Portimão is a town in Portugal, located about 300 km South of Lisbon, near the Algarve coastline, on the West bank of Arade River. Recently, the Portimão Municipality purchased an old industrial plant, located near the river bank, to convert it in the Municipal Museum. The Museum initial design was based on a local geotechnical survey that revealed a top layer of earth fill and alluvium, with a depth varying from 6 to 14m, overlaying stiff Jurassic limestone. So the designer decided to adopt a solution of drilled micropiles using the technology of multiple repeatable post-grout injection.
The contractor to whom the micropiles execution was awarded proposed an alternative solution using a single global post-grout injection. The Designer and the Owner accepted the alternative solution under particular conditions to avoid additional costs due to the change of technology. The paper refers the main aspects of the initial design and of the alternative solution, and the practical consequences of the micropiles execution.
Figure 1. Micropile classification based on type of grouting.
The Designer adopted a solution of drilled micropiles using the technology of multiple repeatable post-grout injection. Some of the micropiles had an unit bearing capacity of 600,00 kN and others of 200,00 kN. For both types of micropiles the designer considered a circular steel casing of Φ 60,3 mm with a thickness of 3 mm. The foreseen bulb extension was of 6,0 m for the 600,00 kN micropiles and of 3,0 m for the 200,00 kN ones. The Designer also specified that the injected bulb should start whenever a soil layer with a S.P.T. value of 60 blows would be attained, and not before.
The initial design was approved by the Owner and the public tender was issued. The Museum construction was awarded to a Portuguese main contractor, Construtora Abrantina, that used, as its sub-contractor, for the micropiles execution, the Portuguese firm Sopecate. The proposal submitted to tender by these two contractors was based on the initial design that was submitted to the public tender and so, on their proposal, it was also considered the execution of micropiles with multiple repeatable post-grout injection.
The tender documents allowed the possibility for the contractors to submit alternative solutions to the initial design that had to be approved by the Owner. Once this approval obtained, if the alternative would lead to a cost reduction, this reduction would be shared in equal parts by the Main Contractor and by the Owner. So the referred contractors decided to submit to the Owner an alternative solution for the micropiles execution.
The alternative solution presented by the referred contractors for the micropiles execution considered a different injection method, the so-called single global post-grout injection. The same unit bearing capacities were used (600,00 kN and 200,00 kN) but the bulb extensions were, respectively, 9,0 m and 4,0 m. In this alternative solution it was not mandatory to start the injection only when a soil layer with a S.P.T. value of 60 blows occurred, being the sub-contractor experience to dictate the adequate soil characteristics to start the bulb injection.
Both designers (the one who executed the initial design and the one responsible for the alternative design), considered the following equation (Coelho, S., 1996) for the lateral shaft resistance R: R = τ. π. D s. L s (1) where L s is the bulb extension and D s is the mean diameter. D s is related to the nominal diameter D n by the following equation: D s = α. D n (2)
From Coelho, S., 1996, and for clays, the value of α varies from 1,8 to 2,0 (for multiple repeatable post-grout injection) and, for single global post-grout injection, α = 1,2. This means that, between both injection methods, we will have a ratio of: 1,8/1,2 = 1,5 So the bulb extension for single global post-grout injection will be 1,5 greater than the one for multiple repeatable post- grout injection. This justifies the difference between the bulb extensions of the initial design and of the alternative design.
The Owner accepted the alternative solution submitted by the Main Contractor but under the condition that if the final cost would be greater than the estimated initial one, the difference had to be taken by the Main Contractor. If there would be a reduction in cost, this reduction would be shared, equally, by the Owner and the Main Contractor, as foreseen in the tender documents.
After the micropiles execution was finished and the contractor submitted the respective invoice to the Owner, there was not an agreement between the two parts. After some time of discussion they agreed to contract a foreign consultant (the author) to define what should be paid by the Owner to the Main Contractor. In a first approach our opinion was that L 1 being the total executed length of a 600,00 kN micropile, the Owner should pay only an extension L 1 obtained from the following equation: L 1 = L 1 – (9,0 – 6,0) = L 1 – 3,0 (3)
due to the difference of bulb extension between the initial design (6,0 m) and the alternative design (9,0 m). On the same way L 2 being the total executed length of a 200,00 kN micropile, the Owner should pay only an extension L 2 obtained from the following equation: L 2 = L 2 – (4,0 – 3,0) = L 2 – 1,0 (4)
In a second approach to define the possible cost reduction obtained during the micropiles execution if D 1 was the depth were the soil had a S.P.T. value of 60 blows, adding to this value the bulb extension defined in the initial design for the 600,00 kN micropiles, a value of T 1 would be obtained which meant: T 1 = D 1 + 6,0 (5) So there would be a cost reduction whenever: L 1 < T 1 (6) The economy would be: E 1 = T 1 - L 1 (7)
and, from equation (5): E 1 = D 1 + 6,0 - L 1 (8) Similarly if D 2 was the depth at which the soil has a S.P.T. value of 60 blows, adding to this value the bulb extension defined in the initial design for the 200,00 kN micropiles, a value of T 2 would be obtained which meant: T 2 = D 2 + 3,0 (9) So there would be a cost reduction whenever: L 2 < T 2 (10)
The economy would be: E 2 = T 2 – L 2 (11) and, from equation (9): E 2 = D 2 + 3,0 – L 2 (12)
For each micropiles type (according to the bearing capacity values), it was necessary to evaluate, for each executed micropile, the value of E 1 or E 2 and see if this value was positive or negative. If the algebraic addition of E 1 or E 2 for the total piles of each type was positive, there was a cost reduction due to the alternative design, to be shared by both parts. If not, there was an over cost that had to be supported by the Main Contractor. In the present case there was a significant economy (about 20%) due to the alternative solution.
An alternative solution to an initial design, presented by contractors based on their experience, is a rather common situation that quite often lead to an overall construction economy. However it is necessary for the Owner to define carefully all the conditions for its acceptance to be sure that the construction quality is not affected and to allow an easy quantification of the respective cost. If both parts do not agree on the final cost evaluation it is better to avoid to go to Court and try to contract an external consultant, accepted by both evolved parts, to decide upon the amount to be paid by the Owner to the Main Contractor.
Coelho, S. (1996). Tecnologia de Fundações, Edições E.P.G.E., Lisbon, 428 p.