Settlement calculation check, Elk Falls Park-and-Ride at OR 403

The submitted preliminary consolidation settlement calculation at boring B-3 was rerun with the procedure workbook on the same data. Settlement comes to 8.4 in. instead of 1.5 in., mainly because Clay 2 is loaded past its preconsolidation pressure but was calculated with C_r.

Settlement_Calc_ElkFalls_PnR_B-3_submitted.xlsx, Nov 3, 20265.0 ft of fill at the OR 403 tie-inCalculation Procedure GT-07 v1.2Prepared Nov 5, 2026

Summary

Settlement · Clay 2

Settlement is 8.4 in., not 1.5 in.

The 625 psf fill load takes every sublayer in Clay 2 (17 to 37 ft) past σ′_p, but Sublayers!J11:J17 applies C_r = 0.07 to the full stress increase. With C_c = 0.75 beyond σ′_p, Clay 2 accounts for 74% of the settlement, and the 4 in. limit in the Geotechnical Design Criteria Memo is exceeded.

The C_c, C_r and σ′_p values and the 4 in. limit are taken from the project documents and have not been checked separately. The result must be verified by the geotechnical engineer of record.

Mitigation at the OR 403 tie-in needs to be evaluated, for example lightweight fill or a surcharge.

Pore pressure

Two errors partly cancel out

The 8.0 ft groundwater depth (Input!C9) comes from observation well OW-2, screened in the crust and read in the dry season on Aug 14, 2026. Vibrating wire piezometers at 12 and 26 ft show hydrostatic pore pressure from 2.5 ft (Geotechnical Data Report, Section 6.3). Correcting only C_r/C_c gives 11.9 in., 42% more than the check calculation.

Stratigraphy

The clay ends at 37 ft, not 31 ft

The submitted calculation stops at the bottom of boring B-3. CPT C-3 shows clay to 37 ft and refusal in dense gravel at 38.5 ft (Geotechnical Data Report, Appendix B). The three sublayers from 31 to 37 ft add 1.6 in., about a fifth of the total.

Sublayer-by-sublayer recalculation

Switch between the submitted calculation and the check calculation, or apply the corrections one at a time. Change the fill height to see what the settlement limit allows. Select a sublayer in the profile or the table to see its values and the formula in the workbook.

Calculation

Corrections

C_r/C_c about σ′_p Pore pressure in the clay Bottom of clay at 37 ft

Fill above existing grade

5.0 ft

–Settlement

–Limit 4 in. at the tie-in

–Share from Clay 2

–Highest fill within the limit

B-3 and C-3: profile, stresses and settlement by sublayer

Existing grade El. 46.5 ft (NAVD 88). Shaded: stress increase beyond σ′_p. Outlined bars show the submitted calculation.

σ′₀ σ′₀ + Δσ σ′_p, consolidation tests Computed with C_c Beyond σ′_p but computed with C_r

Sublayer

	Submitted	Current

From submitted to checked

Corrections applied in checklist order. Dashed line: the 4 in. limit.

QC checklist from the calculation procedure

Calculation Procedure GT-07 v1.2, Section 7. Review of the submitted calculation.

Sublayer table, current calculation

Depth, ft	Layer	σ′₀, psf	σ′_p, psf	σ′₀+Δσ, psf	Index	S, in.	Submitted, in.

Sources and method

Sources

Settlement_Calc_ElkFalls_PnR_B-3_submitted.xlsxSubmitted calculation, Input and Sublayers tabs, 13 sublayers to 31 ft2 tabs
GT-07_Consolidation_Settlement_Procedure_v1.2.pdfCalculation procedure: method, worked example and 10-item QC checklist (Section 7)14 pages
Consolidation_Settlement_Workbook_v1.2.xlsxWorkbook used for the check calculation, same sublayers as the submitted one4 tabs
Geotechnical_Data_Report_ElkFalls_PnR.pdfGeotechnical data report: boring B-3 to 31.0 ft with Shelby tube samples and CPT C-3 (Appendix B), piezometer readings (Section 6.3), index tests. B-3 at N 562,140 ft, E 7,551,860 ft, Oregon State Plane North Zone, NAD83(2011), international feet52 pages
Consolidation_Tests_B-3_ElkFalls_Soil_Testing.pdfIncremental-load consolidation tests per ASTM D2435 at 10, 15, 22 and 28 ft: σ′_p, C_c, C_r, e₀11 pages
C-201_Grading_Plan_Rev_B.pdfExisting grade El. 46.5 ft and finished grade El. 51.5 ft (NAVD 88) at the OR 403 tie-in1 sheet, 22×34
Geotechnical_Design_Criteria_Memo_ElkFalls_PnR.pdfLimit: primary consolidation settlement no more than 4 in. at the OR 403 tie-in6 pages

Method

Every input cell in the submitted workbook was traced to its source and compared with the procedure's QC checklist.
Stratigraphy was taken from boring B-3 and CPT C-3, unit weights from the index tests, and pore pressure from the vibrating wire piezometers at 12 and 26 ft.
σ′_p was interpolated linearly between the test depths. Above 10 ft the 10 ft value is used, and below 28 ft σ′_p is extrapolated with the gradient between 22 and 28 ft.
Primary consolidation settlement was computed in 2 ft sublayers with the one-dimensional C_r/C_c formulation in the FHWA Soils and Foundations Reference Manual (FHWA NHI-06-088): C_r up to σ′_p and C_c beyond it. Parameter selection follows FHWA Geotechnical Engineering Circular No. 5.
The corrections were applied one at a time in checklist order to show what each one does to the result.

Assumptions and limitations

Wide fill: the stress increase is taken as the full fill load throughout the clay. This holds at the center of the fill; settlement is smaller toward the edges.
The crust and the gravel below the clay are assumed to contribute negligible settlement. γ_w = 62.4 pcf.
Primary consolidation settlement at one location. Time rate and secondary compression are not included.
How much each correction changes the result depends on the order in which they are applied. The total is the same.
The check calculation should be reviewed by the geotechnical engineer of record before it is used to select mitigation.
The parameter values (C_c, C_r, the σ′_p profile and OCR) come from the consolidation tests and the Geotechnical Data Report, and the 4 in. limit from the Geotechnical Design Criteria Memo. They have not been assessed separately; the geotechnical engineer of record should confirm that they are reasonable for the site.