Exam Prep Notes: Retaining Walls, Soil Degradation, and Geospatial Tools

Retaining Walls and Their Failure

• Primary Causes of Retaining Wall Failure: Retaining walls primarily fail due to improper building practices or, more specifically, improper engineering based on soil type and water load. This is a crucial distinction, as proper engineering accounts for environmental factors.
• Impact of Soil Type:
  • Sandy Soil: Drains more freely, reducing hydrostatic pressure behind the wall.
  • Clay Soil: Tends to hold water, leading to significant hydrostatic pressure ($P_h$) behind the wall, which can cause the wall to push out or blow out.
• Engineering Requirements and Real-world Application:
  • Any retaining wall over $4$ feet tall must be engineered. An engineer conducts soil sampling to determine proper construction methods.
  • Common Industry Issue: Many contractors in the industry, especially for walls over $4$ feet, often forego engineering, believing they possess superior knowledge. This practice carries $100\%$ liability for the contractor if the wall fails.
  • Large-scale Projects: Major retaining walls at commercial sites like Target or malls are always $100\%$ engineered.
  • Causes of Failure in Large Walls: When these large, engineered walls fail, it is usually due to mistakes by other contractors, rather than the original engineering. An example provided involved a box culvert pipe that was not properly connected, causing water to accumulate behind a wall and lead to its failure.
• Key Factors: Water and soil conditions are the two most significant variables influencing retaining wall stability.

Landslides: Soil Avalanches and Movement

• Definition: Landslides, or "soil avalanches," involve the rapid movement of soil down a slope.
• Causes: While often nature-induced, human activities can significantly contribute to their occurrence.
• Relevance to Retaining Walls: The potential for soil movement down a slope is a critical consideration during the design and construction of retaining walls.

Dynamic vs. Static Loads

• Definition of Loads:
  • Static Load: A constant, predictable force or weight, such as a stable building sitting atop a structure. It is relatively easier to plan for.
  • Dynamic Load: An unpredictable, sudden, or changing force. This is considered more dangerous because it is harder to anticipate and plan for.
• Illustrative Example: Receiving a surprise punch to the stomach (dynamic) is more shocking and harder to prepare for than knowing you will be punched at some point today (static load, as you can brace for it).

Degradation of Soils

• Poor Farming Practices:
  • Media Portrayal vs. Reality: There is a media narrative that farmers and horticulturists are actively ruining soils. However, these professionals derive their livelihood from healthy soil, making it illogical for them to intentionally degrade it.
  • Technological Evolution: Farming practices have evolved significantly over $100$ years, from horse-drawn plows to modern GPS-guided, no-till agriculture. New practices are developed to improve soil health and productivity.
• Destruction of Forests: Practices like slash-and-burn agriculture and general clearing in tropical rainforests and other forested areas lead to soil degradation.
• Soluble Salt Buildup:
  • Cause: Excessive or indiscriminate use of fertilizers without proper soil testing. This leads to an accumulation of soluble salts in irrigated or heavily fertilized soils.
  • Homeowner Example: A common example is a homeowner applying multiple bags of fertilizer annually without soil testing, based solely on anecdotal evidence or videos, which can be highly problematic for soil health.
• Soil Contamination by Man-made Chemicals:
  • Sources: Waste oils, landfills, and other synthetic chemicals are major contributors.
  • Landfills: These sites are permanently altered; they can never return to a natural state. While necessary for waste management (as trash cannot be burned), they represent a complete transformation of the original environment.
  • Catastrophic Examples:
    • Chernobyl: A nuclear meltdown that left the surrounding soils and area permanently altered.
    • Centralia, PA: An underground mine fire has been burning for decades beneath a former town, rendering the area uninhabitable and irreversibly changed. The structural integrity of the ground is also a significant concern.
    • Mining Areas: Many regions with historical mining operations face severe soil degradation.
    • Polluted Rivers: Rivers like the Mississippi have significant pollution, though efforts can be made for cleanup (e.g., Baltimore Inner Harbor).
    • Three Mile Island: Another example of a nuclear incident that necessitated evacuation and had lasting environmental impacts.
• Industry Influence on Soil Degradation: In the landscaping and horticulture industry, the misuse of chemicals and fertilizers, along with fuel spills, are the three most significant ways in which professionals negatively impact soils. Awareness and responsible practices are crucial.

GIS and GPS for Soil and Property Mapping

• Introduction to GIS/GPS: Geographic Information Systems (GIS) and Global Positioning Systems (GPS) are powerful tools that provide extensive geospatial data.
• County Extension Maps and Soil Surveys: Many county extension offices provide online resources, including detailed soil survey maps, which offer a general understanding of soil types in a given area.
• Accessing GIS Data: Most counties in Pennsylvania (and beyond) offer public GIS sites. Searching for [County Name] GIS (e.g., Lycoming County GIS) will typically lead to these resources.
• Information Provided by GIS:
  • Property Details: Owner information, acreage, physical address, assessed values, purchase history.
  • Mapping Layers: Crucially, GIS allows for different data overlays or "layers":
    • Soil Layers: Display primary soil types using abbreviations (e.g., AXB, AB, WBB). These abbreviations can then be referenced on the county's website to understand the specific soil properties (e.g., HHB in one example area).
      • Note: While not perfectly precise for every micro-area, these maps provide an excellent general breakdown of soil types on a site, aiding in initial planning.
    • Property Lines: Distinct yellow lines often indicate property boundaries, which is invaluable for identifying exact land ownership.
    • Flood Plains: Overlays show flood hazard zones (e.g., red/blue for highest hazard, orange for fringe zones, unshaded for no flood zone). This information is vital for dictating plant choices, structure types, and assessing flood risk.
    • Zoning: Information on commercial, agricultural, or residential zoning helps in understanding land use regulations.
• Practical Benefits and Applications:
  • Understanding Soil Conditions: Aids in planning for projects, determining irrigation needs, and selecting appropriate plants.
  • Verifying Property Ownership and Lines: This is arguably the most important tool for landscapers and contractors. It prevents issues arising from incorrect homeowner claims about property boundaries, thereby mitigating liability for trespassing or damaging a neighbor's property. This is particularly challenging and critical in rural areas compared to more defined residential plots.
  • Other Uses: Nursery field crop fertilization and pH control, crop and land development, water movement and drainage planning.
• Additional Useful Tools: The OnX app, popular among outdoors enthusiasts, can also be beneficial for landscapers, offering property line tracking, linear measurements, and the ability to set pins for employee guidance.
• Key Takeaway: GIS sites are free, highly useful resources that provide critical information for the horticultural industry, helping professionals make informed decisions and avoid legal and financial liabilities.