Conventional MSE Wall Design Responsibility 2

[PEStruc]

Hello Everyone,

Just received a project where several types of wall will be used. One of the walls is conventional (standard) MSE. Would like to clarify, who shall provide the design of the concrete panel in terms of the thickness and reinforcement? Also, who should design the steel strip length, number and type. Is that something kind of patent design the manufacturer have and he only the one shall design them or that is the responsibility of the structural engineer? In general, how is the design for standard MSE wall be handled?

Appreciate your input.

Thanks,

 

[EireChch]

I cant say this is the standard or only way MSE walls are designed, buy companies like Tensar or Maccaferri offer a design, supply and build service.

I dont know what you part of the wall you are referring to when you say concrete panel or steel strip length but the above companies would supply a complete service. They would not take responsibility for bearing capacity assessment so the consultant geo would have to confirm founding conditions.

Thats my experience anyways.

 

[PEinc]

Most engineers who design the MSE walls use software the does not consider global stability. They often exclude the global stability analysis. Their designs usually assume that the walls will be built on stable ground. This is not always the case.

http://www.PeirceEngineering.com

 

[EireChch]

PEin - that would be my view too re global stability.

Also, my above comment shouldnt have included "build". Tensar etc dont build the walls (as far as i know)

 

[PEinc]

For a past project of mine, Tensar was the only designer/supplier who seemed to understand the requirement for analyzing global stability. At that time (about 1995), only Tensar included the stability analysis.

http://www.PeirceEngineering.com

 

[PEStruc]

I got a response from SINE Wall and do the design for their patent system ,however the global stability shall be done by the EOR. My question is that the common design procedure? so the prime designer has do do the performance design and the actual design shall be done by the manufacturer?

 

[RFreund]

I would say in most cases in the USA (more specific the Midwest region), it is very typical for the civil engineer to denote an MSE wall as a "Design/Build Element". The civil would show the wall top grade and bottom grade and have a note saying "Contractor to submit retaining wall shop drawings and calculations signed by a licensed engineer..." Typically that engineer (the specialty engineer) will design the wall for external failure modes sliding and overturning along with internal failure modes grid pull-out, soil-grid interaction, facing panels, etc. However, they would not check global stability, settlement or determine an allowable bearing pressure. Essentially the assumption is that the soil around the wall is not "their product". If you want to be ahead of the game see if you can get the geotechnical engineer to provide soil design parameters specific to the wall. This would include: soil unit weight, soil friction angle and cohesion (although cohesion is usually neglected in the wall design, it is sometimes used in the global design). I have also seen where the geotechnical engineer provides the design parameters but does not actually do the global stability. I like this idea but it is not common. I know you have an MSE wall but I will give you a couple references for SRW's (segmental retaining walls) which are similar in nature. Look at NCMA's Design Manual for Segmental Retaining Walls and Allan Block's Best Practices LINK LINK_2. Both of these define the responsibilities of the Civil Engineer, Geotech, Specialty Engineer/Contractor.

Good Luck!

EIT

http://www.HowToEngineer.com

 

[Okiryu]

Not sure if you are part of the design or construction team.

If you are part of the design, you can do a preliminary design in order to produce costs for budgeting purposes. I think that you will need to do this anyway in order to also allow the contractor to cost the project. Your drawings/specs can be performance-based (i.e. request for a minimum FOS). Then the contractor will choose a MSE wall contractor that can do the actual design and build the wall. The design can be reviewed by the design team.

On the other hand, I think that if you want to do the final design of the wall, it should be done by the geotechnical engineer who is part of your design team.

Bottom line, I think it will depend on how the project is managed. In my case, I do preliminary designs following the rules indicated above. It is also fun to be in the construction side, though !

 

[Okiryu]

RFreund, the Allan Block references are good! Thanks for sharing them!

 

[Doctormo]

In the US, the majority of MSE walls with panels and steel reinforcement are for state and federal transportation departments who determine what kinds of retaining walls are required at a location prior to bidding based on geotechnical, structural, aesthetic, and cost requirements. In most cases, this is determined by a geotechnical investigation that considers bearing capacity, settlement, and global stability issues with the proposed forms of retaining wall construction. In simple terms, they attempt to not specify or consider systems that will not work properly at a given location for geotechnical or structural reasons. An example would be to not consider MSE walls where insufficient space exists for their installation or a location where any gravity wall would slide down a hill without considerable subgrade stabilization.

However, there is a segment of transportation projects that are classified as design-build where the design-build team has responsibility for all facets of the project and will have to divide up the responsibilities for things like global stability among the team members. In almost all cases, the geotechnical engineer is the most qualified to do this work since they are in charge of the geotechnical exploration program and will determine what exploration and testing will be done.

The US private sector is completely the opposite of the transportation sector and everyone tries to pass responsibility down to the lowest levels on a project so that a person with a tractor is responsible for getting global responsibility checked on a wall he is building. Of course, there are no borings taken in this part of the jobsite since the geotechnical engineer was never retained to do any more work than the initial investigation where structures were not yet identified (I am being kind here to low bidders).

Back to the OP, proprietary wall systems (panels, blocks, etc) typically design their components for a given project but this requires a lot of trust. Specifications are necessary to determine for example the concrete used in the panels if the owner wants a strong and durable concrete panel. It would be like asking for a design-build driveway and not specifying the concrete material and letting the contractor choose for you. It all looks good for a year or two but what about 10 or 20 years? The same holds true for many aspects of the MSE wall design, you should understand all the key elements to be able to specify then check for conformance to whatever standards required or specified.

