What Designers Need to Verify the Geotechnical Capacity of Boardwalk Foundations

Posted: September 14, 2022

 

In thinking about the proper boardwalk foundation for a project, performing a geotechnical investigation is critical for the overall success of the project. A thorough geotechnical investigation helps prevent change orders after the bid process (allowing for more accurate bids), and it empowers the structural designer to facilitate the best possible boardwalk or bridge.

Well-planned boardwalk foundations ensure a smoother installation process for contractors, a finished product that better aligns with the designer’s original vision, and a happy group of owners!

In this article, we’ll examine what geotechnical engineers need to provide to structural engineers to design a boardwalk or bridge for each type of foundation.

 

Click to download free typical PermaTrak concrete boardwalk sections and  engineering drawings →

 

What Makes For a Thorough Geotechnical Investigation?

A geotechnical investigation specific to the proposed boardwalk structure should be completed early in the design process. 

For maximum usefulness, multiple borings should be taken along the future boardwalk alignment. Soil borings using a drill rig, or hand augers provide the required information on soil characteristics. 

The method of soil exploration (drill rig vs. hand auger) will be determined by the geotechnical engineer based on site access.

 

What Type of Engineer?

Let’s clarify two different types of engineers that we will be referencing throughout this article.

  1. Geotechnical Engineer: This is the type of engineer who will run site-specific geotechnical investigations and write-up a report. Included in the investigation and report will be an analysis of the soil conditions, foundation type recommendations, and associated design parameters for the structural engineer’s use. Designers should consult these geotechnical engineers early on in the project planning process.
  2. Structural Engineer/Boardwalk Designer: This is the type of engineer who utilizes the analysis and data from the geotechnical report to design the boardwalk foundation for strength, serviceability, and embedment depth. When partnering with PermaTrak, our team of structural engineers can, if needed, handle this part of the project and provide free P.E. approved construction drawings.

 

Boardwalk Foundation: Shallow Foundations

Foundation Type Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation
Shallow Foundations Allowable Bearing Pressure, Sliding Coefficient, Soil Pressures Coefficients Owner’s Inspector performs CPT, DCP, PP or visual inspection of subgrade

Prior to installation of a shallow foundation, a site-specific geotechnical report for the boardwalk should provide an allowable soil bearing pressure, provided in pounds per square foot (psf). This allowable bearing pressure is based on the soil conditions discovered during the Geotechnical Engineer’s soil investigation prior to construction. 

Provisions in the project specifications typically require that the owner’s inspector inspects the foundation subgrade prior to casting the shallow foundation. This inspector is often the same geotechnical firm that prepared the geotechnical report during the design phase. 

If the expected soil conditions differ from what has been noted in the geotechnical investigation, the contractor will contact the Geotechnical Engineer and the Boardwalk Designer for reevaluation of the existing soil conditions during construction. 

 

The Typical Process to Verify Shallow Foundation Soil Conditions

Commonly, the verification of soil conditions for shallow foundation is performed through a Cone Penetration Test. The Cone Penetration Test (CPT)* is a versatile sounding method that can be used to determine the materials in a soil profile and estimate their engineering properties. The test is also called the static penetration test, and no boreholes are necessary to perform it. In the original version, a 60 degree cone with a base area of 10 cm2 (1.55 in.2) was pushed into the ground at a steady rate of about 20 mm/sec (0.8 in./sec), and the resistance to penetration (called the point resistance) was measured. The current cone penetrometers measure the following:

  1. The cone resistance (qc) to penetration developed by the cone, which is equal to the vertical force applied to the cone, divided by its horizontally projected area; and 
  2. The frictional resistance (fc), which is the resistance measured by a sleeve located above the cone with the local soil surrounding it. The frictional resistance is equal to the vertical force applied to the sleeve, divided by its surface area—actually, the sum of friction and adhesion.

(*Source: Das, B.M. (2016). Principles of Foundation Engineering, Eighth Edition. Cengage Learning.)

