The importance of carrying out a detailed geotechnical study

Geotechnical studies assist engineers understand the characteristics of the foundation(s) on which infrastructure is to be built. They form the basis upon which decisions are made regarding the type of foundation to adopt, the type of engineering treatment below ground level to make, the cost of the foundation and many others.

On board a flight to Europe recently, I had an interesting chat with Gregoire, a French engineer who specializes in the design of wastewater collection, transportation and treatment systems. He had just completed one of his free-lance assignments in the Middle East and was heading back home.

As we spoke about a number of things, Gregoire mentioned how engineering professionals should not underestimate the importance of carrying out a detailed geotechnical study while designing for infrastructure projects.

“Are you referring to any particular project that you have been a part of?” I asked. ‘Yes’, he replied.

This is Gregoire’s story…

An agency set out to improve the sanitation situation in an area within its jurisdiction. In order to achieve this, a consultant was hired to do a feasibility study and develop a sanitation master plan. The consultant was also required to develop a pre-design for the sanitation system.

After the first consultant had completed the assignment, another consultant was recruited to update the feasibility study, carry out a detailed design and prepare tender documents for construction works. I was contracted as a freelance specialist to assist in the preparation of the detailed design.

The first consultant had identified about four potential areas where wastewater treatment systems could potentially be constructed. Of particular interest to me was Kaitana, one of the four locations selected.

In Kaitana, the consultant proposed a wastewater treatment system comprising primary treatment, secondary treatment with the use of sludge drying beds, and a system of maturation ponds for tertiary treatment. During the inception phase of our consultancy, my colleagues and I were asked to consider a review of this concept.

In order to improve the efficiency of the wastewater treatment facility and to maximise the available space at Kaitana, we revised the sludge handling and treatment method from sludge drying beds to mechanical dewatering at the secondary stage of treatment. For the tertiary treatment stage, we introduced a system of chemical treatment with a series of constructed wetlands to replace the maturation pond system which had been proposed earlier.

After securing the client’s approval of our design concept, we commenced the detailed design.

‘Gregoire, it appears you were bracing yourself for an interesting assignment?’

‘Yes, but there were challenges ahead; the kind that you would not typically be exposed to in a theoretical civil engineering class,’ he answered.

Part A – The challenges with the site at Kaitana

During my desk study, I realised that the first consultant had based their pre-design on assumptions about the nature of the ground conditions at Kaitana, and had not carried out a detailed geotechnical study. I wondered why this had happened and could only think of a few possible reasons;

1. Time constraints.
2. Limitation of funding from the client.
3. Rock-out crops in the surrounding areas, which were visible from Kaitana.

On further investigation, I learnt that the client had achieved firm ground at depths within 7-10 meters in a number of recent projects but had not envisaged the worst case scenario for Kaitana. At the pre-design stage, the first consultant had proposed pile foundations at similar depths for some of the structures.

I realised that the trial pits from which the soil samples were picked did not go beyond a 2 meter depth and the supplementary hand auger method that had been adopted did not go beyond a depth of 7 meters. The soil logs did not convincingly show that the consultant had achieved firm ground. I decided to spend some time within and around Kaitana to appreciate the site much better.

After my reconnaissance survey, I discovered the following:

1. There were a number of people living adjacent to Kaitana but for some reason they all avoided a particular section of Kaitana. This I found to be very suspicious.
2. There was a railway line passing in the peripheral of the site. I imagined that some geotechnical study must have been carried out before the railway line was constructed. My investigation led me to the discovery that there was a borehole at the termination point of the railway line – a distance of about 2 kilometers downstream of Kaitana. I searched for the drilling records for this borehole.

Fortunately, the records were found and they revealed that rock had been hit at a depth of 45 meters below the ground level. With the existing ground conditions now becoming clearer, one could tell that firm ground in Kaitana was much deeper than had been estimated by the first consultant.

With the existing ground conditions now becoming clearer, one could tell that firm ground in Kaitana was much deeper than had been estimated by the first consultant.

Part B – The need for a detailed geotechnical study at Kaitana

One of the outputs of a detailed design is a geotechnical report. To produce this report, we would have to access the site and carry out drilling tests at a number of locations within the site. Within and around the site, people were cultivating some crops and some portions of the site were water logged. It became evident that a geotechnical study would require a specilaised geotechnical company using specialized mechanical equipment to get access to critical areas of the site.

On further analysis of the site conditions, it turned out that this equipment could only get to the critical areas of the site if an access road was built. Obviously the first consultant had not done this because there was no evidence that an access road had been constructed.

After it had dawned on everyone that a detailed geotechnical study was required, my colleagues were asked to produce terms of reference to enable a specilaised geotechnical company price the actual scope and nature of geotechnical work required with some certainty. This was a difficult task because we still couldn’t access the critical areas of the site. And even if we had finally secured a specilaised geotechnical company to do the geotechnical study, an access road would need to be constructed first.

After it had dawned on everyone that a detailed geotechnical study was required, my colleagues were asked to produce terms of reference to enable a specilaised geotechnical company price the actual scope and nature of geotechnical work required with some certainty. This was a difficult task because we still couldn’t access the critical areas of the site. And even if we had finally secured a specilaised geotechnical company to do the geotechnical study, an access road would need to be constructed first

At this point we were running out of time to complete the detailed design and there were no funds to build the access road. The client began to get impatient. We received an instruction from the client to proceed with the detailed design on the understanding that the construction contract would be structured as a design and build contract. The contractor would be required to build the access road and carry out a detailed geotechnical investigation.

