Geology and geotechnics

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For the second time in 2 years I’m working at a wind farm with a potential karst problem.

If you’re reading this post you are probably aware of what is karst – basically, the result of dissolution of rocks such as limestone and gypsum, that can produce beautiful landscaping results (such as caves, sinkholes and dolines) but also several problems for the foundations of structures.

If the size of the cavities is reduced (a couple of centimetres) the will not be a problem because the foundation will not notice them. However in unlucky cases, you can dig and open the foundation and discover a huge hole that can be several cubic meters big.

I’ve heard stories of civil subcontractors who pumped the load of several concrete mixers in the holes without filling them (in some cases you can find a network of connected cavities).

Relocation of the WTG is a solution, but is usually not the best due to several reasons (maybe the adjacent land plot is not available, or you have a tight layout with very close turbines, or there are environmental constraints).

Also, it’s usually not free: depending on your contract, the civil subcontractor could be entitled to ask for a variation order.

Which are the available technical solution?

As far I’m aware of, MASW, electrical tomography and ground penetrating radar (GPR). I’ve used the last one successfully in a project in Jamaica and it worked well.

Basically, the GPR investigation is a non-invasive technique that can detect anomalies (locations, depth and approximate size of cavities) and spot  any inconsistency or variation in the expect soil profile across each site.

Resuming shortly how it works, GPR generate and send electromagnetic waves into the surface with an antenna, moving along the inspected surface.

Whenever a radar pulse strikes a boundary interface of contrasting dielectric (basically, between different materials), a portion of the pulse is reflected back to the surface to a receiving antenna.

The contrast in properties between clay (the weathered material which usually fills the holes) and limestone help to identify zones of subsidence. Conversely, a lack of contrast indicates relatively uniform stratigraphy.

Subsurface profiles will be generated displaying the resulting echoes of individual pulses.

Integrating this technique with standard boreholes will decrease the geotechnical risk of your project.

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I’ve just received a short technical summary of the works performed in Raki (Chile) from Terratest, the company who has made the gravel columns.

Unfortunately it’s in Spanish, but the pictures are beautiful 😉

About 9000 meters of gravel columns have been put in place. The diameter is 80 cm, while the length changed depending on the position (from 9 to 13 meters).

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This is the first post after a long silence (more than 6 months), fortunately due to good news (the birth of my first son, who reduced dramatically my free time).

I want to thank José Ramon, one of our experienced Project Manager, who pushed me to start again 😉

This post is about geotechnical parameters relevant for wind foundation. I’m in debt with Ana from Esteyco, a very good geologist who explained me how this work.

She explained me that the most important parameters that we need define in a geotechnical survey for wind turbines foundation design are 2: bearing capacity and deformability.

 

Bearing capacity is defined by c (cohesion), ϕ (friction angle) and qu (unconfined compressive strength) and it is used in Ultimate Limit State (ULS) design.

In this type of verifications, loads such as extreme wind, abnormal loads or earthquake are used to check the foundation for overturn, slide and collapse (soil failure).

If we are working with soils, we normally define these parameters using in situ tests such as SPT (in sands), CPT, Pressuremeter test and Vane test, or with laboratory tests  such as triaxial (CD, CU, UU) , direct shear test or CBR.

In rock, we could use a compressive strength test, rock quality designation (RQD) or triaxial, although the bearing capacity of rock (even heavily fractured and weathered) is normally so high that these tests are normally not necessary.

Deformability is defined by E (Young's modulus) and G (Shear modulus).

These parameters are used to check if the foundation is compliant with vertical settlement, differential settlement and, above all, rotational stiffness.

These verifications are done with a Serviceability Limit State (SLS) design.

In case of soils, these parameters are defined with in situ test like SPT, CPT, down hole, cross hole (normally not used because we don’t have boreholes nearby normally) and pressuremeter. If we define them with laboratory test, we would use triaxial or oedometer test.

In both case, we prefer field tests to laboratory tests, mainly because it can be difficult to have undisturbed samples with their in situ characteristics.

It is important to highlight that in some cases, we could meet the bearing capacity requirements but not the deformability minimum conditions and vice versa. For instance, soft sandy soils with some combination of WTG and tower can offer a reasonable bearing capacity but an excessive deformability.

In general, the geotechnical study of a wind farm should describe the terrain in order to provide the necessary information to develop the following works:

  • Turbine foundation project: definition of the allowable soil bearing capacity and the most appropriate type of foundation (e.g. shallow, semi-deep, foundation on piles, etc..).
  • Wind farm access roads and hardstands project: determination of the characteristics of subgrade and its possible improvement if necessary.
  • Substation Project: Design of building foundations, along with the design of the earthing network (analysis of the electrical resistivity of the soil).
  • Determination of the suitability of the material from excavation for use in embankments, determination of the angle of stability for slopes.

