Soil Tests & Aggregate Characteristics

• The document presents study notes for the module "Soil Tests for Use in Civil Works" at the "Instituto Tecnológico Padre Segundo Montes"
• The notes for Unit 1, specifically addresses the characteristics of aggregates

Introduction to Soil Mechanics

• Soil mechanics is crucial for all engineering projects as all structures, regardless of size, rely on the ground for foundation

• Economic viability and safety depend on the soil's behaviour

• Structural design precision becomes irrelevant without understanding the soil

• Engineers historically built on rock or compact soils

• Population growth has made these locations scarce, which forces construction in areas with low soil resistance, deformation risks, or filtration issues

• Soil is used to build bases for roads, dams, walls, slopes, and houses

• Soil properties vary by location, requiring study for safe and cost-effective foundations

• Soil Mechanics studies behavior as both a construction material and as engineering support

• Terzaghi defined soil mechanics as applying mechanics and hydraulics to engineering problems dealing with sediments and unconsolidated solid particles from rock disintegration, with or without organic matter

• Soil can have different meanings

• Agronomists view it as the superficial part of Earth's crust that sustains plant life, which is limiting for engineering uses

• Geologists see soil ambiguously as life-giving material, loose and unconsolidated layers from the surface to solid rock, formed by weathering and rock disintegration

• Civil Engineers see soil as uncemented or lightly cemented material accumulations, with varied nature and thickness.

• Soil is defined as any material in the terrestrial crust that is not hard rock

• Properties of interest to engineers are mechanical to allow stability calculations

• Mechanical properties include shear strength measured by cohesion (c) and internal friction angle (φ)

• Soil deformation, gauged by settling(Δa) and consolidation coefficient (Cc)

• Permeability, assessed through the permeability coefficient (k)

Soil Formation

• Earth's crustal rocks disintegrate mechanically and decompose chemically, forming loose materials
• Rock knowledge is essential for understanding how soils form
• Geologists define rock as any coherent crustal material older than one million years old
• Soft materials like clays, shales, and sands are regarded as rocks by geologists and soils by engineers, regardless of age
• Civil Engineers classify rock as hardened material needing drills, wedges, explosives, or brute force to excavate
• Rocks are coherent, hard, rigid deposits which form a part of the terrestrial crush that can be of igneous, sedimentary or metamorphic origin

Rock Classification

• Igneous rocks result from magma cooling, categorized by texture, mineral composition, color, and origin
  • Classified as intrusive (plutonic) if magma solidifies inside the Earth's crust, and extrusive if solidifies after reaching the surface
  • Granite, gabbro, rhyolite, basalt, obsidian, and scoria are examples of igneous rock
• Sedimentary rocks form through weathering, transport, deposition, and cementation
  • Common sedimentary examples include shale, sandstone, and limestone
• Metamorphic rocks result from high-temperature and high-pressure metamorphism of sedimentary or igneous rocks deep within the earth
  • This results in change to texture and mineralogical composition to create rocks like slate, schist, gneiss, quartzite and marble

Soil Formation Processes

• Soils mainly form through mechanical and chemical processes
• Mechanical processes (physical weathering) disintegrate rock without altering the mineralogy of the grains, maintaining parent rock composition
• Magma solidification is an intemperism process
• Mechanical weathering agents: temperature fluctuations, water freezing in rock joints, wind, and rain
• Erosion frees stress after removing a surface layer
• Transport involves gravity, water, and wind
• Deposition is the final step of formation

Physical Change Agents

• Solar heat causes expansion differences, leading to stress and surface layer detachment
• Moving water erodes angular rock fragments, creating rounded "rolled stones" found in rivers and streams
• Rainwater creates cracks and erodes rocks, disintegrating particles
• Wind causes erosion by carrying sand, forming dunes (desert sands) and loess (windblown silt)
• Wind erodes slopes of sandy soils, detaching and carrying away particles

Chemical Change Agents

• Oxidation occurs when rainwater interacts with rocks, causing oxygen in the air to react chemically in the presence of moisture, mainly in iron-containing rocks
• Hydration combines a substance with water to form hydrates of chemically-combined water, creating new minerals.

