Experience Gained During Construction of the First RCC Dam in Bolivia

Francisco Ortega S. 1)

Principal FOSCE, Germany

Christian Gross 2)

Department of Hydraulic Structures CES - Consulting Engineers Salzgitter GmbH, Germany

ABSTRACT "La Cañada Dam", constructed within the framework of the "Comarapa Irrigation Project", will be the first RCC dam in Bolivia. With a height of 51.7 m it will be the highest dam to have been constructed in this country so far. A resume of the conditions found in the remote location of Comarapa and its consequences on the implementation of the construction are given. Special difficulties such as lack of experience, logistics and supply and quality of materials are described. Some particular experiences gained during different stages of the Project are discussed as they may be of interest for the implementation of similar Projects in the region or elsewhere. 1. INTRODUCTION 1.1. Framework of the Project The ‘Comarapa Irrigation Project’ (Proyecto de Riego Comarapa - Saipina - San Rafael), is presently the largest irrigation Project in Bolivia financed by German development funds by means of KfW (Kreditanstalt für Wiederaufbau). It is located near the town of Comarapa, halfway between the cities of Santa Cruz and Cochabamba. It forms part of the Bolivian policy of ‘Fight against poverty’ that is supported by German development politics. The owner of the Project is the Prefecture of Santa Cruz (Regional Authority). The Project comprises the amplification and improvement of an existing irrigation infrastructure by construction of hydraulic structures such as inlet structures, canals, siphons etc. The main component is ‘La Cañada Dam’ with the intention to increase water supply to the irrigated areas. 1.2. Implementation Due to feasibility studies realised in the beginning of the 90's, KfW decided to co-finance the construction of the irrigation system. A contract for Final Design and Construction Supervision was awarded to CES-Consulting Engineers Salzgitter

GmbH, Germany. Malcolm Dunstan & Associates (MD&A) were contracted for specific questions on RCC technology. Final design was realised in 1997/98. As there was no experience of RCC construction in Bolivia, it was proposed to award the construction to an experienced construction company. Accordingly a pre-qualification process limited the number of invited construction companies. However, bid prices of the first and second tendering process were 100% above the cost estimates which originally led to an estimated cost of 7,5 Mio US$ in total, which included the RCC dam. As this amount was not financially feasible for the Project, it was decided to open the tendering to local Contractors. In order to compensate the lack of experience in RCC and in construction of large Projects, the Contractor had to include in his offer experienced international consulting personnel. Tender evaluators considered the curriculum vitae of the proposed Consultants. The third tender evaluation led to awarding the Project to a consortium of three local Contractors (Tauro, S&L, Tai). RCC-Expert for the Consortium was Francisco Ortega (FOSCE) from Germany. The budget amounted to 7,5 Mio US$. 2. TECHNICAL DATA

Figure 1. Isometric view of ‘La Cañada’ RCC dam

Geometry Dam height 51,7 m Crest length 155,0 m Crest width 8,0 m Spillway width 30,0 m Upstream face vertical Downstream face 1:0,20 to 1:0,75 (V:H) Length of stilling basin 27,4 m

Main hydraulic data Design Flood, HQ10.000 320 m³/s Maximum specific discharge at spillway 10,8 m³/s/m Type of spillway channel stepped (h=0,60 m) Reservoir volume 10,0 hm³ Bottom outlet DN 1200 Volume RCC Volume 72.000 m³ Conventional Concrete Volume 5.000 m³ Main design criteria RCC characteristic compressive strength 8 MPa @ 180 days Shear strength across RCC joints 0,5 MPa

