Sedimentary Soils: How Residual Soils Are Moved, Redeposited and Transformed
- MTS DNC ENERGY CONSULTANTS LIMITED
- Jul 26
- 4 min read
Residual soils form in place by weathering of the underlying rock. Once those soils are picked up and moved by wind, water, ice or human activity, they become sedimentary soils (also called sediments).
Engineers often just call them “soils”, but using the term sedimentary soils is more accurate. It immediately signals that these materials have a completely different history — and therefore different properties — from residual soils.
How Sedimentary Soils Form
The properties of a sedimentary soil depend on two main factors:
The source rock
The transporting agent (gravity, water, wind, glacial ice, or modern machines)
Example: Granite weathers into a sandy residual soil that still contains the original minerals of the granite. When this material is transported by wind or water, the softer minerals are gradually worn away. The remaining sediment becomes richer in hard, durable minerals — especially quartz. This is why many river and beach sands are almost pure quartz.
Recycling of Sediments
Sedimentary soils themselves can be eroded again, transported and redeposited as new sediments.
A classic example is a glacial deposit (a messy mixture of sand, silt and clay). When this material is later eroded by a river, the sand, silt and clay can be sorted and deposited separately as alluvium. Each new cycle of erosion and deposition further changes the soil’s particle size distribution and engineering behaviour.
From Sediments to Sedimentary Rocks
If sediments stay buried long enough, two things can happen:
Pressure from overlying layers compresses them
Secondary minerals (especially carbonates) cement the particles together
The result is a sedimentary rock — shale, sandstone or limestone.
These rocks can later be uplifted and weathered again, producing new residual soils. The geological cycle continues.
Random Fill vs Engineered Fill – A Critical Distinction
Soils that have been moved very recently by human activity are called fill. There are two completely different types:
Random Fill
Material dumped without control
Often contains bricks, glass, plastic, timber, metal, or even old newspapers and bedsprings
Has never been properly compressed or cemented by geological processes
Usually weak, highly compressible, unpredictable, and sometimes combustible or contaminated
Building on random fill is one of the most common causes of excessive settlement, cracking and foundation problems. Identification of random fill is one of the most important skills of a geotechnical engineer.
Engineered Fill
Carefully selected material
Placed in thin layers and compacted under controlled moisture and density conditions
Results in a dense, strong, low-compressibility material
Engineered fill is routinely used for:
Building foundations
Highway embankments
Earth dams and levees
Platform construction
When properly designed and controlled, engineered fill is a reliable construction material.
Relative Abundance of Sedimentary Materials
Although sedimentary rocks and sediments form only a thin “skin” on the Earth’s crust, they cover most of the land surface that we actually build on.
A typical geological sequence is:
Deep igneous or metamorphic rock
Layered sedimentary rocks
Residual soil developed on those rocks, or a blanket of younger sedimentary soils deposited by rivers, wind or glaciers
In the last 100–150 years, human activity (especially agriculture) has greatly accelerated erosion. The uppermost layer of sediment on many river floodplains is now “post-cultural” — deposited after widespread cultivation began. These modern sediments often carry agricultural chemicals that can harm fisheries and create “dead zones” near river deltas.
Why This Matters for Engineers and Builders
Material | Origin | Typical Engineering Behaviour | Design Implication |
Residual soil | Weathered in place | Gradual transition to rock | Careful logging of weathering grade |
Sedimentary soil | Transported & redeposited | Properties depend on source + transport | Need for proper classification |
Random fill | Uncontrolled human dumping | Weak, compressible, unpredictable | Avoid or remove / improve |
Engineered fill | Controlled placement & compaction | Strong, dense, predictable | Excellent foundation / embankment material |
Understanding whether the ground under a site is residual soil, natural sedimentary soil, random fill or engineered fill is fundamental to safe and economical design.
Need Professional Help? Contact the Experts!
For Building Services Design – whether it’s HVAC, plumbing, or civil engineering – reach out to Nexus M&E Design for expert solutions tailored to your project needs.
If you require a technical assessment, BER rating, or assistance with SEAI grants, get in touch with the professionals at MTS DNC Energy Consultants for comprehensive guidance and support.
Disclaimer:
The content provided in this post is for informational purposes only and should not be construed as professional engineering, architectural, or surveying advice. While every effort is made to ensure the accuracy and reliability of the information presented, it is not a substitute for a thorough, site-specific inspection or the expertise of a qualified professional. For detailed guidance on foundation issues, structural integrity, or repairs, always consult with a licensed engineer, architect, or surveyor. The authors and publishers are not responsible for any damages or losses resulting from the use or reliance on this information.
