Observation of embankments on soft soil

Installation points, equipment, and parameters to monitor

The complexity of natural ground masses and the theoretical limitations that support the predicting methods (for settlements and their change over time) frequently lead to important biases between estimates and reality. This is particularly applicable to settlement change in time, which is generally significantly faster in reality than predicted by the model theories.

This fact results, in many situations, from the soft soil macrofabric. In fact, the existence of sublayers of higher permeability can considerably reduce the consolidation time, either by the increment of the average horizontal consolidation coefficient or by the reduction of the flow path associated with the vertical consolidation process.

As a consequence, constructions on soft soils need to be monitored in order to adjust the predicted calculations, and to take decisions (if necessary) that lead to the achievement of the design requirements in the most economical and safe way. The measured parameters, the type of equipment used and the point where it is installed are summarized in Table 4.7. Figure 4.35 illustrates a typical instrumented section of an embankment.

The ground surface settlements are generally measured by means of settlement plates. These consist of a base (usually a small square reinforced concrete slab or a steel plate) to which a vertical metallic tube is attached, which can be successively complemented with new portions coupled to the portion immediately underneath. The plates are placed at the

Table 4.7 Equipment, installation points, and parameters to measure for monitoring of soft clay soils.

Measured parameters

Type of equipment

Installation point

Surface settlements

Settlement plates/topography

Ground surface in the loaded area and its neighborhood

Ground horizontal displacements

Inclinometers/inclinometer tubes

Installed in vertical boreholes close to the limit of the loaded area

Pore water pressure

Electric piezometers

Clay layer at several depths

Typical instrumented section of an embankment on soft soil

Figure 4.35 Typical instrumented section of an embankment on soft soil.

initial ground surface and the elevation of the tube top is determined by topographic leveling. After the construction of the embankment and the start of the consolidation process, periodic topographic leveling provides the changes in the tube top elevation, and therefore settlement of the ground surface at each point.

From this description, it is easy to understand that this observation process is quite simple and economic. However, its isolated use is not recommended since it is difficult to do an accurate interpretation of the soil mass behavior based only on the settlement observations, in particular when these are still very far from being stabilized. For instance, the observation of much higher settlements than predicted during a given period after loading can result from very different situations: i) underestimation of the compressibility parameters of the soil; ii) underestimation of the coefficient of consolidation; iii) errors in the evaluation of the boundary conditions of the problem, namely in terms of drainage; iv) existence of significant secondary consolidation settlements; v) the linearity between effective stress and volumetric strain adopted in Terzaghi’s theory (see Figure 4.13); or vi) combinations of two or more of the aforementioned issues.

It is, therefore, highly convenient to combine these measurements with those of pore water pressures in the clay layer at different depths, using, for that purpose, electric piezometers due to the very low permeability of the soil. This matter will be treated in more detail in Chapter 9 (see Annex A.9.3).

In the case of loading in areas of large dimensions, horizontal displacements in the clay mass are only significant under the zones close to the embankment limit. Their measurement is relevant in the context of safety relative to rotational slides of the embankment and foundation soil (as an example, see Figure 2.1). In general, these failures are preceded by quite large horizontal displacements in the foundation soil. The horizontal displacements can be measured along a vertical hole, in which inclinometer tubes are installed. The introduction into these tubes of a device - the inclinometer - allows the measurements of horizontal displacements at each point of the tube. This type of measurement and the inclinometer operation mode will be discussed in Chapter 9 (see Annex A.9.2).

The Asaoka method

Asaoka (1978) developed a recognized method of easy application, which provides an estimate of the final consolidation settlement based on the settlement change observed over a certain time period. The theoretical basis of the method, which will not be presented here, is in agreement with Terzaghi’s one-dimensional consolidation theory.

The method comprises the following steps: i) the change over time of the observed settlement is plotted (Figure 4.36a); ii) a certain instant, £,, and a certain time interval, At, are selected, and, from the previous graph, the settlement values are calculated for f, and for all the other subsequent points, spaced for At, for which observation results are available; iii) from the previously calculated settlements, a graph is constructed, marking on the x-axis the settlement s,_] and on the у-axis the settlement s, (Figure 4.36b); iv) in this graph, the line that best fits the plotted points is drawn and its slope, /?,, is calculated; v) in the same graph, a second line at 45° from the origin is drawn which represents the points where s,_,=s,; and vi) the abscissa of the crossing point of these two lines represents the final consolidation settlement estimate.

The coefficient of consolidation can be obtained from the following equation:

where H has the same meaning as in Terzaghi’s theory of consolidation and /i, is expressed in radians.

Consolidation settlement estimate from observation results (Asaoka, 1978)

Figure 4.36 Consolidation settlement estimate from observation results (Asaoka, 1978): a) settlement versus time curve; b) construction to obtain the final consolidation settlement.

 
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