Deck & gabion wall foundation on helical piers & frost heave

[RattlinBog]

I'm designing a standalone wood deck overlooking a lake. I also need to support a decorative, rock-filled gabion wall next to the deck (about 750 psf/ft). Soils are poor for 30+ ft, so I'm planning on helical piers for gravity and lateral. I'm in a 5 ft frost depth area.

See sketch below. I'm concerned about frost heave on the continuous footing that is supporting the long gabion wall and the back end of the deck. Has anyone found an elegant solution for something like this?

I see a few options:
1) thicken footing to bring bottom down below frost (5 ft below grade) - would rather avoid this...likely expensive and fighting high water table
2) provide insulation around/below footing and extend horizontally in an attempt to prevent frost from reaching footing
3) provide void form below footing to sacrificially crush under frost heave forces
4) do nothing and assume helical piers are able to resist frost heave - I don't believe this is true because frost could still grab the concrete surface

 

[jayrod12]

for a piled foundation we regularly provide void form underneath of the beams to prevent frost heave. So that solution would likely work for you.

 

[W460x68]

I will always provide void form for pile foundations where the grade beam is not below frost depth.

 

[RattlinBog]

Thanks for the feedback. I'll plan to use void form below the strip footing.

Came up with a slightly different concept below. Maybe I'll keep the deck independent of the gabion wall foundation. Support the deck on vertical and battered helical piers only. That way if the gabion wall foundation does heave at all, it at least won't take the deck with it. I'm less concerned about the gabion shifting.

 

[JAE]

1) thicken footing to bring bottom down below frost (5 ft below grade) - would rather avoid this...likely expensive and fighting high water table

FIrst and most sure way to avoid the heave but if you have a water table I understand.

2) provide insulation around/below footing and extend horizontally in an attempt to prevent frost from reaching footing

Usually do this when I have it near/under a heated building. Outdoors might be iffy as you are relying on the shallow earth to provide enough heat from below to avoid freezing. Might be OK but I'd want to research this a bit to ensure I'm not just delaying freezing and not denying freezing.

3) provide void form below footing to sacrificially crush under frost heave forces

Agree with others ... BUT ... void forms deteriorate away and leave an open void - the purpose of course - but then the adjacent earth can sluff down into the void over time and negate the void. In my south Texas days with expansive clays (similar to the freeze issue) we'd always you fill retainer planks around the perimeter of the footing to negate any sluffing. Usually 1 1/2" thick concrete panels - perhaps 18" wide - and leaned against the side of the footing and extended below the void about 4" or more.

4) do nothing and assume helical piers are able to resist frost heave - I don't believe this is true because frost could still grab the concrete surface

Agree with you that you'd have uplift - the question could be put to the pile manufacturer about uplift resistance but freeze uplift is usually thought of as an irresistible force.

 

[DoubleStud]

Just thinking out loud, is it super critical to support the gabion wall? Is it the end of the world if it settles or heaves a little bit since it is very flexible? Can you just put the gabion wall on compacted subgrade and put the deck behind it on helical piers? Last time I did gabion walls was just to cheaply retain dirt and they just sat on dirt.

 

[DoubleStud]

Also I feel like the steel rods from the helical pier is stiff enough to resist lateral loads? The rods are sticking out only a tiny bit next to the gabion wall? It should be pretty stiff? I feel like you don't need angled helical piers?

 

[RattlinBog]

Below is what I ended up with. I decided to make the deck independent of the gabion wall foundation. If the gabion does heave, it should at least no longer affect the deck.

Also I feel like the steel rods from the helical pier is stiff enough to resist lateral loads? The rods are sticking out only a tiny bit next to the gabion wall? It should be pretty stiff? I feel like you don't need angled helical piers?

Perhaps, but I want some extra insurance. I've seen photos of decks sluffing down a bank over time, and I don't want that to happen here. The soil has layers of silt, clay, and peat. Between the water table, frost, and gravity, I could see movement happening naturally from year to year.

 

[jerseyshore]

Also I feel like the steel rods from the helical pier is stiff enough to resist lateral loads? The rods are sticking out only a tiny bit next to the gabion wall? It should be pretty stiff? I feel like you don't need angled helical piers?

Generally with 1.5" or similar small steel shafts the lateral resistance is tiny and that's when they are installed in the ground. I see the guys bend the shit out of them by hand all the time so I would never rely on them laterally when extending above grade. You need a bigger shaft and preferably a grout column to be confident in this setup. The battered pile is critical on decks like this with a steep slope.

 

[RattlinBog]

The battered pile is critical on decks like this with a steep slope.

