7. Metodologia
7.6. Fase VI: Obtención de Datos y Análisis sobre la Oferta
5.2.2.1 Standard Penetration Test (SPT)
The Standard Penetration Test (SPT) is an in situ dynamic penetration test designed to provide information on the engineering properties of soils. The test procedure is outlined in the British Standards (BS EN ISO 22476-3) and in the American Standards for Testing and Materials (ASTM) D1586. The purpose of the test is to provide an indication of the relative density of cohesionless soils such as sands and gravels, but it is also used to recover disturbed samples of silty and clayey soils.
A brief explanation of the test procedure is warranted here. To perform this test, a split spoon sampler is lowered down a pre-drilled borehole until it rests on the layer of soil to be tested.
It is then driven into the soil for a length of 450 mm by means of a 65 kg weight hammer free falling 760 mm for each blow.
The number of blows required to drive the last 300 mm is recorded and this figure is designated the ‘N’ value of the soil. The first 150 mm of driving is ignored because of possible loose soil in the bottom of the borehole from the boring operations. The sampler is then removed from the borehole and its contents examined.
Standard Penetration Tests were performed on the subsoils at the Barre de L’isle Study site during the installation of the standpipe piezometer (BH09-BS-2). The results of the tests are shown in the graph in Figure 5.28a.
The residual soil deposit varies in particle size from a firm clayey silt to a compact silty sand layer and sandy silt. The highly weathered andesite parent rock is very dense.
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5.2.2.2 Rainfall Data
Mean annual rainfall in Saint Lucia varies from 1450 mm at the Hewanorra Airport in the southern part of the island to approximately 3450 mm at Edmond Forest in the upper
highlands. In general, rainfall is closely related to elevation, with the areas at sea level in the extreme north and south of the island receiving the least rainfall (mostly cyclonic, with some convectional rainfall). Adjacent areas in the Eastern Caribbean receive a mean annual rainfall of some 900mm, this being cyclonic. The high interior of the island receives the most rainfall, with a high proportion of this being orographic.
Historically, rainfall is consistently higher for the latter six (6) months of the year with maximum rainfalls being experienced in September to November. Conversely, February to May are the driest months with mean monthly minima varying between 45mm and 170mm.
The total rainfall experienced at the Barre de L’isle Study Site during the period of January 1st , 2009 – January 31st , 2010 was 7850.4 mm. Most of this amount of rainfall occurred during the wet season months of August to December (7366.2 mm). Rainfall for the dry season months of February to July was 484.2 mm. Rainfall data for the month of April, 2009 is not available. Figure 5.28b contains a graph of the rainfall during the period of February 1st, 2009 to January 31st, 2010.
Figure 5.29 to Figure 5.39 show the daily rainfall graphs for the Barre de L’isle Study Site for the period February 1st, 2009 to January 31st, 2010.
5.2.2.3 Standpipe Piezometer Data
A knowledge of excess pore water pressure conditions in the subsoils is required for
calculation of the factor of safety of a slope. The measurement of excess pore water pressure within a saturated zone is most commonly carried out using piezometers installed in
boreholes. The time taken for the piezometer to respond to a change in pore water pressure in the ground should be significantly short to give a meaningful accurate measure of the actual pore pressure. The response time will depend on the rate of change of groundwater pressures due to seasonal rainfall and individual storm events and the accuracy required.
Due to the various factors within the hydrological cycle that affect groundwater flows, and hence piezometer levels, a wide range of piezometric responses can be anticipated. The principle ways in which piezometers may respond to rainfall can be considered as either:
i) a storm response, being short term-hours to days
ii) a seasonal response, being longer term-months to years, or iii) a combination of both effects.
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Monitoring of the instrumentation at the study site provided the first stem in obtaining data necessary for slope stability analysis. Recorded groundwater level indicated a reliable estimate of the effect of pore water pressure fluctuations on the stability of the slope.
Piezometer response to rainfall were recorded for the period of August 1st , 2009 to January 31st , 2010 at the Barre de L’isle Study Site. Because of the isolated location of the site some daily readings were not recorded. A graph of the standpipe piezometer readings is presented in Figure 5.40.
5.2.2.4 Jet Fill Tensiometer Data
The pore-water pressure response to rainfall and infiltration was measured by the jet-fill tensiometers installed at the site. The tensiometers were read frequently although it was not always possible to maintain daily readings due to the remote location of the site.
The tensiometers readings were carried out together with the rainfall and piezometer readings during the period of August 1st, 2009 to January 31st, 2010. The jet-fill tensiometer readings are illusrated from Figure 5.41a to Figure 5.41c.
5.2.2.5 Slope Inclinometer Data
The slope inclinometer installed at the Barre de L’isle Study Site was monitored during the period of June 24th 2009 to November 23rd, 2009. The inclinometer readings indicate that the shear zone is in a clayey silt residual soil layer at a depth of 12.2m (40.0 ft) below existing ground surface. The movement zone lies between a silty sand layer and a sandy silt layer which overlies the weathered andesite bedrock. As discussed in section 5.2.1.1 the silty sand layer is located at a depth of 9.0m below ground surface and it is this author’s belief that the silty sand seam acts as a subterranean spring which is recharged from a source upslope from the study site and discharges in an active spring at the toe of the slope. This would explain the rapid pore pressure response in the standpipe piezometer during periods of intensive rainfall since the silty sand has a greater saturated hydraulic conductivity than the overlying residual soil. Figure 5.42 shows a graph of the incremental movement in the inclinometer during the monitoring period.
5.2.2.6 Surface Runoff/Infiltration
Surface soil infiltration rates represent the critical soil parameter determining how much rainwater will enter the soil and how much will run off for the event of each rain shower. A simple apparatus was used for the measurement of the runoff and infiltration rate at this site due to limited financial resources. The apparatus and the procedure used were described in Chapter 4 under the sub-heading ‘Surface runoff Apparatus’.
The runoff and infiltration tests were carried during the actual rainfall event of October 7, 2009 when a total of 488 mm of rainfall was collected over a 24 hour period. The immediate surface run-off measured over a 60 minute period was 25 mm out of a total rainfall of 45 mm.
The remaining 20 mm or 44 % of the total rainfall would have been temporarily held in the macro-pores in the soil and released over the following hours and days as lateral subsurface water flow through the soil profile. The resulting infiltration rate calculated was 5.6 x 10-6 m/sec.
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5.2.2.7 Saturated Hydraulic Conductivity Test of Residual Soil A falling head permeability test was conducted in the standpipe piezometer installed at the site in accordance with the standards specified by the United States Department of the Navy, Naval Facilities Engineering Command – 1974 (NAVFAC), for a cased borehole with a perforated extension of length ‘L’.
The expression used to calculate the in situ saturated hydraulic conductivity test is explained in Section 4.4.2 of this thesis (In situ Hydraulic Conductivity Test).
The results of the test indicate that the saturated hydraulic conductivity of the residual soils at the Barre de L’isle study site was 9.0 x 10-5 m/sec.
The saturated hydraulic conductivity of the 2.0m thick sand layer encountered at 9.0 m in borehole BH09-BS-2 was estimated from the grain size distribution curve to be in the order of 1 x 10 -3 m/sec (Cedergren, 1977).