Menu
1. introduction
located at Vancouver harbor near the coastline, this project needed
2. In-situ test
based on the scope of work and level of precision needed, different in-situ tests are available to assess the subgrade soil. CPT, DCPT, SPT, PLT are some cases in point. due to significance of the project Evertek professional decided to proceed with CPT. thanks to the continuous profile it provides, CPT could increase the reliability of the analysis; especially in this project owing to presence of loose sandy fill material, there was risk of liquefaction and seismic analysis was of huge importance. Hence, SCPTu (seismic cone penetration test) was opted.
Advantages of CPT | Disadvantage of CPT |
Fast and continuous profiling | High capital investment |
Economical and productive | Requires skilled operator to run |
Results not operator-dependent | Electronic drift, noise, and calibration |
Strong theoretical basis in interpretation | No soil samples are obtained |
Particularly suitable for soft soils | Unsuitable for gravel or boulder deposits |
3.Results
3.1 Field Investigation
by boring two boreholes using a track-mounted drill rig, Evertek engineers could specify different soil layers. the soil basically consisted of loose sandy fill material and the water table was 4 meters below the ground surface.
3.2 Modulus of Subgrade Reaction
Subgrade reaction modulus is the ratio of soil pressure to deflection. This modulus is widely used in structural design of mats and slabs. The structural design is completed by structural engineer of record (SEOR) where he/she utilizes the concept of Beam on Nonlinear Winkler Foundation in order to estimate the pressure of soil as well as shear forces and bending moments induced to the foundation element. Geotechncial engineer of record (GEOR) is responsible to provide appropriate subgrade reaction modulus to characterize the Winkler springs. Appropriate estimation of subgrade reaction modulus would result in appropriate estimation of soil pressure distribution under the mat/slab which would consequently result in accurate structural design of mat/slab.
Unfortunately, there is a widespread misconception in the use of subgrade reaction modulus. Structural engineers often expect to see a single constant value for subgrade reaction modulus in soils report. However, subgrade reaction modulus is not a fundamental soil property. It is a function of (Walker and Holland, 2016):
Geometry of loading surface and type of loading must be provided by structural engineers for appropriate estimation of subgrade reaction. By former parameter, the geotechnical engineer will know to what depth stress bulbs extend, and by latter parameter, the geotechnical engineer will know whether to include consolidation in the calculation of subgrade reaction modulus or not. For example, lift-truck wheel loads (short-term loading) will cause very little, if any, long-term consolidation settlement.
in this case as PLT test was not available Evertek professionals used the indirect method and by modeling the foundation and calculating settlement under unit load, Ks was calculated and reported.
3.3 Settlement
as the governing parameter in settlement of sandy deposits is elastic settlement Bowles, Janbu as well as Mayne and Poulos methods were used to evaluate settlement for service loads. the results were confirmed using software modeling. Also, there are methods in the literature using direct CPT data for settlement calculation that were consistent with Evertek’s. Furthermore, post liquefaction settlement was of great significance and The National Building Code of Canada (NBC) 2020 method was used.
3.4 Bearing Capacity
Bearing capacity is one of the most crucial parameters geotechnical investigation must provide. there are many methods in the literature for estimation of bearing capacity like: Vesic, Terzaghi, Hansen and Meyerhof. also, having had access to CPT logs, engineers could verify their results and come up with the most dependable number both for SLS and ULS.
refs:
[1] National Building Code of Canada (NBC) 2020
[2] Canadian Foundation Engineering Manual (CFEM)
[3] www.wikipedia.com
[4] https://geotechsimulation.com/2018/02/12/subgrade-reaction-modulus-of-footings/
3.5 Seismic Analysis
Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.
Evertek’s investigations showed that Peak Ground Acceleration (PGA) for this site is 0.508g for a probability of occurence of 2% in 50 years (0.000404 per annum) and it would trigger liquefaction. thus, liquefaction hazard was evaluated by different analytical methods and was reported to the client.
4. conclusion
fill material could be prone to liquefaction hazard, unless it is properly compacted or stone columns or wick drains are used to dissipate excess pore water pressure during an earthquake. Hence, in such sites geotechnical investigation is imperative. with a deep familiarity with subgrade materials, structural engineers would be able to design structural parts more economically. Choosing the best in-situ test, and conducting a precise modeling can only be done by relying on years of experience.