Preloading Near Existing Building 1

[GEVANCOUVER]

I am designing a single family house which requires preloading. The ground is 6m peat folowed by 10m very soft silty clayey deposits. The property line is 1.5m from an existing new building founded on timber piles. Embankment fill toe starts from the property line. How can one calculate the maximum fill thickness in the first stage to avoid damage to the neighbouring building.

 

[msquared48]

If the neighboring structure is founded on pile, why are you preloading the site for the new residence? Are you going to use a raft foundation? With the material you describe, I would be very skeptical of a standard spread/strip footing foundation, even with preloading.

Aside from that, I cannot see how preloading would negatively affect the adjacent structure, since it is on piling, unless there is an existing retaining wall involved.

Mike McCann
MMC Engineering

 

[hokie66]

Agree with Mike, assuming the piles are long enough to not be influenced by the preloading fill.

Preloading takes a long time, and I have never known it to be used for a house. It is not a good site, and probably deserves piles.

 

[fattdad]

It seems to me that you'd have to check for downdrag resulting from the settlements associated with the preload. That would be the most-likely "failure" scenario.

(Just to differ somewhat from MSquared48 and fellow Hokie.)

f-d

¡papá gordo ain’t no madre flaca!

 

[GEVANCOUVER]

The house will be founded on timber piles. But the site need to be filled about 1.5m (+settlement) for other reasons. I presume I can call it preloading for engineering assessment of the nearby building. After reaching the design grade, only the areas on the driveways will be preloaded to about 0.7m loose materials.

The existing building is only 1.75 m from the toe of the proposed embankment. Total fill+preload thickness would be about 2.5m.

 

[moe333]

After preloading, the long term secondary settlement of the peat will likely be greater than the house can tolerate. In addition, the preload will induce surface settlement at the adjacent property which will affect drainage and may damage hardscape or other surface features. Stability of the preload fill will also be an issue. Very low shear strengths in the peat would require multiple stages of small embankments with shallow slopes.

In addition, the downdrag of the piles due to the settlement will be an issue, and probably not one that you can estimate/evaluate without knowing all the details of the piles and adjacent soil profile.

All in all, I don't think preloading is a viable option. I think you need to look at piles.

 

[GEVANCOUVER]

Thanks all for the comments. However, please also advise based on my recent message as below:

"The house will be founded on timber piles. But the site need to be filled about 1.5m (+settlement) for other reasons. I presume I can call it preloading for engineering assessment of the nearby building. After reaching the design grade, only the areas on the driveways will be preloaded to about 0.7m loose materials.

The existing building is only 1.75 m from the toe of the proposed embankment. Total fill+preload thickness would be about 2.5m".

 

[fattdad]

My post is irrespective of house foundation. If you are placing 8 ft of fill (i.e., 2.5m) within 6 ft of an existing pile supported house, you should consider downdrag forces acting on the existing structure. If that's o.k. then you're good to go!

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

Depending on the areal extent of the preload, you may still induce surface settlement on the adjacent property. It may be an issue if there is settlement sensitive features along the property line.

 

[hokie66]

Thanks to fattdad for adding downdrag into the parameters. Of course he is correct. Good to see Hokies sticking together. And moe's comments about surface features being affected is also appropriate.

 

[oldestguy]

Watch out for legal ramifications. In this area, we had a case like this and the owner of the adjacent land had no recourse legally when his land was lowered in elevation due to filling work on nearby land. There might even be a lateral movement possibility on the nearby land, in addition to settlement.

In other areas the filling result may be cause for a law suit.


And yes, for the idea of preluding a site for a house, I've done it numerous times with success.

 

[BigH]

Downdrag issue - maybe yes maybe no, depends on what your pile tip is in. (noted already)
Settlement of adjacent property. Likely at the edges - and neighbour's land could settle a bit. (noted already)
Secondary consolidation of peat is important to consider as well as the soft clay. (noted earlier)
Lateral effect of fill on neighbour's house piling. possibility that hasn't yet been mentioned. There is a paper out there I remember seeing a while back that discusses embankment fill adjacent to a pile and its lateral effects.
Oldestguy makes a good point on the legal ramifications. The neighbours property, given the geology, might already have a few problems that have developed. You surely want to know what the base-line condition is and be able to monitor the during preloading condition.
You might consider using wick drains and perhaps get by with a smaller preload thickness for the same amount of loading time to produce the same expected consolidation settlement.
Preloading the Richmond and Burnaby Peat areas are normal practice from my recollection - likely many not even "engineered".
If I can, a case history I am aware of - Burnaby Lake Peat. 25 ft (8 m)of peat over 25 ft (8 m) soft clay then compact sand. Structure put on piles with tips driven into the sand - floor level about 2 m above ground surface. Filled up to reach the floor level (so trucks could drive into the maintenance garage)- would tend to dismiss that the site had been preloaded to any extent. Out the back, piles had been installed for later extension. Over the years, they had to cut off the tops of the "future addition" piles as the groud was settling and hampering the mainteance bay's exit. We investigated and found that they had "topped up" the entrance ramp some 4 or 5 times over the years. We decided to cut out the access ramp (sand and gravel at roughly 130 pcf) and replace it with Elastizell (at 30 pcf unit weight). The earlier gravel ramps basically preloaded the site as the net reduction of pressure was large. Went back 2 years later - no additional movement. I have a copy in storage of the old 1960s report by Fenco to the City of Burnaby on building on the lake peat.

 

[DRC1]

I would be concerned about the effect the preload would have on pore pressures. For soft clays the pore pressure could reduce the cohesion and friction along the piles.
The porepressure vs clay strength should be cheked and the effect of the reduced effective stress in the peat should be checked.

 

[BigH]

DRC1 - depends, eh?, on design philosophy of the piles. I would tend to believe that the piles were driven below into underlying compact sand and that the end-bearing would sustain the loading - likely the adhesion of the piling wasn't counted on - of course, this is a good point if you don't know any details of the neighbour's piling.

 

[escrowe]

You might consider monitoring lateral pressure and movement during construction of the fill,
possibly with inclinometers or pressuremeters at the property line. Lateral concerns me a bit.

 

[GEVANCOUVER]

DRC1, Hi and thanks. Could you give a reference i.e pwp vs C value. The soft organic silts and silty clayey materials have mc > LL are are very soft to firm.

I have also a quastion that why the pwp increases. The water level is almose at ground level and the top 6m of ground is peat. I guess the k of peat should be high enough to prevent any pore water pressure buildup. Please advise.

 

[GEVANCOUVER]

What are the design parameters that should be taken into account for calculating the down-drag forces and lateral effect of fill. One I guess would be the negative skin friction for peat and soft organic silt. What is a typical value for that? For lateral loading, can we assume an equivalent retaining wall at the face of the first rows of the piles in the existing building?

 

[DRC1]

The pore water ppressure build up depends on the permiability of the soil. I am assuming (perhaps incorrectly) that the peat is at least somewhat free draining. I am assuming (a little more confidently) that the clay is not. When pressure is applied to a clay, the pore water acts as a spring and carries the added load. This pressure increase is almost instantaneous. As the pressure is applied the water drains of the soil, and load is transfered to the soil. If the pore pressure exceeds the shear strength of the soil, the soil can fail. this is computed as base heave. So I would be concerened more about the underlying clay than the peat.

As for drag down force, it is typically due to skin friction in the upperportion of the pile. For the peats and soft organics, I would not expect these to be large numbers. Down drag is more of a structural issue than a settelment issue.
Lateral capacity can be checked assuming at least a pinned end and sufficent lateral resitance of the pile cap.