I would like to build some retaining walls around the house to terrace my garden or to expand my driveway area. Do I need a permit for the walls to be engineered?

The building department says I need an erosion control plan to get a NPDES storm water permit or have directed me to the Oregon DEQ for a NPDES permit. Can you prepare an erosion control plan and help with the NPDES permit application?

 A geotechnical engineer has a Professional Engineering license and is registered with the State of Oregon. Oregon has a Professional Engineer’s license as well as a more specialized Geotechnical Engineer license.

 A geotechnical engineer evaluates the subsurface conditions of the site and prepares a report with conclusions and recommendations for design and construction of a project.

You should call a geotechnical engineer early in the project to determine the feasibility of the site or to determine the site preparation and foundation design parameters. Often times, a geotechnical engineer is hired at the same time as a surveyor is preparing the the topographic map of the site. The geotechnical engineer often prepares reports that are necessary for acceptance of the project through the local planning and building department.

Amrhein Associates will provide an estimated time for initial site visits and report completion. It is the responsibility of the contractor to call us back to the site, with a 24 hour notice, for inspections as required by the report recommendations.

A structural engineer addresses structural issues in and around buildings. The structural components include footings, floors, walls and roofs using materials such as, concrete, steel or wood. A geotechnical engineer investigates soil conditions and provides recommendations to the structural engineer so they can design foundation components.

 Back to Top

The geotechnical soil report is intended to define the subsurface conditions and provide geotechnical conclusions and recommendations for design and construction of the project. A geological assessment or engineering geology report may be incorporated into or included as an appendix to the geotechnical soil report for the purpose of providing geologic information for the geotechnical engineer, explaining the implications of the subsurface conditions for appropriate project design and construction. The geotechnical soil report should include the following:

General

• Name, address, and phone number.
• Client for whom the geotechnical soil report was prepared.
• A description of the proposed project and its location.
• A site map of the area at a scale of 1":400' or larger. Geologic conditions, topography, and location of proposed structures are to be shown. A copy of published geologic maps shall also be provided.
• A review of the geologic history and history of prior excavation and fills.
• A field reconnaissance of the site and vicinity.

• A discussion of the engineering aspects of the site and proposed project. The discussion should address foundation types for proposed structures, retaining systems, grading considerations, stability of cut slopes and constructed embankments, settlement of the site and adjacent sites due to existing conditions, proposed surface and subsurface drainage facilities.

GEOTECHNICAL FIELD INVESTIGATIONS

• Planned construction: type of structure and use, type of construction and foundation, floor system, number of stories, estimated structural loads.
• Scope: date of work done, investigative methods, sampling methods, logs of borings or test pits, elevations of borings or test pits for reference of materials and samples to finished grade or footing elevations, identify real or assumed elevations.
• Location of all samples taken, surface and subsurface.
• Groundwater conditions and potential: future natural and artificial seepage effects.

ENGINEERING: MATERIAL CHARACTERISTICS AND TESTING

• Test methods used, type or condition of samples, applicable engineering graphics and calculations, results of all tests, and sample locations of all test samples.
• Unified Soil Classifications of Materials.
• Material competency and strength of existing soils.
• Pertinent engineering geologic attributes (clayey, weak, loose, alignments, fissility, planar boundaries, pervious or water-bearing parts, susceptibility to mass wasting, erosion, piping, or compressibility).
• Effects and extent of weathering (existing and relationship to project design and future site stability, material strength).
• Field moisture content.
• Bearing capacity and/or shear strength of areas affected by future foundation placement (drained or undrained conditions, effective stress or total stress analysis, in-situ or remolded samples must be identified).
• Consolidation or settlement potential.
• Expansion potential.
• Maximum density-optimum moisture parameters of proposed fill material.

  Back to Top

 Why hire a geotechnical engineer?

Many builders, architects, and individual home owners are reluctant to hire geotechnical engineers to perform subsurface investigations, and that is easy to understand. The cost of a house can go up a few thousand dollars with the addition of a geotechnical investigation and report. Despite the cost, there are circumstances where consulting a soils engineer is a wise investment in the project.

• Evidence of previous earthwork at the site, especially filling, requires careful investigation. The site may contain uncontrolled fill soils and debris such as demolition rubble or abandoned vehicles. Any of these things can cause severe settlement problems.
   
• Encountering groundwater or weak soil while excavating for foundations may indicate a potential for settlement in the future. The problem area should be carefully delineated and remedied by a geotechnical engineer.
   
• If you are building a house on a lot with a steep slope, then you need a geotechnical engineer to determine the soil properties and to evaluate the stability of the slopes.
   
• A local history of the presence of clay soils that are susceptible to shrinking and swelling with changes in moisture content also requires careful investigation.

If you are crossing your fingers and hoping the soil conditions won't cause a problem, you're probably right to be uneasy. Call in a specialist.

 Back to Top

Needing a soil test can mean several things. If you have a new site or new project and are preparing an application for planning or building permit, we can conduct a site investigation and prepare a geotechnical engineering report. Our report will describe the site conditions, requirements for grading of the site, and recommendations for foundation design.

 If your house or building is under construction, and the contractor or local building department require an inspection of the excavation or foundation footings, we can conduct a visit to the site to observe the condition of the subgrade and determine its acceptability for placement of the reinforcing steel and pouring of the concrete.

 If soil fill is being placed across your site, we can conduct density tests in the field to determine the soil's density and the adequacy of the compaction by the contractor. Prior to conducting our field density testing, we will need to obtain a sample of the soil being placed in order to conduct a laboratory test that will determine a soil's maximum density that can be obtained over a range of moisture contents. The field density is compared to the maximum density obtained in the laboratory to determine a relative density.

 Back to Top

Landscaping walls built with timbers, stacked blocks or stacked rocks have historically not required permits if less than 4 feet in height. Recently, it is becoming more common for the local building department to require a permit for any wall that supports a slope, driveway, patio or any type of structure. It is becoming common for these walls to incorporate geogrid (open grid fabric) anchors that extend into the soil backfill. For walls constructed with lower quality materials, maximum wall height may be less than 4 feet and you should check with the local building department or a geotechnical engineer with the specifics of your project.

 Back to Top

We can prepare an erosion control plan and complete your NPDES application for your proposed site development. A NPDES permit (1200C) is required for development of any site over 1 acre. This includes sites that are to be developed in phases, and although each phase is less than 1 acre, the total development through all phases will be more than 1 acre. An erosion control plan will detail the location and type of erosion control measures that will have to be installed and maintained on site during construction.

 Back to Top