Pre-excavation grouting – a Nordic focus 08 Jun 2017

Eivind Grøv highlighted advances with PEG

In opening a two-day discussion into the practice of pre-excavation grouting, Professor Eivind Grøv, Chair of the Nordic Grouting Symposium scientific committee and Chairman of the first day of a recent event proceedings, pointed out that; “grouting is a small part of tunnelling overall, but it has great impact. It is the first line of defence for controlling groundwater in underground works. Before you excavate, you block the water,” he said. “All other means of controlling groundwater in tunnelling are more costly, come with pitfalls and are complicated and time consuming.”

Grøv is Chief Scientist with a tunnelling focus at Norway’s research body SINTEF, and an Adjunct Professor at the Norwegian University of Science and Technology (NTNU), in Trondheim. From these positions he has wide experience in taking the Nordic approach to rock excavation and tunnelling to the international arena.

With pre-excavation grouting well established in the Nordic region, Grøv pointed to the Harbour Area Treatment Scheme Stage 2A (HATS2A) sewage project in Hong Kong as an interesting example of a move towards wider use of the strategy. Originally, he said, the plan was to use TBM excavation for the project “but we changed that concept to drill+blast with pre-excavation grouting (PEG), which is an approach that would – and did – avoid issues experienced on the HATS1 project.”

Håkan Stille gave keynote and jacking workshop summary

The rock mass conditions in Hong Kong have many similarities with locations in Norway, reported a symposium paper co-authored by Grøv et al. Having adopted drill+blast excavation with PEG for water control, the Norwegian approach was adapted for extremely strict groundwater ingress limits on HATS2A, and to substantial benefits. The hydraulic head was up to 15 bar and key adaptations to the Norwegian approach were the use of micro-fine cement (MFC) and the supplementary use of colloidal silica, as needed, and combining with non-bleed mix designs.

A contract form was developed that combined traditional fixed price with new elements of unit prices. The risk sharing was not necessarily in line with Hong Kong traditions, but it was an important part in smooth execution of the tunnelling works.

Most of the HATS2A tunnels are below dense urban areas, except for the 4km crossing of Victoria Harbour. The project delivered 94% of the total 20km of tunnel on the project to have substantially lower ingress performance than local limits, and the remainder caused no settlement damage.

Rock types at HATS2A tunnel project, Hong Kong

The ingress limits could only be satisfied using drill+blast, as the method allows efficient, large scale use of probe and grout holes at the required numbers, positions and angles. A total of 480km of boreholes were drilled ahead of the tunnel face. Joints up to 10cm wide were successfully sealed off.

Grøv told TunnelTalk there were many visitors from Hong Kong to Norway to learn more about PEG. Since then, interest in PEG has grown in Hong Kong and more broadly across Asia.

“They are keen on risk reduction,” said Grøv, “and while there are many ways to apply PEG, the basis of the approach is proven.”

PEG was also a feature of a workshop during the symposium event. Grøv said good discussions resulted in re-thinking among working groups to better understand the PEG approach in light of high pressure and the risk of jacking in the rock mass. The question of whether jacking is good or bad “is a key issue these days,” he added.

Micro-fine cement (MFC) joint infill

Grøv noted that PEG demands high data and IT support with information including time, pressure, volume and flow, being recorded at the grouting rigs. “Using this information in real-time is important, to understand what is going on in the rock mass while executing the grouting,” he said.

High pressure grout and jacking discussions

After opening the symposium with the keynote speech, Håkan Stille, Emeritus Professor at Sweden’s KTH, brought the gathering to a close with his review and discussion of workshop views given by tunnellers on challenges with jacking. The workshop had working groups discuss two questions: is jacking of positive or negative effect and, how to balance high pressure and good quality grouting.

To begin his summary, Stille first covered a baseline of terminology as, he noted, “in some of the answers it was indicated that maybe we do not have the same definitions. But,” he added, “all groups seemed to accept, more or less, that jacking occurs when grout pressure exceeds the normal stresses acting over a fracture.”

