embankment

Geotechnical parameters of soil, considering the effect of additional compaction of embankment

Introduction

Development of modern towns and construction of accompanying infrastructure allows to a limited extent for selection of good location for new buildings, in terms of favorable soil conditions. Designers and geotechnicians are compelled to prepare subsoil to new geotechnical conditions. It applies to commercially attractive lots, usually in locations that are difficult in terms of geology and engineering, frequently not meeting safety of facility and its utility function. It forces searching for new technologies and simple methods, as well as change of thinking patterns among engineers, who these days have to follow the interdisciplinary character of their specialty. An additional difficulty for solving contemporary problems of geotechnical engineering is the increasingly aggressive cost optimization.

Authors of works who analyzed the fact that a multi-layer embankment (of sandwich-type) is a real formation of several, separately compacted layers, proposed the following thesis:

  • in order to obtain the required geotechnical parameters during construction of embankment from non-cohesive soil it is not necessary that compacting of every newly built layer meets the required maximum numerical value of compaction parameter, since every new layer added above compacts the lower, bottom layer.

The author of the article presents the following scope of conducted testing works on the test field:

  • selection and application of testing methods, as well as presentation of boundary parameters, selection of backfill material, determination of embankment formation technology,
  • description of selected in-situ testing identifying basic geotechnical parameters,
  • analysis of parameters of direct and indirect measurements, material incorporated into the embankment,
  • analysis of results of geotechnical testing of the bottom layers,
  • analysis, presentation and summary of geotechnical conclusions which result from an analysis of testing performed and mathematical analysis,
  • presentation of possibilities for application of the proposed technology of layer compaction of backfill material during earthworks.

Conducted analytical studies and field tests indicate that the layer incorporated into the embankment is compacted additionally as a result of compaction of every successive, overlying layer. From the point of view of contractor’s practice, it is important to consider in designing and formation of embankments a considerable and beneficial “effect of additional compaction of layers of the already formed embankment.” Results of testing the effect of additional compacting of embankment layers along with mathematical analysis of significance of achieving the effect are presented in the article based on field testing.

Characteristics of the testing area

When it is needed to prepare detailed methods and full program for building an embankment one shall perform proper geotechnical subsoil investigation, determining materials characteristics and physical lithological features of backfill material.
Analyzing results of testing, principal factors were specified, which determined the scope and method for identification of the program of testing, including:

  • geotechnical guidelines and boundary conditions of the facility designed on the embankment,
  • criteria for evaluation and designed values of mechanical parameters of the embankment,
  • selection or designing granulometric composition of proper aggregate to form an embankment,
  • preparation of individual technology for layer performance of earthworks,
  • access to advanced technologies and methods of testing mechanical parameters under in situ conditions,
  • possibly short period of performance and easiness of ongoing inspection of the quality of performed geotechnical works.

These factors specify the area of necessary testing and method for performance of earthworks and implicate the technique for making multi layer embankments without previous compaction of the newly built layer to the state requiring achieving of the boundary, maximum condition of compaction.
Based on our own studies [1,2,3] and guidelines [5, 6], it was assumed during field testing that in the case of using backfill material with granularity index Cu = 3.9, i.e. gravel and sand used universally in road construction, the desired numerical value of dynamic modulus Evd should oscillate around Evd ≈ 35MPa.
An experimental plot was prepared on the testing field, see Fig. 1 and Fig. 2, in order to determine technological factors, i.e. the required minimum of passes, which determine acceleration of works related to compaction and waiting, reduce cost of equipment operation and people, maintaining the required geotechnical quality of the new embankment.

Fig. 1. Example of experimental plot: (a) layout (the dimensions shown in cm); (b) picture of division.
Fig. 2. Testing field – work in progress: (a) first layer stage; (b) second layer stage.

The experimental embankment, see Fig. 3, was divided to several layers (max thickness h = 30cm), which were properly compacted to the required numerical value of dynamic modulus Evd. N = 20 tests of dynamic modulus were performed for each layer in the distance l = 25m from the front testing site.

Fig. 3. Longitudinal section through the test embankment along with separation of individual backfill layers.

