Guest Viewpoint: Geoff Larsen

Thickened aggregate working pad.

My schedule this weekend is particularly full, so in lieu of an original blog post, the following is reprint of an article written by current Construction Specifications Institute - Willamette Valley Chapter president Geoff Larsen, PE, CSI that appeared in the August issue of The Documentor. Geoff is an Associate and Senior Civil Engineer with Mazzetti/BHEGroup’s Eugene office. His article provides an excellent summary about the critical issue of soil conditions, particularly here in the Willamette Valley where clayey, silty soils and the significant concerns that come with them predominate.

The topics of soil conditions and earthwork are far from central to an architect’s education, but my own experience has taught me they can have an outsized impact on a project if not properly anticipated during the design phases. For this reason, it’s important for architects to possess a basic understanding about the issues they present and how they may be addressed in Division 31 of the Project Manual.

Division 31 - Soil Conditions and Earthwork in the Willamette Valley

By Geoff Larsen, President WVC/CSI

At the midpoint of the dry summer construction season, I am reminded of the varied soil conditions that are exposed with excavation and earthwork for projects throughout the Willamette Valley. Although soil conditions are different for each project site, there are some common issues that crop up during earthwork construction for each project. I would like to take this opportunity to reflect on some of those common issues and share some notes and lessons learned related to the Division 31 Earthwork specifications.

 

Division 31 Earthwork Specifications and the Geotechnical Report:

Project earthwork specifications are found in Division 31 and may include multiple sections (i.e. Aggregates, Site Clearing, Grading, Excavation, Fill, Compaction) or may be consolidated into fewer sections (i.e. Earthwork or Earth Moving). The technical details of the earthwork specifications are a subject for another discussion. However, in a nutshell, the earthwork specifications should reflect the recommendations of the geotechnical report and should establish a framework for working with the existing onsite soils (excavation, protective measures, stabilization, etc.) and for placement of imported soils (base rock, drainage aggregate, backfill, etc.). Since the approach to earthwork can change as a result of weather conditions, concealed conditions, and construction methods, it is also beneficial for the Division 31 specifications (in coordination with Division 1) to set a framework for dealing with potential changes to contract time and contract amount.

Sensitive Soils and Protection from Damage:

Soils in the Willamette Valley are often clayey, silty soils that are easily disturbed when excessively moist and are often susceptible to damage from construction equipment. Within the footprint of new building or pavement areas, soils that are excessively soft or that have been damaged by construction equipment will need to be excavated and removed (referred to as “over-excavation”) to reach a firm bearing surface. This over-excavation is then backfilled with stable material (typically imported aggregate). Conducting earthwork during the dry season can help limit the risk of damaging the underlying soils (subgrade) but some soils may be slow to dry out, even during the dry season, which can be problematic for accelerated construction schedules. To address this issue, it is common to provide a thickened rock working pad or cement treatment (see below for further discussion) to protect or stabilize the underlying soils in areas where construction vehicles will operate. For large projects, the cost of protective or stabilization measures may be significant, and the Owner may choose to reduce protective measures to save cost, understanding that there is increased risk of additional cost and time to deal with over-excavation and repair if damage to the subgrade occurs. Regardless of the approach, it is important to establish a structure for subgrade protection within the Division 31 specifications and to communicate the alternatives with the stakeholders. The following are important questions to consider: 

• Is subgrade protection/stabilization (rock pads / cement treatment) defined/required by the specifications? 
• Who is responsible for the cost of repairing damaged subgrade soils when protective measures are not in place (contractor or owner)?
• How is the contract adjusted when unsuitable soil is encountered or when protective measures are followed but the subgrade still becomes damaged and requires repair? Are unit prices utilized?
• How is construction monitored to determine when unsuitable soils are encountered or when the subgrade is damaged and requires repair? When will geotechnical inspections occur?
• How is the extent of subgrade repair (over-excavation) monitored and measured?
• Are there different requirements for wet-weather construction?

Subgrade Stabilization – Cement Treatment:

Cement treatment of native soils involves mixing cement with the upper layer of soil (12” depth is common) and then compacting. Once cured, the cement-soil mixture provides a stabilized surface. Cement treatment can be a cost-effective solution for sites with particularly poor/unpredictable soils, sites with large paved or building areas, or sites where construction will occur during the wet season. Cement treatment can also provide a higher level of cost certainty than conventional aggregate base stabilization due to lower risk of over-excavation and damage from construction equipment. Excavation for footings or utility trenches through cement-treated subgrade can be more challenging, so construction sequencing should be considered carefully.

Unit Prices:

Unit prices can be an effective way to control costs if subsurface conditions are highly unknown or highly variable and the quantity of over-excavation is unknown. The cost of rock excavation is significantly higher than typical soils, so unit prices can also be a good strategy when rock excavation is anticipated. When evaluating unit price bids, it is important to consider the anticipated probable range of excavation quantities when comparing bids. For example, a bid with a low base bid and high over-excavation unit price may result in a higher overall price.

Measurement and Payment:

Measuring soil quantities for repairing unsuitable or damaged subgrade soils can be a point of contention if not properly defined in the contract documents. It is particularly important to be diligent about units of measurement if utilizing Unit Prices for contract adjustments. Typical units of measurement include in-place volume (bank cubic yards) or in-truck volume (loose cubic yards).  Although in-truck quantities seem like a simple way to measure quantities, in-place quantities provide a more definitive and verifiable basis of measurement. Soil changes volume (increases) when excavated, so there is always a difference between in-place and in-truck quantities. For large projects with potentially large areas of unsuitable soil removal (over-excavation), it may be necessary to verify in-place quantities using survey equipment.

Geotechnical Construction Administration Services:

As projects move toward the construction phase, it is important to have the appropriate geotechnical and testing services in place to monitor earthwork once construction is underway. The native soil directly below building foundations is a critically important interface and it benefits all parties to ensure proper subgrade inspection by the Geotechnical Engineer prior to pouring foundations. The contract documents should be clear about who is responsible for procuring the testing and inspection services.

Proactively thinking about these issues has helped me on past projects to set expectations, provide cost control, and has led to successful projects. If you are a Specifier, Architect, Contractor, or Engineer, I hope these notes can help you bypass predicaments related to earthwork and set your projects up for success.

Geoff Larsen, PE, CSI

Enjoy the rest of your summer (and the dry construction season)!