QUANTITY OF BOREHOLE LOG PER AREA

[Erasa]

Hi Engineers,
Can someone tell me the minimum distance between borehole logs or number per area, any standard reference?

 

[EireChch]

The below is from EC7 Part 1. It is very general guidance. Often the type of structure dictates the site investigation. Geology and local knowledge is also very important. Without knowing what your site investigation is for its hard to give a more specific answer.

B.3 examples of recommendations for the spacing and depth of investigations
(1) The following spacing of investigation points should be used as guidance:
− for high-rise and industrial structures, a grid pattern with points at 15 m to 40 m distance;
− for large-area structures, a grid pattern with points at not more than 60 m distance;
− for linear structures (roads, railways, channels, pipelines, dikes, tunnels, retaining walls), a
spacing of 20 m to 200 m;
− for special structures (e.g. bridges, stacks, machinery foundations), two to six investigation
points per foundation;
− for dams and weirs, 25 m to 75 m distance, along relevant sections.
(2) For the investigation depth za the following values should be used as guidance. (The reference
level for za is the lowest point of the foundation of the structure or structural element, or the
excavation base.) Where more than one alternative is specified for establishing za, the one which
yields the largest value should be applied.
NOTE For very large or highly complex projects, some of the investigation points generally extend to greater
depths than those specified under B.3 (5) to B.3 (13).
(3) Greater investigation depths should always be selected, where unfavourable geological
conditions, such as weak or compressible strata below strata of higher bearing capacity, are
presumed.
(4) Where structures under B.3 (5) to B.3 (8) and B.3 (13) are built on competent strata, the depth
of investigation can be reduced to za = 2 m, unless the geology is indistinct, in which case at least
one borehole should be taken down to a minimum of za = 5 m. If a bedrock formation is
encountered at the proposed base of the structure, this should be taken as the reference level for za.
Otherwise, za refers to the surface of the bedrock formation.
(5) For high-rise structures and civil engineering projects, the larger value of the following
conditions should be applied (see Figure B.1 a)):
− za ≥ 6 m;
− za ≥ 3,0bF.
where bF is the smaller side length of the foundation.
(6) For raft foundations and structures with several foundation elements whose effects in deeper
strata are superimposed on each other:
za ≥ 1,5⋅bB
where bB is the smaller side of the structure, (see Fig. B.1 b)).

 

[dik]

It depends on the variation of the borehole data... if the soil is uniform in both strength and profile, then few test holes are required.

Dik

 

[Erasa]

Thanks very much.
The site is a Telecom site.

 

[EireChch]

We will need more information than that...

Are you constructing a utility line? a building?

How long is the line? or what is the footprint of the building?...

How big is the site? Is it a flat site? Are there retaining walls? Are you cutting/filling for roads etc?

What geology are you expecting?

 

[oldestguy]

I have had this question many times. I answer "At least one boring". Then have the field crew call the geotech engineer daily or more often or have him on site for all borings. All jobs and sites are different and do not confine the investigation to any strict standard.

 

[dik]

I like at least 2 borings... if the same OK... else, more maybe.

Dik

 

[Erasa]

Again, thank you all.
EireChech, i will be constructing a mat foundation for a mast; size say 10x10m and the site is a flat terrain.

 

[EireChch]

It is normal to limit investigations to depths at which the expected increase in stress due to foundation loading will be less than 10% of the existing overburden pressure. Using Bouissenq distribution that would be 2b for a square foundation. Based on that I would drill 2 boreholes to 20m depth (2xb). If you new the area well and the geology you may get away 1 borehole in the centre.

 

[killswitchengage]

EireChch gave an excellent point of view , use 2 times B and work from there on. Anyway,past experience on local geology is a must

 

[Okiryu]

If you need to provide seismic site class you may also need to consider to drill to 100 ft (30 m) down.

 

[Erasa]

Thanks, i really appreciate this.

 

[fattdad]

if the borehole spacing exceeds 200 ft, the data set is likely going to be a limitation.

If you are working an entire, "Site" you should have at least 3 borings.

The depth should extend throughout the seat of settlement, loosely taken as the depth where the improvements will not return more than a 10 percent stress increase to the original sigma V'.

For any pier footing that's more than 40 ft wide, we'd have at least 2 holes.

Any bridge founded on rock will extend at least 10 ft below rock with RQD >50 or such that we know we are below the anticipated depth to scour.

For retaining walls and sound walls, we'd drill (or require the designer) every 100 ft.

In terrain with acid-producing rocks, we'd drill more frequent core holes if the rock is dipping.

For cut slopes, we'd drill below the proposed toe elevation by at least the cut height (depth)?

There are too many site variables to have a cookbook for all eventualities. For example a site that needs ground improvement, we may need more frequent borings and deeper target depths.

We use CPT and DMT along with SPT.

Knowing the geology can prevent the need to drill to 100 ft just for site class. After 3 or 4 decades in one market, we sort of get that!

f-d

ípapß gordo ainÆt no madre flaca!

 

[delagina]

I usually request 5 for small stations/sites... 4 on corners, 1 in the middle.. if I have an important foundation, I'd request one on that as well..
cost of mobilization is much higher than adding a couple of bore holes..

 

[EireChch]

Delagina - in my experience the cost of mobilisation is only a couple of hundred (say £500), where a 20m borehole is probably £1500. Therefore adding boreholes would be very expensive