3. CONSTRUCTION 3.1. Particular conditions on site The construction of La Cañada RCC dam had some specific conditions different from those found in other Projects. Bolivia is among the poorest of the countries in South America, but has great potential for civil engineering and construction. In many regions the most basic necessities are still not covered and financial resources are still very limited. For this reason, the construction of a large dam like this one has been a great challenge for all parties involved. As discussed above, the first obstacle the Project had to face was the limited financing available. Only the use of national resources, including materials and equipment made the construction of the dam economically feasible. An RCC alternative was found to be the most economic solution for La Cañada site. However, the lack of experience not only in RCC technology but also in construction of large Projects, has been one of the most difficult challenges to deal with during the planning and construction of the RCC dam. Secondly, the lack of availability of required equipment and plants for the Project throughout the whole country created further problems. For example, some capacity units larger than usual were required. Such equipment is seldom in Bolivia and more often than not, second or third hand re-conditioned equipment is operated by less skilled personnel. However it was found more economic to work with the existing resources in the country and allow for a large list of stand-by equipment, than to import equipment from abroad. Now, at the final stage of the construction, this is still our thinking even after having suffered problems during the production of aggregates. However, for further Projects the decision for every case must be analysed in detail at the planning stage and is likely to depend mostly on the size of the dam and the daily cost of non-production. Long distances and transport are important limitations in the Bolivian economy; the road system is still poorly developed. Even interconnections between major cities are

still without pavements. This is due, on one hand, to geological instabilities of some zones of the Andean mountains, and on the other hand to the relatively high costs of maintenance. 3.2. Programme of construction The site-installation and preparation of the job prior to the start of placing RCC in the main dam has taken one year; from August 2000 to July 2001. During the first three months of this period, the general planning and design of the main equipment ran in parallel with the installation of the auxiliary plants and camp, including the construction of a 9 km-long access road from the closer town to the dam site. The main activities developed during this year period were the following: mix design, installations of main plants, production and stockpile of 30% of the aggregates, excavation and foundation treatments, river diversion, structural concrete on the foundation, and finally, the training programme of the personnel on site. The RCC started in the dam in the middle of August 2001, and has taken seven months to complete, including three long breaks of two months in total. Two of those breaks were necessary to recover the aggregate production and the third one was over Christmas. If the aggregate production had not been a limiting factor, the dam could have been built probably in half the time it actually took to build, i.e. within three to four months. That rate was well proven at the beginning of the placement; in the first month of construction with the stockpile of aggregates at full capacity, the maximum output with an average rate of two layers per day was achieved, even at the beginning of the ‘learning curve’. This is quite unusual and denotes a good training prior to starting placement in the dam but at the same time, not enough resources were available to increase the capacity as the dam was raised. 3.3. Materials and mix design During the first year of construction, prior to placing RCC in the main dam, a detailed mix programme was undertaken. The background information available before starting those final trials was very limited, as the results available from previous trials of the design stage were somewhat confusing and could never be completed due to limited financing. As a result, a complete analysis, including investigation of materials, three laboratory mix programmes and a full-scale trial was necessary during the construction stage. The objective was to find an optimised RCC mix that would meet the design criteria and the specification of the awarded contract at the same time, and obviously within the available funds. No doubt if the trial mix programme during the design stage could have been completed before the Contract was awarded, this would have proved more economic for the Project. The use of cementitious materials from local manufacturers proved to be the most economic option for the Project. The addition of the transportation cost to the high taxes for imported materials made any other alternative unfeasible. Therefore, cementitious material production is practically in the hands of monopolist companies. Regional consumption is very low and the closest factories are located 250 km away. Cementitious materials are filled into big bags (1,0 to 1,5 m3) and transported by intermediate trucks (25 to 40 t). Total travel time for a Sucre - "La Cañada Dam" - Sucre is 60 hours. However, as the road crosses several dry watercourses, it may

happen that intensive rainfall restricts the possibilities of unhindered traffic. In addition, the unions of transportation services are very strong in Bolivia, so further unknown risks have to be considered. In order to guarantee some continuity in the cementitious materials supplied, a storage system consisting of four silos with a total of 1000 t (2 for cement, 2 for pozzolans) was implemented. This installation guaranteed one week of independent material supply for an average production. Different sources of aggregates were used in several stages during the construction period. The original gravel pit located in the future reservoir was excavated as far as the conditions on site permitted, and in addition, several alternative areas out of the Project area were investigated and explored as an aggregate source. Therefore several alternative mixes for different materials were investigated and used in the dam. The poor quality of the raw material for aggregates has highly determined the mixture proportions. Some properties are included in Table 1. Aggregates (1933 kg/m3) (fine ag.:33%) Cement (140 kg/m3)

