بکارگیری فشارسنج در سنجش تنش در محل خاک رس Glaciolacustrine Seattle

ABSTRACT In Situ Lateral Stress Measurement in Glaciolacustrine Seattle Clay Using the Pressuremeter

An extensive geotechnical exploration program was conducted along the alignment of the proposed State Route (SR) 99 Tunnel Project. The roughly 2.6-km-long bored tunnel is designed to run north-south below downtown Seattle (Fig. 1) at depths ranging from approximately 30 to 85 m below the ground surface. At 17.7 m in diameter, the SR-99 tunnel would be the largest bored tunnel in soil in the world.

The SR-99 tunnel exploration program included over 120 pressuremeter tests (PMT), which were used to estimate in situ soil stressdeformation parameters along the tunnel alignment. Many of the tests were conducted in a very stiff to hard glaciolacustrine clay known as Seattle clay. This unit is historically knownfor deep-seated slope failures and many of these failures have been attributed to the release of high, locked-in lateral stresses. In the early stage of the PMT program, there were indications of a high in situ lateral stress from a visual inspection of the pressuremeter data. During the final 10 PMTs in this clay, the lateral stresses were considered in more detail; this is the focus of this paper.

In situ lateral stresses, sh0, were a primary focus of the pressuremeter testing program as they impact the earth pressure balance machine face pressure design, tunnel liner design, and are used in finite-element modeling for settlement estimation. Owing to the hard consistency of this unit and the potential for cobbles, neither selfboring pressuremeter nor dilatometer testing was feasible and, therefore, prebored pressuremeter testing was used. Because most published procedures for estimating sh0 involve examination of the initial portion of the pressuremeter stress-strain curve and because this portion of the curve is the most impacted by preboring-related disturbance (Marsland and Randolph 1977), estimation of in situ sh0 in Seattle clay was particularly challenging.

In this study, sh0 was primarily estimated using constitutive model, curve-fitting techniques using pressuremeter data and using a heretofore unpublished, experimental method that relies on iterative creep testing. These methods both make use of the postyield and unloading portions of the pressuremeter stress-strain curves. Due to the higher stresses at these latter test stages, the annular zone of stressed soil is larger than at the beginning of the test and, therefore, a larger percentage of the soil response is thought to be derived from the intact soil beyond the annulus of soil most disturbed by the preboring process. As such, it is reasoned that these methods are less impacted by borehole disturbance than other methods that rely on the initial stages of the tests.

The pressuremeter-derived in situ lateral stresses presented here were compared to results of previous studies in Seattle clay and to stresses estimated using relationships between the overconsolidation ratio(OCR5 effective vertical yield stress, s9vY, divided by the current effective vertical stress, s9v0) and the coefficient of lateral earth pressure at rest (K05 effective in situ lateral stress, s9h0, divided by s9v0), which assumes the clay has undergone one-dimensional, laterally constrained loading and unloading (hereafter referred to as simple loading and unloading). The in situ lateral stresses in the Seattle clay were estimated to be significantly higher than what would be expected by assuming a simple loading and unloading stress path due to glaciations and rebound. Deformational features commonly encountered in Seattle clay indicate its stress history also has included significant lateral shearing. Thememory of this shearing within the fabric of the claymay influence the in situ stress state and response to lateral unloading.