Davis Septic Design
Soil Types & OSCAR-II Septic Installation in King/Snohomish Counties: A Case Study
For homeowners in King and Snohomish counties, the ground beneath their feet tells a story. It’s a story of ancient glaciers, rich deposits, and dense, compacted soils that can make installing a standard septic system a significant challenge. When a septic system fails here, the solution isn’t just about replacing a tank; it’s about understanding the unique soil science of your property and choosing a technology engineered to work with it.
This guide will walk you through the critical link between local soil types and your septic system choices. We’ll then go behind the scenes on a real project, following the installation of a modern OSCAR-II system from the initial soil test to the final performance check, showing you how today’s technology solves yesterday’s problems.
Soil Science & Septic Design
Before any septic design begins, we must first “read the land.” In Washington, the soil itself is legally considered the final component of the treatment process. Its ability to safely filter and disperse wastewater dictates everything that follows.
How does soil type affect septic choice?
Your property’s soil acts like a natural filter and sponge. It needs to absorb treated wastewater (effluent) at a steady rate—not too fast and not too slow.
- If soil is too dense (like clay), effluent can’t soak in. It pools, surfaces in your yard, and causes a system failure.
- If soil is too coarse (like clean gravel), effluent can pass through too quickly without adequate filtration, potentially contaminating groundwater.
This “percolation rate,” along with the usable soil depth, determines whether you can use a simple, conventional gravity system or if you need an advanced engineered system to overcome the site’s limitations.
Reading soil logs—what to look for
A soil log is a professional report created by a licensed designer after digging a test pit. It’s the scientific basis for your septic design. Key things we look for include:
- Restrictive Layer: This is the most critical factor. It’s a layer of dense glacial till, hardpan, or bedrock that water cannot easily pass through. Its depth determines your vertical separation.
- Seasonal High Water Table: We identify this by looking for “mottling”—patches of rust-colored soil. This tells us the highest point groundwater reaches in the wet season. The bottom of your drain field must be a set distance above this mark to ensure it never gets flooded.
- Soil Texture: We document the layers of sand, silt, and clay, which informs the system type and size.
Gravel, clay, sand: pros and cons
| Soil Type | Pros for Septic Systems | Cons for Septic Systems |
| Sand / Sandy Loam | Excellent percolation. Ideal for simple, cost-effective conventional gravity systems. | Can be rare in areas dominated by glacial deposits. |
| Gravel | Drains very quickly. | Can drain too quickly, posing a risk to groundwater if effluent is not highly treated first. |
| Clay / Glacial Till | None. | Extremely slow percolation. Causes effluent to pool and surface. The #1 reason conventional systems fail in our region. |
The OSCAR-II System Explained
When a site has dense glacial till or a high water table, we need an engineered solution. One of the most effective and versatile options is the OSCAR-II system.
What is an OSCAR-II?
OSCAR stands for On-site Sand Coil Area Recharge. It is a low-profile treatment and dispersal system. An OSCAR-II unit consists of coils of drip tubing laid out on a bed of specialized sand medium. Effluent from the pump tank is slowly dripped onto the sand, which provides a final, high level of treatment before the water is gently absorbed into the native soil below.
Advantages over conventional systems
The OSCAR-II is a problem-solver. Its key advantages include:
- Handles Poor Soils: It’s specifically designed to work on sites with dense clay or shallow restrictive layers.
- Small Footprint: The treatment unit is compact, making it perfect for smaller lots or properties with limited space due to trees, outbuildings, or setbacks.
- Excellent Water Treatment: The sand medium provides robust filtration, making it an environmentally sound choice for protecting groundwater.
Suitability for local soil conditions
The geology of King and Snohomish counties was heavily shaped by the Vashon Glacier, which left behind vast deposits of highly compacted, dense till. This soil is notoriously difficult for conventional septic systems. The OSCAR-II directly addresses this challenge, making it possible to safely develop and repair homes on properties that would otherwise be unsuitable.
Step-by-Step Installation Walkthrough: A Woodinville Case Study
The Scenario: A homeowner on a 0.5-acre wooded lot in Woodinville had a failing septic system from the 1990s. Their yard had soggy patches, and a recent inspection confirmed the drain field was no longer functioning.
