Not only did the tight work spaces pose a challenge, but the elevations of adjacent working benches also varied, resulting in additional time to move the rig and equip- ment from bench to bench. The grout for the rigid inclusions was pumped from a ready- mix truck through hoses attached to the rig. In some cases, the concrete pump had to be placed up to 150 ft (46 m) away from the installation zone, which meant the grout had to be pumped through a series of hoses. In addition, some of the low elevation working areas were at risk of flooding. Due to the proximity to marshes and high-water tables, flooding of the working pads occurred often. After flooding, the work zones had to be dewatered and the working pad recompacted. Subsurface Conditions Not only did the site configuration provide challenges, but the surficial soft soils and thick layers of compressible soils also provided additional design challenges. Due to the expanse of the project site, the ground surface elevation varied greatly, ranging from El. 8.2 ft to El. 55.8 ft (El. 2.5 m to El. 17 m). In general, the soil profile consists of silt and clay layers that increase in strength with increasing depth. The upper soil layers typically consist of a loose-to-medium dense sandy fill from the ground surface to El. 27.9 ft (El. 8.5 m). The fill was underlain by a medium stiff silt/clay to about El. -12.1 ft (El. -3.7 m), a stiff silt / clay to El. -24.9 ft (El. -7.6 m) on the south and El. -29.9 ft (El. -9.1 m) on the north, underlain by the stiff silt/clay bearing layer. The sandy fill was only present in certain areas across the site and much of that fill was excavated for the construction of the CSEs, leaving the soft clayey soils close to the surface. Ground Improvement Design The design of the rigid inclusions evaluated the soil-structure interaction (SSI) along the length of the elements, the performance of the LTP at the top of the inclusion and the load carrying capacity of the rigid inclusion below its neutral axis (i.e., point of maximum load). Finally, the design assumptions were compared to the measured settlement data to verify that the rigid inclusions performed as designed. Flooding in work zones Layer Med. Stiff Clay / Silt Med. Stiff Clay / Silt (undrained) Stiff Clay / Silt (drained) Stiff Clay / Silt (undrained) Stiff Clay /Silt I-14 Backfill – Sandy Fill I-15 MSE Wall Backfill Working Platform Load Transform Platform (LTP) Elev. at Bottom ft (m) -3.7 -3.7 -7.6 -7.6 -9.1 --- --- --- --- Unit Weight, pcf (kN/m3) 18.1 ᵞ 18.1 19.6 19.6 19.6 18.8 19.6 19.6 19.6 Soil properties of the in-situ soil The various working pad elevations presented an operational challenge since minimum slopes had to be maintained for rig accessibility and stability. The slopes also presented a design challenge because the elevations of the working pad directly influenced the elevations of the load transfer pad (LTP) and, thus, the geometry of the axisymmetric finite element models. The majority of the interchange site will be raised to a higher elevation than the existing grade, though the amount of fill placed and working elevations vary across the site. Due to the many possible design and loading scenarios, the changes in the eleva- tion of the working pad were the starting point for the design of the CSEs. Models were constructed for each continuous working pad elevation. Then, analyses were performed at the locations of the maximum and minimum applied load, and interpola- tion was used to estimate the performance of the intermediate loading conditions. Finite element software, Plaxis, was used to estimate the load distribution between the rigid inclusions and the soil and to estimate the post-construction settlement of the rigid inclusion system. For this analysis, the rigid inclusion-reinforced soils and LTP were simulated with axisymmetric unit cell models in Plaxis. Cohesive soils were modeled as undrained soils and granular soils were modeled as drained. All soils used the hardening soil material model. The rigid inclusion was modeled as a soil volume with linear-elastic properties. DEEP FOUNDATIONS • JAN/FEB 2019 • 77 Friction Angle, (°) 27 0 30 0 0 34 / 35 34 38 38 ϕ Dilatancy (°) 0 0 0 0 0 5 4 8 8 Angle, ψ Cohesion, c psf (kPa) 0.1 35.9 0.1 71.8 143.6 0.0 0.2 0.2 0.2 Cc 0.42 0.42 0.23 0.23 Cr 0.075 0.075 0.025 0.025 0.17 0.0025 0.17 0.0025 --- --- --- --- --- --- Init. void ratio, e0 1.0 1.0 0.7 0.7 0.7 --- --- --- ---
