend on local c l i m a t i c conditions. In regions subject to drought, the downstream slopes are protected with gravel, rubble, or rockfilm, similar to the upstream slopes. Special attention is paid to areas where the dam faces join the side abutments, as here flow concentrations from heavy rainfall occur. The experience with rockfill (rfprap) slope protection in Canada was well presented in the papers by N. L. Iverson and A. S. Ringheim et al. (I3-38 and D-39). These papers contain an amply d e t a i l e d description of a design method for riprap protection, which, among other things, was applied in designing the structures of the Churchill Fails hydroelectric station and several other projects. The method of strengthening gravel or rockfill (riprap) protection facings by the penetration of a bituminous mastic into it cannot fail to attract attention; this method has already been applied at several dams in Czechoslovakia and France (D-V; D-23). The method developed by the firm "Ingstav Brno" is used to construct protective layers of pebbles 3 to 7 cm in size, or '/ to 15 cm for layers of 60 to 80 cm thickness. After impregnation with the bituminous mastic, such a layer performs as an elastic m a t e rial when subjected to shock loading, and as a plastic m a t e r i a l under long-term loading. T h e constructional merit of this protective treatment is that all the operations are carried out from start to finish under full mechanization, thereby sharply raising the efficiency of the work. Thus, for instance, the time input per cubic meter o f this t r e a t ment is 3t/a times Iess ~han for a riprap protection of equal thickness. Of great interest is the method of protecting the downstream slopes of dams, described also in the paper of M. Brusek (D-2a). It comprises the simultaneous a p p l i c a t i o n onto the prepared slope surface of a mixture of grass seed, water, c h e m i c a l fertilizers, and organic matter by means o f a mobile pumping station mounted on a truck. This raises the output more than 10 times as compared with the performance of placing turf on the slope. In conclusion the writers note that nearly all the papers on Question 42 are concerned with a c t u a l problems o f constructing earthen and e a r t h - a n d - r o c k f i l l dams, and undoubtedly are of interest to specialists working in this field.