Distribution of residual ingress limits

Workshop participants agreed that injecting grout at high pressures leads to faster flow rates. Stille added that jacking implies an increase in a fracture aperture but then that “depends on the capacity of the jack, so to say – and that depends on the size of the jack, the area, or in this case it is the spread of the grout and the excess grout pressure.”

“This,” said Stille “is, I think, the background to this whole question.”

Following the baseline discussion, Stille delved more into the results of the workshop discussions, noting that for the first question all working groups “seemed to consider that jacking may have negative effects,” although they said it may be beneficial in some circumstances.

Most working groups believed there were two main negative effects; loss of control and higher grout consumption. Some groups raised concerns over opening up new, larger water channels, such as beyond the grout zone. There was also discussion around high pressure grouting in relation to increased risk for workers.

A jacking workshop group

Circumstances where jacking could be of benefit include the during the excavation of deep and rural tunnels and not as beneficial on shallow tunnels in urban areas. This is related to better penetrability of the grout, the working groups reported. But, Stille added, some groups noted this requires special conditions, such as establishing and using reasonable stop criteria to maintain control and prevent high grout consumption.

Stille said: “If you do not have stop criteria or you do not use them, then you are not in control, and then you have high grout consumption, and then you are back in negative effects. But given that we have control and stop criteria, we can use high pressure for rural and deep tunnels.”

Questions raised by the groups and meriting further discussion, he noted, included:

• understanding what happens with spread and penetration of the grout into finer fractures;
• limited experience of jacking; and, the need to get more experience from the field;
• determining the stop criteria to use, and indicators to monitor, to keep jacking under control.

Distribution of residual ingress limits

In discussing how to balance high pressure and good quality grouting, Stille said the working groups indicated that high pressure at the beginning is good to get faster flow of grout, and faster tunnelling operations, with suggestions that grouting pressure should be close to the jacking pressure and some groups indicating that brief periods of higher pressure at the outset could be allowable.

Stille reported that almost all groups suggested monitoring should be used to control jacking, and that stop criteria are mandatory. He added his own view that “I do not like stop pressure” in this area of grouting, as determining the stop pressure is a challenge and flow increases with increased pressure. He noted that some groups suggested volume control and other methods of control, while others suggested adapting rheological properties to the actual situation on a project.

Erik Johansson provided a welcome to Finland for the 2019 symposium

There were also discussions among groups about a range of further aspects, such as:

• geological site investigations to include fracture orientation;
• education;
• the use of the GIN grouting intensity number. Stille noted his view, and questioned “do we need it?” He said many problems have been reported with the method, that it depends on the grout being used and the rock mass, but he added that GIN is an area for further discussion;
• stable grout including a question raised about bleeding. Stille said that, for him, it is not an issue, as the information from a standard measurement has nothing to do with what happens in a narrow fracture;
• special conditions, such as parallel tunnels. It was noted by one working group that in such circumstances it is not just the risk of jacking in the rock but leakage to other tunnels, and jacking in them needs to be examined;
• contracts. One working group discussed contractual issues in relation to high pressure grouting and jacking. The contract between the client and contractor is very important and “a very essential part of this method,” Stille commented;
• is jacking reversible. A group raised the question of whether reducing pressure would see the rock go back, and Stille said this could be an area for further discussion.

The workshop summary concluded with a lively Q&A session, some of which focused on the practicalities and experience of flow and pressure measurement and controls, especially as performed by operators to react to, and minimise, jacking. A focal point for the discussions was a flow-pressure chart from a project in Sweden. All delegates at the packed workshop briefing gave Stille a strong round of thanks for his effective summary of the many discussions and views.

Held every three years, the next Nordic Grouting Symposium is to be hosted in Finland, in 2019. Northern Europe’s tunnelling experience and expertise, however, and that of Norway, in particular, will be in the spotlight in mid-June when Norway hosts the WTC 2017 conference, in Bergen.