Excavation and support of cross passages on Westside 1

Jeff Brandt, Kerwin Hirro, Peter Dietrich, Norbert Fuegenschuh, Mike Cole and Joseph DeMello The Los Angeles County Metropolitan Transportation Authority’s (LA Metro) Purple Line Extension will add nearly 15 km (9 miles) of underground heavy rail to the Los Angeles Metro Rail System. The project is broken up into three different sections and upon their completion will connect the existing Wilshire/Western Station in Koreatown to the Veterans Administration hospital in Santa Monica, CA and include the addition of seven more subway stations. The goal is for all three sections to be fully operational prior to the 2028 Summer Olympics in Los Angeles. LA Metro’s Section 1 of the Purple Line Extension is a $1.6 billion, design-build contract in partnership with a joint venture of Skanska-Traylor-Shea (STS). The Section 1 segment ties into the existing Wilshire/Western station, providing connection to the existing D-Line and the LA Metro Rail System. Section 1 extends the system west 6.27 km (3.9 miles) under the Wilshire Boulevard Corridor, passing directly adjacent to the renowned La Brea Tar Pits. The project includes two 6.7-m (22-ft) diameter tunnels and the construction of three underground subway stations. Along the tunnel alignment, smaller, 4.87-m (16-ft) diameter cross-passage tunnels were included to provide emergency egress and facilitate tunnel-support-system function. GeologySection 1 of the Purple Line Extension was divided into four separate tunneling drives with various soil conditions encountered throughout. The twin tunnel boring machines (TBMs) were launched heading east from the Wilshire/La Brea Station and tunneled 2.9 km (9,570 ft) toward the existing Western/Wilshire Subway Station, terminating at a retrieval shaft built for the reception and disassembly of the TBMs. After completion of Reach 1, the TBMs were disassembled, transported back to the Wilshire/La Brea Station, and reassembled for launch to the west. Reaches 2, 3 and 4 connect La Brea station to Fairfax and La Cienega Stations before termination of the tunnels in Reach 4 approximately 137 m (550 ft) west of the La Cienega Station. The tunnel profile depth of cover ranged between 18.2 and 30 m (60 and 100 ft) from the surface level and was situated within the San Pedro and Fernando Formations, with some sparse zones through the Lakewood Formation.The project’s geotechnical data report (GDR) defined the San Pedro Formation as containing both fine-grained dense sand and silty sands with medium-to-coarse-grained sands, as well as the inclusion of some stiff silts. The San Pedro Formation also contained cobbles and gravelly sand layers as well as shell fragments (Parsons Brinkerhoff, 2014). Core sample recoveries along the alignment were typically incomplete, indicative of low cohesion. Along with a high permeability, fast raveling to running conditions were to be expected. The Fernando Formation typically underlies the San Pedro Formation and contains stiff-to-hard siltstone and claystone, yielding unconfined-compressive-strength (UCS) testing results between 25 and 135 pounds per square inch (psi) and noted stand-up time tests beyond several hours. The Lakewood Formation would be sparsely encountered, characterized by poorly graded sands, silts, clays and some clayey sand layers. Groundwater measurements for the project indicated that all the tunnel and cross-passage profiles would be below the water table, and to expect both perched and semiperched groundwater conditions, as well as the presence of locally confined aquifers (Parsons Brinkerhoff, 2014).Common to the Southern California geological profile, the GDR was clear in anticipating potentially active and inactive faults in the vicinity of the alignment, posing potential difficulties with both hydrocarbons and gassy conditions. The Los Angeles Basin is home to many oil fields, and the Section 1 tunnel alignment passes through both the Salt Lake Oil Field and the South Salt Lake Oil Field according to the California Division of Oil, Gas, and Geothermal Resources’ (CalGEM, 2020) records.The hydrocarbons and other notable petroleum byproducts such as methane or hydrogen sulfide gases, were expected to migrate through any available pore space and along any geological fractures, particularly near the La Brea Tar Pits. In contrast to their gassy state, the hydrocarbons are also characterized along the alignment as zones of asphaltic and tar-impacted sand lenses or discrete pools and seeps of a viscous tar and oil (Parsons, 2019). The large void spaces in the coarse-grained San Pedro Formation led to zones of “asphalt-impacted soils” as well as the zones between Fernando and San Pedro Formation where seeps could most readily travel. The project was initially classified by Cal/OSHA as gassy due to the expected presence of methane, hydrogen sulfides and volatile organic compounds (VOCs) (California Code of Regulations, 2022). As the TBM tunneling encountered levels of gas