1. Some geotechnical aspects of harbor structures in karst formations
Alexandre Santos-Ferreira1, Cláudia Santos2, Liliana Ribeiro3 & Cláudia Rocha4
1 Sénior Engineer, IPTM, I.P., CICEGe - Faculdade de Ciências e Tecnologia, Univ. NOVA de Lisboa,
2 Trainee at IPTM, IP./post-graduate student at FCT - UNL, 3 Trainee at IPTM, I.P., Almina, Minas do Alentejo, SA, Aljustrel, Portugal,
INTRODUCTION
Defining the correct location and extent of karst features is frequently difficult. Consequently constructing in these types of formations can involve some geotechnical problems regarding foundation stability and strength.
reliable results safety assurance
Site investigation careful planning accurate karst characterization costs in plan control
minimized unexpected situations future costly remedial interventions avoidance
to attain
to aim
In Portugal, some harbors are located at geology sites characterized by alternated layers of limestone and marls, often presenting karsification.
Both Figueira da Foz and Ericeira have harbor structures founded in karst formations, so these cases were analyzed and discussed in the light of the geotechnical interventions that were performed.
CASE A: QUAY WALL EXTENSION IN FIGUEIRA DA FOZ HARBOR
CASE B: ANCHORED RETAINING WALL IN ERICEIRA
In Figueira da Foz harbor, the second stage of the construction works in plan was comprised of a 240.7m quay wall extension in the alignment of the existing one, creating a m gross freight quay.
Ericeira harbor presents a high cliff shore, with some small beaches at their foot, namely, Fisherman’s beach (in red in Figure 1B). The town is just on the top of the cliffs and, at north harbor, is less than 1m from the cliffs crest.
The proposed foundation solution for the extension included three alignments of reinforced concrete piles.
According to the design and as performed in the first stage of the quay wall construction, the piles should surpass the karst zone and should be embedded in the sound limestone formations, in an adequate length to guarantee a proper bond to the bedrock.
Figure 1A. Figueira da Foz’s harbor location and the study area.
Figure 1B. Ericeira’s harbor location, study area within the harbor jurisdiction limits, cliffs general view and the south access road to Ericeira’s fishing harbor prior the stabilization works. Numbers represent the stabilization works as described next.
GEOTECHNICAL CHARACTERIZATION
Since the local geology is prone to stability issues, the cliffs have been subject to stabilization works (since the XIX century). The performed interventions are described next, following the numeration in the figure:
Local geology:
(a) Alluvial units – Modern: fluvio-marine formations, with variable thickness, of medium to coarse sands and, at the base, fine to medium muddy sands
(C2-3) Turonian limestones – Cretacic: compact fossiliferous limestones presenting vertical fractures and karsification with clayey filling material
For the design stage, the site investigation included:
Construction of a stone blocks masonry gravity wall founded on a sandstone layer, in the central area, to stabilize the road on top of the cliff (1853):
Building of a new wall outside the original one, in reinforced concrete, deep anchored in the sandstone layers to control occurring displacements ( ).
Construction of a 15 m high concrete gravity wall for stabilization purposes ( );
Stabilization of the cliffs upper half in its highest section with the five stone masonry buttresses (1936);
Building of the two main accesses to the Fishermen's beach which presented adequate behavior until some years agora when signs of degradation started to show, mainly in the south access (5b) (end of XIX century);
roto-percussion boreholes,
super heavy dynamic probing (DPSH)
and systematic SPT tests every 1,50m
in land and at an hydraulic embankment.
All the boreholes had a 100 mm diameter.
The survey data analysis enabled the geotechnical zoning of the area: GU1 to GU3 zones, from the least favorable to the most favorable mechanical characteristics, respectively).
The attained bedrock RQD values were quite high, varying from 60% to 90%, classifying it from fair to good. The occurrence of vertical fractures is a negative factor for the piles installation.
Since 2008, a systematic monitoring and a stability analysis of all the cliffs under harbor jurisdiction begun. The performance of a risk analysis allowed the identification of priority rehabilitation works, respectively:
the main access to the harbor, at the north of the quay wall: out of the scope of this analysis;
and the south access : the subject of this case study (rightmost image in Figure 1B).
GEOTECHNICAL CHARACTERIZATION
Local geology:
(C2 b’’) Praia do Peixe Sandstones – Lower Cretacic (Albian): alternating strata of coarse sandstones (from 0.3 m to almost 1 m thick) and purplish marls (0.1 to 0.5 m thick). These sets overlay a grayish-pink sandstone clayish bank, very fine-grained.
The marl erosion is the main process under which the cliffs become unstable. Then, according to the discontinuities, faults and absence of support, blocks fall. Also, in some locations the underground water dissolves the sandstone, forming a not well-developed karst. In Fisherman’s beach south access, the sea waves’ action at the cliff base, on the support strata of the stone gravity retaining wall, lead to a much more significant marl erosion and bigger karst cavities.
As the base of the cliff was filled with sandstone blocks from old falls, it:
helped to protect the cliff from direct wave action, and
prevented an accurate assessment of the real conditions of the retaining wall’s foundation.
Anyway, the visible part was sufficiently degraded to consider that there was a high risk for the structure.