Type: Ordinary Portland Cement (ASTM Type I) Type: partially crushed gravel (sandstone, limolite & shale) split in four sizes: 50/20-20/5-5/2,4-2,4/0 Pozzolan (100 kg/m3) Relative Density (SSD): 2530 kg/m3 Type: Natural Pozzolan (Class N, ASTM C618)

Absorption: 3,5 to 5,0% Free water (138± kg/m3) Sand equivalent (raw): 33% From river, no treatment

% Los Angeles lost: 41 to 51% Admixtures: water retarder tested

RCC mix VeBe time: 15± sec Theoretical air-free density: 2358 kg/m3

Characteristic compressive strength: 8 MPa @ 180 days Average compressive strength: >12 MPa @ 180 days Table 1. Summary of materials and RCC properties During aggregate processing 20 to 25% of the raw materials were rejected by means of a thorough washing: pre-washing at the point of extraction, scrubbers, spray bars over deck screens, screw classifier with water and post-washing on the truck before dumping on the stockpiles. The resulting aggregates were of great variability and poor quality. Therefore a significant margin was defined between the average and characteristic compressive strength of the RCC mix. The crushing process on a limited installation (operated in extremely low technical conditions) of a poor quality raw material has been the critical activity during construction. During laboratory tests several sets of mixes with similar workability were tested. After optimisation of the aggregate processing and gradation, proportions of cement and natural pozzolan were achieved in the final mixture just within the limits of an accepted mix in terms of economy within the Contract specifications (see Figure 2).

Figure 2. Average cylinder compressive strength of the RCC mixes at 180 days Once the mix had been selected, a full-scale trial commenced. The objective of this trial was both to analyse the RCC properties (mainly at horizontal joints) and the training and selection of personnel and equipment simultaneously. Those objectives were successfully achieved. The full-scale trial was an optimum opportunity to introduce the RCC in Bolivia for the first time, and by the time work on the main dam had started, all major concerns regarding placement and mix performance had been solved. The full-scale trial could be cored and tested at the design age (180 days). Different exposure time between layers and joints treatment could be investigated. More than 93% of the joints were classified as well-bonded. Some results are included in Table 2 and further details may be found in reference[1].

Matrix 5 x 10-11 m/sec Permeability (in situ test) Horizontal joint 1 x 10-10 m/sec Specimens 16,0 MPa Compressive strength of RCC Cores 13,3 MPa Indirect, specimen 1,7 MPa Indirect, core (joints) 2,1 MPa (1,0 MPa) Direct, specimen 1,4 MPa

Tensile strength of RCC

Direct, core (joints) 0,6 MPa (0,1 to 0,5 MPa) Angle of friction 41º to 44º Shear strength at joints Cohesion (Figure 3) 1,0 to 2,5 MPa

Table 2. Test results of the full-scale trial @ 180 days

Figure 3. Relationship found between exposure time and cohesion at horizontal joints 3.4. Power Supply An important factor to be considered in rural areas of developing countries is the availability of sufficient electric power. Power systems in rural areas are sometimes still fairly non-existent. Such is the case in Comarapa. Basic power consumption is about 600 kW, peak hour consumption (18:00 - 21:00) is approximately 1.300 kW. The network's peak production is about 2.200 kW, so the power available for the construction site is limited during peak local consumption to 900 kW, whereas 1.500 kW are necessary. The Electricity company limited the construction site by indicating a pause of the crushing units during peak residential consumption in the evening. All alternative solutions such as independent additional generators etc. proved to be unfeasible due to the costs and availability of the equipment. 3.5. Concrete production and transportation system Two identical concrete plants have been installed on a platform on the right abutment upstream and near to the top of the dam. The total output of both plants is 2 x 60 m3/hour. Forced-action horizontal twin-shaft batch mixers have been selected. An excellent uniformity in quality and production has been achieved. For the transportation of the concrete to the point of placement, two stages have been applied (see Figure 4). During a short initial stage a direct transportation with dump trucks from the concrete plant to the point of placement was used. For the rest of the dam a double circuit of trucks (one external and one in the dam) was implemented. At this stage the transfer between both circuits was made by means of a half-round steel