Is there a quantifiable way to figure out how many battered piles you need for a given deck size? I currently show 2 battered piles for a 40 ft long deck. I have horizontal 2x6 bracing nailed to bottom of joists to creat a bit of a diaphragm too. I don't know exactly what lateral loads to put on this, so I estimated 100 plf lateral for now. I'm getting less than 2 kip in each battered pile.

Is there a better way to design a deck for lateral? No seismic here. 107 mph ultimate wind.

 

[jerseyshore]

Is there a quantifiable way to figure out how many battered piles you need for a given deck size? I currently show 2 battered piles for a 40 ft long deck. I have horizontal 2x6 bracing nailed to bottom of joists to creat a bit of a diaphragm too. I don't know exactly what lateral loads to put on this, so I estimated 100 plf lateral for now. I'm getting less than 2 kip in each battered pile.

Is there a better way to design a deck for lateral? No seismic here. 107 mph ultimate wind.

There are some good threads on here for deck lateral loading. 4-10 psf is the most common range I've seen used. But for me it's not so much just purely the deck lateral loading, but also just how confident you are with small shaft helical sticking up out of the ground. Especially as the ground below erodes over time.

I went to a house recently with a deck over a cliff like that and the tieback helicals (which were retrofits) were installed very shallow to get far away from any potential erosion areas.

Idk what your spacing is but at a bare minimum regardless of load capacity I would have at least 3 tiebacks (one each end and one in the middle).

 

[RattlinBog]

Idk what your spacing is but at a bare minimum regardless of load capacity I would have at least 3 tiebacks (one each end and one in the middle).

Currently I have (2) tiebacks as shown below. If I were to bump up, I'll likely go with (4) for symmetry. What do you think?

I'm also specifying 35 ft minimum embedment for all anchors to get beyond the poor soil.

 

[jerseyshore]

Currently I have (2) tiebacks as shown below. If I were to bump up, I'll likely go with (4) for symmetry. What do you think?

I'm also specifying 35 ft minimum embedment for all anchors to get beyond the poor soil.

View attachment 14608


View attachment 14609

With that layout I'd be inclinded to just do all 6 spots.

Is there a reason why you are running beams front to back and joists side to side? Typically we try to do it the other way around.

 

[RattlinBog]

Is there a reason why you are running beams front to back and joists side to side? Typically we try to do it the other way around.

Yeah, it's a bit unconventional. A couple reasons for the layout. I'm designing for 100 psf LL due to possibility of public assembly, and it's difficult to get a 2x12 to work at a 12 ft span. I also don't have enough clearance from grade to top of deck elevation to bear the joists on top of the beams for an overhang. I believe the layout I have is a bit more efficient with material use.

 

[jerseyshore]

Yeah, it's a bit unconventional. A couple reasons for the layout. I'm designing for 100 psf LL due to possibility of public assembly, and it's difficult to get a 2x12 to work at a 12 ft span. I also don't have enough clearance from grade to top of deck elevation to bear the joists on top of the beams for an overhang. I believe the layout I have is a bit more efficient with material use.

Gotcha. Makes sense.

I would do the 6 helical tie backs. If it was my deck I would want one on each. Because you know if you only do 4, the two that you don't brace back will end up having more erosion and it will cost 5x more to retrofit them.

I never mess around with decks because they are one of the few areas that have a high probability of actually seeing the full design load (or more). Plus exposed to weather etc. Now put a deck on the side of the cliff and you better believe I am over designing the shit out of it. I want to see one of those choppers everyone is always telling me about landing on this thing

 

[RattlinBog]

Yeah I agree. I should probably design for a dance party load too. Ha

 

[jerseyshore]

Yeah I agree. I should probably design for a dance party load too. Ha

I've said it before, but my mindset is if people are going to pile on for fireworks. I've seen and experienced some crazy balcony and deck loading scenarios when people are packed outside watching fireworks so that's how I justify going 60 psf LL min. Especially here for houses along the shore, 100 psf LL is not uncommon for these massive waterfront houses.

 

[gte447f]

Is there a better way to design a deck for lateral? No seismic here. 107 mph ultimate wind.

I have asked myself the same question in the past, and I have settled on 12 psf of plan area. There is basis for 12 psf lateral load in ASCE 7 lateral loads specified for reviewing stands, grandstands, and bleachers, and there is also basis for 12 psf as an upper bound for decks based on research into this specific topic. Additional information can be found in a 2014 Structure magazine article titled "Lateral Loads Generated by Occupants on Exterior Decks". I am too lazy to link the article, but you should be able to Google it.