A two-step option (short cut) was proposed to the client and later adopted;

Step 1 – A quick geophysical investigation was carried out to get a feel of the depth and an indication of the nature of the soil layers on site.

Even though this was not very conclusive, the results indicated that the nature of the soil layers up to a depth of 30 meters seemed very weak.

Basing on the geophysical investigation, we were able to prepare terms of reference for the specialised geotechnical company.

Step 2 – Based on the site conditions and the results obtained in step 1 above, we opted to drill on the sides of the Kaitana site where we had full access. It turned out that rock was obtained between 23 and 25 meters.

Based on the results from step 1 and 2, the depth to firm ground (rock) at the critical areas of the Kaitana site was estimated.

‘Gregoire, your instincts must have kicked in the moment your team received the geotechnical data for the borehole at the termination point of the railway line?’

‘Certainly.’

Part C- The design and tender documents

We proceeded with our detailed design. Based on the results of the geotechnical investigation, end bearing piles were recommended for some of the structures i.e. the sludge digesters and sedimentation tanks. For other structures, raft foundations on consolidated soil were recommended.

The question that came up was, ‘What would be the best option to quicken the soil consolidation process?’ Vertical drains together with soil pre-loading were proposed. However, the consolidation platforms would have to be monitored from a period within 6 months to a year.

Preparatory foundation works to be done prior to construction were packaged into the tender documents as a design and build contract. This meant that the access road to the critical areas of the site would be done by the contractor, who would then carry out a site specific (targeted) geotechnical investigation.

Part D – The construction works

A contractor was secured through an international competitive bidding process. The access road to the site was built and the contractor proceeded to do a detailed geotechnical study.

The contractor’s efforts (site specific (targeted) geotechnical investigation) revealed that firm ground (rock) existed at a depth of about 30 meters. At the center of the site, the tests revealed a 2 meter layer of sand between 10 and 12 meters deep. This layer of sand contained water in it. With the presence of water in this layer of sand, seismic issues had now come into play. There was a likelihood that the piles bored through this area would break as a result of horizontal hydraulic pressure.

The contractor proposed to utilise skin friction within the sand layer such that the piles bored through this layer of sand would not be required to go as deep as 30 meters. He also proposed to insert concrete into the layer of sand in order to make it firmer.

An option of driving stainless steel piles right down to the bottom of the rock also came up. We were now faced with three pile options i.e. concrete piles, friction piles and steel piles. Driving down steel piles to a depth of 30 meters would be very noisy for the surrounding inhabitants while friction piles would be shorter and cheaper but with a risk of them failing due to seismic issues. On the other hand, the concrete piles were bound to be very expensive.

Irrespective of the pile option to be selected, the use of vertical drains to assist in the soil consolidation process was a must. Unfortunately, the contractor did not have this equipment available and would have to import it.

Unfortunately, the ownership of the site on which the pre-design was based wasn’t confirmed as fully owned by the client during the preparation of the feasibility and pre-design study, and at the time I was engaged to update the feasibility study and produce a detailed design, the land ownership issue was still pending.”

While the client, contractor and consultant were busy trying to figure out the best way forward for the foundations, a court injunction was served to the client for trespassing on the land at Kaitana. In order not to delay the project any further, the client began to consider relocating to an alternative site.

While the client, contractor and consultant were busy trying to figure out the best way forward for the foundations, a court injunction was served to the client for trespassing on the land at Kaitana. In order not to delay the project any further, the client began to consider relocating to an alternative site.

Part E – Moving to a new site

Considering the court injunction and the numerous unresolved issues related to the best choice of piles for the foundations, a decision was taken to shift the wastewater treatment facility to another location. Obviously a fresh geotechnical study was required at the new location.

Surprisingly, it also turned out that a combination of end bearing piles and friction piles were required for some of the waste water treatment units at the new site.

Currently, construction works are ongoing at the new site albeit with lessons learnt by all parties involved in the project.

Part F – Lessons learnt

1. Gregoire is a process engineer who is now fortunate to have a very good understanding of geotechnical issues. In case you are part of a multidisciplinary project team, you will always have the opportunity to improve your competence in other professional fields which might be related or unrelated to your preferred field of practice. You need to be prepared and willing to seize such an opportunity.

2. Sometimes the best solution to a technical challenge lies right in front of us. For one reason or another we tend to ignore our intuition and yet we already have the answers.

3. As a consultant, your professional judgement about a matter like this can at times be ignored by your client because of a host of issues. Whereas you might also be wrong, it is important to try and protect your reputation at all times.

4. Geotechnical studies should preferably be done at the detailed design stage. However, some might argue that this is not desirable because the cost of the design consultancy might become unnecessarily “expensive”. For this reason, certain construction contracts are structured to include confirmatory geotechnical investigations before the commencement of physical construction works; usually as a means to mitigate any unforeseen geotechnical issues that may come up as the construction works progress.

5. Trying to avoid a thorough geotechnical study simply postpones the inevitable costs. You cannot run away from the actual costs because you will eventually incur them.

6. When unforeseen geotechnical issues emerge during construction works, the contractor and consultant will most likely require a time extension to their construction and supervision contracts respectively. This means that the client will have to spend more money to complete the project.

Conclusion

Doing a detailed geotechnical study should be a must. It is simply a disciplined way to proceed prior to commencement of detailed designs and subsequently construction works especially where large sums of money are being sunk to develop infrastructure.

©. The Builders’ Garage 2016. Permission to use this article or quotations from it is granted subject to appropriate credit being given to thebuildersgarage.com as the source.