7.4 Soil Data, Boring Logs, and Unit Conversions
Key Takeaways
Boring logs are point-in-time observations, so WRE decisions must weigh spatial variability, groundwater timing, and sample quality between borings.
USCS classification starts with percent passing the No. 200 sieve, then uses gradation for coarse soils and Atterberg limits for fine soils.
Groundwater levels on logs are time-sensitive; a 24-hour stabilized reading can differ from a drilling reading or seasonal high.
SPT N-values indicate relative density or consistency but are affected by hammer energy, gravel, and depth and need correction.
The WRE exam mixes SI and US Customary units, so soil questions require clean conversion among pcf, psf, tsf, kPa, and kN/m3.
Reading Soil Data Like a WRE Engineer
Under the April 2024 WRE specification, soil classification and boring-log interpretation sit in the Materials reporting group, while Soil Mechanics carries lateral pressure, consolidation, compaction, bearing capacity, settlement, and slope stability. In practice these are linked: a boring log may be the only warning that a sewer trench crosses soft clay, a basin outlet rests on loose sand, or a pump-station excavation will hit groundwater.
What a Boring Log Can and Cannot Prove
A boring log records observations at one location and time. It may carry soil descriptions, Unified Soil Classification System (USCS) symbols, sample depths, standard penetration test (SPT) N-values, groundwater observations, lab water content, dry density, Atterberg limits, grain-size data, and drilling remarks. It does not prove the same soil exists everywhere between borings, which is why spacing and judgment matter.
| Log item | Useful meaning | Caution |
|---|---|---|
| Soil description | Visual-manual classification clue | Subject to sampler and logger judgment |
| USCS group symbol | Engineering-behavior shorthand (SW, ML, CH) | Quality depends on lab vs field method |
| SPT N-value | Relative density or consistency | Affected by energy, gravel, depth; needs N60/(N1)60 correction |
| Groundwater note | Water observed during/after drilling | Seasonal and delayed response differ |
| Atterberg limits | Plasticity of fine soils | Must be tied to fines content |
| Unit weight / water content | Phase-relationship input | Confirm wet vs dry basis |
Groundwater notes deserve special care. A note of water at 6 ft during drilling and 9 ft after 24 hours may reflect drilling disturbance, perched water, low permeability, or delayed stabilization. Excavation planning must not assume the site is dry below 9 ft, and a dry-season boring may not represent spring groundwater near a channel or basin.
Classification Workflow
Use a structured approach instead of guessing from a name:
- Check percent passing the No. 200 sieve. More than 50 percent fines means fine-grained behavior; 50 percent or less means coarse-grained.
- For coarse soils, split gravel from sand and judge gradation (well-graded W vs poorly graded P) and fines.
- For fine soils, plot liquid limit and plasticity index on the plasticity chart to separate lean clay (CL), fat clay (CH), silt (ML/MH), and organic soils.
- Connect classification to WRE performance: permeability, erosion risk, compressibility, bedding suitability, and compaction response.
- Use lab and field data together; a single description like silty sand is not enough for a settlement or stability decision.
A clean sand may be permeable and good for drainage, yet vulnerable to running ground below the water table in an excavation. A plastic clay has low permeability but high consolidation settlement. A gravelly soil may be strong but hard to sample and classify from ordinary split-spoon data.
Unit Conversions That Prevent Wrong Answers
Because the exam mixes SI and US Customary units, soil conversions often involve force per area and unit weight:
- 1 tsf = 2,000 psf, approximately 95.8 kPa.
- 1 pcf is approximately 0.157 kN/m3.
- 1 ft = 0.3048 m and 1 in = 25.4 mm.
- Vertical soil stress is unit weight times depth: pcf x ft = psf.
- Water unit weight is about 62.4 pcf or 9.81 kN/m3.
- 1 psi = 144 psf, and 1 kPa is about 20.9 psf.
SPT N-Value Interpretation
SPT N-values are a frequent log entry and a quick consistency screen. Approximate correlations (uncorrected field N) help sanity-check a description:
| N (blows/ft) | Sand relative density | Clay consistency (approx) |
|---|---|---|
| 0 to 4 | Very loose | Very soft |
| 4 to 10 | Loose | Soft to medium |
| 10 to 30 | Medium dense | Stiff |
| 30 to 50 | Dense | Very stiff |
| Over 50 | Very dense | Hard |
Field N must be corrected to N60 for hammer energy and, for sands, to (N1)60 for overburden before use in design correlations. A low N near a planned excavation flags running ground or low bearing; a description of soft clay with N = 2 should immediately raise settlement and slope concerns.
Worked Unit Conversion
A report lists a clay undrained shear strength su = 1.2 tsf. Convert for a problem given in SI: 1.2 x 2,000 = 2,400 psf, and 2,400 psf x (1 kPa / 20.9 psf) = about 115 kPa. If a problem then asks for the vertical effective stress at 15 ft in soil with gamma = 118 pcf above a water table at 5 ft (gamma_sat = 122 pcf below): total stress = 5 x 118 + 10 x 122 = 590 + 1,220 = 1,810 psf; pore pressure = 62.4 x 10 = 624 psf; effective stress = 1,810 - 624 = 1,186 psf. Labeling each term as total, pore, or effective prevents the classic mix-up.
Exam Strategy
Before calculating, label every value as wet, dry, total, effective, allowable, ultimate, gross, or net, then convert units in a single line while preserving the basis. If a log shows soft clay from 8 to 20 ft with groundwater at 7 ft, a trench or footing item is not pure geometry; it is a soil-behavior problem carrying groundwater, sampling uncertainty, and constructability consequences. Treat one boring as a sample, not the truth for the whole site, and let the weakest credible layer and the highest credible water level drive the conservative answer the exam expects.
A boring log for a proposed 12 ft deep sewer trench notes groundwater at 6 ft during drilling and at 9 ft after 24 hours. What is the best interpretation for exam purposes?
Assume the trench is dry because the later reading is below the pipe zone.
Use the 7.5 ft average as a permanent design water table for all seasons.
Ignore the groundwater notes because only classification affects trench work.
Recognize groundwater observations are time-sensitive and plan for verification or possible dewatering.
A small equipment footing for a basin outlet has an allowable bearing pressure of 2.5 tsf. What is this value in psf?
250 psf
1,250 psf
10,000 psf
5,000 psf
Sections you finish are checked off in the contents.