beyond that in the initial GBR profile, the project was reclassified along Reach 2 and Reach 3 as extremely hazardous. These challenging geological conditions were all paramount in considering the approach for both the design and the execution of cross-passage construction. Cross-passage designAs the Section 1 design-builder, STS was partnered with Parsons to complete the design and construction of the cross passages. The tunnel alignment of approximately 5.47 km (3.4 miles) requires cross passages spaced at intervals between 228 and 243 m (750 and 800 ft), resulting in a total of 23 cross passages, designated as CPB 4 through CPB 26. The cross passages were designed with a typical interior dimension to accommodate the project-defined systems layout and code compliance, ultimately requiring a minimum interior width of 3.2 m and height of 2.9 m (10 ft 6 in. by 9 ft 6 in.). The twin tunnels are typically separated by a pillar of material approximately 5.18 m (17 ft) wide and thus resulted in the average interior space of the cross passages ranging from 4.1 m to 4.9 m (13 ft 6 in. to 16 ft 1 in.) long (Parsons, 2016). The cross passages were initially designated by four different ground classifications that would then require different programs for both excavation and initial support lining of the cross passage; however, as the ground treatment program was further developed, a fifth ground-condition classification was developed. According to the cross-passage design report, the ground classifications for the cross-passage profile are as follows: Ground Class 1 — Profile is within the coarse-grained San Pedro Formation.Ground Class 2 — Profile is at the intersection of both the coarse- and fine-grained San Pedro Formation.Ground Class 3 — Profile is in a mixed face of both the San Pedro and Fernando Formations.Ground Class 4 — Profile is entirely within the Fernando Formation.Ground Class 5 — Profile is within the tar-impacted San Pedro Formation. All locations within the San Pedro formations were designed with the anticipation of localized depressurization and ground treatment that would be required to limit groundwater seepage to less than 50 gal/min. Jet grouting was assumed as the model for limiting the seepage and providing soil stability in all Ground Classes 1, 2 and 3. Initially, the cross passages within the tar-impacted sands, Cross Passage (CPB) 17, CPB 21, CPB 22 and CPB 23, were thought to be subject to ground improvement via jet grout, but the tar-impacted soils were not able to be washed thoroughly for treatment and the addition of Ground Class 5 was necessary. The design for Ground Class 4 assumed that ground improvement was not required, and only local relief wells may be necessary for sufficient gas and water depressurization (Parsons, 2016). Considerations were made at the design phase to move the cross passages into more favorable ground conditions, including even lowering the tunnel alignment in its entirety to better locate cross passages within the preferred Ground Class 4 designation. Ultimately, Reach 1 included CPB 4 through CPB 15; Reach 2, CPB 16 through CPB 21; Reach 3, CPB 22 through CPB 25; and Reach 4, CPB 26.The 23 cross passages were excavated by the sequential excavation method (SEM) after the completion of both TBM drives in any given reach. The SEM method requires in-depth practical experience, extensive geotechnical understanding and engineering as well as skilled execution. To ensure successful excavation of the challenging ground conditions of the Section 1 project, STS enlisted the assistance of BeMo Tunneling GmbH (BeMo) at the early design phase as well as throughout the entire excavation operation, employing its services as the lead SEM engineer for the project. Using BeMo’s unique and exceptional experience in the industry, the excavation of all 23 cross passages was planned to intricate detail including excavation sequencing, ground support measures, instrumentation and monitoring concepts, basic toolbox items, as well as contingent and emergency response procedures. Prior to any excavation, pre-excavation probing was required to verify the ground conditions and determine the groundwater conditions at the location, ensuring that the design parameters for the location were accurate. Any changes in condition were recorded in the probing log and shared with the engineer of record for review. With all information gathered and ground conditions confirmed, prior to any excavation, a general required excavation and support sheet (RESS) was produced by the SEM engineer and reviewed with the tunnel manager, the engineer of record, the surveyor and the owner’s representative tunnel manager to agree on the process of excavation and ground support. The general RESS was documented and used as a standard for a daily RESS sheet to be reviewed and signed off by all parties to document the previous excavation advance’s ground conditions, support systems installed, and any changes that the SEM engineer could require. With