7.4 Soil Data, Boring Logs, and Unit Conversions

Reading Soil Data Like a WRE Engineer

The April 2024 WRE specification lists soil classification and boring log interpretation under Materials, while Soil Mechanics lists lateral pressure, consolidation, compaction, bearing capacity, settlement, and slope stability. In practice, those topics are connected. A boring log may be the only clue that a proposed sewer trench crosses soft clay, a basin outlet sits in loose sand, or a pump station excavation will encounter groundwater.

What a Boring Log Can and Cannot Prove

A boring log is a record of observations at one location and time. It may include soil descriptions, Unified Soil Classification System symbols, sample depths, standard penetration test N-values, groundwater observations, lab water content, dry density, Atterberg limits, grain-size data, and remarks about drilling difficulty. It does not prove that the same soil exists everywhere between borings.

For WRE projects, groundwater notes deserve special attention. A note such as 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 should not assume the site is dry below 9 ft. Similarly, a boring drilled during a dry season may not represent spring groundwater near a channel or basin.

Classification Workflow

Use a structured classification approach instead of guessing from a name:

1. Check percent passing the No. 200 sieve to separate coarse-grained from fine-grained behavior.
2. For coarse-grained soils, decide gravel versus sand and evaluate gradation and fines.
3. For fine-grained soils, use liquid limit and plasticity index to distinguish lean clay, fat clay, silt, or organic behavior.
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 such as silty sand is not enough for settlement or stability.

A sand with little fines may be permeable and useful for drainage, but it may also be vulnerable to running ground in an excavation below groundwater. A plastic clay may have low permeability but high consolidation settlement. A gravelly soil may be strong but difficult to sample and classify with ordinary split-spoon data.

Unit Conversions That Prevent Wrong Answers

The WRE exam uses both SI and US Customary units. Soil mechanics 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.
• Pressure from soil is commonly unit weight times depth, such as pcf x ft = psf.
• Water unit weight is about 62.4 pcf or 9.81 kN/m3.

Exam Strategy

Before calculating, label every value as wet, dry, total, effective, allowable, ultimate, gross, or net. Then convert units in one line and preserve the basis. If a boring log gives soft clay from 8 to 20 ft and groundwater at 7 ft, a foundation or trench problem is not just a geometry problem. It is a soil behavior problem with groundwater, sampling uncertainty, and WRE constructability consequences.