The company responsible for the geotechnical study should collect all available geological information about the area (geological maps, photo geology, and visual inspection). The purpose of this phase is to provide a first approximation of the type of materials to be able to find and confirm the type of field and laboratory tests more suitable, and if necessary to propose other type of tests or additional studies.

With this initial information, it must be prepared an exhaustive proposal for the geotechnical survey detailing all the work to be executed and the time schedule.

The type of field tests to be performed will depend on the geological nature of the materials in the area.

 Field tests

In soft soils:

Boreholes drilling to a depth of 30 meters, including standard penetration tests (SPT) every 2 to 3 meters in not cohesive layers, extraction of undisturbed samples in cohesive layers, determination of the water table and graph with the geological and geotechnical profile, photographical report of the samples and of the boreholes, diameter and type of drilling at each depth, rate of recovery of specimens, presence of water.

The number of boreholes depends on the type of materials, being at least:

  • In favourable terrains, where is possible to predict homogeneity and continuity of the layers: boreholes in 40% of the positions of the wind turbines.
  • In geologically complex areas: boreholes in 100% of the positions of the wind turbines.

Trial pits using a backhoe. Includes sampling for posterior laboratory test, photographic report, carrying out of the appropriate in situ tests and classification of soils for engineering purposes using unified soil classification system. Also shall be indicated the position of the water table and a subjective estimate of the consistency and permeability.

The trial pits to be done are the following:

  • In all the WTG position where hasn’t been done a borehole: a trial pit in every position, up to the foundation depth or the bedrock.
  • In the access roads: a trial pit every Km, or where it is considered to be representative. The required depth is from 2 to 3 meters.
  • In the substation area: a trial pit in the building area, with an additional pit in the switchgear area if it is considered necessary. The required depth is from 2 to 3 meters.

In hard soils and rock:

Drilling up to a depth of 30 meters with continuous rotary drilling. Alternatives geophysical methods are acceptable (e.g. Seismic refraction profiling) but they must be technically justified. This is the case in situation where the area is clearly homogenous and made of solid rock.

In karst areas or when the presence of underground cavity is suspected an exploration of the ground using geophysical methods (micro gravimetric georadar, etc.) must be performed in order to determine the position of the cavities.

Resistivity tests. In order to provide information for the design of the grounding system of the substation electrical resistivity tests of the subsoil shall be conducted in the whole area of the substation.

The tests will be normally realized using Wenner four-electrode method arrangement or equivalent array as per ASTM D6431 - 99 (2010).

Laboratory tests

With the specimens obtained during the field works at least the following test will be done:

Classification tests:

  • Determination of the particle size distribution by sieving.
  • Determination of the Atterberg limits (liquid limit, plastic limit, plastic index).
  • Determination of the natural water content.
  • Modified Proctor test to determine the relationship between water content and dry unit weight of soils (compaction curve).
  • Soil classification.

Mechanical tests:

  • Direct shear test.
  • California Bearing Ratio (CBR).
  • In plastic, expansive or poorly consolidated clays: triaxial test.
  • In rock: geological classification of the sample as per ASTM D5878 - 08 using a suitable system of classification for Engineering Purposes. Rock quality designation, rock mass rating.

Chemical tests:

  • Determination of environmental aggressiveness and corrosion risk for concrete: pH and sulphate content in topsoil and water.

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Floating roads are a solution developed in Scotland during the construction of several Km of internal wind farms roads on very poor, organic materials such as peat.

It is an interesting constructive technology, developed on a very compressible, decayed material normally considered as the worst possible subgrade for road construction. In a normal highway project, peat would be removed

Usually floating roads are stabilised haul roads with one or two layers of geogrid. This approach leads to several advantages:

 

  • Reduced road thickness (and subsequently less weight)
  • Better distribution of pressure
  • Untouched surface layer, lesser impact on vegetation
  • Less construction material needed
  • Usually cheaper

 

The suggested approach to floating roads construction is based on six steps:

 

  1. Carrying out a detailed survey to define the hydrology of the area and the paet type (according to the “Von Post” system, there are 10 possible classification for this material depending on the decomposition level)
  2. Identifying the value for in situ peat strength
  3. Estimate the expected traffic loading
  4. Design the road
  5. Monitoring the construction
  6. Recording action and outcomes for future projects

 

Among the design assumptions, it’s relevant to remark that a rut with a depth up to 10 cm is to be expected.

The first step is an in depth site investigation, where several parameters are recorded: peat depth and classification, side slope angle, hydrology and permeability. Many in situ test are available, from probing and sampling to more sophisticated techniques such as ball penetrometer or Mexecone penetrometer and ground penetrating radar. After, in laboratory, water and organic content can be defined, together with vane testing and direct simple shear tests.

