Question 4 of 6: Field and Laboratory Program for Consolidation Settlement

Question 4: Field and Laboratory Program for Consolidation Settlement (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Estimating the ultimate consolidation settlement under a new fill requires, first, an accurate picture of the subsurface stratigraphy, and second, the compressibility parameters of every compressible layer within that stratigraphy. Both come from a staged field-and-laboratory program, not from either alone.

Desk study and reconnaissance. Before any drilling, existing geological maps, aerial/LiDAR imagery, and any prior borehole logs near the site are reviewed to anticipate the likely stratigraphy (e.g. a buried soft marine clay, glaciolacustrine silt, or peat) and to plan borehole locations and depths efficiently.

Field exploration — boreholes and in-situ testing. A grid of boreholes (spacing and count set by site size and expected stratigraphic variability) is advanced across and beyond the fill footprint, to a depth well below the zone of significant stress increase — typically to where the fill-induced vertical stress increase (estimated via Boussinesq/2:1 methods) falls below about 10% of the existing effective overburden stress, or to a stiff/incompressible stratum if shallower. Standard Penetration Test (SPT) blow counts are logged continuously to build the stratigraphic log and flag granular layers; a field vane shear test is run in any soft clay to get an independent, low-disturbance estimate of undrained shear strength for a companion bearing-capacity check; and a Cone Penetration Test (CPT) sounding, where available, gives a continuous, high-resolution profile of layer boundaries and relative stiffness between borehole locations. Standpipe piezometers are installed to establish the current groundwater table and any excess/artesian pore pressure — essential because the settlement analysis works in terms of EFFECTIVE stress, both before and after the fill is placed.

Undisturbed sampling. In every cohesive (clay/silt) layer identified, thin-walled Shelby-tube samples are pushed (not driven, to minimize structural disturbance) to recover intact specimens for laboratory consolidation testing; SPT split-spoon (disturbed) samples from the granular layers are retained for index/classification testing only, since granular soils consolidate too fast, and too little, to warrant oedometer testing.

Laboratory testing. Index tests (natural water content, Atterberg limits, grain-size distribution, specific gravity, unit weight) classify each stratum and give a first empirical estimate of compressibility. The critical test is the one-dimensional oedometer (consolidation) test on each undisturbed clay/silt sample: it directly yields the preconsolidation pressure $\sigma_p'$ (via the Casagrande construction, hence the overconsolidation ratio $OCR=\sigma_p'/\sigma_{vo}'$), the compression index $C_c$ and recompression index $C_r$ (the slopes needed for the settlement MAGNITUDE calculation), and the coefficient of consolidation $c_v$ from the time–settlement curve of each load increment (needed for the settlement RATE, via the time factor $T_v$).

Together, the borehole/CPT stratigraphy fixes which layers compress and their thickness and drainage-path length; the piezometer data fixes the initial effective-stress profile; and the oedometer results on the Shelby-tube samples supply every parameter the one-dimensional consolidation equation needs to turn "how much extra stress does the fill add" into "how much, and how fast, will the ground settle."