DESIGN OF AN NATM TUNNEL FOR MISSION VALLEY LIGHT RAIL - EAST EXTENSION
David Powell PhD CEng MIMM  Mott MacDonald
David Field MEng MSOA CEng MICE  Hatch Mott MacDonald
Richard Hulsen MEng CEng MICE  Mott MacDonald
DESIGN OF AN NATM TUNNEL FOR MISSION VALLEY LIGHT RAIL - EAST EXTENSION
David Powell, David Field, Richard Hulsen
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. The project will require construction of a
330m long (1080 feet), 11.24m wide (36.9 feet), single twin track tunnel through Stadium Conglomerate,
a poorly cemented dense sandy gravel with cobbles. Key technical issues are discussed including the
ground conditions and predicted ground response, the use of sequential excavation methods to control
deformations and specific construction concerns such as instrumentation and monitoring. Management of
the construction process to minimize risk is particularly important in this environment and the strategy
selected to control the works is also considered.
PROJECT DESCRIPTION
The original method of construction planned for the Mission Valley Light Rail Tunnel section was a
Tunnel Boring Machine (TBM) and a precast concrete segmental lining installed behind the cutter head.
Changes to the alignment to significantly reduce the impact on University buildings also substantially
reduced the length of the tunnel section, Figure 1, and led to a re-evaluation of the proposed construction
method.
The revised alignment transitions from cut and cover construction into mined tunnel at Scripps Terrace
and passes underneath the steep escarpment adjacent to the Women s gym with a maximum of 16.0m
(52.5 feet) cover, Figure 2. Due to a constant grade of 4.2% towards the Station, the amount of cover
along the tunnel gradually reduces from this maximum and under campus athletic facilities and important
utilities at Campanile Drive is as low as 6.0m (19.7 feet).
The ground conditions and the reduced mined tunnel length, much of which lies above the water table,
supported the use of sequential excavation methods and support techniques commonly used in the New
Austrian Tunneling Method (NATM). This approach allowed a switch from single track twin bores to a
single bore twin track configuration, Figure 3. Cost savings were realized with this change and, despite
the increase in span to 11.24m (36.87ft), the NATM approach when applied with appropriate risk
management was expected to be effective in controlling the construction risks and impact on the
University facilities.
Figure 1  Alignment
Figure 2 - Profile & Geology
Figure 3 - Typical Cross Section
GROUND CONDITIONS AND EXPECTED GROUND BEHAVIOUR
The entire length of the NATM tunnel will be mined in a lightly cemented dense sandy cobble
conglomerate that forms part of the Stadium Conglomerate Formation. Although extensive information is
available on the geological setting around the San Diego State University Campus, there is only limited
experience of tunneling in the Stadium Conglomerate formation. It was important therefore to carefully
assess the worst credible ground conditions likely to be encountered during excavation.
Subsurface Hazards
While the Stadium Conglomerate is generally uniform in composition, due to the high energy
environment during deposition there are sufficient variations laterally and with depth to give rise to some
local concerns during tunneling. These include the presence of large boulders (or clasts), layering with
discrete contact surfaces and grading variations and the presence of groundwater conditions above the
crown for part of the tunnel length. These are discussed below.
Clasts. The Stadium Conglomerate Formation contains a significant number of clasts that will be
encountered during construction. These are usually known as  Poway Clasts and are generally
composed either of metavolcanic or quartzitic materials with an average unconfined compressive strength
of 200MPa (29,000 psi). The table below indicates the likely quantity and grading of the clasts.
Clast Size Baseline Average Number of Clast/m3
Range Cumulative of Clast Material
(mm) Clast Volume
(% of excavated material)
76 to 150 12% 2,000
150 to 300 8% 230
> 300 1% 95
Table 1 - Clast Distribution in Stadium Conglomerate.
