图3 左图是位于阿尔卑斯山脉奥勒河上的Grand-Maison土石坝，于1988年投入使用，是世界上最大的水坝之一，高140米，坝宽550米。剖面图中显示的芯层（粘土质防水层）和河床中的防水帷幕起到了防水作用。这些防水设施外加上下游土石堆填，确保了坝体的稳定性。右图是大坝和水库的照片。该大坝是一个污水处理厂（抽水蓄能电站，装机容量1820兆瓦，年发电量300千兆瓦时）的上部，可以在非高峰时段将未使用的电能以水力的形式储存起来。 [来源：左，法国大坝和水库委员会，2012，大坝技术]；[右，杜歇昆汀（Douchet Quentin）GFDL（http://www.gnu.org/copyleft/fdl.html）或CC BY-SA 3.0（http://creativecommons.org/licenses/by-sa/3.0）]通过维基共享。（图1左：coupe transversale横截面，plate forme aval下游平台，rembial de pied aval下游堤防，drain exutoire引流管排水，recharge aval en éboulis下游充填碎石，parement en enrochements抛石壁板，altuvions冲积层，moraine底碛，drain incline排水坡度，galerie sous fluviale河道下廊道，voile de drainage排水罩，voiles d’injection喷射帆，630m environ约630米，CRISTALLIN: gneiss结晶状:片麻岩，retenue normale: 1695 NGF正常保留1695 NGF，filter fin过滤结束，zone principale en enrochements主要填岩区，protection en enrochements choisis在选定的岩石中进行保护，niveau inimal of exploitation normale:1951最低正常运行水平:1951，batardeau amont: 1575上游码头:1575，decharge amout垃圾填埋体积，zone intermediaire en eboulis中间碎石区，galerie d’injections et de controic注射和控制回廊，contact lias/cristallin接触束/晶状）
图4 采用三轴试验测试深层土体的刚度和强度。左图为测试原理（未显示计量）。右图是三轴腔室的照片。圆柱形试样在受约束应力p（模拟深度）的条件下进行压缩（轴向力F代表临近结构的作用），从发生小变形直至破坏。 [来源：马可波罗（Marc Boulon）]（Compression verticale纵向压缩，Pierre poreuse多孔岩体，Membrane élastique étanche防水弹性膜，Pression p de cellule autour de l’échantillon样品所受围压p，Cellule sous pression p压力仓，Echantillon de sol岩土样品，Drainage ou mesure de pression interstitielle排水或孔隙压力测量）
图7 由钢筋混凝土和钢制成的里昂-安提里翁斜拉桥于2004年投入使用，该桥横跨希腊地震非常活跃的佩特拉海峡，距离水面65米，其海床主要由软粘土厚层组成。桥全长2883米，以4个直径90米、墩间最大跨度560米的桥墩为基础，并结合了预防措施。桥墩下的粘土由30米长的金属桩进行加固，从而防止了土墩组件的旋转。这些金属桩上有一个易熔颗粒层（玄武岩块），能够允许在大地震时桥墩发生相对的水平位移（不可逆滑动）。左图是高架桥的景象。右图是地基土体的改良原理。 [左，来源：大卫毛尼克斯（David Monniaux）（自己的作品）[GFDL（http://www.gnu.org/copyleft/fdl.html），CC- By -sa -3.0 （http://creativecommons.org/licenses/by-sa/3.0/）或CC- By -sa 2.0 en （http://creativecommons.org/licenses/by-sa/2.0/fr/deed.en）]，通过维基共享。[右，马可波罗（Marc Boulon）]（65 m 65米，pile 地桩，blocs de basalte玄武岩块，pieux桩，substrat argileux泥质基质）
The very diverse soils on our planet require the expertise of specialized engineers to carry out civil engineering projects, in conjunction with other specialists. This specialist highlights the soil properties to be taken into account and characterizes them with appropriate tests, so that the foundations of civil engineering structures are sufficiently stable, with a safety reserve. Particular attention is paid to design tools for modelling the soil-structure interaction during the life of a structure. The inspection of the surrounding site provides a permanent record of the condition and possible movements of the supporting soil of a building throughout its life. Today, at the cost of soil improvement and reinforcement work, very large structures can be built in areas that were once considered unsuitable for any particular location. Current construction methods, which are less and less disruptive in urban areas in particular, make it possible to push the limits of what is possible beyond what was once imaginable.