Step 1: Soil log testing and feasibility
Our first step was to dig two test pits. The soil log revealed just 18 inches of usable topsoil before hitting a dense, water-restricting layer of glacial till. This immediately ruled out a conventional system. Given the lot’s size and the homeowner’s desire to preserve several mature trees, a large mound system was not ideal. The OSCAR-II was identified as the perfect solution due to its small footprint and suitability for shallow soils.
Step 2: Permitting timeline and submittal
With the feasibility confirmed, we created a complete design package. This included the soil log data, detailed plans for the new tanks, the pump system, and the OSCAR-II unit specifications. The package was submitted to Public Health — Seattle & King County. Because the design was thorough and used a pre-approved, modern technology, the permit was issued in just under four weeks.
Step 3: Installation coordination with the contractor
Once the permit was in hand, we coordinated with the homeowner’s chosen certified installer. We held a pre-construction meeting on-site to walk through the design, stake out the exact locations for all components, and confirm the installation schedule. This ensures that the designer’s plan is executed precisely by the installation team.
Problem-Solving Mid-Project
Even with perfect planning, real-world construction can present challenges.
Typical delays and solutions
The most common installation delays are caused by weather and supply chain issues. Heavy rain can flood excavations, while specific components like pumps or control panels can sometimes have lead times. We mitigate this by ordering critical parts as soon as the permit is approved and by building buffer days into the installation schedule.
How installation challenges were handled
During our Woodinville project, an unexpected summer downpour occurred after the excavation for the new tanks was dug but before they could be installed. The hole partially filled with water.
Our Solution: The installer acted quickly. They used a pump to dewater the excavation, allowed it to dry for 24 hours, and then laid a fresh, stable base of crushed rock. This ensured the tanks were set on a solid, level foundation, preventing a minor weather event from turning into a major project delay.
Post-Install & Performance
The job isn’t done when the last patch of dirt is smoothed over. Proper maintenance is key to a long system life.
Service requirements and maintenance
An OSCAR-II is an advanced system that requires routine professional service. This includes an annual inspection by a certified maintenance provider to check the pump, floats, and electrical components, and to flush the drip lines. This is a simple, proactive step that ensures the system functions at peak performance for For homeowners in King and Snohomish counties, the ground beneath their feet tells a story. It’s a story of ancient glaciers, rich deposits, and dense, compacted soils that can make installing a standard septic system a significant challenge. When a septic system fails here, the solution isn’t just about replacing a tank; it’s about understanding the unique soil science of your property and choosing a technology engineered to work with it.
This guide will walk you through the critical link between local soil types and your septic system choices. We’ll then go behind the scenes on a real project, following the installation of a modern OSCAR-II system from the initial soil test to the final performance check, showing you how today’s technology solves yesterday’s problems.
Soil Science & Septic Design
Before any septic design begins, we must first “read the land.” In Washington, the soil itself is legally considered the final component of the treatment process. Its ability to safely filter and disperse wastewater dictates everything that follows.
How does soil type affect septic choice?
Your property’s soil acts like a natural filter and sponge. It needs to absorb treated wastewater (effluent) at a steady rate—not too fast and not too slow.
- If soil is too dense (like clay), effluent can’t soak in. It pools, surfaces in your yard, and causes a system failure.
- If soil is too coarse (like clean gravel), effluent can pass through too quickly without adequate filtration, potentially contaminating groundwater.
This “percolation rate,” along with the usable soil depth, determines whether you can use a simple, conventional gravity system or if you need an advanced engineered system to overcome the site’s limitations.
Reading soil logs—what to look for
A soil log is a professional report created by a licensed designer after digging a test pit. It’s the scientific basis for your septic design. Key things we look for include:
- Restrictive Layer: This is the most critical factor. It’s a layer of dense glacial till, hardpan, or bedrock that water cannot easily pass through. Its depth determines your vertical separation.
- Seasonal High Water Table: We identify this by looking for “mottling”—patches of rust-colored soil. This tells us the highest point groundwater reaches in the wet season. The bottom of your drain field must be a set distance above this mark to ensure it never gets flooded.
- Soil Texture: We document the layers of sand, silt, and clay, which informs the system type and size.
Gravel, clay, sand: pros and cons
| Soil Type | Pros for Septic Systems | Cons for Septic Systems |
| Sand / Sandy Loam | Excellent percolation. Ideal for simple, cost-effective conventional gravity systems. | Can be rare in areas dominated by glacial deposits. |
| Gravel | Drains very quickly. | Can drain too quickly, posing a risk to groundwater if effluent is not highly treated first. |
| Clay / Glacial Till | None. | Extremely slow percolation. Causes effluent to pool and surface. The #1 reason conventional systems fail in our region |
The OSCAR-II System Explained
When a site has dense glacial till or a high water table, we need an engineered solution. One of the most effective and versatile options is the OSCAR-II system.