Cohesionless Material. During tunnel excavation layers or lenses of poorly graded sand dominated
matrix lenses up to 600mm (23.6 inches) in thickness are predicted to occur within the Stadium
Conglomerate. Although most of these are likely to occur above the tunnel crown, some will be present
in the crown or sidewalls of the tunnel. If encountered below the groundwater table, these layers could
lead to fast raveling and/or free flowing conditions that locally represent a serious tunneling hazard.
Groundwater Table. The initial section of the bored tunnel will lie just below the water table, Figure 2.
Observations from large diameter boreholes designed to allow inspection of the layering at depth
indicated seepage at the contact of layers, and particularly at the base of poorly graded sand layers.
Between these layers seepage was not observed and the overall interpretation from pumping tests
suggested that the zones of saturation represented semi-confined aquifers. The principal concern is the
possibility that these saturated layers have the potential to create fast raveling and/or running ground
conditions during excavation. The overall horizontal saturated hydraulic conductivity is considered to
range from 2x10-6 to 2x10-5 cm/s.
Ground Behavior. In order to provide some indication of the likely ground behavior, use has been made
of observations from local cuts for foundations, test pits with horizontal enlargements to monitor and
assess raveling and Terzaghi s classification (Terzaghi 1950) for soils for tunneling. In general, the
Stadium Conglomerate falls into the category of firm to slow raveling when above the groundwater table.
Below the groundwater table it will behave in a similar manner in the 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 and the design of the
primary linings is based on the ground having sufficient stand-up time for advances of up to 1.0m to be
excavated safely. The test pit excavations and the enlargements indicated that this was likely to be
several hours and adequate to allow an 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 relatively recent in North
America although it has been used successfully in rock tunnels since the 1970 s, e.g. Washington Metro
and L.A. Metro. It is also proposed for several current high profile projects such as Seattle s Link Light
Rail and Devil s Slide Road Tunnel in California.
While several notable and widely reported collapses have occurred on projects where the NATM has
been used, a careful examination of the problems suggests that either the application was inappropriate
for the ground conditions or the management and control procedures employed during construction were
less than adequate (Health & Safety Executive 2000). Ensuring that the design intent is not compromised
is especially critical for shallow tunnels in soft ground in high risk urban areas where tight excavation
and support sequences are required to control settlements.
Based on the predicted ground conditions, and precedent practice elsewhere in similar tunnels, e.g.
Taiwan High Speed Rail system, three excavation and support types were selected. The expected
distribution of the types along the tunnel length was related 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 frequency 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
From Station 4+591 to 4+745 the tunnel crown will be above the water table and firm to slow raveling
conditions are predicted. The sequence will use a split top heading although a full top heading can be
completed before the invert of the primary lining is constructed. Full closure of the lining is achieved
within 4.0 m (13.1 feet) of the face.
Figure 4 - Support Type 1
Support Type 2
The phreatic surface is present above the tunnel crown from Scripps Terrace portal to Station 4+591.
Where poorly graded sand-dominated matrix lenses and layers are present this could lead to either fast
raveling or flowing ground conditions. To reduce the level of risk associated with this behavior, a side
gallery and enlargement sequence has been specified to reduce the span in each of the top headings. With
the possibility of flowing conditions it is safer to fully support one sidewall of the tunnel before enlarging
to the full section. In addition, routine probe holes with slotted liners have been specified to drain the
ground ahead of the face and reduce hydrostatic pressures. Full closure of the lining is achieved within
4.0-8.0 m (13.1 - 26.2 feet) of the face.
Figure 5 - Construction Sequence 2
Support Type 3
At Campanile Drive from Station 4+745 to 4+799 the cover to the tunnel will be less than 6.0 m (19.5
feet) and it will underpass a number of utilities. The clearance between the utilities and the crown of the
tunnel is less than 4.0 m (13.1 feet) in some cases. In order to maintain a suitable cover to span ratio of
at least 1.0 for each stage of the excavation, a twin side gallery and central pillar arrangement has been
selected to tightly control surface settlements and any potential impact on the utilities. Full closure of the
lining is related to the pillar removal sequence.
