Tag Archives: geotechnical investigation

Geotechnical Risk Management

In our first ever post in Geotechpedia’s blog we tried to answer the most common question among professionals in the Geotechnical engineering industry “Geotechnical Investigation data, always not enough?”. In the specific post, it has been mentioned that “some of us proudly state “I saved so much by reducing the geotechnical investigation” but all this immediately changes when something goes wrong”. We are all aware of how limited or inadequate Geotechnical Investigation (GI) can affect both a project’s schedule and budget. In the following lines, we are trying to quantify these effects, provide guidance on how to easily create a risk scoring matrix and attributed risks, typical geotechnical risks and related mitigation measures.

Cost and time effect

Back in 1748 Benjamin Franklin stated “Time is money” in his “Advice to a Young Tradesman”. This quote finds application in all business sectors and the engineering one couldn’t stand out as an exception. In every project, delays are translated into cost and as such we are going to examine the estimated cost effects of delays deriving from inadequate Geotechnical Investigation on a project’s total construction budget. The most common chart when discussing the risk management in geotechnical engineering is presented below (Figure 1). It is obvious that for low values (1% approximately) of Geotechnical Investigation cost / tender cost (adjusted values), the total increase in construction cost may vary between 2% and 98% with an average value of 15-25%. When the Geotechnical investigation budget is slightly increased (adjusted Geotechnical Investigation cost / construction tender cost values between 2 and 4%) then the total increase in the construction cost drops to a typical range of 2% to 25% with an average value of 5-10%; meaning that an increase of 1-2% on the construction tender cost for additional Geotechnical Investigation signalizes a significant drop of approximately 25 to 50% (absolute values) in the total construction cost.

Figure 1: Total increase in construction cost related to adjusted Geotechnical Investigation cost / construction tender cost (source: UK Highways Agency projects (1994))

 Typical Risk Scoring Matrix

During the tendering procedure of a project, a risk assessment needs to be undertaken in order to evaluate the geotechnical risks at an early stage and propose mitigation measures. The following table (Table 1) presents a typical and simple scoring matrix that can be used in this kind of assessments and Table 2 details the specific risks associated with geotechnical works and categorizes them into probability of occurrence and cost/time impact.

The purpose of the following matrix is to help rank the key risks on site.

Table 1 Risk scoring matrix

Scores of 1-5 are allocated to the probability and impact in order to quantify and rate the risk rating. The risk scoring matrix should be used in conjunction with the priority action table detailed below (Table 2).

Table 2 Priority action table

Typical Geotechnical Hazards and recommended mitigation measures

Successful implementation of the suggested mitigation measures will assist with managing and reducing known risks to acceptable levels.

Table 3 below presents typical risks/hazards, related impact on construction budget and proposed mitigation measures.

Table 3 Risk/hazard assessment and proposed mitigation measures

In general, Geotechnical Risk Management gains supporters through the Projects Manager’s community since experience has proved that inadequate or incomplete Geotechnical Investigation during the tendering stage can have a severe impact on a project’s schedule and overall cost. Moreover, managing geotechnical risks also helps to increase safety levels in siteworks.


We need to keep in mind that geotechnical risk cannot be avoided and ignored but it can be managed and mitigated.

Taking all the above into consideration it is recommended that a detailed Geotechnical Investigation program is proposed at early stages of each project, following an in-depth desk study of all available information and site walk-over surveys.

It must be highlighted that the above post and its recommendations are to be read in conjunction with site specific available information and with critical thinking. In all cases, the Designer should set strict guidance for adequate Geotechnical Investigation in line with project specifications and international standards.

Useful References

[1] BS5930:1999, British Standard Code of practice for Site Investigations

[2] EuroCode 7 – IS EN 1997-2:1997 (Part 2, Annex B3)

[3] Clayton, C.R.I. (2001) Managing geotechnical risk, Thomas Telford.


Proper amount of geotechnical investigation

The situation is like this: A major problem occurs in a bridge abutment and significant differential settlement between abutment and road is observed. The Owner decides to investigate the situation and assigns the job to a joint collaboration between a University and a Geotechnical Consultancy Firm.

The collaboration requests the execution of three boreholes to a depth of 30m, execute a number of consolidation tests some in a private laboratory and some in the laboratory of the University, together with other appropriate soil tests such as gradation, shear strength etc.

The collaboration provides a report in which it is stated that the previous geotechnical investigation did not evaluate properly the soil conditions because only one (1) drilling of 20m depth was executed in the abutment and did not evaluate properly the thickness of a compressible clay layer. Also based on the 15 consolidation tests executed by the collaboration, the coefficient of consolidation and the preconsolidation pressure estimated by the previous Geotechnical Consultant were optimistic. The previous consultant had executed three (3) consolidation tests.

So the conclusion of the report was that the problem was due to the optimistic evaluation of settlements made by the previous Geotechnical Consultant which had executed only one (1) drilling of twenty (20) meters and limited consolidation testing.

It is very easy to come to a “correct” solution after a significant amount of geotechnical investigation (money spent) has been executed in an area where a problem has occurred. The problem is known, some geotechnical information is available and the investigation can be targeted appropriately.

But how easy is this to be done from the start of a project? Most people involved in geotechnical investigation know how difficult is to persuade the Owner, Contractor etc to execute even the minimum required investigation not to mention the increased difficulty to persuade for additional investigation in an area where a hint of geotechnical problem is speculated.

In the fast track way that most projects are executed these days how can the geotechnical investigation and design produce “accurate” results?

How can we persuade the Clients that less is not more in geotechnical investigation and design? Food for thought.