chute covered with a flexible rubber band. This chute system was placed on the right abutment at a 45º inclination. A similar system was used first in China[2] and was successfully experienced in La Cañada in terms of both the quality of the mix and the production: No segregation was observed for the 15 seconds-VeBe time workable RCC mix and an output of ca. 120 m3/hour was achieved with one chute. The required maintenance of the system did not recommend any additional chute for this Project. However to meet the short construction programme (see section 3.2) a second stand-by chute would have been necessary.

Figure 4. Transportation systems for RCC

Figure 5. La Cañada RCC dam under construction

3.6. RCC placement The RCC has been spread with D4 and D5 type dozers and compacted with large 10-ton single drum vibratory rollers and small 2,7 ton double drum vibratory rollers. No other treatment apart from continuous water curing has been applied to the RCC horizontal surfaces younger than 30 hours. For older joints, a conventional exposed aggregate finish by means of high-pressure water jet has been applied. The use of mortar as a bedding mix in some cold joints was directed as an additional factor of safety. The main concept of the design of the RCC structure was simplicity. This was most beneficial in La Cañada as it was the first RCC experience in the country. All intakes, adits, and outlets had been taken to the abutments, and a simple all-RCC dam design was successfully achieved. In the full-scale trial the GEVR (grout-enriched vibratable RCC) was tested and accepted as the perfect complement to a rich RCC mix to be used as interface concrete against the rock abutment and against the forms (see Figure 6). At the same time this was the simplest way to obtain an excellent finish for the dam facings without producing and transporting a different concrete to the dam than the RCC. The use of just one type of concrete for the dam made the whole procedure more simple and easy to learn for the non-experienced teams.

Figure 6. GEVR typical finish in La Cañada

Figure 7. Formation of transverse joints

Figure 8. Formation of drainage system

Both faces of the dam have been formed. The Contractors have manufactured themselves the formwork for the dam facings and its design was based on the continuity of RCC. Several 2-lifts high panels conform the module. The lower element is raised as the rest are fixed by anchor systems to the concrete. The much lower labour costs at the job site when compared with other sites have made this activity extraordinarily cheap (less than half the cost of previous experiences in Europe[3]). The transverse joints have been formed by insertion of small steel plates on the fresh, fully compacted RCC using an insertion plate mounted on a hand set, pneumatic hammer (see Figure 7). Again the low cost of labour force has made this procedure extremely cheap.

Another peculiarity has been the formation of the drainage system in the dam. A holed PVC pipe enclosed in fine gravel has been placed manually in a formed cylinder in the fresh RCC after compaction was finished (Figure 8). This may avoid additional costs for further drilling. A rather strict quality control programme has been established during dam construction. The combination of the high variability found in the quality of the aggregates and the rest of conditions discussed above had recommended such control beyond the standard procedures in Bolivia. The parameters are within the expected limits: coefficients of variation between 12 and 18% are obtained, and that is self-explanatory of a relatively great success at La Cañada (Figure 9). Figure 9: Some historical data of quality control during dam construction 4. ADDITIONAL EXPERIENCE 4.1. Liquidity of the Contractor The present Bolivian regulations for disbursement are highly restrictive and involve a number of authorities in order to reduce corruption. However, the fulfilment of these obligations takes up important time. This proved to be a major difficulty for the Contractor. It has to be considered that an underestimate of the necessary pre-financing led to a serious situation of insolvency for some time. Only the fortified efforts of all parties resolved the problem that had arisen. The lack of liquidity led in some moments to almost a complete stop of construction activities