Figure 6 - Construction Sequence 3
Additional Support Measures
In addition to the sequences and support requirements for each of the support types, additional support
measures have been specified. It is recognized from the site investigation information that local factors
such as clasts and poorly graded sand layers 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 the advancing excavation.
This is a prudent safety measure that protects the workforce by controlling any potential for ground loss
due to boulders or running ground as well as preventing the face from drying out.
At various stages of the work, stoppages may occur that result in the face standing for longer periods than
normally experienced during construction. To minimize the risk of instability, the bench/invert will be
completed up to the face and the tunnel face domed and supported by a temporary headwall. This same
procedure will be implemented for a change in the construction sequence.
Spiling and Canopy Tubes. Around 15% of the lenses/layers of poorly graded material are predicted to
be greater than 600 mm (23.6 inches) in thickness. Whilst these are not expected within the tunnel
excavation, they could be as much as 2.4 m (7.9 feet) in thickness and would represent a stability concern
in relation to the face and the adjacent shotcrete lining. If local instability develops, the layers will
require temporary support to the heading in advance of the excavation and either spiling or canopy tubes
have been specified for this purpose.
Probe Drilling. Probe holes drilled ahead of the face have been specified for the entire 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 and detection of the potential for hydrostatic pressures.
Pressure relief holes may also be required in the temporary shotcrete face support to prevent local
instability.
Infill Shotcrete. Although clasts of up to 600 mm (23.6 inches) have been predicted for the entire length
of the tunnel, the overall percentage encountered during excavation is expected to be small, as
highlighted in Table 1. These clasts may be partially exposed in the excavated surface of the tunnel and
remain stable even after excavation with mechanical methods. The integrity of the primary lining is
unlikely to be affected by clasts intruding up to 100 mm (4 inches) providing that they do not impede the
placement of lattice girders. Where clasts exceed this dimension or are not sufficiently stable they will be
removed and the excavation made good with infill shotcrete.
DESIGN APPROACH AND METHODOLOGY
The design of the primary and secondary linings is based on the Stadium Conglomerate having sufficient
stand-up time for advances of up to 1.0 m (3.3 feet) to be excavated and supported safely. The excavation
sequences specified, Figures 4, 5 and 6, are designed to control strains in the ground so that as much as
possible of the ground load bearing capacity is used and the strains are maintained at levels that prevent
yielding.
Numerical methods of analysis (FLAC Version 3.4) were used to model the different excavation stages
and predict the performance of the linings in terms of the stresses and corresponding deformations. Three
sections along the tunnel route were chosen to represent fully the range of ground conditions that could
be encountered during construction, particularly the maximum potential hydrostatic pressures and the
maximum and minimum ground cover to the exterior of the tunnel primary lining.
The input parameters for the analyses to model ground and lining behavior are summarized in Table 2.
Throughout the mesh a dry unit weight Å‚d of 17.65 kN/m3, a porosity of 0.286 and a tensile strength of 0
MPa were adopted. In addition, the initial tangent modulus assumed small strain stiffness with a non-
linear reduction as the stress levels changed and strains increased. The Duncan and Chang (1970)
method was used to represent the non-linear behavior. To verify the results of the plate loading tests
carried out during the site investigations, the tests were modeled with FLAC to derive the stiffness values
indicated in Table 2.
Level Effective Effective Angle of Initial Elastic Poison
(mOD) Internal Cohesion Dilation Tangent Ratio
Angle of Intercept È (º) Modulus ½
È ½
È ½
È ½
Friction c' (kPa) Eut (MPa)
Ć' (º)
Ć
Ć
Ć
Gl - 131.4 40 5 6 400 0.35
131.4 - 42 15 8 800 0.35
126.4
126.4 - 44 15 10 1600 0.35
121.4
121.4 - 44 10 10 2400 0.35
base
Table 2 - Soil Parameters adopted for the Analyses
Each analysis used drained parameters and the estimated non-linear stress-strain ground response was
derived from the plate loading tests and from review of the down-hole geophysical testing. In addition, a
range of Ko values (0.5-1.0) was applied to check the sensitivity of the headings and primary lining to
variations of the in situ stresses.