Soil Profile Formation

• Rock turns into boulders, gravel, sand, silt and clay due to mechanical breakdown and chemical reactions
• Transport and deposition by streams, oceans, wind, and glaciers erode and move soil
• The materials settle into deposition sites

Influence of Transport

• Transport and deposition cause erosion, which wears down the Earth via mechanical action
• Being lifted or dragged erodes particles, reducing their overall size and easing transport

Soil Deposits

• Aeolian Deposits are sediments moved by wind consisting of small particles, accumulating where wind velocity drops, with the wind sorting particle sizes
• Glacial Deposits are rocks of all sizes and various sediments transported by glaciers which fall from the valley sides
• Alluvial Deposits is loose soil and rock dust which are detached by rain hitting earth
  • The resulting pluvial and fluvial deposits of these particles are transported in a water current.
  • Decrease the current slope and the materials are deposited, losing speed
• Lacustrine Deposits are lake deposition in both fresh and salt water, generally being small material at the bottom
  • In seawater, particles with colloidal size flocculate faster, quickly precipitating to lake bottoms

Rock Cycle

• The rock cycling process is continuous, extremely slow, and geological where rocks change through the categories of igneous, sedimentary, and metamorphic
• Cycle begins when magma surfaces via volcanic eruption where it forms extrusive or intrusive igneous rocks depending on surface or in-crust cooling
• The rocks can melt in future eruptions or get metamorphed by pressure and temperature into metamorphic rocks
• Pyroclastic material ejected is spread, weathers with environment conditions

Soil Types

• Gravel consists of loose rock fragments over 2mm in diameter, transported by water, causing edge wear which ranges from 2 mm to 76.2 mm
• Sand is made up of fine particles from artificial rock crushing, which contains relative proportions of clay and gravel
  • Clean sands don't contract when drying, are nonplastic, less compressible than clay, and are instantly compressed when loaded
  • Diameter usually ranges from 0.05 mm to 2 mm
• Silts are fine-grained soils with no/low plasticity, unsuitable for supporting load when loose and saturated
  • Silt can be clear to very dark grey, permeability is very low, compressibility is very elevated which makes them poor when cementing and commonly measures between 0.005 to 0.05 mm

Clay

• Clay: Solid particles smaller than 0.005 mm in soil, become plastic when mixed with water
• Clay exhibits high compressibility, making it poor for foundations
• Advances in soil studies have improved clay to better the properties

Talpetate

• Talpetate: Hard, dark brown material of sands/silts/clays with clay or calcium carbonate cement
• Talpetate compacts with a very hard result after consolidating under large leads
• This makes it suitable as foundation

Definitions of Soil Types

• Residual Soils: Weathered rock remnants that haven't been transported which is sandy or gravel-like in nature with concentrations of oxides from leaching

  • Examples: laterite and kaolinite

• Weathering Profile: Different material properties, which originate mechanically and decompose chemically which depends on local variables of the local rock structure, terrain, erosion environments, groundwater habits, and climate

• Inherited Structures: Joints/cracks/faults as structural defects from original rock

• Transported Soils: Removed from the formation site, and deposited to form unconsolidated sediment such as wind blown sand etc

• Alluvial Soils (alluvium): Materials laid down with areanas and gravels from streams and rivers, presented irregularly in discontinuous ways with good gradation.

• Cohesive Soils: Clay particle content gives cohesion and plasticity.

• Non-Cohesive Soils: Rounded particles, not laminar without plasticity such as areanas or gravels.

• Organic Soil: Mostly vegetation-based mixture with various decay levels and mineralization levels originating through lakes or swamps, creating loose texture at touch with darker color and odor

• Turf: Exclusively organic, very spongey, easy to compress and can burn with many engineering problems from high compressibility.

• Loess: Air based silt of particles, which is uniform in size and low density made of vertical holes which were made out of root. Loess compaction is difficult, and foundations can sag if soils get too saturate.