7.4 Soil Data, Boring Logs, and Unit Conversions

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.

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:

1. Check percent passing the No. 200 sieve. More than 50 percent fines means fine-grained behavior; 50 percent or less means coarse-grained.
2. For coarse soils, split gravel from sand and judge gradation (well-graded W vs poorly graded P) and fines.
3. 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.
4. Connect classification to WRE performance: permeability, erosion risk, compressibility, bedding suitability, and compaction response.
5. 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:

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.