Monday, July 30, 2012


What is a Semi-Walkout basement? Well, I just do not know what else to call it. It is a walkout basement that comes out below grade. These may have interior steps that lead to the door, or open to a recessed area. These are built in water traps. The frost gets down to the weeping tile, and can produce issues. These warm weather southern designs work where there is free draining soils, no ground water within some distance, or no (little) frost penetration, or dry climates.

Wet year there are issues. Here, we can get storms that will dump 4 inches of water in one hour. The design storm. That is over 2 Imperial gallons per square foot. Now we need drains that will handle that amount of water. If you have a 500 square foot area, you need a pump system to handle 1000 gallons imperial per hour.

Any drain system with pump(s) is high maintenance. And then there is the issue of what to do with the water, and what about winter? Are you below the ground water level? if yes, will you be pumping water in the winter? now what about freezing and ice buildup? Ice kills lawns. Ice kills trees.   

Pavements over Organic Soils

Clients and non geotechnical engineers frequently want to build pavements over organic contaminated cohesive soils. Organic contaminated soils have two outstanding characteristics; they loose considerable strength when they become saturated, 1 or 2 orders of magnitude, often essentially flowing, and they are not volume stable. Much of their dry strength is due to negative pour pressure developed in the organic material when dry, aka, desiccation cementing.

Build over unknown amounts of peat organic topsoil, A horizon material, or even B horizon organic contaminated clays or silts, is possible as long as the client will accept unknown amounts of settlement. Peat will commonly settle to 25 percent of it's original volume. CNR estimates the amount of settlement over time equal to the depth of fill placed on it. Building over peat is a subject unto it'self.

Road pavements and parking lot pavements are dependent on good drainage for a good life. 2 percent grade in 2 directions is functionally the minimum grade that one can expect will maintain drainage with the typical amounts of settlement that we see in parking areas. Even this will not compensate for buried topsoil, poorly compacted trench backfill and local overloads, rutting, and the like. Any thing less than 2 percent guarantees birdbaths over time.

Pavements at 1 % is less than the paver accuracy, so birdbaths are guaranteed upon completion of paving.

To compensate for buried organic fill, at least in one direction, the grade should be increased. I would suggest 4%, however, this may be reduced and reduction will result in shorter pavement life. Anywhere water will puddle will shorten pavement life as asphalt is permeable to water, especially if the AC has low density or is cracked as a result of rutting or settlement.

For pavements to function fairly well, it is recommended that over peat or burred topsoil, a minimum of 0.9 metres of clean clay fill be placed. For heavier traffic, trucks, 1.2 is recommended. ATU requires 1.5 metres for primary highways. This assumes that 0.3 metre lift will support compacters, to achieve 98% cSPD.

It is the opinion of the author that these may be reduce 0.3 or 0.45 metres where organic contaminated clay is the offending organic. All these depths are below subgrade elevation. Standard 300 mm subgrade preparation is assumed, or cement stabilized. If the client wants to use 150 mm subgrade, then only reduce 0.3 metres, so the minimum is 0.6 metres.

These are typical rules of thumb. Specific information and detailed analysis may alter these suggestions.

Friday, July 27, 2012

Zero skin friction zones

These are general rules of thumb, first approximations, and are often broken for various reasons.

The upper portion of a skin friction has no reliable skin friction due to drying and shrinkage of the soil away from the pile, frost penetration, and potential for some fill.

Fill is unreliable, as it has two characteristics, it has unreliable strength, and unreliable settlement characteristics, most of the time. Shallow engineered fill, after one year may be suitable for light structures.

Typical recommendations for skin friction piles, drilled, cast in place concrete, are from 0 to 1.5 metres below final grade, 0 skin friction. This provides protection from drying and/or frost action. This requires a relative level site after stripping, plus/minus 1 metre, and competent native soil.

Occasionally we use 0 to x.x metres below current grade, where we have fill, or a significant slope to the site. The cause of this requirement should also be noted in the report. Occasionally elevation may be used of odd cases.

Generally, we prefer only one condition for a site, however two or more may be used if there is an clear reason, such as an old basement, utility line fill, or other special situation.

In the case where there is fill, there may also be down drag. Down drag is a load placed on the pile from fill settlement. Typically for old competent shallow fill, -10 kPa is the downdrag value assigned. New loose fill should be atleast -25, typically -20 kPa is used for fresh compacted fill. These are typical for 1 to 3 metres of fill.

Often, where less than one metre of fill below floor slab elevation, the fill is just part of the zero friction allowance and is ignored. With say 1 to perhaps 3 metres of fill, the first 2 or 3 metres of pile friction resistance, below the fill, is discounted to deal with a bit of down drag. This requires a fairly uniform condition across the site.  This requires good friction in the upper soils. Note that the surface soils can also be soft, and have lower value for other reasons, or it is specified and must be included in the structural design. That is useful  where the site has know fill areas.

