Design of a NATM Tunnel for Mission Valley Light Rail - East Extension

3

Chapter 1

DESIGN OF A NATM TUNNEL FOR MISSION VALLEY LIGHT RAIL—EAST EXTENSION

David Powell

Mott MacDonald

David Field

Hatch Mott MacDonald

Richard Hulsen

Mott MacDonald

ABSTRACT

The Mission Valley Light Rail will loop through San Diego State University (SDSU)and to minimize the impact of construction most of the works will be underground. Theproject will require construction of a 330-m long (1080-ft), 11.24-m wide (36.9-ft),single twin track tunnel through Stadium Conglomerate, a poorly cemented densesandy gravel with cobbles. Key technical issues are discussed including the groundconditions and predicted ground response, the use of sequential excavation methodsto control deformations and specific construction concerns such as instrumentationand monitoring. Management of the construction process to minimize risk isparticularly important in this environment and the strategy selected to control theworks is also considered.

PROJECT DESCRIPTION

The original method of construction planned for the Mission Valley Light RailTunnel section was a Tunnel Boring Machine (TBM) and a precast concrete segmentallining installed behind the cutter head. Changes to the alignment to significantlyreduce the impact on University buildings also substantially reduced the length of thetunnel section, Figure 1, and led to a re-evaluation of the proposed constructionmethod.

The revised alignment transitions from cut and cover construction into minedtunnel at Scripps Terrace and passes underneath the steep escarpment adjacent tothe Women’s gym with a maximum of 16.0 m (52.5 ft) cover, Figure 2. Due to aconstant grade of 4.2% towards the Station, the amount of cover along the tunnelgradually reduces from this maximum and under campus athletic facilities andimportant utilities at Campanile Drive is as low as 6.0 m (19.7 ft).

The ground conditions and the reduced mined tunnel length, much of which liesabove the water table, supported the use of sequential excavation methods andsupport techniques commonly used in the New Austrian Tunneling Method (NATM).

4 2001 RETC PROCEEDINGS

This approach allowed a switch from single track twin bores to a single bore twin trackconfiguration, Figure 3. Cost savings were realized with this change and, despite theincrease in span to 11.24 m (36.87 ft), the NATM approach when applied withappropriate risk management was expected to be effective in controlling theconstruction risks and impact on the University facilities.

GROUND CONDITIONS AND EXPECTED GROUND BEHAVIOUR

The entire length of the NATM tunnel will be mined in a lightly cemented densesandy cobble conglomerate that forms part of the Stadium Conglomerate Formation.Although extensive information is available on the geological setting around the SanDiego State University Campus, there is only limited experience of tunneling in theStadium Conglomerate formation. It was important therefore to carefully assess theworst credible ground conditions likely to be encountered during excavation.

Subsurface Hazards

While the Stadium Conglomerate is generally uniform in composition, due to thehigh energy environment during deposition there are sufficient variations laterally andwith depth to give rise to some local concerns during tunneling. These include thepresence of large boulders (or clasts), layering with discrete contact surfaces andgrading variations and the presence of groundwater conditions above the crown forpart of the tunnel length. A discussion of these hazards follows.

Figure 1. Alignment

DE

SIG

N O

F A

NA

TM

TU

NN

EL

5

Figure 2. Profile and geology


Clasts.

The Stadium Conglomerate Formation contains a significant number ofclasts that will be encountered during construction. These are usually known as“Poway Clasts” and are generally composed either of metavolcanic or quartziticmaterials with an average unconfined compressive strength of 200 MPa (29,000 psi).Table 1 indicates the likely quantity and grading of the clasts.

Cohesionless Material.

During tunnel excavation layers or lenses of poorlygraded sand dominated matrix lenses up to 600 mm (23.6 inches) in thickness arepredicted to occur within the Stadium Conglomerate. Although most of these are likelyto occur above the tunnel crown, some will be present in the crown or sidewalls of thetunnel. If encountered below the groundwater table, these layers could lead to fastraveling and/or free flowing conditions that locally represent a serious tunnelinghazard.

Groundwater Table.

The initial section of the bored tunnel will lie just below thewater table, Figure 2. Observations from large diameter boreholes designed to allowinspection of the layering at depth indicated seepage at the contact of layers, andparticularly at the base of poorly graded sand layers. Between these layers seepagewas not observed and the overall interpretation from pumping tests suggested that thezones of saturation represented semi-confined aquifers. The principal concern is thepossibility that these saturated layers have the potential to create fast raveling and/or

Figure 3. Typical cross section

Table 1. Clast distribution in Stadium Conglomerate

Clast Size Range(mm)

Baseline AverageCumulative Clast Volume(% of excavated material)

Number of Clast/m

3

of Clast Material

76 to 150 12% 2,000

150 to 300 8% 230

> 300 1% 95

DESIGN OF A NATM TUNNEL 7

running ground conditions during excavation. The overall horizontal saturated hydraulicconductivity is considered to range from 2x10

–6

to 2x10

–5

cm/s.