this system, the SEM process allows for broad adaptation of different “toolbox” measures to ensure that the excavation is completed safely. Typical to many SEM designs, the cross passages include an initial supportive liner, a high-density polyethylene (HDPE) water- and gas-tight membrane with redundant grouting capabilities, and a final cast-in-place concrete liner. The prescribed initial support for the cross passages on Section 1 was designed based on the system carrying a minimum soil load equivalent to that exerted by a height of soil above the opening or twice the diameter of the opening, whichever is smaller, and hydrostatic pressures (Parsons, 2016). Based on this analysis and the individual cross-passage ground-classification designation, a design was produced for the initial lining support using a steel-fiber-reinforced-shotcrete layer either eight or 10 inches thick with the inclusion of steel-lattice girders sized and spaced as necessary. Ground improvement from the surfaceCross passages with a designation of Ground Class 1, 2, 3 and 5 required some form of ground improvement due to the presence of soft ground or mixed-face conditions under full hydrostatic head (Parsons, 2016). Ground-treatment options that were considered included, but were not limited to, soil-cement mixing, chemical injection, jet grouting, ground freezing and localized depressurization via deep wells. The tunnel alignment runs under Wilshire Boulevard, one of the most trafficked noninterstate roads in Los Angeles, and the long-term street closures required for depressurization or deep-well operations proved to be extremely difficult to obtain, particularly in union with the project’s other traffic closures. A ground improvement program that could operate independent of the TBM schedule and operation was preferred. Further, as cross-passage excavations could be performed as quickly as in a matter of weeks or last a duration of months, a “one-and-done” treatment program that could be deployed, installed and demobilized was required, eliminating the consideration of ground freezing and depressurization at many locations. Through analysis of ground conditions, the amount and pressure of ground water present, restrictions on schedule and applicability of installation from the surface compared to treatment from within the tunnel and not without cost consideration, jet grouting was chosen by STS as the most preferred ground-treatment solution. The San Pedro Formation was readily erodible by the jet-grout method. Additionally, the jet-grouting method allowed the installation of the jet-grout block treatment ahead of the schedule of the advancing TBM tunnels, completely removing the schedule constraint of ground treatment from the cross-passage excavation schedule once the tunnels were built. A specialized jet-grouting subcontractor was selected to install the jet-grout blocks along the alignment. The subcontractor designed a jet-grout program with 3-m (10-ft) columns at 2.4-m (8-ft) spacings on center, providing complete coverage of the entire cross-passage excavation profile. The intent was to create a jet-grout block with minimum dimensions of 12.1 m (40 ft) wide and centered, at a minimum, along the centerline of the twin running tunnels for complete coverage. The jet-grout blocks were designed to extend 3 m (10 ft) above the crown of the cross passage and approximately 2.4 m (8 ft) below the invert of the cross-passage profile. The jet-grout block-design parameters for this project required a minimum compressive strength of 300 psi by UCS testing methods (Parsons, 2016).The subcontractor performed a series of test blocks on the project prior to beginning its jet-grout program with results that yielded acceptable results to the engineer of record (Parsons) in the Ground Classes 1 through 3. The testing in the San Pedro Formation impacted with tar (Ground Class 5), however, was not favorable. It must be noted that the jet-grouting method was shown to be possible in the tar-impacted sands, but the cost and effort to make the block perform as specified was beyond the limits available to STS. Ultimately, 13 cross-passage locations were chosen to have the jet-grout treatment. The remaining four locations in Ground Class 5 would require additional ground improvement efforts to ensure safe excavation. Cross-passage excavationPrior to cross-passage excavation, probe drilling was completed after completion of TBM mining to verify the ground conditions at each cross-passage location. Shut-off valves were mounted to the intrados of the precast concrete tunnel liner and drilled through, retrieving the soil cuttings. The approximate flowrate was measured and relevant photo and video documentation was taken before closing the valve to document pressure. Typically, probe holes were located at the center of the cross-passage profile, as well as two locations to each side near the crown. However, as many of the probing results did not yield enough data to fully consider the condition