Introduction
Case study: Stockholm bypass
Rock class | Q-value | Rock quality | Description of rock mass |
---|---|---|---|
I | Q > 10 | Very good | Sparsely fractured or large blocky granite, gneiss-granite, pegmatite or occasionally slaty gneiss. Mainly rough fracture surfaces with no or little fracture filling. Average block edge length > 2-m. Three or fewer fracture sets. |
II | 4 < Q ≤ 10 | Good | Large or medium blocky granite, gneiss-granite, pegmatite or moderate slaty gneiss. Mainly rough fracture surface with little fracture filling. Average block edge length 0.6–2-m. Three or more fracture sets. |
III | 1 < Q ≤ 4 | Fair | Medium to small blocky granite, gneiss-granite pegmatite or slaty gneiss. Fracture surfaces are rough to smooth, with moderate fracture filling. Average block edge length of 0.2–0.6-m. |
IV | 0.1 < Q ≤ 1 | Poor | Small blocky to crushed, metamorphic granitic rock mass or heavily slated gneiss with mineral-filled fractures. Average block edge length < 0.2-m. |
V | Q ≤ 0.1 | Very poor | Tectonically heavily affected, disjointed rock mass, fracture and crush zones. Mainly smooth, polished fracture surfaces filled with large amounts of soft minerals. |
Rock class | Rock quality | Bolt | Sprayed concrete thickness (mm) | Decision year | ||||
---|---|---|---|---|---|---|---|---|
Spacing (m) | Length (m) | |||||||
Roof/haunch | Wall | Roof/haunch | Wall | Roof/haunch | Wall | |||
I | Q > 10 | Selective (1.5) | Selective (1.5) | 4 | 4 | 50 | 0 | 2014 |
II | 4 < Q ≤ 10 | 2.0 | Selective (1.5) | 4 | 4 | 50 | 0 | 2014 |
III | 1 < Q ≤ 4 | 1.7 | 1.7 | 4 | 4 | 75 | 50 | 2014 |
IV | 0.1 < Q ≤ 1 | 1.5 | 1.5 | 4 | 4 | 75 | 50 | 2014 |
V | 0.01 < Q ≤ 0.1 | 1.5 | 1.5 | 5 | 5 | 200 | 200 | 2017 |
VI | Q ≤ 0.01 | Individual assessment | 2017 |
Methodology
Measurement while drilling data
MWD parameter | Penetration rate (m/min) | Percussive pressure (bar) | Feed pressure (bar) | Damping pressure (bar) | Rotation speed (rpm) | Rotation pressure (bar) | Water flow (L/min) | Water pressure (bar) |
---|---|---|---|---|---|---|---|---|
Maximum | 3.89 | 178 | 66.9 | 73.8 | 495 | 80.9 | 207 | 44.9 |
Mean | 1.74 | 147 | 38.8 | 47.2 | 288 | 56.2 | 176 | 22.9 |
Median | 1.73 | 151 | 40.7 | 46.4 | 289 | 56.6 | 177 | 23.4 |
Minimum | 0 | 0 | 0 | 0 | 0 | 4.4 | 0 | 0 |
Tunnel mapping and support requirements
Analysis of combined data
Results and discussion
MWD parameter residuals’ distributions
Actual Q-values’ distributions
Rock support
Numerical comparison between MWD, Q-values and rock support
Multilinear regression
Mean: R | West wall | West haunch | West roof | East roof | East haunch | East wall |
---|---|---|---|---|---|---|
RQD–MWD | 0.22 | - | 0.32 | 0.14 | 0.26 | 0.00 |
Jn–MWD | - | - | - | - | - | - |
RQD/Jn–MWD | 0.26 | 0.32 | 0.44 | 0.41 | 0.24 | 0.30 |
Qbase-value–MWD | 0.30 | 0.42 | 0.51 | 0.51 | 0.32 | 0.35 |
Q-value–MWD | 0.32 | 0.42 | 0.50 | 0.50 | 0.36 | 0.35 |
Sprayed concrete thickness–MWD | 0.26 | - | - | - | - | - |
Bolt spacing–MWD | 0.36 | - | 0.36 | 0.14 | 0.32 | 0.33 |
-= No correlation found with MLR model |
Levenberg-Marquardt method
Holistic visual interpretation
Normalized MWD data versus RQD and Qbase-value
Normalized MWD data versus geotechnical mapping
Fracture area | Section mapped | Section MWD | Description |
---|---|---|---|
I | 11708–11716 | 11707–11715 | Large area, across the tunnel, seen at same sections in mapping and MWD data |
II | 11716–11722- | 11715–11722 | Large area, west side of the tunnel, seen at same sections in mapping and MWD data, the continuation of area I |
IIIa | 11722–11730 | 11726–11733 | Large area, west-east across the tunnel, west side of the tunnel, IIIa seen at same sections in mapping and MWD data, a continuation of areas I and II |
IIIb | 11724–11732 | 11732–11741 | Large area, west-east across the tunnel at 70° dip, east side of the tunnel, IIIb as seen 5-m–10-m further north in the MWD data, on the edge of the grouting fan, weak rock mass observed by geologist and contractor, the continuation of areas I and II |
IV | 11743–11746 | 11740–11747 | Medium area, at the centre of tunnel roof, seen at same sections in mapping and MWD data, the continuation of areas I–III |
V | 11765–11772 | 11760–11767 | Small area, at the east side of the tunnel, seen at same sections in mapping and MWD data |
VI | 11774–11779 | 11777–11778 | Small area, at the centre and east side of the tunnel, seen at same sections in mapping and MWD data |
Section | Rock mass condition based on MWD | MWD factor index | Mapped Qbase | Bolt spacing (m) | Sprayed concrete thickness (mm) |
---|---|---|---|---|---|
11700–11707 | Fair rock mass quality | 55–120 | 1.3 | 1.5 | 100 |
11707–11716 | Weaker, more fractured rock mass, but increased Qbase | 100–150 | 1.8 | 1.25 | 200 |
11716–11721 | Improved quality, especially east side | 50–100 (east) 100–160 (west) | 2.4 | 1.4 (west), 1.5 (east) | 100 (east), 150 (west) |
11721–11730 | Improved rock mass quality in the centre of the roof | 60–100 (east) 80–140 (west) | 0.8 | 1.25 | 150 |
11730–11742 | Fractures across tunnel roof | 100–170 | 3.9 | 2.0 | 100 |
11742–11780 | Small fracture areas, fairly homogeneous MWD fracture index | 60–120 | 8.9 | 2.0 | 75 |
Concluding remarks
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The penetration rate and rotation pressure residuals at Tunnel 201 in the Stockholm bypass have a semi-normal distribution;
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The data collected for the Q-system and the rock support to be installed show an incremental normal distribution because the data were recorded in increments;
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The multilinear regression showed a moderate to a strong correlation between the mean MWD values and the Q-parameters;
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The multilinear regression showed no correlation between the residual MWD values and the Q-parameters;
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The Levenberg-Marquardt method demonstrated the predictive capability of MWD residuals for the Q-system parameters and the rock support in tunnelling, albeit with low precision;
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The holistic visual approach showed good results; the normalized MWD parameters and their indices have a good resemblance with the RQD and the Qbase-value;
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The holistic visual approach using the MWD combined normalized penetration rate and rotation pressure (fracture index) can reliably distinguish more of the fractured areas observed in the geotechnical mapping;
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The holistic visual approach showed MWD data can be applied to predict rock support requirements in tunnelling because of the correlation of the fracture index with the installed rock support;
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The holistic approach using MWD data should be a tool incorporated in the observational method for rock support assessment.