Figure 2A. Geotechnical zoning of Figueira da Foz’ harbor area
The karstification assessment was performed during the boreholes execution and it was observed that, in some points, the karst is residual and, in others points, it surpasses the boreholes terminus.
DESIGN OVERVIEW
The quay wall extension design comprised the execution of seven 30x16 m modules and an extra closing module 30.7 m long, in order to create a m structure.
Each module included a set of 15 reinforced concrete piles spaced 6.0 m in the longitudinal direction and 8.0 m in the transversal direction.
The foundation solution considered:
in A and B alignments:  1.0 m piles,
in C alignment:  0.8 m piles
and the pile embeddedness in the bedrock was set to be, at least, 3 m for each pile, after surpassing the karst zone.
DESIGN OVERVIEW A B C
C A
In 2012, as the retaining wall central part and the south access collapsed. It was urgent to begin the rehabilitation works at once, so an adequate survey and even the assessment of the mechanism of failure was not possible in that phase.
Though some basic decisions could be established:
The support wall had to be totally rebuilt;
There was not enough space, considering the distance from the retaining wall to the cliff so, another solution was necessary;
Remove all the blocks from the base of the retaining wall, to be sure of the foundation conditions;
During the reconstruction it would be absolutely
Figure 3A. Geotechnical zoning of Figueira da Foz harbor area
MAIN RESTRAINS
necessary to understand how did the wall fail;
Maintain the appearance of the retaining wall, integrating
it in the area; recover, at least in part, the new wall with stone block, and to rebuild its top, in the failed area;
The wall is 19m high in its highest section. The selected solution consisted in a reinforced concrete wall, with a thickness from 0.5m at the top, untill 0.8m at the base, with no footing, but with 105ground anchors, 23m long, sealed in stable sandstone strata.
Figure 2B. Aspects of the south access’ of retaining wall in
Constructing over karst environments may involve uncertainty, risks and is often associated to higher construction costs. Even the most extensive site investigation program has limitations to accurate assess the karst conditions in site, mostly due to its variable nature.
The geotechnical survey and the first construction stage results were considered for design purposes. The boreholes data presented the average karst length as 1.65 m; considering also the RQD values, 1,60 m to 1,80 m karst length was considered adequate.
This assumption, based on boreholes with 100 mm diameter, can underestimate the real karst length, as the piles with 800 mm and 1000 mm diameter would likely intersect karst cavities that the boreholes had probably missed.
MAIN RESTRAINS
The crane had to be installed in the access itself so, to assure its stability, it was constructed a base founded in micropiles;
Limited access to most trucks to work site for the construction materials and equipment supplying.
EXECUTION CRITERIA AND DISCUSSION
The average karst lengths obtained during the second stage construction works were 2.95 m in the most inner piles alignment (A alignment) and m in the intermediate alignment (B alignment).
It was possible to notice that the top of limestone
The foundations of the old wall were inexistent in several sections, so the entire wall was risking collapse. In order to perform a base to ensure the crane stability, 8 micropiles were executed and the cleaning of the base of the retaining wall was performed.
The design set that the new wall was built starting by its lowest section, followed by the installation of part of the ground anchors; as for the failed section, it was decided that it would be cleaned only after the new wall was built in both of its sides.
Table 1A.. Project lengths
layer fitted well the estimated profile in design. The top of the limestone layers presented a slight tilt towards the river and so the piles in B alignment presented an average length a little longer than the piles in A alignment. Considering both alignments, the total piles length increase due to karstification was less than 5% of the total foreseen length. This increase was due mainly to two factors:
The natural variability of the geological formations and its karstification prevented an exact prediction;
In design, the karst estimation was based on boreholes with 100 mm diameter, while during construction the piles had 1000 mm diameter.
It can be stated that the geotechnical model assumed in design was accurate. The difference between the predicted and the real karst thickness was quite small, and no increase in accuracy was expectable by extending the number of executed boreholes prior to the design phase, during the site investigation.
Figure 3B. Aspects of Ericeira’s south access prior to the rehabilitation works
The drilling presented some problems related to collapse of the drill holes and to karst cavities. The technical solution adopted was to fill them with grout and then repeat the drilling operation. Also related to the karst cavities was the increase of the ground anchors sealing grout volume used, and the increase in the sealing bulb volume.
All ground anchors were submitted to load tests and seven of them were equipped with load cells. For a good monitoring, some topographic survey marks were installed as well as two inclinometers.
Figure 4B. Ericeira’s south access after the conclusion of the rehabilitation works
CONCLUSIONS
From the two cases discussed, some facts can be highlighted, namely, the karst importance in the design and execution of pile foundation is difficult to correctly foresee; however, this constraint can be minimized with a sound knowledge of the area and the use of information from previous similar construction works. Also, the sea waves may have a relevant effect in the evolution of a cliff of alternating layers of sand or limestone and marls. For old gravity retaining walls, the waves can affect these structures in several ways: weakening the cement, increasing the rate of the marl layers erosion and of the dissolution of the sandstone calcium matrix, eventually removing the foundation support layer of the retaining wall. Given the pointed out facts, in these formations, a regular and accurate monitoring system should be implemented.