The geotechnical study
Why knowing what’s underground is mandatory for sustainable design.
Knowing what’s underground is as important as knowing what’s above, and this is what a geothecnical study helps us understand. It’s a mandatory step for a truly sustainable building.
Many people try to avoid this step just for saving few hundreds euros but in this post we’ll understand why doing a properly-made geotechnical study it’s the only way for respecting the environment, saving a lot of money and living in a safe environment. Let’s begin.
Samples collected after the geotechnical drilling
Prices vary according to the type of building, number of excavation needed (in my case 2), soil type and particular necessities of the owner and architect.
I was told that a basic, properly-made geotechnical study would have costed about a thousand euros, but from the companies of Fuerteventura I received an average quotation of 1800€. Too much.
A few days after Guayarmina gave me the contact of Javier, a professional geotechnician from Gran Canaria. I called him and, since he already had all the machines and equipment in Fuerteventura, he made me a good offer before going back to Las Palmas.
The invoice of the geotechnical study
At the end, as you can see from the invoice below, I paid 1284€ for making the geotechnical study. It seemed a lot to me too but then I realized it’s not, especially if we compare it with what we can save once we have it done. Let me tell you why every cent is worth it.
Why a proper geotechnical study is good and convenient
There are three main reasons why making a proper geotechnical study is good.
Knowing well how strong our land is helps us build a well-dimensioned foundation system, using just the amount of material that is needed for what we’ll build above it, avoiding the big waste of material coming from an over-sized foundations, thus avoiding a big impact on the environment.
It’s actually very logical, oversizing the house foundation means wasting much more money than needed because you’ll use much more material and labour. How much?
Let’s consider a 200sqm house like mine and let’s consider two cases; the first with a 40cm high concrete slab, the second with a 60cm (oversized) one; the difference in volume is 200m2*0,2m= 40m3 of concrete. At an average price of 60€/m3 [2016, http://www.reformas-servicios.com/precio-metro-cubico-hormigon-espana/] this means 40*60 = 2400€ saved. This is just for the material, not even considering the labour!
It’s a commom practice of many builders saying that the geotehnical study is not important and that you could save the money but that’s not true. Now, at least, you know why.
But there is more: doing a true geotechnical study is the only way for obtaining the seguro decenal (10-year insurance) for the house. And this paper is mandatory if we want to sell the house in the future. This translates in a much higher value of the house, and I am talking of many thousands. I extimate at least a 5000€ increased value of the house if you include a proper geotechnical study with your project.
Talking about safety, the geotechnical study ensures the structural resistance of the house and the absence of dangerous/toxic/hazardous gases coming from underground (like Radon). How much would you pay for your health?
Let’s see how to make it, which are the steps and how long does it take to have everything done.
Step 1. The drilling
It’s needed for collecting the samples.
Javier came to my piece of land with all the equipment for taking the samples from underground.
Mind to make clear at the very beginning that you have to do a proper study, for being compliant with the CTE (the spanish building code) in order to be able to get the 10-years insurance (seguro decenal).
Of course, I had no problem with Javier, he already knew that. Keep in mind that for a residential house like mine it is necessary making 2 underground excavations plus the laboratory tests.
Photogallery of the works
It took approximately 4 hours and it was really exciting to see the composition of each layer of the ground where I’ll build the house. A great way to lower the impact of the building itself 😀
Step 2. The final report from the laboratory
It took approximately a week for receiving the final report, with the results form the laboratory that investigated on the samples of the excavations. Of course the entire document is available in the open archive of the project, for reading and/or for direct download.
The complete geotechnical study (click on the image for the download)
It came out that the first impression we had with Javier and Juan, while drilling, were correct: it’s a great soil! The result gave an average resistance of 2,5kg/cm2, with a quite uniform soil composition. Awesome. No harmful gases have been detected and nor any kind of aggression from sulfates. Awesome again!
Before closing the post I want to write a coulpe more words for giving the contact of Javier. I’m having nothing from recommending them, I am just sharing my experience that was absolutely flawless. Thank you Javier, Juan and Elvira!
Now that I know well all that I have around, under and above the ground, it’s time to start thinking serioulsy about the building process, the materials and the techniques.
Let’s move on!