Ground Behavior.

In order to provide some indication of the likely groundbehavior, use has been made of observations from local cuts for foundations, test pitswith horizontal enlargements to monitor and assess raveling and Terzaghi’sclassification (Terzaghi 1950) for soils for tunneling. In general, the StadiumConglomerate falls into the category of firm to slow raveling when above thegroundwater table. Below the groundwater table it will behave in a similar manner inthe short term but is expected to degrade to fast raveling within a period of a few hours.

In terms of ground response during construction the material is extremely stiff andthe design of the primary linings is based on the ground having sufficient stand-up timefor advances of up to 1.0 m to be excavated safely. The test pit excavations and theenlargements indicated that this was likely to be several hours and adequate to allowan initial layer of shotcrete to be applied to maintain excavation stability.

CONSTRUCTION SEQUENCE AND SUPPORT TYPES

The use of the NATM in soft ground such as Stadium Conglomerate is relativelyrecent in North America although it has been used successfully in rock tunnels sincethe 1970s, e.g., Washington Metro and L.A. Metro. It is also proposed for severalcurrent high profile projects such as Seattle’s Link Light Rail and Devil’s Slide RoadTunnel in California.

While several notable and widely reported collapses have occurred on projectswhere the NATM has been used, a careful examination of the problems suggests thateither the application was inappropriate for the ground conditions or the managementand control procedures employed during construction were less than adequate (Health& Safety Executive 2000). Ensuring that the design intent is not compromised isespecially critical for shallow tunnels in soft ground in high risk urban areas where tightexcavation and support sequences are required to control settlements.

Based on the predicted ground conditions, and precedent practice elsewhere insimilar tunnels, e.g., Taiwan High Speed Rail system, three excavation and supporttypes were selected. The expected distribution of the types along the tunnel length wasrelated to key constraints and issues such as:

�

Variations in the amount of cover above the crown

�

The likely stand-up time and other relevant characteristics such as the fre-quency of boulders, poorly graded sand layers and the groundwater table

�

The location of surface structures and utilities

�

Estimated volume losses and predicted settlements

�

The type of equipment used for this type of construction

�

Interfaces with the cut and cover and SDSU station.

Support Type 1 (Figure 4)

From Station 4+591 to 4+745 the tunnel crown will be above the water table andfirm to slow raveling conditions are predicted. The sequence will use a split top headingalthough a full top heading can be completed before the invert of the primary lining isconstructed. Full closure of the lining is achieved within 4.0 m (13.1 ft) of the face.



The phreatic surface is present above the tunnel crown from Scripps Terrace portalto Station 4+591. Where poorly graded sand-dominated matrix lenses and layers arepresent this could lead to either fast raveling or flowing ground conditions. To reduce thelevel of risk associated with this behavior, a side gallery and enlargement sequence hasbeen specified to reduce the span in each of the top headings. With the possibility offlowing conditions it is safer to fully support one sidewall of the tunnel before enlargingto the full section. In addition, routine probe holes with slotted liners have beenspecified to drain the ground ahead of the face and reduce hydrostatic pressures. Fullclosure of the lining is achieved within 4.0–8.0 m (13.1–26.2 ft) of the face.


At Campanile Drive from Station 4+745 to 4+799 the cover to the tunnel will beless than 6.0 m (19.5 ft) and it will underpass a number of utilities. The clearancebetween the utilities and the crown of the tunnel is less than 4.0 m (13.1 ft) in somecases. In order to maintain a suitable cover to span ratio of at least 1.0 for each stageof the excavation, a twin side gallery and central pillar arrangement has been selected

Figure 4. Support type 1

Figure 5. Construction sequence 2


to tightly control surface settlements and any potential impact on the utilities. Fullclosure of the lining is related to the pillar removal sequence.

ADDITIONAL SUPPORT MEASURES

In addition to the sequences and support requirements for each of the supporttypes, additional support measures have been specified. It is recognized from the siteinvestigation information that local factors such as clasts and poorly graded sandlayers or lenses could influence the stability of the unsupported excavation at the face.The range of measures available to the contractor, if required, include:

Temporary Face Support.