Other soil verification methods include Dynamic Cone Penetrometer Testing and or use of a Pocket Penetrometer*. The Dynamic Cone Penetration Test provides a measure of a material’s in-situ resistance to penetration. The test is performed by driving a metal cone into the ground by repeated striking it with a 17.6 lb (8 Kg) weight dropped from a distance of 2.26 feet (575 mm). The penetration of the cone is measured after each blow and is recorded to provide a continuous measure of shearing resistance up to 5 feet below the ground surface. Test results can be correlated to in-situ density, resilient modulus, and bearing capacity.

(*Source: https://pavementinteractive.org/reference-desk/pavement-management/pavement-evaluation/dynamic-cone-penetration-test/)

A pocket penetrometer is a small handheld gauge which contains a telescoping rod which can be pushed into the soil. The distance the rod goes into the soil corresponds to a compressive strength on the dial.

Common shallow footings used to support boardwalk foundations include: precast piers and cast-in-place footings with a circular column.

     

Shallow Footing Examples

 

Boardwalk Foundation: Helical Piles / Piers or Screw Piles:

Foundation Type Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation
Helical Piles/Piers or Screw Piles Boring Logs with Blow Counts (SPT N-Values) Static Load Testing

Helical piles, sometimes referred to as “screw piles” can serve as a great foundation for your boardwalk project.

Made up of one or more helix plates attached to a central steel shaft, helical piles require smaller installation equipment and are generally friendly to the environment.

The helical piles’ strength is achieved through soil bearing at each helix, which is attached to the lead section of helical pier. The lead section of a helical pier is typically a steel tube or square bar. They are installed quickly with light construction equipment, and tend to be a cost-effective solution for both PermaTrak and timber boardwalks.

Helical pier (screw pile) installation process from Keller-NA

 

For more information about helical piles, check out our article, “3 Things All Engineers and Architects Should Know About Helical Piles.”

Prior to installation, the geotechnical report should include boring logs with blow counts for each one of the samples (SPT N-Values). This is called the General SPT or Standard Penetration Test Procedure. Helical piers are considered a specialty foundation system and the detailed design is left up to a helical pier contractor and their professional engineer. Some examples of companies in this space include Foundation Technologies/Chance and Pier Tech Systems.

General SPT (Standard Penetration Test Procedure)

A thick-walled tube (referred to as a split-spoon sampler) is driven into the ground with a drop hammer. The hammer weighs 140 pounds and the drop is 30 inches. The test involves driving the hammer three/four consecutive increments of 6 inches into the undisturbed soil. The SPT N-value is taken as the sum of the blows for the 2nd increment (6 to 12 inches) and 3rd increment (12-18 inches), representing the number of blows per foot.

This is a common and relatively inexpensive test that allows the Geotechnical Engineer to analyze the geotechnical parameters for the soils and provide recommendations. With the SPT N-value for each soil stratum shown in the boring logs, the helical pier designer is then able to input these parameters into the helical pier software and design the helical pier appropriately.

Once the helical piers are installed, one method of verifying that the installed pile can withstand the design pile load (provided by the boardwalk designer) is through a static load test. A static load test is an American Society of Testing and Materials (ASTM) load test. During construction, after a pile has been installed, a load test is set up on site to verify whether the pile is installed sufficiently to resist the design pile load. 

Typically, the requirements for how many static load tests are required would be determined by the owner (or owners' representative) prior to construction. PermaTrak can provide guidance for helical pier specifications which includes standard helical pier submittal requirements, installation tolerance and load testing requirements. 

 

Boardwalk Foundations: Drilled Shaft & CFA (Auger-Cast Piles):

Foundation Type Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation

Drilled Shaft & CFA Piles

Allowable Skin Friction and/or Allowable End Bearing Pressure Of Soils Owner’s Inspector performs CPT, DCP, PP or visual inspection of subgrade

Drilled Shafts and CFA (Continuous Flight Auger) Piles are both cast-in-place deep foundation elements that can be used to support various structures, including boardwalks. These foundations rely on skin friction and/or end bearing of the soil in order to achieve the required capacity for the structural system that it supports above. 

Drilled shafts and CFA piles are essentially the same in functionality once installed, but primarily differ in their construction method.

Drilled-shaft installation process from Keller-NA.

CFA (Auger Cast) installation process from Keller-NA.

Similarly to shallow foundations, prior to the installation of a drilled shaft or CFA (Continuous Flight Auger Pile), a site-specific geotechnical report for the boardwalk would provide an allowable end-bearing pressure. 

