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The Los Angeles (LA) Metro Purple Line (D-Line) Extension project requires the design and construction of deep station excavations and tunnels for rail transit from downtown to west LA. The tunnel alignment for Reach 2 of the Westside Purple Line Extension 1 construction transects naturally-occurring tar-infused soils, which have been known to cause challenging construction conditions in southern California, as well as many other locations around the world. Two stations in similar geology but located within and outside tar soils were compared. The soil investigations of the tunnels and station excavations consisted of subsurface exploration including deep soil borings, Cone Penetration Testing (CPT), seismic velocity measurements, pressuremeter testing, and gas measurements, among others. The results of CPT and shear-wave velocity testing provide extensive data in tar soils unique to Southern California and an opportunity to increase our understanding of four-phase soil materials and the effects of tar on soil behavior interpretation and engineering properties. CPT correlations for conventional (non-tar-infused) soils were found to be inadequate for tar soils in the Los Angeles basin. The CPT based Soil Behavior Type Index (SBTn) determined in tar soils suggested the presence of much finer-grained material than determined from laboratory testing and field observations. Additionally, the presence of tar soils amplified the difference between CPT correlations for shear wave velocity (Vs) and direct Vs seismic CPT measurements. 

Empirical methods for estimating tunneling-induced ground movements have been widely adopted in the tunneling industry. The transverse surface settlement profile can be described by a Gaussian curve or a modified Gaussian curve whose maximum value and trough width are related to volume loss. Volume loss in turn is related to soil type, tunnel geometry, and construction techniques. Several empirical equations have been developed based on the Gaussian curve and the assumptions of (1) trough width dependency on tunnel depth and ground condition; and (2) volume loss dependency on the ground type and construction techniques. For Earth Pressure Balance Machine (EPBM) tunneling, a volume loss of 0.5% in granular soils and 1%–2% the soft clay has been assumed in the past as an initial estimate. However, with complete filling and pressurization of both the shield (overcut) gap and the grouted tail gap around the lining, volume losses below 0.1% to 0.2% are being achieved in the alluvial granular and clay soils on current Los Angeles Metro tunneling projects. The LA Metro K Line Crenshaw/LAX transit project, tunneled from 2016 to 2018, has provided an opportunity to acquire and organize data on compatible data management systems, and evaluate the extensive field monitoring data for ground conditions specific to predominately granular soils in Old Alluvium. These data allow for the improvement of current empirical methods and correlations for predicting surface settlement induced by EPBM tunnels. The approximately 1-mi (1.6-km)-long, 20.6-ft (6.5-m)-diameter twin tunnels were excavated by an EPBM in a dense sand layer overlain by a silt/clay layer. The cover-to-diameter ratio was consistently about 2. The settlements and volume losses are observed to be heavily dependent on the face/shield pressures. In general, maintaining continuous pressures can significantly reduce settlements. An equation for estimating the volume loss based on the measured EPBM shield pressures is proposed. This equation can be used with the existing empirical methods to estimate the surface settlement profile transverse to the longitudinal axis of the tunnel. 

Estimating excavation-induced ground surface displacements in urban areas is needed to assess potential structure damage. Empirical settlement distribution models have been widely used to estimate the zone of influence and ground response behind braced excavation walls. Three underground station excavations, part of the Los Angeles Metro’s K Line Crenshaw/LAX Transit Project, offer a unique opportunity to collect field instrumentation data to improve estimates of ground deformations. One excavation employed cross-lot braces and soldier piles and wood lagging while the other two were supported by cross-lot braces and stiffer Cutter-Soil-Mixing (CSM) walls. For the excavations with stiff support systems and relatively small wall movements, upward surface displacement or heave governed the ground surface response, while surface settlement was measured at the excavation with the more flexible wall system. This heave behavior is often masked by settlement caused by relatively large wall movements, and is thus commonly disregarded. By idealizing the excavation unloading as an upward strip load at the ground surface, the Boussinesq solution for elastic upward movement can be used in combination with a settlement component resulting from lateral wall movements to estimate the magnitude and distribution of excavation-induced surface displacements.