of the cross-passage ground condition, particularly in the jet-grouted zones, additional probe drilling was performed with up to nine probes from each tunnel being performed to adequately map the ground. At many locations, the probing results indicated to STS that the jet-grouting campaign did not improve the ground conditions to levels that would provide a safe excavation without further ground improvement. STS chose to begin cross-passage excavation at locations within Ground Class 4 to properly train the excavation crews and thoroughly test the equipment and systems required for working in a gassy/extra-hazardous environment. After installation of the necessary propping support cages to allow removal of sections of the precast tunnel liner, excavation began with the top heading. The cross-passage design drawings provided by the engineer of record (Parsons) and the SEM engineer’s RESS typically provided for a top heading excavation followed by the bench excavation. In the Fernando Formation and properly treated ground, the entire excavation and installation of the initial liner could be completed in roughly three weeks, working continuously for two 12-hour shifts, four days per week. However, as soon became the challenge of the project, the remaining 19 cross passages required much more effort and engineering to safely complete. Cross-passage excavation in incomplete jet-grout blocksAfter successfully completing several of the Type 4 cross passages, CPB 14 was next but had to be skipped due to concerning probe-drilling results. CPB 14 was a cross passage treated with jet grout, but the probing results yielded flowing, untreated sands and running water beyond 20 gal/min. While the subcontractor responsible for the jet-grout program and the engineer of record (Parsons) reviewed CPB 14 data and developed a remediation program, STS continued the cross-passage excavation at CPB 15. Immediately upon removing the precast tunnel liner, the Lakewood formation was encountered. Fine sand pockets intermingled with jet-grout columns occupied the crown with stand-up times of less than one hour. However, when applying shotcrete to these sand pockets, the sand would begin to flow and the face would lose stability. An application of a low-air-pressure, eggshell-thin layer of shotcrete was required to provide initial stabilization of the ground before adding a layer of wire mesh anchored into the nearby jet grout to provide adhesion for the following layer of shotcrete. Unfortunately, as the excavation proceeded below spring line, the competency of the jet-grout columns began to fail, and the columns did not overlap to provide improved ground conditions and effective water cut-off. The lower half, or bench, of the excavation of CPB 15 yielded flows of sands and water with much higher pressure and volume. Pockets and veins of flowing sands with flow rates beyond 200 gal/min were encountered where the jet grout was missing. The largest inflows came from under the running TBM tunnels, boiling into the excavation from below where the flow could not be stopped without damaging the tunnel liner. Small-section sheet piles had to be driven between the cross-passage excavation and the TBM tunnel liner using the hammer of the mini excavator. The ground loss and intake of water during this procedure was extremely concerning to the project management team. Once the excavation was secured with a sealing layer of shotcrete, compensation grouting was performed and further attempts at stabilizing the ground began by cementitious and chemical injection.After failed attempts of grouting with both ultra- and microfine cementitious grout, a permeation chemical grouting campaign was developed with the jet-grout subcontractor and its chemical injection consultant. A two-part acrylate resin was injected into the cohesionless soil, creating a gelatinous mass that could stem or reduce water flows. As the pressure required to permeate the silts and sands was increased, the layers of shotcrete would rupture, and the entire cycle would need to be repeated until enough of a grout curtain was created before continuing. To prevent the shotcrete from rupturing, the invert was braced with breasting boards and shoring prior to applying additional layers of shotcrete. Drainage pipes were inserted to relieve water pressure away from injection zones but caused issues as the chemical could readily find this path of least resistance and would not penetrate the ground. Three-inch-diameter well points were strategically installed to use negative pressure to draw the chemical grout into desired areas of the excavation.After grouting, additional verification probe holes were drilled before continuing excavation. If required, grouting campaigns would continue, often for several shifts before acceptable results were achieved. After grouting, excavation would have to continue using hand tools as the vibrations from larger equipment would fragment the grout curtain, and the work would have to be repeated. Along