What is an OSCAR-II?
OSCAR stands for On-site Sand Coil Area Recharge. It is a low-profile treatment and dispersal system. An OSCAR-II unit consists of coils of drip tubing laid out on a bed of specialized sand medium. Effluent from the pump tank is slowly dripped onto the sand, which provides a final, high level of treatment before the water is gently absorbed into the native soil below.
Advantages over conventional systems
The OSCAR-II is a problem-solver. Its key advantages include:
- Handles Poor Soils: It’s specifically designed to work on sites with dense clay or shallow restrictive layers.
- Small Footprint: The treatment unit is compact, making it perfect for smaller lots or properties with limited space due to trees, outbuildings, or setbacks.
- Excellent Water Treatment: The sand medium provides robust filtration, making it an environmentally sound choice for protecting groundwater.
Suitability for local soil conditions
The geology of King and Snohomish counties was heavily shaped by the Vashon Glacier, which left behind vast deposits of highly compacted, dense till. This soil is notoriously difficult for conventional septic systems. The OSCAR-II directly addresses this challenge, making it possible to safely develop and repair homes on properties that would otherwise be unsuitable.
Step-by-Step Installation Walkthrough: A Woodinville Case Study
The Scenario: A homeowner on a 0.5-acre wooded lot in Woodinville had a failing septic system from the 1990s. Their yard had soggy patches, and a recent inspection confirmed the drain field was no longer functioning.
Step 1: Soil log testing and feasibility
Our first step was to dig two test pits. The soil log revealed just 18 inches of usable topsoil before hitting a dense, water-restricting layer of glacial till. This immediately ruled out a conventional system. Given the lot’s size and the homeowner’s desire to preserve several mature trees, a large mound system was not ideal. The OSCAR-II was identified as the perfect solution due to its small footprint and suitability for shallow soils.
Step 2: Permitting timeline and submittal
With the feasibility confirmed, we created a complete design package. This included the soil log data, detailed plans for the new tanks, the pump system, and the OSCAR-II unit specifications. The package was submitted to Public Health — Seattle & King County. Because the design was thorough and used a pre-approved, modern technology, the permit was issued in just under four weeks.
Step 3: Installation coordination with the contractor
Once the permit was in hand, we coordinated with the homeowner’s chosen certified installer. We held a pre-construction meeting on-site to walk through the design, stake out the exact locations for all components, and confirm the installation schedule. This ensures that the designer’s plan is executed precisely by the installation team.
Problem-Solving Mid-Project
Even with perfect planning, real-world construction can present challenges.
Typical delays and solutions
The most common installation delays are caused by weather and supply chain issues. Heavy rain can flood excavations, while specific components like pumps or control panels can sometimes have lead times. We mitigate this by ordering critical parts as soon as the permit is approved and by building buffer days into the installation schedule.
How installation challenges were handled
During our Woodinville project, an unexpected summer downpour occurred after the excavation for the new tanks was dug but before they could be installed. The hole partially filled with water.
Our Solution: The installer acted quickly. They used a pump to dewater the excavation, allowed it to dry for 24 hours, and then laid a fresh, stable base of crushed rock. This ensured the tanks were set on a solid, level foundation, preventing a minor weather event from turning into a major project delay.
Post-Install & Performance
The job isn’t done when the last patch of dirt is smoothed over. Proper maintenance is key to a long system life.
Service requirements and maintenance
An OSCAR-II is an advanced system that requires routine professional service. This includes an annual inspection by a certified maintenance provider to check the pump, floats, and electrical components, and to flush the drip lines. This is a simple, proactive step that ensures the system functions at peak performance for decades. The septic tank will still require pumping every 3-5 years, just like any other system.
This annual checkup is vital for ensuring decades of reliable performance. For more on what you can do as a homeowner between professional visits, read our guide on essential septic maintenance tips to protect your investment.decades. The septic tank will still require pumping every 3-5 years, just like any other system.
This annual checkup is vital for ensuring decades of reliable performance. For more on what you can do as a homeowner between professional visits, read our guide on essential septic maintenance tips to protect your investment.
