GROUND IMPROVEMENT SITE INVESTIGATON

Transcription

1 GROUND IMPROVEMENT SITE INVESTIGTON November 2009 Lankelma ltd old Harbour arn old Harbour Lane Iden, East Sussex TN31 7UT U.K. T: +44 (0) E: Gardline Lankelma bu Dhabi T: +971 (0)

2 ontents 1. Introduction 3 2. Role of the PT 3 3. ompliance testing 4 4. Settlement of deep compaction fills 5 5. PT plant 6 ppendix Soil behaviour charts ppendix Influence of compressibility on normally consolidated, uncemented, unaged predominantly quartz sands ppendix Plant capabilities ppendix D Soil parameters ppendix E ase studies 2

3 Depth (m) 1. Introduction In general terms ground improvement may be considered to be when the engineer forces the soil to adapt to the project requirements by altering its natural state, rather than changing the engineering design in response to the natural limitations of the soil. n overview of ground improvement techniques includes: Densification vibro and dynamic compaction; blasting and compaction gravity. onsolidation preloading; vertical drains; electroosmosis and vacuum consolidation. Reinforcement soil nailing; piles; stone columns and fibre reinforcement. Weight Reduction wood; flyash; slag; tyres and geofoam. hemical Treatment soil mixing; lime columns and jet/fracture grouting. Thermal Stabilisation ground freezing; vitrification iotechnical Stabilisation brush matting; bush layering Geosynthetics geotextiles; geogrids and geomeshes. number of these techniques readily lend themselves to investigation and compliance testing with the PT. 2. Role of the PT qc (MPa) For noncohesive sands and silty sands requiring densification by the techniques outlined above, the PT has been found to be one of the best methods to monitor and document the effect of densification due to the continuous and repeatable nature of the PT process and data (see figure 1) before compaction o after compaction For shallow compaction the PT can also be useful in checking the variability of a fill compacted in layers, or in checking whether unsatisfactory material has been left below a fill Figure 1 3

4 In the improvement of cohesive soils by means of surcharge, with or without vertical drains, the primary task is the monitoring of the rate of dissipation of generated excess pore water pressures, as well as the assessment of general variations of hydraulic conductivity of the soil. For these activities a cone penetrometer with an additional pressure transducer (piezocone) is required. selection of guidelines to the zone of soil behaviour where vibrocompaction techniques are most applicable are given on the PT soil behaviour charts presented in appendix. 3. ompliance Testing The required effect of any deep compaction technique can be set directly in terms of measured cone resistance, or in terms of equivalent relative density. Other analytical approaches that utilise the PT platform for compliance testing of deep compaction projects include the full displacement push pressuremeter and the seismic cone. The shear wave velocity obtained from the seismic cone, like the pressuremeter, can be directly related to the small strain shear modulus (G o ), and is therefore a direct measure of the soil stiffness. Hence, a compaction specification of compliance criterion could also include a minimum normalized shear wave velocity, or the pressuremeter limit pressure. further advantage in earthquake prone geographic regions, is that the shear wave velocity can be used as an additional measure of liquefaction potential, especially in silty sands. 4

5 4. Settlement of Deep ompaction Fills The settlement analysis is fundamental to the design of most compaction applications. The analysis requires a knowledge of the soil compressibility, that is, the soil modulus and preconsolidation stress. Since the Factor of Safety against bearing capacity failure is usually high for foundations on coarse grained soil, the designer is interested in a modulus, E 25, for an average applied stress limited to a value equal to about 25% of the estimated ultimate bearing resistance. The modulus can be obtained directly from the seismic cone shear wave velocity, or the full displacement push pressuremeter, or indirectly from the average cone tip resistance as follows: E 25 = α qt where E 25 = secant modulus for a stress equal to about 25% of the ultimate stress. α = an empirical coefficient qt = cone resistance simple approach promoted by the anadian Foundation Engineering Manual (FEM 1992) states that the ratio between E 25 and qt is a function of both soil type and compactness and is presented on Table 1. Table 1. α = E 25 / qt from static cone penetration tests. Soil Type α = E 25 / qt silt and sand 1.5 compact sand 2.0 dense sand 3.0 sand and gravel 4.0 The above values of E 25 apply to a settlement analysis that can be assumed to behave as linearly elastic media. 5