with the permeation grouting, the RESS was updated to account for shorter round lengths, smaller pockets, inclusion of drainage mats, forepoling ahead of the excavation, additional probing, and ultimately, more layers of shotcrete.In reflection, the experience gained at CPB 15 provided valuable insight for future RESS planning, including permeation-grouting programs, as well as reinforcing the importance of contingency and toolbox items being immediately accessible during excavation. CPB 15 was jet grouted and no flowing materials were encountered during probing. However, the bench excavation of this cross passage alone took nearly two months to complete. STS made the decision to reprobe all the previous cross passages and all future cross passages a minimum of nine times to gain further information before opening the excavation. Jet-grouted locations were no longer considered to be safe enough for excavation without additional grouting efforts, and the jet grout could not be relied on to have fully overlapping columns. Cross-passage permeation grouting prior to breakout and excavationWith the experience and lessons learned from CPB 15, a permeation-grouting program from within the tunnels was deployed by the jet-grouting subcontractor at all jet-grouted locations that did not meet their specified performance criteria. Ultimately, any probing that indicated nonoverlapping columns of jet grout and produced flowing sands or water beyond 4 gal/min required some additional grouting effort. The subcontractor developed a grouting program that consisted of a chemical-grout curtain that was then reinforced with steel spiling, installed from both running TBM tunnels to fully encapsulate the cross-passage profile. Within the cross-passage profile, a 0.9-m (3-ft) grid in the interior was drilled and grouted to stabilize soils and cut off any paths of flowing water. The new ground-improvement operation from within the tunnel proved to be extremely challenging for STS’s schedule and was a main reason why this method was not considered initially when ground-improvement options were considered. During the drilling and grouting operation, the equipment blocked the tunnel and strategic planning was required to ensure all other daily activities could still take place. Additionally, to supply the permeation-grouting operation while supporting base-scope work led to large schedule issues and major constraints. Thousands of gallons of grout were injected into each of the cross-passage locations requiring treatment, and the campaigns could take nearly eight weeks to complete.After both jet grouting and the additional permeation grouting was completed, STS could not consider a cross passage safe for rapid excavation. In locations where no flowing material was encountered in probe drilling, additional steps were taken in case problematic areas were missed during the investigation. Smaller pocket sizes were usually the first revision to the RESS. Instead of the two pockets seen in the Fernando, four or more pockets were typical. Two pockets would be excavated above the wedge, and a pocket on either side of the wedge for each leg of the lattice girder would complete the initial round. The two pockets above the wedge could be reduced in half, allowing the top sections to be handled and adequately supported first before moving on to middle and lower levels. These extra steps made the excavation take much longer than previously planned but allowed for safe and consistent progress. Cross-passage excavation in tar-impacted soilsIn contrast to the permeable sands and flowing water of the cross passages in Reach 1, the introduction of tar-impacted soils in both Reach 2 and Reach 3 added another difficult challenge to overcome. The tar created cohesive sands that would slowly ravel but would quickly succumb to air slack and transform into fast raveling, eventually collapsing in a similar manner as a pile of hot-mixed asphalt, even when simply being disturbed by the movement of the mini excavator. The tar also limited the permeability of the sands such that all cement and chemical-grout treatments proved ineffective at strengthening or binding the soils. Efforts were made to both cool and freeze the ground with some moderate success, but the application to the entire excavation profile was not feasible with the level of effort required. The tar-impacted soils had the additional quality of restricting the penetration of the shotcrete into the substrate, causing layers of shotcrete to remain susceptible to delamination, collapsing off the crown, face and walls before a round of excavation could be completed.The SEM engineers modified the RESS to provide additional lateral and overhead support to the ground prior to excavation and forepoling prior to advance. A roof canopy of R51 hollow steel bars and a second, offset row of R32 bars at 0.3-m (1-ft) centers were installed above the excavation profile from both tunnels and grouted with microfine cement. A curtain of bars was also