**3.6 Reinforcement of the natural Foundation of the Highway by partial soil exchange and replacement by a drainage structural layer**

Sometimes, when the layer of low bearing capacity of the natural foundation (subgrade) is not very thick (2–3 m), then it is possible to replace all of it with another, betterquality material, imported from the deposit. When the layer is deeper (**Figure 17a**), and the total removal of the layer or the use of reinforced piled embankment is uneconomical (discussed in Item 4.4 of the Chapter "Challenges in the Construction of Highways in the Brazilian Amazonia Environment: Part I: Identification of Engineering Problems"), it is possible to partially replace the layer by introducing a layer of coarse crushed stone material (locally known as *rachão*, with a particle size of 100–250 mm, i.e., cobble grain size classification) over a layer of clean sand (**Figure 17b**). The first layer acts as a structural reinforcement, while the second layer acts as a draining layer, although both layers are very permeable and provide a significant increase in the shear strength of the natural foundation layer. Sometimes, geotextile is placed between the natural subgrade and the sand layers, and between the layers of the *rachão* (high stiffness geotextile must be used so as not to be ripped by the friction with the crushed stones) and the compacted embankment, to avoid filling the voids of these layers. However, geotextile is not introduced between the sand and *rachão* layers, as it is intended that there be "needling" of the sand particles in the voids of the rock granular material, allowing the internal structure of the grains to become more rigid by particle-to-particle contact.

**Figure 18** shows the phases of partial excavation of the soft clay layer for the execution of an urban connection corridor in the city of Manaus, Amazonas (Point 11 of **Figure 1**). The thickness of the *rachão* layer – despite being a bearing capacity problem and being predefined in the design – is usually confirmed in the field by local experience in passing a fully loaded truck (82 kN of load for a rear single axle with dual wheels) several times over the surface of the layer, and no visible deformation is observable to the naked eye. In general, this thickness varies between 60 and 100 cm and may reach an even greater value, depending on the site conditions and embankment surcharge. Obviously, this is an onerous solution for locations where stone material is scarce and therefore is limited to short stretches of a road.

#### **Figure 17.**

*Partial replacement of natural foundations with low support capacity by drainage layers and greater shear strength in road construction.*

#### **Figure 18.**

*Construction of an urban corridor with partial replacement of the natural ground with low bearing capacity. a) Partial excavation of the layer; b) placement of the coarse crushed stone material (rachão); c) placement of the embankment layer for road construction; d) appearance of the rachão layer after partial implementation of the compacted earthen embankment (photos: NS Campelo).*

## **3.7 Construction of highways in the** *Terra Firme* **and** *Várzea* **regions**

Item 1 described that the Amazon Basin encompasses a vast area of lowland and relatively flat lands, with numerous watercourses crossing them. The highlands, which are not flooded, constitute the *terras firmes*, while the lowlands areas flooded by flood river pulses constitute the *várzeas*. In the *terras firmes,* there is a predominance of yellow-red latosols, and horizons of *piçarras* are found with relative ease, albeit with reduced volume, given the low thickness of these layers. In addition, the subgrade in these regions usually has more permeable material

*Challenges in the Construction of Highways in the Brazilian Amazonia Environment: Part II… DOI: http://dx.doi.org/10.5772/intechopen.105017*

and, together with the more rugged relief, allows better surface and deep drainage of the highways.

However, in *várzea* regions, there are several complications for highway construction. Starting with the impermeable natural subgrade (silty or clayey soil), formed mainly by argisols and gleysols, with low bearing capacity (mean N72 less than 3 strokes/30 cm, which could meaning an allowable bearing capacity of soil somewhat less than 30 kPa, at depths ranging from 0 to 4 m in most cases), an absence of horizons of lateritic concretion, a deficiency of quality material to constitute the road embankment (generally, the existing soils consisting of expansive clay minerals, even without the presence of organic matter), an absence of rocky material, and a natural drainage basin forming a tangle of watercourses with approximately 10–15 m of vertical fluctuation between the maximum flood and minimum ebb levels, etc.

**Figure 19** shows two Brazilian federal highways that cross the State of Amazonas. **Figure 19a** shows a stretch of highway BR-174, with a total length of 3,320 km, which connects the States of Mato Grosso, Rondônia, Amazonas, and Roraima and from this to Venezuela and the rest of the Americas and Caribbean (see **Figure 7** of the Chapter "Challenges in the Construction of Highways in the Brazilian Amazonia Environment: Part I: Identification of Engineering Problems"); only 210 km of this highway cross the state of Amazonas. Throughout almost its whole length, deposits of *piçarras* and stone material are found; it cuts the *terras fimes*, with the top of the highway located between 50 and 200 m altitude; it crosses a more dispersed natural drainage basin, with lower frequency of larger watercourses. **Figure 19b** illustrates a stretch of highway BR-319, with a total length of 885 km, of which 820 km is in the state of Amazonas and 65 km is in the state of Rondônia (see **Figure 7** of the same Chapter), connecting the city of Manaus to the city of Porto Velho in the North–South direction and from there to the rest of the country. The highway crosses *várzeas* for much of its length (approximately half the length, in the direction Manaus–Porto Velho), with deposits of lateritic concretions and rocky material only in the last 200 km of the highway. The top of the highway is located at elevations between 25 and 70 m altitude and crosses a more concentrated natural drainage basin, with a higher frequency of small and large watercourses.

