The observational method in geotechnical design

As in accordance with Eurocode 7 Part 1, the hen prediction of geotechnical behavior is difficult, so it may be appropriate to apply the approach known as the “observational method,” in which the design is reviewed during construction.

This method is applied by following this procedure:

• limits of acceptability of the values of certain quantities representative of the behavior of the structure-soil complex must be established;
• it must be shown that the chosen solution is acceptable with respect to these limits;
• alternative solutions (if the stated limits are assumed to be achieved), congruent with the project, must be provided, and their economic burdens defined;
• an adequate in-progress monitoring system, with associated control plans, must be established such that one of the planned alternative solutions can be adopted in a timely manner if the stated limits are reached.

The first two points refer to the definition of the limits of acceptability

Acceptability limits are those values of certain quantities representative of the behavior of the structure of soil complex

The remaining three points, however, can be traced back to the monitoring of the work-ground complex described in the following section.

Monitoring of the work-ground complex

Monitoring of the work-ground complex and interventions is done through the installation of appropriate instrumentation and in the measurement of significant physical quantities: displacements, stresses, forces, and pressures interstitial -before, during, and/or after the construction of the project work.

Monitoring should verify the correspondence between design assumptions and observed behaviors and check the functionality of the artifacts over time.

Design hypotheses ↔ Observed behaviors.

Under the Observational Method, the purpose of monitoring is to confirm the validity of the adopted design solution or, if not, to identify the most suitable one among the other solutions envisaged in the project.

If provided, the monitoring program should be defined and explained in the Geotechnical Report.

Historical evolution of the observational model

The father of the Observational Method is Terzaghi who, in 1945 wrote:

Earth Mechanics, for what we know nowadays, provides us with a third method that we can call the observational method. The procedure to be followed is as follows:

• Base in design on whatever information you can get.
• Make a preliminary list of possible differences between reality and the assumptions made.
• Then calculate, based on the original assumptions, several quantities that can be measured

at the site.

In 1969 Peck revisited the observational method and proposed:

1. preliminary exploration sufficient to establish, at least in general, the nature, stratigraphy, and properties of the deposit, but not necessarily detailed;
2. definition of the most probable conditions and the most unfavorable conceivable deviations from those conditions; geology often plays an important role at this stage;
3. sizing and design of works based on assumptions of soil behavior under the most likely conditions;
4. selection of strategically important quantities to be measured during construction and calculation of their expected value under the assumed assumptions;
5. calculation of the values of the same quantities under the most unfavorable conditions expected on the basis of available subsurface data;
6. prior definition of interventions and changes in the original design for any possible significant deviation of the experimental findings from the values expected from the calculation;
7. measurement of the chosen quantities and timely evaluation, through their interpretation, of the actual site conditions during construction;
8. Modification of the design to suit the actual conditions…

From the considerations conducted by Peck on the observational method, it is noted how the probabilistic criterion of knowing the terrain and assessing its physical and mechanical characteristics arises as early as the preliminary exploratory stage.

In the 1960s, the New Austrian Method (NATM; Rabcewicz, 1964, 1965) in tunnel construction became widespread, which was an important stage in the development of the Observational Method.

The NATM revolutionized the philosophy of excavating and supporting tunnels, based on the experience gained in the construction of Austrian Alpine tunnels.

Among the principles of NATM introduced by Mueller (1978), (there are 22 in total, of which only 7 characterize it substantially compared to other tunnel construction methods), an important role is played by the measurement of the deformation and tensional state around the tunnel: the purpose of the measurements is to obtain from experience in the construction of the first sections of the tunnel the information about the behavior of the rock mass and, therefore, to continuously update the sizing of the supporting structures.

During the development of NATM, important measurement tools were refined and have become commonly used.

Fig. 1 – Example of measurement equipment of a NATM tunnel

NATM favored the development of the Observational Method because of the great interest that was paid to the behavior of the rock mass, which itself became a fundamental supporting element of the created void, mobilizing residual resistance.

The flexible support works provided by the NATM had the sole task of controlling the closure of the cable and regulating the mobilization of the residual resistance of the rock mass.

Currently, in-process measurements are routine in the project scheduling of every job, and modern computers are capable of recording (even remotely) and facilitating the timely interpretation of measurements of quantities of all kinds.

On-site measurements must be carefully planned in advance and their success is linked to the possibility of interpreting them correctly through the calculation model adopted. It is precisely on the basis of the calculation model assumed for the simulation of the rock mass and the realized structures that the measurement campaign must be defined in detail.

Measures in progress

The purpose of in-progress measures is to obtain adequate safety factors during and after construction, at minimum cost (project optimization).