However, in addition to the allowable end bearing pressure value, the Geotechnical Engineer would also provide a skin friction value for each soil stratum found during the geotechnical investigation. This value is typically provided in pounds per square foot (psf) and acts over the circumference of the circular shaft or CFA pile. 

The Structural Engineer/Boardwalk Designer utilizes the skin friction and end bearing values provided by the Geotechnical Engineer to design for the required diameter and embedment depth of the foundation to resist the applied pile loads. 

During construction, the contractor verifies that the soil conditions referenced in the geotechnical report reflect the soil conditions at the exact location of the installed foundation. If so, the contractor will install the foundations to the minimum embedment depth noted on the plans by the Structural Engineer/Boardwalk Designer. If not, the contractor will contact the Geotechnical Engineer and the Boardwalk Designer for re-evaluation. 

 

Boardwalk Foundation: Driven Piles

Foundation Type Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation

Driven Piles

Allowable Skin Friction and/or Allowable End Bearing Pressure Of Soils Geotechnical Engineer Provides Driving Criteria Through WEAP Analysis and/or PDA Testing With CAPWAP Analysis

 

Common driven piles used to support boardwalk foundations include: FRP (Fiber Reinforced Polymer) Piles, Steel H-Piles, Steel Pipe Piles, and Round Timber Piles.

 

Driven Steel H-Pile installation process from Keller-NA.

 

While fairly uncommon and not typically used for boardwalk foundations, concrete piles are another driven pile option. Driven concrete piles offer durability and longevity, and they are typically manufactured in circular or square shapes. For PermaTrak precast concrete boardwalks, the PermaTrak cap attaches similarly to other concrete options with a steel dowel and epoxy connection.

Just like for drilled shafts and CFA piles, the Geotechnical Engineer provides the allowable end bearing and/or skin friction values to the boardwalk designer for an estimated pile depth to be determined. In some cases for driven piles, the Geotechnical Engineer may even provide the embedment depth required to resist the applied pile loads. In this case, the Structural Engineer would still be responsible for providing the minimum embedment required for lateral stability.

For driven piles, the Geotechnical Engineer is responsible for providing criteria to allow the pile driver/installer to verify that an installed driven pile can withstand the design pile load. To determine this criteria, the Geotechnical Engineer performs a geotechnical analysis that is provided to the pile driver prior to construction, before the piles are installed. The most common geotechnical analysis software used for this analysis is called GRLWeap, and provides the pile driver/installer with criteria to know when they have met the vertical load required for the installed pile. 

Basically, the pile driver needs to know “how many times do I need to drop the pile driving hammer on this pile before I know I can stop and move on to the next pile?"

Pile with marketings to indicate when vertical load requirement has been met.

The GRLWeap analysis provides exactly that, by outputting the required number of times the pile driver needs to drop the pile driving hammer on the pile without advancing the pile more than one foot (referred to as blows/ft). 

A sample GRLWeap analysis output determining pile driving criteria.

 

Geotechnical Investigations and Analysis: Don’t skip this step!

Foundation Type Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation Information Provided to Structural Engineer by Geotechnical Engineer to Design Foundation

Shallow Foundations

Allowable Bearing Pressure, Sliding Coefficient, Soil Pressures Coefficients Owner’s Inspector performs CPT, DCP, PP or visual inspection of subgrade

Helical Piles/Piers or Screw Piles

Boring Logs with Blow Counts (SPT N-Values) Static Load Testing

Drilled Shaft & CFA Piles

Allowable Skin Friction and/or Allowable End Bearing Pressure Of Soils Owner’s Inspector performs CPT, DCP, PP or visual inspection of subgrade

Driven Piles

Allowable Skin Friction and/or Allowable End Bearing Pressure Of Soils Geotechnical Engineer Provides Driving Criteria Through WEAP Analysis and/or PDA Testing With CAPWAP Analysis

 

By passing on the right information, data, and analysis, geotechnical engineers help structural engineers efficiently design and plan the appropriate foundations for the boardwalk or bridge project.

For landscape engineers, engineers or other designers new to this process, we would love to help. Drop us a note about your upcoming project by visiting our “Contact” page, and our team of engineers will reach out to you.

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Topics: Boardwalk Design