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Holding up traffic

ECSMGE CONFERENCE

The importance of geotechnics for transportation infrastructure is widely underestimated by the public. In fact geotechnics and engineering geology greatly influence the alignment, planning, design, and maintenance of all traffic and transportation arteries.

In so doing geotechnics not only facilitates personal tranpsort, but also the supply of modern society with goods and the disposal of the resulting waste and sewage via roads and highways, railways, subways, harbours and waterways, airports, pipelines, water supply systems, including construction of tunnels and galleries, bridges, retaining structures, landslide stabilisation, ground improvement, etc.

Geotechnical considerations play an important role in overall project feasibility including calculated risk and safety aspects, construction, operation, and maintenance costs, and even aspects of possible repair and restoration.

The requirements of local and international transportation infrastructure within the densely populated countries of Europe force geotechnical engineers to develop solutions which often approach the borders of feasibility. Building in unstable, heterogeneous, or soft soil and rock includes a significantly higher calculated risk than is experienced by other branches of civil engineering.

Consequently, a proper design requires not only a firm theoretical knowledge but also comprehensive experience and engineering intuition. In most cases, sophisticated theoretical models and calculations simply feign an accuracy which in practice does not exist. Statistical investigations do not really solve the problem either. In contrast parametric studies are essential for a reliable risk assessment and to follow the concept of most probable and most unfavourable conditions.

This involves design issues which need to be ruled out during construction or even in the long term according to the observational method. Unstable terrain requires a 'semi-empirical' design method, based on comprehensive monitoring, and pre-planned safety measures which allow for future strengthening.

Highways and railways along unstable slopes

Several highways and railways in the Alps have in recent years been constructed along unstable slopes which previously would have had been considered unsuitable. These works have been carried out using the engineering philosophy of observational method.

Figure 1 shows a part of a 20km long highway section where 75% of the alignment is running on slope bridges and aqueducts. Nevertheless, the visible part of this highway represents only 20% of the construction costs, whereas the other 80% are invisible, ie foundations, retaining structures, and prestressed anchors (in the case shown up to a single length of 120m).

In mountainous regions, the ground parameters frequently exhibit wide scattering (even within a small area) to such an extent that geotechnical design procedures provide only border values and serve for reference only. The mean design value can only be a 'most probable' value and has to be validated by the observational method.

Steeply inclined slopes, seepage flow, and, moreover, seismic aspects have to be considered. The results of evaluating slope stability or the calculated

lateral pressure on retaining structures are less influenced by the method of calculation than by the assumption of relevant soil/rock properties, seepage

flow conditions and seismic parameters.

This is the reason why sophisticated design methods are generally much less informative than parametric studies involving geological variability, ground- water conditions, and specific construction measures.

The optimal solution for slide stabilisation and retaining structures can frequently be achieved step by step in connection with taking insitu measurements. It would be economically unjustifiable to construct most expensive protective structures, while throughout assuming and superposing the most unfavourable parameters.

'Calculated risks' have to be accepted in the design of roads through valleys in mountainous areas where hillsides with a slide potential extend over a distance of several kilometres.

To reduce construction costs and to save time, the application of supplementary construction methods, typically ground anchors, should be considered. Such measures are - even in connection with local remedial works - less costly than an 'absolutely safe' design which seeks to avoid the possibility of additional measures later. In any case (and whatever your designer tells you) 'absolute safety' cannot be provided under such extreme topographical and geotechnical conditions.

In such cases, flexible retaining structures have proved effective. They are adaptable step by step, both technologically as well as economically, to the locally prevailing slope pressures, slope movements, and ground conditions.

This practical approach is based on continuous measure- ments and observations of the retaining structure, the surface and the subsoil/rock surface during the entire construction period (eg by geodetic survey, extensometers, and inclino- meters, monitoring anchors, earth/rock pressure cells). After completion of construction, subsequent random monitoring is recommended.

Calculations and theoretical considerations are only the basis for the first design and for interpreting the obtained measurement results. Retaining structures built using this 'semiempirical' design method have stood up under most difficult conditions for more than 25 years.

In many cases of ground engineering under difficult conditions this philosophy provides the only technical solution - not to mention the cost savings. The potential to make modifications during construction and to strengthen the structure at any time, even after construction, is a fundamental requirement respectively of the observational method and the semi-empirical design method.

Both involve the concepts of the most probable and most unfavourable conditions. They make design a creative process that seeks 'high quality simplicity' and avoids over-complication. Be aware that high-quality simplicity does not imply the reasoning behind 'simple' practices is overlooked - in contrast over-simplification, sometimes through so-called high-tech mechanistic calculations, can cloud engineering judgment.

Risk assessment in connection with creeping ground and progressive failure of slopes is especially critical if statically sensitive bridges have to be constructed. Monitoring should begin as early as possible before starting construction. Numerous measurements over a period of 25 years have revealed that a creeping pressure acts on retaining structures and foundations in such unstable zones which exceeds widely the earth pressure at rest but hardly approaches the passive boundary value (Brandl, 1993).

Figure 2 shows an example of the range of retaining measures required to resist the lateral forces for a bridge pier in an unstable steep slope. The reinforced concrete beam on top of each pair of caissons gives the ability to install additional anchors if long-term monitoring indicated a need for subsequent strengthening.

Figure 3 shows the cross section of a slide-prone slope where a highway had to be constructed. To minimise the slope cut, a 'semi-bridge' was designed. Its foundation is much deeper than the visible part above ground surface.

Silty slope deposits of mica schist required an intensive anchorage. The calculation of the 17m to 22m high anchored element wall illustrates the extreme influence of the ground's shear properties on the required anchor forces. Even in the 17m high wall section the necessary anchor forces for achieving a safety factor of one varied by 1,000kN if the friction angle is varied by only one degree.

In reality the internal friction exhibited a scatter of 15degrees and, moreover, it could drop to the residual shear value. Use of even a small amount of cohesion in the design also exhibited a strong influence on the results, leading to a great difference between most probable and most unfavourable conditions.

This example is very characteristic of the advantage of the observational method or semi-empirical design respectively over the fully engineered design method. In this case the half-bridge was constructed with multi- anchored caissons with remote monitoring of the anchor forces. Tubes installed in the top zone of the caissons and in the retaining walls makes subsequent strengthening possible, rapidly and at all times.

References

Brandl H (1993). Installation, monitoring and design of caissons. Proceedings 2 International Geotechnical Seminar (BAPII), Ghent. AA Balkema, Rotterdam, pp. 3-20.

Brandl H & Adam D (1997). Sophisticated continuouscompaction control of soils and granular materials. Proceedings XIVth International Conference on Soil Mechanic and Foundation Engineering, Hamburg. AA Balkema, Rotterdam, pp. 31-36 (Vol. 1).

Brandl H (1999). Risks and responsibilities of Geotechnics in highway, bridge, and slope engineering. Proceedings, XIVth International Conference on Soil Mechanics and Foundation Engineering, Hamburg. AA Balkema, Rotterdam, pp. 2595-2607 (Vol.4)..

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