Polymer Drilling Slurry

[SoilRocks]

Hey, Can anyone tell me their experience in using polymer drilling slurries. My assumption was that we would get a reduction in skin friction. The manufacturer claims an increase in skin friction. Any experience out there? Thanks much.

 

[BigHarvey]

There is no significant change in skin friction between a pile or diaphragm wall drilled under bentonite slurry or polymer slurry.
Polymer slurries should not been used in very permeable soils where the cake would not have time to form. In soils without fines it can be necessary to add a few kg of bentonite per m3 as an additive .
In all cases a polymer slurry should be checked like a bentonite slurry.

 

[Zambo]

I think the manufacturer is claiming an increase in skin friction because the alkalinity of the concrete will breakdown the polymer whereas the bentonite will remain at the concrete/soil interface. I doubt that this is a critical design issue, and if increased skin friction is required then maybe shaft grouting is a way forward.

Zambo

 

[SoilRocks]

Hey guys, thanks for the posts. The soils are generally fine grained, and around here we typically case holes, if needed. The contractor on a job would like to add a polymer + water as a drilling mud/slurry to keep the shaft from caving. While we don't typically use muds/slurries to keep holes open around here, I'm open to it if it doesn't decrease our shaft capacity. Any backup or papers you know of that would help? Also, BigHarvey, when you mention "checking" the polymer slurry, what do you mean? Viscosity? Thanks again!

 

[Mccoy]

Soilrocks,
as far as I know polymer muds do make up a mudcake on borehole walls, and this "slippery" layer is supposed to reduce skin friction, exactly as it happens in bentonite slurries.
Nevertheless, since mud chemistry can vary so much, the producer might be able to sell a formula where the mudcake dissolves under particular conditions, like zambo suggests (I remember I once enquired for such a product, but costs were prohibitive for drilled shafts. You might want to ask for the details).
Reese suggests a reductive alpha coefficients of 0.3, if memory serves me right, for skin friction when bentonite slurries are used. Big Harvey maybe observed mudcake is not an issue with low-density slurries, but there is a high variability in mudcake thickness and conditions (some people claim the bucket scratches the cake altogether), but then why suggesting a reductive coefficient at all ? Maybe he has more info on the subject.
Please note that a chapter in Bowles deals with skin friction and Reese's work on drilled shafts.

 

[Zambo]

It is a while since I worked on bored piles, but I think that the terminology in this thread could be causing a confusion. A polymer mud is a combination of polymer and bentonite and does act like bentonite. It is only when using polymer only (just looks like wall paper paste) that the alkalinity of the concrete breaks down the polymer.

As far as cost goes polymer is more expensive by weight than bentonite but manufacturer's claim higher re-use and a benefit that sand can be removed by settlement tanks instead if a sand removal machine.

Zambo

 

[BigHarvey]

Polymer can be used alone or with a limited quantity of bentonite if the excavated soil has a very low fine content in order to help build a cake along the walls of the excavation. The greatest advantage of slurry polymer is that you do not need to remove the sand from the slurry before concreting : it will settle in the bottom of the excavation after leaving the slurry at rest for half an hour. You then just have to clean the bottom of the excavation before concreting. If you put more than 10 kg/m3 of bentonite in a plymer slurry you lose this property, it is then more appropriate to use a bentonite slurry. As far a cost is concerned you put 30 times less of polymer than bentonite biut the cost of the polymer is 30 times the price of the bentonite ... ( don't forget it is often the same people who sell both products ! )

 

[Focht3]

Hmmm,

Yes, at one point Lymon Reese did provide for a reduction in alpha for slurried piers, but I think the recommendation was a qualified one. (And I recall the reference dating from the 1960's, but can't recall which paper.) What reference are you referring to, [blue]Mccoy[/blue]?

Dr. Reese is an eminently practical engineer - in many ways he is more results-oriented than most engineering professors. I suspect that his current opinion would be that slurry processed piers are equivalent to cased piers provided that the work is done in an expeditious manner, the mud doesn't become too thick and the subsurface conditions don't create a circumstance where a heavy mud cake would form quickly. As I recall my grad student days at Texas, in the early 1980's Lymon felt that a thin film of bentonite was not a significant problem for most piers.

It is also important to remember that the concrete mix must have high flow characteristics in order to provide the best pier possible. I highly recommend the use of a flow box to evaluate the concrete mix if you are worried about obtaining the maximum skin friction from your production piers.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Mccoy]

Focht3,
the reference is drawn from Bowles, your favourite textbook ( ).
The reference is the following:
Reese et al., 1976, "Behavoiur of drilled piles under axial loading", JGED, ASCE, vol 102, n.5,May, pp.493-510

The alpha reductive coefficient for Su along the pile shaft is reported to be 0.5 for dry holes or light slurries, and 0.3 for denser slurries, where removal of mudcake is uncertain.

This makes perfect sense and agrees with what you and BigHarvey said.
Looking up at Fang's treatise (Foundation Engineering Handbook), though, Kulawhy's article on drilled shafts, mention is made of more recent work (Reese and O'Neill, 1988, FHWA-HI-88-042 report, but not to bentonite slurries in connection with the alpha concept. Rather, he presents a graph with the alpha adhesion factor correlated to Su, with a mean non-linear trend given by:

alpha = 0.21 + 0.26(Pa/Su) <= 1.0, hence not related to the presence, the absence or the type of drilling fluid.

It may be that the early studies of Dr. Reese overrated the importance of slurries, or simply that conditions vary so much that, in some cases, a significant reductive factor is appropriate (according to Kulhawy this is the case with higher values of Su, > 1 tsf).

Just one more concept: the mudcake phenomenon and its plastering effect take place in permeable formations, with the presence of a gradient creating an outbound flow from the borehole to the surrounding soil. In impermeable layer this shouldn't occur. There might be though the presence of a thin film due to electrical forces beween soil and mud components, or other factors. Also, with a wet hole, there may be some clay softening effect which decreases Su and is translated into a lower alpha-value.
The model appears to grow of increasing complexity. Time is sure a main factor, mudcake and electrical phenomena need it to manifest properly.
To soilrocks, at this point I don't know wether a cased hole would give you a significantly higher lateral resistance with respect to a wet, uncased hole. In my area the introduction of bentonite slurries in lieu of steel casing increased contractors' profit by a substantial amount.
If the project is a large one it might be a good idea to experiment with a few load tests.
I find Focht3 and BigHarvey's observations very sensible.
By the way, do you guys calculate your drilled shafts based only on lateral resistance (minimum settlement) or on a combination of lateral and base resistance (higher settlements, hard to define precisely)? That is probably a tricky issue when load tests are not available.

 

[Focht3]

We use a lot of drilled shafts in Texas - lots of good load test data. As a result, all of the good geotechnical firms use both friction and bearing in designing "typical" drilled shafts. Atypical situations require special consideration -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Prataung]

May I share my experience in Bangkok Clay, typical soil profile is alternate layer of clay and sand. Originally, we have extensively used bentonite slurry for driling bored pile (auger type), 30-60 m depth. The result show kstan(d) ranging about 0.25-0.5 for friction and small end bearing (Nq ~ 6-15).

Recently (3-5 years period), pure polymer and also polymer with some bentonite have been used in drilling bored pile. Main reason for this would mainly be contractor's benefit on less construction time and less pollution compared with old fashion slurry.

In design points of view, several instrumented pile load tests indicate higher unit skin friction obtained. It is probably due to no or less cake at the interface between concrete and soil. However, we still keep monitoring more test results before changing the coefficient of skin friction in design.

Hope this information could provide you an additional backgroud for polymer application.