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Building with the Subsurface for realizing cost-efficient infrastructure
Construire avec le sous-sol pour réaliser une infrastructure à coût avantageux
Venmans A.A.M.
Deltares, P.O. Box 177, NL-2600 MH Delft, The Netherlands
ABSTRACT: The paper introduces the concept of ‘Building with the Subsurface’ for optimising constructions to profit from
subsurface conditions. The concept fits in the framework of Value Engineering. A case study for a road on soft soil illustrates the
concept. Subsoil heterogeneity is expressed in sets of discrete synthetic subsoil profiles, suitable for geotechnical design calculations
with conventional tools. The case study shows that the uncertainty in the whole life cost of the road ranges between ±10% and ±30%,
depending on lithology and sensitivity of the construction method to subsoil uncertainty. Adding local site investigation to subsoil
data from public sources reduces the uncertainty.
RÉSUMÉ : L'article introduit l'idée de ‘Construire avec le Sous-sol’ afin d'optimiser les constructions en profitant des conditions du
sous-sol. Le concept s'inscrit dans le cadre de l'Ingénierie de Valeur. Le cas d'une route construite sur sol compressible illustre le
concept. L'hétérogénéité du terrain est exprimée par des jeux de profils synthétiques discrets qui conviennent aux calculs
géotechniques menés avec des outils conventionnels. L'étude de cas montre que l'incertitude sur le coût de la route pendant toute sa
durée de vie varie entre ±10% à ±30%, selon la lithologie et la sensibilité de la méthode de construction aux incertitudes liées au sous-
sol. Ajouter aux données du sous-sol de source publique une reconnaissance de sols locale réduit l'incertitude.
KEYWORDS: Subsurface model, Value Engineering, geological uncertainty, cost estimate, roads, soft soil, piled embankments.
1 BUILDING WITH THE SUBSURFACE AND VALUE
ENGINEERING
Reduction of subsoil related risks has been an important issue
for the past few years in the Netherlands. The potential
economic benefit is estimated at 1.5 % of the construction
sector turnover (van Staveren, 2006). However, entrepreneurs in
the construction sector are easier stimulated by opportunities
than by problems. Value engineering is an acknowledged
method for identification of options for cost reduction. Value
engineering saved between 6 and 8% of the total construction
costs in U.S. highway projects in the past five years (US DoT
FHWA, 2011). This paper focuses on the potential of the
subsurface to realize cost savings in infrastructure construction.
The lithology, engineering properties and hydrology of the
subsoil determine the feasibility of construction methods and
their costs. Critical parameters may concern foundation depths,
the continuity of an impervious layer or geotechnical properties.
These critical parameters are often not adequately and
systematically mapped in the current Dutch site investigation
practice. The usual approach is based on CPT’s with typical
centre-to-centre distances of 100 m, and soil sampling in
borings even wider apart. Although this will provide a general
idea of the soil profile and properties, heterogeneity will still
cause substantial uncertainty. The more expensive way out is to
use construction methods that are robust with respect to
geological heterogeneity, such as piled embankments.
Three key elements of realizing more cost-effective
infrastructure are (1) knowing the potential heterogeneity, (2)
knowing its impacts on construction methods and costs and (3)
reducing the impacts. This concept is called ‘Building with the
Subsurface’, i.e. optimising constructions to profit from
subsurface conditions. The impacts of subsurface uncertainty on
construction costs are made explicit during the process. This
allows informed decisions to be made on additional site
investigation and finally, the selection of a construction method
on the basis of costs, and uncertainty in costs. A Value
Engineering / ‘Building with the Subsurface’ study can be
performed in any project stage, but will be most rewarding in
the feasibility stage. Usually the alignment or corridor will have
been set in this stage. Choices regarding construction methods,
materials and mitigation of impacts on the surrounding area are
still open. The main outputs of the feasibility stage will be cost
estimates, a time schedule for construction, and
recommendations for mitigation of impacts.
Table 1 illustrates activities of the ‘Building with the
Subsurface’ concept in the context of Value Engineering. This
scheme is applied in the following virtual case study, using
actual geological and geotechnical data.
2 EXAMPLE: FEASIBILITY STUDY OF A ROAD
2.1
Preparation phase
A 2x2 lane road is to be constructed in the soft soil area around
Rotterdam Airport (Figure 1). The time available for
construction may be ½, 1 or 2 years, to be decided later.
The study should identify the alignment of the road running
approximately north-south in the 10 km
2
area, the whole life
costs and their uncertainty. Whole life costs are the sum of
construction costs of earthworks, drainage and pavement and
costs of subsoil related maintenance. Construction methods will
be selected on the basis of the 90% upper limit of their whole
life cost. Also, the 80% confidence interval of the cost estimate
should be within ±15% of the average value. ‘Uncertainty’ is
thus expressed as the half width of the 80% confidence interval.
2.2
Information Phase
The elevation of the road surface will be 0.5 m above ground
level. The 1:50,000 geological map indicates that the subsoil
consists of 15 to 20 m of Holocene soft peat and clay over
Pleistocene sands. Peat was excavated in part of the area, and
