Zsolt Bencze Analysis of the test results obtained on the slag embankment of the motorway M6 (Diploma project)

Transcription

1 Zsolt Bencze Analysis of the test results obtained on the slag embankment of the motorway M6 (Diploma project) 1. Background Partly due to the European Union endeavours and specifications, the use of industrial byproducts has been revived also in Hungary. Our former experience has shown that the majority of the furnace by-products is fully appropriate for road construction. The slag deposits (piles) of the former metallurgic plants (blast furnaces) constitute huge landscape wounds close to several cities, nevertheless, the use of this by-product has not spread widely. Several Hungarian technical directives specify the use of blast-furnace slag in roads ÚT :2002; ÚT :2003; ÚT ). These specifications have been rendered more strict lately, partly justified, partly unjustified. Owing to a country-wide survey performed by the Institute for Transport Sciences (KTI), the main characteristics of domestic slag are known. A ministerial resolution related the tender announcement of the construction of the motorway M6 section between Érdi tető and Dunaújváros specified the use of the slag produced by Dunaújvárosi Vasművek (Dunaujvaros Iron Works) in the embankment. Detailed preparatory works were carried out before the building of slag into the embankment of the motorway, for the first time in Hungary. Then the experience gained were analysed, which was followed by the monitoring of an experimental section, and finally the application of slag started in the construction of the motorway M6. 2. The diploma project and its main goals I joined to the quality control activities of the slag embankment construction of the motorway M6 as an employee of KTI. My task was to investigate the slag built in the motorway and to record the test results for analysis. Besides, I had to be in communication with the Nievelt Laboratory, the quality manager of the project. The main objectives of my diploma project were to analyse the experience obtained during the slag construction, the experience on the revision of the Road Technical Specification ÚT :2002 The general rules of the geotechnical design of roads which started in autumn

2 2005, the special quality problems of the built-in slag embankment and the requirements on this secondary material. The compilation of the qualification documentation of the motorway section containing slag embankment was also among my tasks. So, I had the possibility to analyse comprehensively the construction of the slag embankment in the motorway M6 and to draw conclusions for similar projects with the use of a huge quantity of slag in motorway embankment. 3. Available data As it was mentioned, the preliminary test results had proved the suitability of slag for embankment material. Rather detailed test series were carried out on the slag pile considering the limited time and money available. However, the survey did not comprised the whole deposit area (waste pile). The tests related just to the surface, and in such a case no information could have been given about the whole depth of the huge waste piles. The breaking of the slag material into pieces was relatively intensive during the Proctor-test (needed for density measurement) but this was not considered as an important quality parameter for the present application area of the slag. The swelling properties of the material were not investigated carefully enough. This quality parameter might cause some problem with the slag performance, since the non-homogenised slag which is susceptible to breaking to pieces may change its volume under the influence of high traffic load. It can lead to subsidence causing pavement failure. During the slag cementation, the volume of the material can swell by 3-4 % resulting eventually the rise of pavement surface. It was unclear which of these properties could have a dominant role in a huge slag embankment. The eventual movement of large embankments can be detected just after years of their execution. 2

3 Site of KTI shaft sampling Figure 1 Slag pile in Dunaújváros and its sampling sites. 4. The tests performed and the relevant specifications, new data analysis The preparatory tests before the construction of the slag embankment on the motorway M6 were carried out to promote the building of almost 1 million m 3 embankment. A limited test series was performed on a trial embankment constructed in the slag pile. Their measuring results were relied on when decisions were made on the technology applied in motorway construction. During this technological experiment, several (150, 250 and 400 mm) compaction layer thicknesses were investigated. It was proved that some 94 % density and homogenous bearing capacity can be attained in the case of 400 mm layer thickness. The 3