 

[Doctormo]

ps: As a side issue, the NCMA Design Manual 3rd edition "attempts" to define responsibilities of the parties in a construction project (generally private sector projects). While this is good in concept, the plans and specification of a project govern responsibilities as well as legal precedence for items not defined. There are many private sector specifications that refer to the concept of "delegated design" where the Engineer/Architect clearly denies any responsibility for a wall design in the specifications and places all responsibility on the contractor's wall designer, almost to the point of where the wall designer is responsible for fixing mistakes in the Engineer/Architect's drawings at no cost to them. Every project has its own issues and specification requirements or lack thereof. Designers have to be clear in their scope of work of what is included and what is not. The general contractor is responsible for meeting all the requirements of the project, not every supplier or subcontractor who routinely exclude portions of work from their scope.

pss: I think there is a difference in doing a global stability analysis and running global stability software. Any engineer can run software, a global stability analysis requires knowledge of a site, local soils, groundwater and the establishment of short and long term strength properties to make a reasonable assessment of stability. This is almost impossible to do well unless one is the geotechnical engineer and has been charged with acquiring the necessary information for a given retaining structure. This does not mean it is not done based on assumed soil conditions quite often but the results are only a guide and may or may not reflect the actual stability of wall structure.

Sorry to hijack the thread but this topic never goes away.

 

[RFreund]

Some great points by Dr.Mo. I too wish things were more clearly defined. I can't speak too much about true MSE walls which i usually only see on public projects, but for SRW walls in the private sector I think things are getting better, slowly. I'd like to see a couple things improve:
[ol 1]
[li]Global stability considered early in the project and a set of guidelines for when global stability is not required. (i.e. some sort of soil/height/slope criteria). Allan block has attempted to define this and I appreciate that.[/li]
[li]More designs done prior to releasing drawings for bid and include an "or equal" clause. Atleast this way the contractors are bidding on the same thing and other constructability issues have been considered.[/li]
[li]Have borings done specifically for the wall and include friction angle, unit weight and cohesion design parameters for design (no equivalent lateral loads).[/li]
[/ol]

EIT

http://www.HowToEngineer.com

 

[fattdad]

(didn't read all replies)

From my experience, the consulting geotechnical engineer has the responsibility for external and global stability and the wall-system manufacturer has the responsibility for the internal stability.

External stability relates to the stability of the reinforced zone when acted on by the external forces.
Global stability relates to the stability of the reinforced zone as it induces shear in the adjacent soil.
Internal stability relates to the forces acting within the reinforced zone.

I don't think of the latter as a design-build component. I think of it more as a shop drawing.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Doctormo]

fattdad - Your observation is consistent with the DOT market. However, there are some DOT's and and many transit agencies that attempt to push all soil stability responsibility onto the wall designer via the contract specifications which is why all bid documents have to be reviewed. I have been involved in bid transportation projects (not design build) where a geotechnical engineering firm has to be retained by the contractor/wall designer as the Owner did not do any of the required work to evaluate the stability of the soils involved including bearing capacity and settlement much less global stability. The work requires actual borings, soil testing, and recommendations thus a pretty big expense that has to be added to the contractor's bid. Not common with DOT's but occurs sometimes.

Design-build transportation projects many times will have very limited geotechnical information and the full scope of geotechnical work is part of the design-build package. This makes it very difficult to prepare the cost estimates when no geotechnical issues or parameters have been established and it is beyond me how the contractors actually put the numbers together without huge cost contingencies.

The private sector is the opposite and treats everything as design-build for the most part with minimal geotechnical information available and everyone else is responsible for all soils issues on the project other than the owner. The number of private projects that are completed from preliminary geotechnical reports (defined as a handful of borings on a large site since there is no site plan developed yet) is quite amazing and one has to wonder what happened to good engineering practice to follow through with the real soils investigation. The minimum required to obtain the permits is all that is required in the private sector.

Just from my experiences...

 

[fattdad]

(Not to derail the conversation, but to respond to Doctormo.)

Out typical d-b advertisement will include some subset of geotechnical data. Perhaps 30 to 70 percent of what's expected by my program. The d-b imagines their bid price and gets an award. The responsibility to complete the geotechnical requirements of the program then shift to the d-b, who has to follow our book of words on minimum data requirements. They have 180 days to get their own compliant geotechnical engineering study completed.

When their geotechnical study is completed, we enter into scope validation where we will consider type 1 work orders - i.e., those that develop from incomplete information. After 180 days the d-b no longer has access to type 1 work order adjustments.

There is always recourse for type 2 work orders.

Our d-b projects consider risk management in that light.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Doctormo]

(also not intended to derail the conversation that appears to have stopped)

fattdad - 50 states and probably about that many different approaches. Sometimes it is a funding issue when money becomes available and a project is pushed forward on a D-B basis ahead of the engineering thus minimal data. I also refer to other transportation projects like mass transit that many times fall under a different section of the DOT (they used to be called highway departments). They are typically handled different than highway D-B projects. I have seen all kinds and some are much worse than others.

 

[BigH]

In my view the overall designer is responsible foe ensuring tbe global stability and settlement are acceptable and the internal stability belongs to the wall designer. Worked a projdct in India where the designer didn't understand this and figured RECO was fully responsible for all - 11 m high RECO wall on 6 m of very soft clay. Caused issues in construction where we had to install pvd and stage load. Over 1000 mm settlement during construction.

This seems the logical responsibility scenario but as we see far too many in the world are not logical.