Down drag can be reduced to -5 kPa by using a double polly rapped sone-tube as a friction reducing smooth surface.

Crap fill, as truck dumped winter fill, uncontrolled truck fills should all be considered as poor, and are unreliable for pile friction and slab support.  

Values and recommendations from previous projects, found in the boiler plate report, must be customized to the current situation. Any missing recommendation sections must be added. 

Friday, July 20, 2012

Engineering Education

Engineers are taught the new, hot, sexy, stuff at university, not the basics. That is becoming an issue. We have people that know about the great stuff, but not how to do the basic stuff of every day work. Sure, when they get the chance at the big stuff, they can do it, but not simple stuff, or they are just uneconomical to do the simple stuff.

Desperation cause us to make poor short term decisions. Hiring unskilled but educated workers is one such alternative that pisses off the existing experienced workers. Now, are you just turning workers, or advancing the company?

Is there need of "engineering lite" books, but there are some of these, but they prescribe ways of doing things, but not the rational, the mathematics behind the methods. But I see a need. Or perhaps videos, documentary videos.

I took a job one time out of desperation, which was not great, but I got to explore paving over ice rich permafrost, which melted out as expected, resulting in wavy undrained surface, as well as pavement failures. Such is life. Doing thing for the wrong reasons, but getting some results.

Wednesday, July 11, 2012

why is my road falling apart?

The story:  Underground constructed in 2007, roads 2007, no houses yet (2012), asphalt is ACB, numerous cracks, corner pushes.

ACB is a base coarse asphalt, not intended to be used as a surface course. It is open, and will allow water to penetrate. In addition, there is considerable segregation and exposed rock areas typical of cold placement of truck box corners and hopper wings. The asphalt had variable asphalt content, often to the low end. Densities were adequate.  

Separate the mechanical damage, such as the location where the crane or picker truck lifted something heavy, without adequate pads. Two rectangular spots where the rectangular pads were in contact with the AC, and is punched the structure.

Through out the project, there were issue with lift thickness, and the contractor was not cooperating with control of lift thickness. This was all reported to your man on site, but nothing changed.  

Separate trench settlements, where underground settled. Density is not enough to control; lift thickness must also be controlled. We struggled with the contractor over lift thickness when we were on site, but we were not full time. This was reported numerous time to Mr. xxx, your man. Nothing happened, the contractor continued with thick lifts.   A typical compactor has difficulty to compact the bottom portion of a 300 mm lifts, the maximum allowed. We cannot test more than 8 inches deep. A nuclear density machines density reading has surface bias, 87.5 percent of the value is in the top 50% of the depth, nearly all of the moisture reading is in the top inch. Trench settlement typically creates two longitudinal cracks, one at each side of the trench, somewhere from the straight wall to the edge. Often it shows up as three cracks where two trenches are involved. Services, and other lines that cross, can be identified by settlement in there location.

Trench settlement can also be identified by "manhold growth", which is settlement around the manholes. In several locations, it is likely that vertical ends in the backfill were created near manholes. This results in a loose column of backfill which can settle.

The lots were not graded to drain water away. There are ponds where the houses will be in portions, and water drains the rear or the curbs in locations. When subdivisions are left, it is recommended that the rear of curbs be backfilled to encourage water to drain across or away, not stand and soak in.

Traffic is more than minor. During the inspection 5 loaded trucks were noted, one loaded semi-trailer use the the 97A/89 route twice,  and on both trips, did backing practice on 97A/89 corner ( the only purpose I could think of, but I could be for other reasons)  The other truck was a tandem, hauling topsoil on Grand.

On numerous corners, the asphaltic concrete has failed by diagonal vertical shear. This is a common issue with poor compaction and pushed hand placing producing a shear plain in the asphalt during placing, which never gets knitted together without considerable rubber rolling. Thermal expansion of the mono sidewalk curb and curb produces a lateral forces on the asphalt, and the asphalt faults along the pre-existing weak plain / shear plain. 

There are several areas that have typical failures of missed cement stabilization or deep gravel infill into some trench, producing a water source for frost heave. These can be investigated by a trench, should it be desired. This involves cutting a trench about 2 feet wide, three feet deep, and 10 feet long through the problem for evaluation. Testholes often miss the issue, as these may be small. In several of the locations, these may be gas or electrical crossings, off a bit from where the plan indicates.

There are a number of areas that appear to have structural pavement failures. This can result from a few truck overloads,  traffic volume, thin structure, or some issue with the structure. It is recommended that testholes be cored, and hand augers be conducted for evaluation. Asphalt thickness, gravel quality and thickness, cement stabilized holidays, and other issues can be identified. In some cases, these are occurring at trench settlements crack locations, which will permit the ingress of water and soften the subgrade and below.