When all the data is collected, normally is the geogrid manufacturer who will design the cross section of the road, deciding the type of grid (there are several dimensions possible – they must match with the aggregate size), the number of layers and the height of the compacted stones.

The designer will use semi empirical rules, using as inputs shear strength of the peat, weight of the road and expected number of equivalent axes.

It is standard practice to use at least 2 layers of geogrid: a lower one directly on the existing material, and an upper layer on the top of the embankment, used as a support for the controlled granulometry crushed gravel.

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Here you have a real world example of geogrid use.

We are in a wind farm in southern Spain, and thanks to previous experience with this technology the client decided to use a Tensar Triax geogrid TX160.

This is a triangular geogrid (the “old school” version was square). It seems that the triangular geometry guarantee a better distribution of the loads

The same geotextile  has been used in many wind farms (more than 100) around the world, with several big project in Germany and UK where in some cases more than half a million square meters have been used. Often the soil was peat (turf), with a very low CBR (even less than 1).

The saving in crushed stone using a geogrid can be around 30% to 40%, so with a price around 2-3 €/sqm it is normally a cost effective solution.

When we opt for this solution we’ve had helpful feedback and hints from Tensar, as they can study the existing info and provide a design based on the soil conditions and the available materials. There are several models available with a different triangular size, so it is not easy to choose the right model.

We haven't had any problem during the first half of the civil works circulating with machinery and trucks, but when we started moving the narrow track crane on the internal roads between the pads significant damages appeared:

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Every month I have the opportunity to read several geotechnical survey made for wind farms all around the world.

There are enormous differences between countries, not only in the formal presentation but also in the type of tests used, that often depends on tradition and historical reasons.

For instance reading a geotechnical study made in Peru I discovered the Peck’s Cone (“Cono de Peck”): it is a variation of the standard SPT, introduced in the ’70 in the country.

The most used correlation is:

 

Nspt = 0,5 c

 

Where:

Nspt = SPT value (number of blows for a 30 cm penetration)

c =  Number of blows for a 30 cm penetration using Peck’s cone

It is used instead of the SPT because it is quicker and cheaper: there is no need to extract the point and store the soil, as the perforating cone is closed. Actually it uses the same machine of the SPT, but is more similar to a DPSH test (but among the other differences, the cone of the DPSH has a 90º angle while the Peck’s Cone has a 60º angle).

On the downside, it has several problems: it is a “blind” test, because you don’t know the materials you are perforating, and there are many available proposed correlations (in addition to Nspt=0,5 c I have found Nspt=1 c and others).

I haven’t been able to find an international standard for this test, so I wouldn’t recommend it.
You can read more about the test (in Spanish) in this study.

 

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This is a debate we are living each time we have to build internal roads on soils with a low (<5) to very low (<2) CBR.

Basically when the others alternatives (mainly soil substitution and soil improvement) are not feasible we are adopting two different solution, either a strong geotextile with reinforcement and separation properties or a geogrid (coupled with a thin geotextile used as a filter if necessary).

Presently both of them are working well, but only after many years we will know which one works best. I'm hearing very different opinion on the subject, so there is not an universal consensus.

As geotextile, both woven and nonwoven alternatives seem reasonable. Both of them provides separation of the aggregate from the subgrade and have high tensile strength and modulus, adding reinforcement to the foundation soil. Right now the woven solution is widely preferred.

As woven geotextile we have used the US250 from US Fabric, with the following properties:

PROPERTYTEST METHODENGLISHMETRIC
Tensile StrengthASTM D-4632250 lbs1,112 N
Elongation @ BreakASTM D-463215%15%
Mullen BurstASTM D-3786450 psi3,102 kPa
Puncture StrengthASTM D-4833100 lbs445 N
CBR PunctureASTM D-6241900 lbs4,005 N

And as nonwoven, something like the US Fabric US 160NW  looks like the best option:

PROPERTYTEST METHODENGLISHMETRIC
Weight - TypicalASTM D-52616.0 oz/sy203 g/sm
Tensile StrengthASTM D-4632160 lbs711 N
Elongation @ BreakASTM D-463250%50%
Mullen BurstASTM D-3786305 psi2,103 kPa
Puncture StrengthASTM D-483390 lbs400 N
CBR PunctureASTM D-6241410 lbs1,824 N

Regarding geogrid, it has been used in several wind farms all around Europe. I had a meeting with the representative from Tensar, and their product looks interesting.

It is a triangular net, providing support to the stone aggregate. It works equally well in every direction.

We have used it in Spain, and it have been used in several other projects in UK and Romania.

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