Figure 1. Offshore wind turbines 5 MW, seabed at 30 m, 110 m off water, 110 m emerged, on the left weight base of diameter 30 m, on hard rocky soils (granite); on the right foundation on driven and/or drilled monopile of diameter 6 m, on soft rocky soils (chalk). [Source: A. Puech, 2008, Marine Geotechnical Engineering Course, ENSHMG]Let’s talk about construction first: All major land-based structures (dams, bridges, viaducts, high-rise towers, silos, petroleum and chemical reservoirs, power generation plants,…) require foundations. As for underground structures (tunnels, galleries, underground factories, gas storage tanks,…) they must support the action (commonly known as pressure) of the ground. Finally, offshore structures, which are subject to marine elements, draw their stability from their support on the seabed (weight structures, figure 1) or from their anchorage on the seabed (floating structures, jackets, figure 2, or monopiles, figure 1, oil pipelines).
Figure 2. On the left, one of the jackets elements supporting an offshore platform, up to 300 m of water, size and weight comparable to those of the Eiffel Tower. On the right, jacket equipped with flotation ballasts towed on site; pile (diameter 2 m, length 50 to 100 m) guide sleeves for nailing on the sandy seabed. [Source: A. Puech, 2008, Cours de géotechnique marine, ENSHMG]But soil is a “living” material, likely to evolve over time under the influence of various natural and anthropogenic phenomena, both planned and unexpected. We will therefore also have to deal with soil reinforcement in contact with the structure, and/or remediation of any disorders that may occur.
A specialized engineer (the geotechnical engineer and more often a team of geotechnicians) is in charge of the interaction between soil and structure (soil-structure interaction). He is of course in close contact with the team responsible for the work itself. In the rest of this text on soils, we will use the term engineer to refer to the geotechnical engineer.
When the idea of a major civil engineering structure, useful in principle, comes up, part of the preliminary project consists in closely examining the entire environment it will undergo and modify. We must therefore consult the annals of local natural phenomena likely to affect the deformations and stability of the construction over time (rain, snow, drought, flood, storm, freeze-thaw, earthquake, explosion,…). But a broad impact study (physical, hydraulic, ecological, socio-economic,…) is also essential to assess the repercussions of the structure on the nearby and distant site. The interests of individuals and the general interest are often in conflict. For example, the installation of the Aswan dams has completely changed the conditions of agriculture in Egypt: beneficial effects in the Upper Nile Valley (irrigation), but disastrous on the Lower Valley (salinization of land, lack of fertile annual alluvium).
So, if there is nothing fundamentally wrong with construction, we enter the project phase (the precise design of the structure and its interaction with the ground).
Figure 3. The Grand-Maison earth and rockfill dam on Eau d’Olle river in the Alps, commissioned in 1988. One of the world’s largest dams, 140 m high, 550 m long at the top. Above section showing the core (clayey waterproofing), the waterproofing curtain in the river bed, completing the waterproofing, the rockfill refills, upstream and downstream, ensuring stability. On the right, photo of the dam and reservoir, seen from below. This dam is the upper part of a WWTP (pumped energy transfer station, 1820 MW, 300 Gwh/year), which allows the unused electrical energy to be stored in hydraulic form during off-peak hours. [Left, source: Comité français des barrages et réservoirs, 2012, Technologie des barrages] ; [Right, Douchet Quentin, source : GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia CommonsUnder the term soils, we mean soils and rocks, located under the vegetable and/or organic terrestrial layer. They are all natural materials, all different from each other, by their mineralogy, their granulometry, their possible cementation, and finally by the whole history of their formation. However, they are grouped into large classes with neighbouring properties, gravel, sand, silts, clays, more or less hard, more or less tectonized rocks. Water is almost always present, saturating the soil (under the roof of the water table), or accompanied by air (unsaturated soil) above the water table. In an earth dam (Figure 3) the materials are carefully selected according to the areas.