A temporary shotcrete layer to support the face of theadvancing excavation. This is a prudent safety measure that protects the workforce bycontrolling any potential for ground loss due to boulders or running ground as well aspreventing the face from drying out.

At various stages of the work, stoppages may occur that result in the face standingfor longer periods than normally experienced during construction. To minimize the riskof instability, the bench/invert will be completed up to the face and the tunnel facedomed and supported by a temporary headwall. This same procedure will beimplemented for a change in the construction sequence.

Spiling and Canopy Tubes.

Around 15% of the lenses/layers of poorly gradedmaterial are predicted to be greater than 600 mm (23.6 inches) in thickness. Whilstthese are not expected within the tunnel excavation, they could be as much as 2.4 m(7.9 ft) in thickness and would represent a stability concern in relation to the face andthe adjacent shotcrete lining. If local instability develops, the layers will requiretemporary support to the heading in advance of the excavation and either spiling orcanopy tubes have been specified for this purpose.

Probe Drilling.

Probe holes drilled ahead of the face have been specified for theentire length of tunnel to relieve any potential hydrostatic pressure ahead of the face.Installation of these holes will be implemented after logging of the face features anddetection of the potential for hydrostatic pressures. Pressure relief holes may also berequired in the temporary shotcrete face support to prevent local instability.

Infill Shotcrete.

Although clasts of up to 600 mm (23.6 inches) have been predictedfor the entire length of the tunnel, the overall percentage encountered during excavation isexpected to be small, as highlighted in Table 1. These clasts may be partially exposed inthe excavated surface of the tunnel and remain stable even after excavation withmechanical methods. The integrity of the primary lining is unlikely to be affected by clasts

Figure 6. Construction sequence 3


intruding up to 100 mm (4 inches) providing that they do not impede the placement oflattice girders. Where clasts exceed this dimension or are not sufficiently stable they will beremoved and the excavation made good with infill shotcrete.

DESIGN APPROACH AND METHODOLOGY

The design of the primary and secondary linings is based on the StadiumConglomerate having sufficient stand-up time for advances of up to 1.0 m (3.3 ft) to beexcavated and supported safely. The excavation sequences specified, Figures 4, 5 and6, are designed to control strains in the ground so that as much as possible of theground load bearing capacity is used and the strains are maintained at levels thatprevent yielding.

Numerical methods of analysis (FLAC Version 3.4) were used to model thedifferent excavation stages and predict the performance of the linings in terms of thestresses and corresponding deformations. Three sections along the tunnel route werechosen to represent fully the range of ground conditions that could be encounteredduring construction, particularly the maximum potential hydrostatic pressures and themaximum and minimum ground cover to the exterior of the tunnel primary lining.

The input parameters for the analyses to model ground and lining behavior aresummarized in Table 2. Throughout the mesh a dry unit weight

γ

d of 17.65 kN/m

3

, aporosity of 0.286 and a tensile strength of 0 MPa were adopted. In addition, the initialtangent modulus assumed small strain stiffness with a non-linear reduction as thestress levels changed and strains increased. The Duncan and Chang (1970) methodwas used to represent the non-linear behavior. To verify the results of the plate loadingtests carried out during the site investigations, the tests were modeled with FLAC toderive the stiffness values indicated in Table 2.

Each analysis used drained parameters and the estimated non-linear stress-strainground response was derived from the plate loading tests and from review of thedown-hole geophysical testing. In addition, a range of Ko values (0.5–1.0) was appliedto check the sensitivity of the headings and primary lining to variations of the in situstresses.

The following steps typically were used in the analyses to model the constructionprocess:

�

Excavation of the ground

�

Establish steady state flow conditions

�

Relaxation of the ground to represent deformations in advance of support

�

Installation of the primary lining

Table 2. Soil parameters adopted for the analyses

Level(mOD)

Effective Internal Angle of Friction

φ

' (°)

Effective Cohesion Intercept c' (kPa)

Angle of Dilation

ψ

(°)

Initial Elastic Tangent ModulusEut (MPa)

Poison Ratio

ν

G1–131.4 40 5 6 400 0.35

131.4–126.4 42 15 8 800 0.35

126.4–121.4 44 15 10 1600 0.35

121.4–base 44 10 10 2400 0.35


�

Analysis of the model and solving to equilibrium

�

Installation of the secondary lining

�

Analysis of the model and solving to equilibrium.