The following steps typically were used in the analyses to model the construction process:
" 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
" 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 ensure that the design complied
with Ultimate Limit State and Serviceability Limit State requirements. In addition to static loadings
derived from the construction process, the performance of the secondary lining was checked for
compliance with the dynamic loading predicted for the ODE and MDE.
MANAGEMENT OF THE CONSTRUCTION PROCESS
Tunneling is not a deterministic process and therefore carries higher risks than most other forms of civil
construction. A recent and important development in the tunneling industry has been the introduction of
management systems that ensure the safety of personnel and the public as well as exercising control over
costs and schedule.
Recent high profile collapses where the NATM has been used, for example Heathrow Express,
highlighted the importance of implementing management procedures throughout the project process. This
failure was strongly related to poor risk management, especially quality control in forming construction
joints, and the main conclusion from the lengthy proceedings instituted by the UK s Health and Safety
Executive was that effective site supervision is essential. This served to emphasize one of the main
concerns of soft ground NATM tunneling, best practice has to include provision for the designer to
actively follow through into the construction phase. This not only provides an owner with reassurance
that any necessary adjustments can be made before problems arise, it also promotes the development of
management and organizational systems that ensure key decisions on issues such as changes to the
excavation sequence or support are monitored progressively and kept under constant review.
Management Approach
The owner, the Metropolitan Transportation Development Board (MTDB) of San Diego, has been
proactive in requesting an Engineer s Design Representative (EDR) to maintain continuity of the design
through the construction phase. This is an important role and management systems and procedures have
been implemented in the 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 confrontational relationship
between the construction supervisor and contractor and promotion of a single team approach with the
very clear objective of safe construction.
Such an approach is important where the Designer is not the Construction Manager, as is typical in North
American contract practice. There will always be numerous assumptions built into any tunnel designs
that are not always easy to convey or obvious to the Construction Manager without the facility of regular
communications 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 is a willingness for all parties to
work together even though the risks and contractual arrangements are not always fully compatible with
this objective.
A single team approach, where key engineering decisions are made through assessment and evaluation of
information on performance on a routine basis during construction has demonstrated clear benefits in
terms of reducing risks related to safety and quality control. The single team approach does not, however,
mean that lines of responsibility are blurred as each organization is required to appoint experienced staff
to understand the engineering as well as contractual risks. In this context, the EDR will represent the
Designer s interests and has as a principal role in the interpretation of the Contract Documents for the
Construction Management and Contractor s Teams. Experience of similar type of construction and the
response expected during the progress of the works also provides the client with the reassurance 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 from the Geotechnical
Interpretative and Baseline Reports (GIR & GBR). The site investigation data indicates reasonably
uniform ground conditions along the alignment although there is layering, poorly graded bands, boulders
etc., that could influence the ground response locally. Even with designs based on conservative
assumptions, there are always factors in this 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 consolidated 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 and the additional support
measures are designed to provide sufficient flexibility during construction for the contractor to respond
to the actual conditions encountered. However, verification of the design performance during the early
stages of construction is essential and instrumentation and monitoring requirements are directed to
achieve this.
The assumptions made at the design stage will be updated progressively during construction through the
EDR role. Support types will be verified and sequences adjusted as necessary to suit the actual ground
conditions encountered. Notwithstanding the possibility of unforeseen conditions, having developed the
design on the basis of lower bound conditions, the verification process should be viewed positively with
the aim of benefiting the client in terms of cost and program while also ensuring that the contractor is
properly compensated.