2. Important soil properties and their characterization
The essential tool of the soil engineer in charge of a civil engineering project is mechanics. Therefore, its most examined properties are its mechanical and hydraulic properties, namely its rigidity (modulus of elasticity), its strength (cohesion and friction), its dilating or contracting tendency to rupture, its permeability, and its reaction to hydration/dehydration. The anisotropy of these properties is always considered. The pore pressure in a soil leads the engineer to consider the total stresses and the effective stresses, the latter being those actually supported by the soil skeleton.
Laboratory tests directly provide hydro-mechanical data. On the other hand, in situ tests can only be interpreted by correlations with hydro-mechanical parameters, with a certain uncertainty. Soundings, penetrometer tests, pressuremeter tests provide local information (according to a vertical), while well-conducted seismic tests provide information about the ground in its mass, highlighting its heterogeneities. Many other techniques are available to characterize the subsoil layers: electrical conductivity, gravimetry, radar, which also help to detect cavities and discontinuities – faults, fractures –.
3. Design tools for soil
Classically, we talk about the structure in project (the bridge, the dam, the power plant,…), and the ground that must support it or even constitute it (earth dam, for example). During the life of the structure, the soil-structure interaction is permanent.
Equipped with the characteristics of the local soil (§ 2), the engineer evaluates the service and exceptional loads of the structure on the ground. Then the project is defined and the structure is completely dimensioned by ensuring one or more safety factors, obtained by estimating failure scenarios by increasing the loads or by reducing the soil characteristics. National and international standards (including Eurocodes, Eurocode 7 for soils) have been developed and gradually refined to assess safety, taking into account duly recorded and meditated historical disorders and accidents. But the actual deformations of the ground and the structure, before any failure, are also relevant in terms of the health of the structure.
Figure 5. Meshing of 2 structures in finite elements. On the left, a dam (very simplified) and its valley. On the right is a power station and its nearby site. The method consists in writing the mutual equilibrium and deformations of a large number of small volume elements, each made of materials with their own rigidity and strength. [Source: MESTAT, P., (1997), Finite element meshes, advice and recommendations, BLPC 212, 39-64]The engineer has tools at his disposal to model how works a structure and the surrounding site, using conventional or more advanced methods. It should be noted that these methods are constantly being improved, thanks to the dialogue between professionals and researchers. Traditional methods are mainly oriented towards safety. They assume the plastic rigid ground, i.e. not deforming before it breaks up suddenly. More recent numerical methods (in particular the finite element method, Figure 5) provide the soil with complete constitutive laws that reflect deformations up to rupture, and the rupture itself. They provide access to both the safety assessment and the deformations of the structure in service and the ground (typically a dam and its nearby valley). Today, classical and recent methods coexist in the profession.
We have just mentioned numerical modelling (finite elements) as a good predictive tool for the engineer. But the prediction can only be satisfactory if the hydro-mechanical data that feed it are representative. However, the initial characterisation of the soils (§ 4), at the time of the project, is always approximate, simply because of the heterogeneity of the subsoil. For example, miners digging a tunnel or a gallery tell you that they really only know the ground they are crossing when they excavate it, when they are driving it. This is where the power of finite element numerical modelling can be harnessed. The phasing of the work (the successive stages of construction) is simulated, the results of which are compared with on-site measurements during this work, from the beginning (the virgin site), of the modifications of the hydro-mechanical variables of the soil (displacements, stresses, interstitial pressures,…). This provides the basis for an inverse analysis, allowing the soil project parameters to be corrected as the construction progresses. This results in a more realistic definitive simulation of the structure’s behaviour in service and under exceptional loading. This so-called observational methods also make it possible to rethink the initial project, in case it has been too bold, to the point of no longer meeting the safety criteria.