due to strikes and lack of basic materials (fuel, steel etc.). In order to prevent these problems it is recommendable to leave some flexibility and establish mechanism for recognising some costs before RCC placement start (for example stockpile of aggregates, site installations,…). 4.2 Thermal analysis The mix to be used in the dam body was defined during the trial mix programme. However, the materials and proportions were different from those investigated in the final design stage. In the final design, a maximum placing temperature of 26,5 deg C had been specified and maximum joints spacing of 17 m. Therefore the unit price did not consider any pre-cooling systems for the RCC mix. However during the construction stage a decision was taken to investigate, in more detail, the characteristics of the materials that were actually used. A thermal study including heat of hydration and adiabatic temperature rise has been executed. Measured values showed an adiabatic temperature rise of 21,7 deg C, which is something common for this content of cementitious materials. A new thermal-stress analysis defined a new joint spacing of about 10 m for the same placing conditions. This was assumed as the additional cost of transverse joints could be assumed while the implementation of cooling systems to reduce the specified placing temperature would not have been feasible in terms of economy and logistics. Measurement of thermocouples installed in 3 layers is shown in Figure 10, and confirm the laboratory records. The untypical form of curve in first level is due to a stop of placement just after installing the thermocouples.

20,0

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6/9/01 26/9/01 16/10/01 5/11/01 25/11/01 15/12/01 4/1/02 24/1/02

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No placement Average placement of 1 layer/24 hours No placement 1 layer / 24h

Figure 10: Development of temperature monitored in 3 different layers of the dam 5. CONCLUSIONS Various aspects of technical and economical problems that occurred during the construction of ‘La Cañada Dam’ within the framework of Comarapa Irrigation

Project have been described and some applied solutions have been explained. Recommendations have been given to prevent these and other difficulties arising in the future due to the nature of the type of Project in remote areas. It is obvious that these are partly regional experiences, however most of them apply to similar conditions in other developing countries. In terms of total Project costs it has been proved that the implemented methodology of relatively small local Contractors combined with experienced engineering support can achieve an attractive result for the Project owners. The RCC technique, whose popularity is due to its simplicity, high speed of construction and economy, is suitable for such an adaptation, as even small Contractors are familiar with some of the implemented equipment. Consultants, Designers and Contractors do have to adapt RCC technology to local conditions. This is even more important in developing countries, where other restrictions, in addition to the more commonly known restrictions apply. A successful implementation of RCC technology under these conditions depends highly on simplicity of the design. REFERENCES 1. Ortega, F. and Somdalen, B. (2000). ‘Financing, design & construction techniques

for the first RCC dam in Bolivia’ Proceedings of the International Workshop on Modern Techniques for Dams-Financing, Construction, Operation, Risk Assesment, ICOLD 69th Annual Meeting, Dresden, Germany, (2) 296-312.

2. Shen, D., Mao, M. and Xiao, L. (1999). ‘Characteristics of concrete deep-trench belt conveyor and negative-pressure chuting system and its application in Jiangya RCC dam’ Proceedings of the International Symposium on Roller Compacted Concrete Dams, Chengdu, China, (2) 663-673.

3. Martín, J., Marsellá, J.A., and Ortega, F. (1995). ‘Use of RCC for Cenza dam construction’ Proceedings of the International Symposium on Roller Compacted Concrete Dams, Santander, Spain, (2) 951-967.

AUTHORS 1. Francisco Ortega S.

Member of the Spanish Committee on Large Dams (SPANCOLD) Consultant & RCC expert FOSCE, Glinde 1c, 23843 Bad Oldesloe, Germany Tel.: +49 4531 670115, Fax: +49 4531 670116, E-mail: fosce@online.de

2. Christian Gross

Department of Hydraulic Structures CES Consulting Engineers Salzgitter GmbH Nord-Süd-Straße 1, 38259 Salzgitter (Bad), Germany Tel.: +49 5341 823-0, Fax: +49 5341 823-199, E-mail: grs@ces.de