The results of the analyses were checked in accordance with ACI–318 to ensurethat the design complied with Ultimate Limit State and Serviceability Limit Staterequirements. In addition to static loadings derived from the construction process, theperformance of the secondary lining was checked for compliance with the dynamicloading predicted for the ODE and MDE.

MANAGEMENT OF THE CONSTRUCTION PROCESS

Tunneling is not a deterministic process and therefore carries higher risks than mostother forms of civil construction. A recent and important development in the tunnelingindustry has been the introduction of management systems that ensure the safety ofpersonnel and the public as well as exercising control over costs and schedule.

Recent high profile collapses where the NATM has been used, for exampleHeathrow Express, highlighted the importance of implementing management proceduresthroughout the project process. This failure was strongly related to poor risk management,especially quality control in forming construction joints, and the main conclusion from thelengthy proceedings instituted by the UK’s Health and Safety Executive was that effectivesite supervision is essential. This served to emphasize one of the main concerns of softground NATM tunneling, best practice has to include provision for the designer to activelyfollow through into the construction phase. This not only provides an owner withreassurance that any necessary adjustments can be made before problems arise, it alsopromotes the development of management and organizational systems that ensure keydecisions on issues such as changes to the excavation sequence or support aremonitored progressively and kept under constant review.

Management Approach

The owner, the Metropolitan Transportation Development Board (MTDB) of SanDiego, has been proactive in requesting an Engineer’s Design Representative (EDR)to maintain continuity of the design through the construction phase. This is animportant role and management systems and procedures have been implemented inthe contract documents to allow the designer to form part of the supervision team.What is important in this process is avoidance of the more traditional confrontationalrelationship between the construction supervisor and contractor and promotion of asingle team approach with the very clear objective of safe construction.

Such an approach is important where the Designer is not the ConstructionManager, as is typical in North American contract practice. There will always benumerous assumptions built into any tunnel designs that are not always easy toconvey or obvious to the Construction Manager without the facility of regularcommunications and discussions of key issues. This type of approach, often viewed as“Partnering”, worked well on the LA Metro and will always be successful where there isa willingness for all parties to work together even though the risks and contractualarrangements are not always fully compatible with this objective.

A single team approach, where key engineering decisions are made throughassessment and evaluation of information on performance on a routine basis duringconstruction has demonstrated clear benefits in terms of reducing risks related tosafety and quality control. The single team approach does not, however, mean thatlines of responsibility are blurred as each organization is required to appoint


experienced staff to understand the engineering as well as contractual risks. In thiscontext, the EDR will represent the Designer’s interests and has as a principal role inthe interpretation of the Contract Documents for the Construction Management andContractor’s Teams. Experience of similar type of construction and the responseexpected during the progress of the works also provides the client with thereassurance that the project can be completed on time and budget.

Design Risk Management

The input parameters and assumptions used in the design have been derived fromthe Geotechnical Interpretative and Baseline Reports (GIR & GBR). The site investigationdata indicates reasonably uniform ground conditions along the alignment although there islayering, poorly graded bands, boulders etc., that could influence the ground responselocally. Even with designs based on conservative assumptions, there are always factors inthis type of ground that are difficult to assess accurately prior to construction, such as:

�

How certain is it that the Site Investigation has identified the worst credible conditions?

�

Are there unforeseen local planes of weakness or pockets of wet poorly consol-idated ground that could affect installation of support?

�

Are the predicted deformations ahead of the face realistic for the support types?

�

Are the predicted volume losses realistic?

�

Although the ground conditions suggest that the face will be stable are there local factors that could affect safety?

The excavation and support sequences have been selected to meet such risks andthe additional support measures are designed to provide sufficient flexibility duringconstruction for the contractor to respond to the actual conditions encountered. However,verification of the design performance during the early stages of construction is essentialand instrumentation and monitoring requirements are directed to achieve this.

The assumptions made at the design stage will be updated progressively duringconstruction through the EDR role. Support types will be verified and sequences adjustedas necessary to suit the actual ground conditions encountered. Notwithstanding thepossibility of unforeseen conditions, having developed the design on the basis of lowerbound conditions, the verification process should be viewed positively with the aim ofbenefiting the client in terms of cost and program while also ensuring that the contractor isproperly compensated.

Construction Management Procedures

Control of the construction process is related to the predicted and actualperformance of the excavations. Three principal activities are required to ensure thatthe control is effective:

�

The use of Key Performance Indicators (KPI’s)

�

The implementation of daily site meetings to review the KPI’s

�

The facility of emergency Review Meetings should KPI’s exceed trigger and limit values.