Construction Management Procedures
Control of the construction process is related to the predicted and actual performance of the excavations.
Three principal activities are required to ensure that the 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 a deterministic process.
However, on projects in the UK such as Heathrow Express and the Jubilee Line Extension for London
Underground Limited these worked well and they are compatible with the aims of the GBR. For the
Mission Valley project lining and ground 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 the Serviceability and Ultimate
Limit State for the primary and secondary linings.
The daily site meetings will review the instrumentation and monitoring results and look at related issues
such as the excavation sequences, additional support measures and quality control. The KPI s will also
be viewed in terms of the trends and the response by the site team if trigger values are exceeded will take
a balanced view of both 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 will consist of the EDR, CM
and Contractor s representatives.
The instrumentation array for in-tunnel deformations, and the trigger and limit values and the
deformation trends defined for such as array are presented in Table 3.
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 -30 m to 0 m Daily
0 m to +30 m Daily
Key Performance Indicator Frequency of Reading
Distance from Face Frequency
+30 m to +60 m Twice Weekly
> +60 m Weekly
Piezometers General Weekly
-15 m to 0 m Daily
0m to +30 m Daily
Table 3 - Frequency of Key Performance Indicator (KPI) Monitoring
Quality Control Procedures
The need for quality control throughout the construction process and, most importantly, during the
installation of the initial support is fundamental. To achieve the specified shotcrete performance, Table 4,
will require clear guidance on the mix design requirements, site trials to prove the mix design and
application processes, and qualifications of key staff.
Age of Specimen Design Strength
8 hours 4 MPa (580 psi)
24 hours 7 MPa (1015 psi)
72 hours 14 MPa (2030 psi)
28 Days 25 MPa (3750 psi)
Table 4 - Specified Shotcrete Strength Development
Specific mix design issues will include pumpability, workability and cohesiveness. Additives and
admixtures, for example silica fume, have proven benefits in achieving these requirements without
affecting either the early and long-term strength or durability characteristics. At 10% by weight of
cementitious material, silica fume, has been 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 equipment specified for the tunnel
works will be required to provide assurance that the specified mix design is workable and its short and
long term strength characteristics are achievable. A series of test panels, both vertical and overhead, and
laboratory and site testing will be used to satisfy performance requirements.
Shotcrete performance will be thoroughly monitored throughout the tunnel construction. Recent failures,
as mentioned previously, have clearly demonstrated the need for strict inspection and quality control
procedures. The intention is that the EDR will assist the Construction Manager to strengthen the
management team to ensure that the contract requirements are met.
The use of shotcrete in an underground environment calls for experienced shotcrete applicators capable
of placing well compacted, void free shotcrete. The need for experienced and qualified shotcrete
applicators has been recognized in North America with the introduction of the ACI 506.3R - Guide to
Certification of Shotcrete Nozzlemen. This will be replaced shortly by ACI C660 - Certification of
Shotcrete Nozzlemen. Moreover, the introduction of the American Shotcrete Association Nozzlemen
Certification Program in 1999 will benefit projects such as Mission Valley by setting training standards
and creating a pool of trained applicators for the industry.
REFERENCES
1. Terzaghi, K., (1950). Geologic Aspects of Soft Ground Tunneling, Chapter 2. Applied Sedimentation.
P.D.Trask (ed.), John Wiley and Sons, New York.
2. Health & Safety Executive, (2000). The collapse of NATM tunnels at Heathrow Airport. HSE Books.
ACKNOWLEDGEMENTS
The authors wish to thank Dave Ragland of MTDB for his constructive comments and encouragement in
preparation of the paper. Siegfried Fassman of BRW, the Project Manager for detailed design of the
SDSU Loop, and John Hawley, Hatch Mott MacDonald s engineer-in-charge for the NATM section,
have provided valuable advice and assistance in developing the bored tunnel design and their guidance
and inputs during the design process are gratefully acknowledged.