16.2 Concrete, Soil, and Geosynthetic Materials
Key Takeaways
- The April 2024 WRE Materials area explicitly includes soil classification, boring-log interpretation, soil properties, concrete, piping materials, test methods, and specification conformance.
- Soil classification is not just a geotechnical topic on WRE; it drives trench stability, bedding, compaction, infiltration, liner selection, retaining walls, and erosion-control performance.
- Concrete questions usually test durability, nonreinforced versus reinforced behavior, curing, water-cement (w/c) ratio logic, and suitability for water or wastewater exposure.
- Geosynthetics appear through sitework functions — separation, filtration, drainage, reinforcement, erosion control, and containment — even when the word geosynthetic is absent from the prompt.
- Material conformance answers come from comparing the measured property to the specified acceptance criterion, never from intuition about whether the material 'seems strong.'
Materials Beyond the Pipe Barrel
The WRE exam does not treat materials as a standalone memorization chapter. Materials explain why a trench sloughs, why a pond liner leaks, why a concrete tank spalls, why a filter clogs, and why a retaining-wall backfill drains or builds pressure. The April 2024 specification names soil classification, boring-log interpretation, soil properties, concrete, piping materials, and material test methods — all tied to sitework, hydrology, wastewater facilities, and construction conformance.
Soil Properties to Read From the Problem
Most soil items hand you enough data to classify behavior. Look for grain-size distribution, Atterberg limits, moisture content, dry density, groundwater elevation, standard penetration test (SPT) N-values, and notes such as fill, organics, loose sand, stiff clay, or refusal. Under the Unified Soil Classification System (USCS), the dividing sieve between coarse and fine is the No. 200 (0.075 mm): coarse-grained soils retain more than 50 percent on it; fine-grained soils pass more than 50 percent through it.
| Property or clue | WRE consequence | Typical exam use |
|---|---|---|
| High-plasticity clay (CH) | Low permeability, shrink-swell risk, slow drainage | Pond liner support, trench backfill, settlement |
| Clean sand or gravel (SP/GP) | High permeability, low cohesion | Dewatering, filter design, infiltration BMP, bedding |
| Organic or soft soil (OL/OH/Pt) | Compressible and weak | Embankment settlement, unstable cut, poor foundation |
| Dense granular fill | Good drainage, compactable | Pipe bedding, structural backfill, underdrain envelope |
| Groundwater above trench invert | Buoyancy, instability, inflow | Dewatering, bedding migration, pipe flotation |
A frequent calculation is relative compaction:
Relative compaction = (field dry density / laboratory maximum dry density) x 100 percent.
If trench backfill has a field dry density of 112 pcf and the standard Proctor maximum dry density is 120 pcf, relative compaction = 112 / 120 x 100 = 93.3 percent. A 95-percent specification means the lift fails and must be recompacted, even if the pipe itself is acceptable.
Concrete in WRE Facilities
Concrete appears in channels, manholes, junction boxes, culverts, tank slabs, foundations, headwalls, spillways, and treatment basins. Distinguish nonreinforced concrete, which relies on compressive strength and mass, from reinforced concrete, which uses steel to resist tension and control cracking.
For durability, focus on exposure. Water and wastewater structures face wet-dry cycling, freeze-thaw, sulfate attack, low pH, hydrogen-sulfide corrosion of the crown, chloride exposure, and grit abrasion. Lower water-cement ratio (often w/c below about 0.45 for severe exposure), proper curing, adequate cover, air entrainment where freeze-thaw applies, and sulfate-resistant cement all improve durability. On the exam, a low compressive-strength break, excessive slump, missing curing, or wrong exposure class is a specification problem, not a hydraulic one.
Geosynthetics by Function
Geosynthetics earn their keep by delivering one defined function in a thin manufactured layer. Do not mix the functions:
- Geotextile separation keeps subgrade fines from pumping into aggregate.
- Geotextile filtration passes water while retaining soil at drains, swales, and outlet protection; specified by permittivity and apparent opening size (AOS).
- Geocomposite drainage provides an in-plane flow path behind walls or below caps.
- Geogrid reinforcement improves soil or aggregate mass stability; specified by tensile strength.
- Geomembrane containment limits seepage from ponds, tanks, landfills, or contaminated areas.
- Erosion-control blankets / turf reinforcement mats protect channels and slopes until vegetation establishes.
A geotextile under riprap at a channel outlet is a filter/separation layer. A geomembrane under a detention pond is a seepage barrier. A geogrid in retaining-wall backfill is reinforcement, not a filter. When the prompt gives permittivity, AOS, tensile strength, puncture resistance, or interface friction, match that property to the specified function.
Calculation and Conformance Workflow
- Identify the required property: gradation, plasticity, density, strength, permeability, slump, air content, thickness, tensile strength, or opening size.
- Convert units before comparing.
- Compare the measured test result to the acceptance range.
- Decide the consequence: accept, reject, retest, recompact, redesign, or add protection.
That workflow blocks a frequent PE mistake — answering from material preference instead of the acceptance criterion stated in the problem.
Reading a Boring Log Quickly
A boring log is a vertical record of what the drill found. Scan it top to bottom for four things the exam cares about. First, the groundwater table depth (and date measured), because it sets dewatering, buoyancy, and bedding-migration concerns. Second, the soil sequence — surface fill, then native strata — since a soft or organic layer beneath a foundation drives settlement. Third, SPT N-values, a rough strength and density index: N below about 4 in sand is very loose, N above about 30 is dense; in clay, low N means soft and compressible.
Fourth, refusal, which marks rock or a very dense layer and may control pile tip or excavation limits.
| Boring-log clue | Immediate WRE interpretation |
|---|---|
| Water table at 6 ft, trench invert at 10 ft | Dewatering and flotation/buoyancy check required |
| 8 ft of fill over native clay | Suspect uncontrolled fill; settlement and bearing risk |
| Organic silt (OL) layer at depth | Long-term consolidation settlement of structures above |
| SPT N = 2 in sand | Very loose; liquefaction/settlement concern, poor bearing |
| Refusal at 14 ft | Rock or hardpan; affects excavation cost and pile design |
Bedding Classes and Why They Matter
Buried-pipe load capacity is not a property of the pipe alone — it depends on the bedding. Rigid pipe (RCP) is rated by D-load (the three-edge-bearing test) multiplied by a bedding factor: better bedding (Class A concrete cradle, Class B shaped granular) supports more load than dumped backfill (Class D). Flexible pipe (PVC, HDPE) relies on the surrounding compacted soil to limit ring deflection; the Iowa formula links deflection to the modulus of soil reaction, so poor haunch compaction is the usual culprit when a flexible pipe deflects.
On the exam, a buried-pipe failure with adequate wall strength almost always points back to bedding or haunching, not the pipe material itself.
A trench backfill specification requires 95 percent relative compaction. A field density test gives dry density = 112 pcf, and the laboratory standard Proctor maximum dry density is 120 pcf. What is the correct conclusion?
A stormwater pond must minimize seepage into contaminated underlying soil. Which geosynthetic function is most directly needed?