Most designers dislike defining KPI’s because structurally tunnel design is not adeterministic process. However, on projects in the UK such as Heathrow Express andthe Jubilee Line Extension for London Underground Limited these worked well andthey are compatible with the aims of the GBR. For the Mission Valley project lining andground deformations will be monitored and trigger and limit values will be specified for:


�

In-tunnel deformation arrays

�

Borehole extensometers and inclinometers

�

Surface settlement monitoring points

�

Piezometers

In terms of procedures, the trigger and limit values are compatible with theServiceability and Ultimate Limit State for the primary and secondary linings.

The daily site meetings will review the instrumentation and monitoring results andlook at related issues such as the excavation sequences, additional support measuresand quality control. The KPI’s will also be viewed in terms of the trends and theresponse by the site team if trigger values are exceeded will take a balanced view ofboth the actual values and the trends. If there are real concerns in terms of the trends,the contract empowers the CM to convene emergency review meetings that willconsist of the EDR, CM and Contractor’s representatives.

The instrumentation array for in-tunnel deformations, and the trigger and limitvalues and the deformation trends defined for such as array are presented in Table 3.

Quality Control Procedures

The need for quality control throughout the construction process and, mostimportantly, during the installation of the initial support is fundamental. To achieve thespecified shotcrete performance, Table 4, will require clear guidance on the mix designrequirements, site trials to prove the mix design and application processes, andqualifications of key staff.

Specific mix design issues will include pumpability, workability and cohesiveness.Additives and admixtures, for example silica fume, have proven benefits in achievingthese requirements without affecting either the early and long-term strength or

Table 3. Frequency of Key Performance Indicator (KPI) monitoring

Key Performance Indicator Frequency of Reading

Distance from Face Frequency

In-Tunnel Deformation

0 m to +30 m Daily

+30 m to +60 m Twice Weekly

> +60 m Weekly

Inclinometers & Extensometer

–30 m to –15 m Twice Weekly

–15 m to 0 m Daily

0 m to +30 m Daily

+30 m to +60 m Weekly

> +60 m Weekly

Settlement Monitoring Points


0 m to +30 m Daily

+30 m to +60 m Twice Weekly

> +60 m Weekly

Piezometers

General Weekly


0 m to +30 m Daily


durability characteristics. At 10% by weight of cementitious material, silica fume, hasbeen specified to promote:

�

Increased mix cohesiveness and increased bond to substrate

�

Reduced rebound

�

Increased shotcrete strengths

�

Reduced permeability and hence increased durability

Comprehensive site trials of both the mix and applicators using the equipmentspecified for the tunnel works will be required to provide assurance that the specifiedmix design is workable and its short and long term strength characteristics areachievable. A series of test panels, both vertical and overhead, and laboratory and sitetesting will be used to satisfy performance requirements.

Shotcrete performance will be thoroughly monitored throughout the tunnelconstruction. Recent failures, as mentioned previously, have clearly demonstrated theneed for strict inspection and quality control procedures. The intention is that the EDRwill assist the Construction Manager to strengthen the management team to ensurethat the contract requirements are met.

The use of shotcrete in an underground environment calls for experiencedshotcrete applicators capable of placing well compacted, void free shotcrete. The needfor experienced and qualified shotcrete applicators has been recognized in NorthAmerica with the introduction of the ACI 506.3R—Guide to Certification of ShotcreteNozzlemen. This will be replaced shortly by ACI C660—Certification of ShotcreteNozzlemen. Moreover, the introduction of the American Shotcrete AssociationNozzlemen Certification Program in 1999 will benefit projects such as Mission Valleyby setting training standards and creating a pool of trained applicators for the industry.

REFERENCES

Terzaghi, K., (1950). Geologic Aspects of Soft Ground Tunneling, Chapter 2. AppliedSedimentation. P.D.Trask (ed.), John Wiley and Sons, New York.

Health & Safety Executive, (2000). The collapse of NATM tunnels at Heathrow Airport.HSE Books.

ACKNOWLEDGMENTS

The authors wish to thank Dave Ragland of MTDB for his constructive commentsand encouragement in preparation of the paper. Siegfried Fassman of BRW, theProject Manager for detailed design of the SDSU Loop, and John Hawley, Hatch MottMacDonald’s engineer-in-charge for the NATM section, have provided valuable adviceand assistance in developing the bored tunnel design and their guidance and inputsduring the design process are gratefully acknowledged.

Table 4. Specified shotcrete strength development

Age of Specimen Design Strength

8 hours 4 MPa (580 psi)



28 Days 25 MPa (3750 psi)