Therefore:

• Safety control: in most cases, completed underground structures have excessive degrees of safety; on the other hand, during the construction phase there have been occurrences in the past disasters that emphasize the importance of continuous safety monitoring; measurements can be seen, in this case, as a warning system that makes it possible to become aware of the risk in advance and, therefore, to prepare appropriate countermeasures; in this regard, the recording of measurements over time, especially of the derivative of the measured magnitude over time, is essential;
• The determination of material properties and, if possible, the undisturbed stress state (obtained through back analysis techniques);
• The determination of the physical and mechanical characteristics of the rock mass at this stage is particularly interesting because it relates to the problem in true magnitude, as opposed to the preliminary tests, which, on the other hand, involve a limited portion of the rock mass;
• The verification of the validity of the structural choice with reference to the construction method;
• Optimization of the geometric and mechanical characteristics of the support structure and the timing of site operations are conducted on the basis of on-site measurements; the original design is continuously updated until the experimental finding is satisfactory (both from a safety and economic point of view);
• The comparison of theoretical predictions with actual structural behavior;

Since the computational model developed cannot perfectly simulate real conditions, there is always some difference between the results of the calculation and the measurements taken; even an accurate parametric analysis can only derive backward the mechanical and physical parameters that make the deviation between the results of the calculation and the measurements taken minimally.

CALCULATION RESULTS ≠ MEASUREMENTS

When the deviation becomes significant, it is necessary to reconsider the assumptions assumed in the calculation model, based on assessments of rock mass behavior found during construction

Example of application of the Observational Method for an excavation

The method consists of the following points:

1. Exploration to build the geological and geotechnical model of the deposit;
2. Identification of the most likely conditions and the most unfavorable conceivable deviations from model conditions;
3. Sizing and design of works based on assumptions of soil behavior under the most likely conditions;
4. Selection of quantities to be observed during construction and calculation of their expected value under the assumed assumptions;
5. Calculation of values of the same quantities under the most unfavorable expected conditions based on available subsurface data;
6. Prior identification of interventions and changes in the original design for any possible significant deviation of the experimental findings from the values expected from the calculation;
7. Measurement of the chosen quantities and timely evaluation, through their interpretation, of the actual site conditions during the construction time (in progress);
8. Modification of the design to suit actual conditions.

Fig. 2 – Conceptual outline observational method

For each selected quantity (item d), the difference between the values measured during excavation and those predicted in the design must remain within pre-determined attention levels.

Exceeding these levels (thresholds) identifies the interventions provided for in (f), which are classified under the term “countermeasures” if they have the effect of moving the work away from conditions assessed as hazardous (line hazardous conditions), or instead can be called “rationalizations” if they allow for less onerous design solutions, resulting from observed behavior that is less burdensome than expected.

The Geotechnical Report and the Observational Method

As also described in Eurocode 7 (Geotechnical design- Part 1: General rules UNI . UNI ENV 1997-1), the geotechnical report must include a plan for supervision and checks. The report should clearly identify those parts of the work that require testing during construction or maintenance after completion.

If the required verifications were carried out during the work, they should be reported in a supplementary report.

In relation to supervision and controls, the geotechnical report should state:

• The object of each set of observations or measurements;
• The parts of the structure to be checked and the locations where the observations are to be made;
• The frequency with which measurements should be performed;
• How the results are to be evaluated;
• The range of values within which the results should be considered;
• The period of time during which inspections should be continued after the work is completed;
• Individuals responsible for performing measurements and observations, interpreting results, and monitoring and maintaining instruments.

Supervision of the construction process and the quality of construction, as well as monitoring the behavior of the structure, during and after construction, must be carried out according to the specifications prepared in the geotechnical report.

The supervision plan contained in the geotechnical report must identify acceptable limits for the results that will be obtained during the supervision.

It is appropriate for the supervision plan to indicate the type, quality, and frequency of supervision activities, which in turn correlate with:

• The degree of uncertainty of design assumptions;
• The complexity of soil and loading conditions;
• The potential risk of rupture in progress;
• The possibility of introducing changes to the project or making corrective actions during construction

Fig. 3 – Operational flow diagram for programming a plan of measures

The application of the observational method at a glance

1. Justification for use: uncertainties that can only be resolved during the execution phase
2. Modalities:
   1. Some significant quantities should be identified
   2. Define the limits of acceptability of these quantities
   3. Acceptability of the planned solution in relation to the limitations of point 2
   4. Ongoing monitoring

What are the advantages of the Observational Method?

In conclusion, in addition to those already reviewed, the advantages of the method are:

• Optimize the use of natural resources
• Better understanding of soil-structure interaction (e.g., due to earthquake impact, wind loads, etc.)
• Efficient use of underground spaces
• Improvement of health and safety
• Reduction of natural and man-made environmental risks
• Knowledge based on high-tech constructions
• Good reputation for the geological and geotechnical field

Excerpted from the lecture of the Master’s Degree Course in Engineering Geology of the Center for GeoTechnology of the University of Siena: “The Observational Method” by Prof. Eros Aiello

Ground Investigation Testing – GIT

GIT is the software to calculate the estimate of the significant geotechnical volume according to: Lancellotta and Calavera, Eurocode 7. The significant volume to be investigated is the mass of soil within which the effects of the intervention are felt.