Should I always get a soils report? 1

[DrewBur]

We're doing structural design and calculations for additions to homes - Sesmic Zone 4.

We have always obtained soils/geotechnical reports in order to do our design. Most of the homes have been on some sort of hillside.

We have a couple of additions coming up that are on flat sites - pretty simple design. My question is "Should I ALWAYS get a soils report for this type of work?".

I can make conservative assumptions regarding the seismic cofficients, foundation pressure, lateral bearing pressure, etc.

I like to get the report, but I hate to ask the client to pay $2000 to $4000 for a report if it's not needed.

I would like to know what others think.

 

[Mattman]

I ALWAYS get a soils report for commercial or industrial work. I get a soils report whenever possible for residential work---but for residential work, it is often not possible. When the owner objects too strenuously, and when the project seems straightforward enough, I give my favorite soils engineer a call. He has decades of experience, and knows the soils for almost any site in my area that I can mention. If he has a good handle on the soils, he'll give me a quick summary of them/recommendation. If he doesn't know the site, or if he recommends a report due to variations in that area, or special conditions, I'll insist that the owner get a soils report.

I'm in a good situation, however. I don't live in a seismic zone (Chicago), and I have a soils engineer that I trust, with whom I have a good enough working relationship that he'll give me free recommendations when he can (even though he probably gets less than half of my testing work, due to owners price-shopping).

 

[Ginger]

Drewbur

In the UK, geotechnical engineers have been banging the drum for years regarding SI. The points which have finally begun to sink in are as follows:-

1. Yes SI can be costly (especially if you waste money getting the wrong type of investigation specified).

2. The potential savings which can be made due to having good SI will usually be greater than the cost of the investigation ( and often by a large factor).

3. The risk of your structure failing due to unforseen ground conditions is massively reduced. Insurers hate risk by the way.

4. If there is some problem with the ground and you have not undertaken any SI then you risk being sued for negligence.

Try these with your client and see if he/she wants to take the risk!!

Regards Andy Machon

 

[dlew]

DrewBur,

Normally we do not get soil investigation in case of an addition to a existing one or two story high residential building, unless we have a specific reason, per example, some unexplained cracks in walls, suspect bad subsoil condition, or the new foundations will be heavily loaded. In many cases we have access to the soil report prepared for the original building.

For new construction, where there is not existing information of the subsoil, we always order a soil investigation. First, because we need the information to do a good and economical design; secondly, because the liability to which we will get exposed if we do not obtain soil information. We are in Florida, where seismic design does not control, and where soft layers of soils are common.

 

[100islands]

DrewBur,

I you have soil investigation report of adjacent properties or areas, it is not necessary to conduct SI unless your constructing a 3 or more storeys high building.

Also, for one or two storey buildings, the presumptuous allowable bearing capacity from local building codes can be adopted but watch out for areas near rivers as these are susceptible to liquifaction during earthquakes.

 

[dougantholz]

100islands,
I was reading along and your post caught my eye. It is UNETHICAL to use a soils report for an adjacent property, if the property is owned by different party, without that property owner's consent. This goes for if you are the structural engineer who has a copy, the geotechnical engineer who wrote the report for just up the street, or the building developer who sold the property. The soils report is intelectual property and the owner has all rights unless the geotech has a contract stipulating they can use these borings for other projects.
Now if you are doing a site for a company that owns a large tract of land, and they have a soils report for another building on their property, then its alright to use it because the owner gives you permision (even if it isn't written).

 

[100islands]

dougantholz,

Sorry if I was not able expound my post. I presumed that DrewBur can get a copy of the SI report legally/ethically. Of course, he cannot use the report per se (even if he was able to get the permission of the SI report owner, etc.)for his project but he can have a good feel on the soil bearing capacity that he will use in his design.