installed along each side of the cross-passage profile walls. There was concern that because the tar prevented permeation of the grout to form a full canopy, the bar would not hold back the running sands. This concern proved accurate; however, the spiling bars not only created a shorter arch span for the soils and increased stand-up time, but they offered additional anchoring connections for shoring and shielding plates during excavation, which proved invaluable in preventing ground collapse. A further addition to the RESS toolbox was to use face dowels through the face of excavation. Hollow fiberglass IBO rods were drilled from one running tunnel through the cross passage and into the opposite tunnel on 0.9-m (3-ft) centers. The rods were anchored with steel plates and nuts in the tunnel opposite of the advancing side while steel channel, plates and nuts were used to provide compression on the excavation side. As the excavation advanced in small pockets, the steel channels were removed, the excavation would advance, and then the channels were reinstalled and tightened, compressing the face. The rods could then be cut back to the face of excavation to provide room for the next pocket excavation.The constant movement and squeezing nature of the tar-impacted soil caused the shotcrete to crack and rupture, requiring additional bracing to the girders and shotcrete prior to advancing. The extra weight of the top-heading shotcrete and girders also caused the pore space within the tar-impacted soil below the excavation to collapse, and the entire cross passages would sink. To anchor the cross passage in position, lateral spiling and shotcrete “elephant feet” were installed outside the limits of the excavation.RESS revisions eventually included shortening the lattice-girder spacings from 0.9 m (3 ft) to 0.45 m (18 in) and modifying the heading from top and bench headings into four headings — top, top-middle, middle-bench and bench. Smaller hand-excavated pockets, 0.45 m × 0.45 m (18 in. × 18 in.), were required to eliminate the vibrations caused by the heavy mini excavator. Pockets could not be opened adjacent to one another or the vibrations from impacting the previously excavated round would damage the shotcrete and material would ravel through while the shotcrete cured. This methodical alternating of small-excavation pockets, although time consuming, eventually allowed the excavation of each face to proceed at a predictable, safe pace. A successful 0.6-m (2-ft) round, including excavation, top-heading girder installation, spile installation and full shotcrete application would be completed in three shifts. For comparison, the entire top heading of a cross passage in Fernando soils could be completed in less than eight shifts, but a similar-sized excavation in the tar-impacted sands required more than 20 shifts. ConclusionSection 1 of the Westside Subway Extension had several challenging ground conditions to overcome to ensure a successful project. While preparing for flowing sands, gas and tar-impacted soils posed a significant challenge, the use of the sequential excavation method for the cross-passage excavation allowed the tunneling crews to quickly adapt and respond to unknown and ever-changing scenarios. The installation of a ground- improvement system could not be guaranteed in such challenging conditions, and every possible solution to potential problems had to be considered and made available to safely complete the project. While pre-excavation planning and ground-improvement methods were used to limit risk, it was seen on this project that additional tactics were required once the ground was exposed. ReferencesCalGEM. (2020). California Geologic Energy Management Division (CalGEM). Retrieved from California Geologic Energy Management Division (CalGEM). Well Finder. Online geographic information system. : https://maps.conservation.ca.gov/doggr/wellfinder/#/.California Code of Regulations. (2022, September 23). 8 CA ADC T. 8, D. 1, Ch. 4, Subch. 20 Tunnel Safety Orders. Division of Industrial Safety, Subchapter 20 Tunnel Safety Orders. California, United States: Barclays California Code of Regulations.Parsons. (2016, June). Cross Passage Design Report. LACMTA DU5 Tunnel Cross Passages AFC Submittal. Pasadena, CA, USA: Parsons.Parsons. (2016, 02 29). Jet Grouting Ground Improvement Design Calculations. Westside Subway Extensions Project, Extension 1 Design Calculation Report. Pasadena, CA, USA: Parsons.Parsons. (2019, 03 14). Tar-Impacted Soil Cross Passage Analysis - Tool Box Items. AFC Submittal - Supplemental Information. Pasadena, CA, USA: Parsons.Parsons Brinkerhoff. (2014, November 3rd). Geotechnical Baseline Report. LACMTA C1045 Westside Subway Extension Project, Section 1. Los Angeles, CA, USA: Parsens Brinkerhoff.Purple (D-Line) Extension Transit Project. (2022, 12). Retrieved from Metro.net: https://www.metro.net/projects/westside/.

Excavation and support of cross passages on Westside 1

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