4. The auscultation of structures and soils
The measures accompanying the construction of the structure, on the ground and on the structure itself, have just been mentioned (§ 3). But a structure and its site have a very long life, after construction. For large structures, as well as for ongoing and identified risk situations (landslides, rockfalls,…), programmed measurements of site hydro-mechanical variables are common, constituting the auscultation. To be useful, this approach requires rapid interpretation and dissemination in real time. Thus, on a landslide, an acceleration of movements without modification of loads means a rapid evolution towards sudden rupture, and must trigger the alert of threatened populations. The structures (and their sites) commonly ausculted are dams (and the slopes of their valleys), power plants, bridges and viaducts (for which differential settlements are feared), tunnels and galleries (for which limited convergence is ensured, resulting from the movement of the faults crossed, or from the alteration of the surrounding rock. The devices installed are extensometers, inclinometers, settlement, pore pressure and groundwater level sensors, topographic survey tools,..
In the past (on the scale of the century(s)), the engineer had at his disposal rough measuring instruments (theodolite for displacements, level for inclinations,…). We are nowhere near these proven measurement technologies. New technologies have a prominent place in geotechnical engineering. Today, topographic movement measurements are carried out quickly, automatically and precisely using GPS. Extensometric and inclinometric measurements use optical fibre. In tunnels or galleries, all guidance and convergence measurements are based on laser techniques. UAVs and image analysis techniques are used to monitor the condition of large facings (dams, bridges, etc.). And many other new technologies are expected to be included in the panel of auscultation tools.
5. Soil improvement and reinforcement
Figure 6. A high retaining wall made of “reinforced earth wall”. Each wall level consists of joined reinforced concrete “scales” anchored by friction in the upstream embankment by means of metal rods. The backfill is built as the wall is erected. This technique makes it possible to create aesthetic, stable, sub-vertical and draining walls. [Source: E. Lucas, P. Sery, A. Tigoulet, D. Brancaz, 2008, Les ouvrages récents de grande hauteur en sol renforcé, Compte rendus JNGG 2008, Nantes]Soil can be reinforced preventively, or deformations, expected or not, can be corrected under or in the vicinity of a structure. There are many methods for preventive improvement. Let us quote the compaction, which is a hardening of the ground, practiced by rollers possibly vibrating, or by dynamic action (falling of heavy masses on the ground, explosions at ground level). In the case of very fine waterlogged soils, drainage and consolidation are chosen, by laying drains, or an electro-osmosis system, or by using atmospheric pressure by vacuum under a waterproof surface membrane. But the geotechnician must always be patient! Reinforcement by geotextile sheets, micropiles, nailing, is very common. In particular, nailing, using steel bars sealed in a borehole, passive or active (tensioning after sealing), is widely used to stabilize rock slopes and suspicious tunnel walls. Along roads and highways, armed embankments are often found (Figure 6).
The most widely used technique worldwide to compensate for differential settlement is the injection of cement grout under foundation areas with excess settlement. But another original technique has recently developed. The worrying and increasing inclination of the Tower of Pisa and Mexico City Cathedral, built on a thick layer of clay, has been treated, in part, by under-excavation, or clay extraction under the highest foundation area. The recovery of these buildings was not intended, but rather the stabilization of their inclination (in addition to technical prudence, the tourist manna must be protected…).
Piles are the preferred foundation methods in soft ground, drilled or driven, commonly reaching several metres in diameter and a hundred metres in length in offshore. Cyclic loads are actively studied due to the so-called cyclic degradation phenomenon. To the extent that the capacity of the foundations is almost unlimited, provided that the price is paid, the structures themselves change in nature. For example, self-stable cable-stayed bridges and viaducts (Millau, Rion-Antirion,…) take precedence over suspension bridges
Today, on land, we are experimenting with geothermal piles and structures, with a dual function, foundation and heat exchanger.
Many other innovations are to come, which will mobilize the new generations..
References and notes
Cover image. The Grand’Maison dam by Douchet Quentin[GFDL or CC BY-SA 3.0], via Wikimedia Commons