6 5. PT Plant variety of PT units are available for deep compaction verification and compliance testing purposes. Plant type selection should assess whether wheeled or track mounted units are appropriate for the surface traffic conditions. 6

7 ppendix Guideline for soils suitable for vibrocompaction techniques Soil classification for deep compaction based on the EslamiFellenius chart Soil classification for deep compaction based on PT data

8 Influence of compressibility on normally consolidated, uncemented, unaged predominantly quartz sands (after Jamiolkowski et al 1985) ppendix

9 ppendix ll Lankelma PT units can deploy the following devices: one penetration testing Friction cone cone tip and friction sleeve resistance Piezocone cone tip and friction sleeve resistances and porewater pressure Seismic cone as the piezocone, but with the measurement of shear wave velocity and hence the small strain shear modulus, Gmax Soil moisture probe as the piezocone, but with the additional insitu water content, temperature and soil conductivity measurement Environmental probes a variety of probes are available including fuel fluorescence detection, resistivity, conductivity and temperature Shear vane equipment Lankelma can deploy a Geonor penetration shear vane for the assessment of insitu undrained shear strength Push full displacement pressuremeter to assess insitu soil stiffness Sampling Soil sampling fixed piston MOSTP samples, thin wall Shelby tubes, push windowless samples Geotechnical instrumentation Instruments include conventional standpipes and standpipe piezometers, vibrating wire piezometers, gas monitoring wells, inclinometers See our website for further details.

10 ppendix D Soil parameters and ground types The applicability and usefulness of insitu tests pplicability: = high; = moderate; = low; = none * ф = will depend on soil type Soil parameter definitions: u = insitu static pore pressure ф = effective internal friction angle s u = undrained shear strength m v = coefficient of compressibility c v = coefficient of consolidation k = coefficient of permeability G 0 = shear modulus at small strains σ h = horizontal stress OR = overconsolidation ratio G o Soil Parameters OR / Hard rock Soft rock Gravel σ h Penetrometer Dynamic Mechanical Electric (PT) Piezocone (PTU) Seismic (SPT/SPTU) Standard penetration test (SPT) Vane testing Soil type Profile / u *ф / / / k / / Ground type Sand Silt lay Peat PT cone penetration testing: c v m per day m v ontinuous profile S u Soil characterised insitu Instantaneous results Minimal soil disturbance High quality, repeatable results Electronic fast, flexible data transfer

11 ppendix E Two case studies 1. Ground Improvement at Great Yarmouth, UK The outer harbour, Great Yarmouth, is currently under construction a project undertaken by a Van Oord/am Nuttall joint venture. With a view to expanding Great Yarmouth s port operational capabilities, the outer harbour will service a whole range of ships including freight ferries and container ships. 1,600,000m3 of dredged sand The project involves the dredging of 1,600,000m3 of sand from in and around the harbour to allow the reclamation of around 17 hectares of land. urrently, dredging and placement of the sand has been completed by Van Oord in the surrounding areas of the harbour. This material has been hydraulically pumped into areas surrounding the quay walls to depths of over 12m. Levelling of the reclaimed land started in late May Pre and post compaction Lankelma s role in this project is to test the level of pre and post compaction of the placed material. This will provide the client with an indication of what further works may be required to minimize settlements to structures that are constructed on the site. The PT has proved to be an ideal technique for collecting this form of information. It readily provides verification and specification compliance of the placed sand, and allows the rapid identification of any weak zones within the fill that may require further compaction. Track truck excellence Testing was carried out by our combination track truck vehicle UK15. This rig excelled in this instance; wheeled operations allowed quick mobilization and rapid movements around staging areas, whilst track deployment of the unit was necessary for traversing the reclaimed land due to the loose surface sand. The use of both trafficking modes meant for rapid and safe testing. 2. Ground Improvement at a port in the UE Gardline Lankelma carried out PTs at one of the new major ports that are being developed in the Gulf region. The port is based in the UE, with newly reclaimed land stretching 4km out from the shore Testing 24/7 Operations were soon increased to 24/7 operations to cope with the high volume of testing that was needed, and to keep up with the dredging activities. Piezocone tests were carried out pre and post vibrocompaction, and also to test the strength of the gravel columns. 7,518m of tests were completed.

12 Dynamic compaction circa 1629