4 density values measured were in the % range which can be considered too wide for a motorway construction. If the slag density has a so wide tolerance, the embankment can not be sufficiently homogeneous. Several mistakes of principle were made during the slag pile experiment. A major part of the measurements were carried out in the first layer, although, the results were modified by the influence of subgrade beneath. In addition, some important characteristics like swelling and cementation were not even mentioned, though they can be important in the coming years. The determination of the organic matter content of slag by ignition does not necessarily provide the critical value since some of the mass loss can come from the formation of slag. The tests of the samples taken during slag pile experiment proved that its physical and mechanical properties allows the increase of original organic mass content limits. A preliminary trial section was built along the motorway. The control tests performed there allowed the finalisation of the increased 90 % slag density limit value. In addition, the diploma project emphasized how severe responsibility was taken by the contractor when having won the construction tender. The completion deadline of the motorway was extremely short considering the general European practice. The number of the quality tests required on the slag embankment was several times higher than that on the traditional (soil) embankments. The technological directives were completed just after finishing the construction. The uncertainties of the geodetic survey were characteristic for the whole project. The already mentioned possibility of the eventual mistake when measuring the organic matter content of the slag was not investigated at all. The partly foreign Kkt. (contractor consortium) wanted to adapt the higher slag density values in Hungary which are common in the German and Austrian practices. That is why, it had initiated an increase in the limit value which was proved later by the mass of the actual slag test results. The measuring results obtained have proved that slag is a high quality construction material. The problems of its breaking into pieces and its magnetizability have not arisen during its building in. The breaking into pieces due to dynamic load will not play an important role later since only two sections (between km and ; as well as the connecting road no. 6219) had entirely slag embankment up to the frost protection layer. (These section can be taken as excellent trial sections in the future). 4

5 The analysis of the density measuring results has shown that it was not necessarily a right decision to specify 0 % tolerance for the 90 % density limit value based on the test results of the trial section. This specification had been approved by the Independent Engineer. However, the results of the high number of measurements performed during the construction have proved that it had been a right decision. If the density values on the trial section had been calculated based on average maximum dry bulk density, the actual quality could have been evaluated right on the spot. However, then slightly lower density values would have been given than the realistic ones. Relative frequency (%) All measurements Trial section Figure 2 Comparative analysis of the test results obtained on the trial section and on the motorway Due to climatic effects, the cementation process of slag has been started proved by the increasing E vd (dynamic E-modulus) values. The special distribution of E vd -values measured by light falling weight deflectometer can be contributed to the slag properties and the technology applied. The swelling due to the cementation and the corrosion of the eventual ferriferous particles in the slag were not investigated by the contractor or the client; that is why, I was dealing with this topic in laboratory. The results obtained here are not transferable to high embankments since there slag is not soaked in water for several weeks and its load is much higher than that in a measuring-tube. 5

6 In the critical bearing capacity zone, slag was mixed by silty fine sand; its consequences can not be predicted. Will the fine sand be cemented by slag or will the silty fraction make an obstacle to the cementation. Table 1 Total number of quality tests performed on the slag embankment Test type Soil mechanical tests Radio-isotop density tests Light falling weight bearing capacity tests Total number of tests Main conclusions and proposals in the project Ignition loss of slag does not include only the organic matter content. If the same dynamic and static bearing capacity limits are to be specified, their relationship should be analysed already in the trial section phase. If a correlation can be found between them, the dynamic measuring method is to be preferred due to its much higher testing speed. The radio-isotopic density measurement is not an out-dated technique. The experts who are against it ignore that the majority of the domestic device pool has an isotop close to its half-life period. (So, their accuracy is lower in the shorter measuring time range). The Proctor-test generally used for the determination of density needs a special carefulness due to it relative complexity. This challenge can not be readily met if the measuring laboratory has a very tight schedule. When trial sections are built, the eventual quality deviation of the material used should also be taken into account. If all of the quality parameters of the slag to be used is known beforehand, the construction of the motorway embankment can be carried out quicker and at a higher quality. The density measuring points and the sampling points for comparison purposes should be harmonised. If, for example, density measurements are required at the level -1.0 m, 6

7 then also sampling sites have to be pointed out, otherwise misleading results can be given using Proctor-results from distant samples. If there is no sampling site in a density measuring point, the closest sampling of the same slag layer should be selected for reference purposes. This sample has obviously closer characteristics to the material in density measuring point than the samples taken from a lower or an upper slag layer. If 90 % density is specified for the slag embankment, it is not advisable can be even detrimental to specify a 97 % limit at a level -1.0 m. At this level, the specification of 95 % can be taken more logical in order to ensure a smoother bearing capacity transition from the earthwork to the pavement structural layers. As a summary, it can be stated that so huge measuring data mass could be gathered during the quality control of the slag embankment of the motorway M6 that it can be utilised: partly for the updating of the present specifications to slag embankment construction, and partly for the compilation of new calls for motorway tenders when slag embankment is also envisaged. 7

8 Dunaújváros Centrum junction with road no Junction of motorways M6 and M8 Figure 3 Overview of the sections of the motorway M6 where slag embankments were built 8