Due to the period of river flooding in the *várzea* region, the highways function as "earth dam", dividing the drainage basin between the two sides. In fact, the situation is more aggravated because, while in the conventional dams only the upstream side

#### **Figure 19.**

*Different reliefs cut by highways in the state of Amazonas. a) Wavy, in the "terra firme" region. b) Flat, in a "várzea" region (photos: NS Campelo).*

is subject to the variation in the external water level, in these highways both sides are influenced by the flooding and ebbing water levels. The ebb period is the most critical because, as the embankment is formed mainly of clayey soils, the dissipation of pore-pressures from the interior of the massif is slow and, if accompanied by a rapid decrease in the water level of the surrounding watercourse, may result in a phenomenon analogous to the slope failure by "rapid drawdown", observed in conventional dams (**Figure 20**). Thus, any of the highway margins may be subject to slope failure, and there may even be a rupture of both sides. Therefore, there is a combination of factors that can lead to the rupture of these road slopes: large increase in the external water level; slow dissipation of pore-pressures from the interior of the clayey/silty compacted embankment; and natural subgrade formed by soil stratum of low to medium bearing capacity.

Large river floods have become more frequent in the Amazon Basin, leading to increasing maxima water levels year by year. Thus, in some regions of the BR-319 highway, overtopping occurs (**Figure 21**), and in the ebb cycle, partial rupture of the slopes may occur in the higher embankments when conditions are favorable (**Figures 22** and **23**).

**Figure 22a** (point 12 of **Figure 1**) is located near 23 km of highway BR-319, while the other occurrences are in a range between 20 km and 60 km.

Souza [7] and Souza et al. [8] studied the phenomenon of *terras caídas* ("fallen lands"), which is the rupture of riverbanks, commonly in floodplain regions, under the ebb of "white-water" rivers (classification due to Sioli [9]), locally called "águas barrentas" ("muddy waters"). Despite being a natural phenomenon, the same

**Figure 20.**

*Lowering of the river water level on both sides of the road.*

#### **Figure 21.**

*River flooding causing overtopping at some places on federal highway BR-319 (photos: NS Campelo).*

*Challenges in the Construction of Highways in the Brazilian Amazonia Environment: Part II… DOI: http://dx.doi.org/10.5772/intechopen.105017*

**Figure 22.** *Rupture of road slopes, after the river ebb cycle (photos: NS Campelo).*

**Figure 23.** *Rupture of road slopes, after the river ebb cycle (photos: NS Campelo).*

concept of hydraulic rupture can be applied to road embankments, as described in **Figure 20**. **Figure 24** shows the phenomenon of "fallen lands" near the city of Manaus (Point 13 of **Figure 1**).

This phenomenon causes damage to the riverine population, as a large amount of land detaches from the massif, reaching the residences (wooden *palafitas*) and the urban and rural roads that border the riverbank of the communities and cities. It can also cause small "tsunamis" when a rupture of land occurs, causing the sinking of small boats anchored nearby.

#### **3.8 Behavior of tropical soils under suction pressure and the laterization process**

As reported in Items 2.8 and 2.9 of the Chapter "Challenges in the Construction of Highways in the Brazilian Amazonia Environment: Part I: Identification of Engineering Problems", tropical soils present some behaviors dictated by the pedogenetic process of laterization and soil suction when in an unsaturated state. Together, they significantly increase the resistance of cutting (mainly) and embankment slopes (in relation to the matric suction) in the case of shear resistance against rupture and erodibility processes.

There are records of cutting slopes existing for more than 50 years that are almost vertical (**Figure 25a** and **b**), which, even without anti-erosion protection – by vegetation cover (grasses or native vegetation) – or even without the existence of any superficial drainage of rainwater (in a region of high rainfall, above 2,500 mm per year, see **Figure 2b** of that Chapter), remains in a stable condition, except for some non-lateritic stratum (saprolitic), where it is vulnerable to erosion (**Figure 25b** and **c**), or when it is sandy or silty lateritic soil (**Figure 25a**). **Figure 25a, b**, and **c** corresponds to Points 14 to 16 of **Figure 1**, respectively. Other times, the distinction between the

lateritic and saprolitic horizons is given by the existence of a "stone line" of lateritic concretions, as shown in **Figure 26a**, on state highway AM-070 (Point 17 of **Figure 1**).

In some cases, a thin film (1–3 mm) of lateritic crust forms on the surface of the slope, of dark red color, which acts as an anti-erosion protectant, allowing the stability of the slope against the deleterious action of the rains for decades, even with almost vertical slopes. **Figure 26b** and **c** shows these thin films (Points 18 and 19 of **Figure 1**, respectively) that occur in the city of Manaus.