Methodology and test plan

Fill material

Basic criteria used in the choice of fill material include:

  • Cu and Cc parameters – granulometric properties of the aggregate,
  • ρds parameter – specifying the quality of aggregate and immutability of its grain size,
  • wn ≈ wopt parameter – possibility of maintaining constant moisture content during supply,
  • standard methods for controlling compaction measurements,
  • issue related to freezing of aggregate – and the zone of building in soil,
  • availability of soil in good and repeatable quality,
  • efficient technology of construction of the earth embankment – selection of proper type of equipment used fo r compaction,
  • economic factor – low cost, essential for the investor.

Preliminary qualifying laboratory tests of soil usefulness were performed, including granulometric analysis andProctor compaction testing. The results of obtained physical parameters are shown in Table 1.

Considering positive results of geotechnical usefulness of soil and possibility of supplying the desired volume of at least M = 15,500 tons of sand with regular grain-size distribution, the fill material was approved by the Investor and used for construction of the test embankment, followed by the main embankment.

Technology of earth embankment construction

The final formation of the embankment was made based on documentation compiled at the testing field, boundary conditions and general principles of embankment formation:

  • layered construction of the embankment, sandwich type, thickness of individual fill layer h = 0.3m,
  • fill material – aggregate, fraction 0-4 mm,
  • mechanical compaction with the use of plate compactor with 500 kg in weight,
  • verification of mechanical parameters, in each layer at least 1 test per 1,000 m2,
  • mechanical parameters were accepted as follows:
  • Is ≥ 0.95; in zone from -1.0 m-2.0 m
  • Is ≥ 0.97; in zone from – 0.2 m-1.0 m
  • Is ≥ 1.00; in zone from – 0.0 m-0.2 m
  • testing of compaction measurements of the embankment with the use of field methods in relation to calibrated direct laboratory methods and indirect field methods,
  • possibility of building the embankment with the use of sandwich method, i.e. not achieving the full required reference value of parameter in the compacted layer – the lower layer (bottom) achieves the required parameter after compacting the overlying fill layer.

Examples of results of testing the increase of dynamic modulus Evd, depending on the number of compactor passes according to technology chosen, are presented in Fig. 4.

Fig. 4. Increase in the value of dynamic modulus, in compaction technology – I.

Proper moisture content of embedded soil was determined with the use of Proctor method and amounts to wopt ~11.00 %. When soil indicated low moisture content, proper amount of water was added to it, in order to obtain Wopt.

Results and analysis of the field tests

Based on results of direct measurements of the condition of compaction, statistical analysis of the final tested physical relationships was conducted. The purpose of this analysis was verification and searching for formalized relationships in relation to the formulated testing hypothesis, i.e. that:

  • achieving of the designed values of mechanical parameters of compaction of a multi-layer embankment is
  • possible without the necessity of maximum compaction of every fill layer to the designed value. Mathematical analysis included:
  • determining the minimum sample size – nmin,
  • examining the structure of the population (the mean, the variance and the standard deviation were calculated),
  • determining the type of function,
  • determining the maximum calculation error,
  • conducting tests of statistical significance of means,
  • conducting tests for correlation coefficients – r.

The results of testing the dynamic modulus Evd, in the layer before and after compaction, are presented in Fig. 5, specifying also mathematical relationship between the dynamic modulus before and after compaction as well as correlation coefficient.
Table 2 below shows characteristic values of measurement results of the dynamic modulus Evd in the fill layer with thickness h = 30 cm before its compaction (Measurement I) and after its compaction before another new layer (Measurement II). Statistical parameters for analysis calculated based on information in Table 2.

Further studies included information resulting from geotechnical requirements related to quality:

  • the expected value m was assumed as the design value Evd = 30MPa,
  • the value of permissible error of the measurements at 􀀧􀀃= ± 2.0 MPa,
  • confidence level a= 0.95.
Fig. 5. Relationship of the dynamic modulus Evd, in the layer before and after compaction.

According to the requirements of Polish Standard [4,5] and national instructions [6,7,8,9] design included in individual fill layers proper value of compaction index Is. Correlation between compaction index Is and dynamic modulus Evd was determined for the needs of quality control of work and the condition of compaction of the embankment. Verification of correlation between parameters of compaction of layer before additional compaction was performed and determined with the use of the following linear function:

Empirical function (1) makes possible determination of the required minimum value of dynamic modulus Evd, layer before additional compaction, in order to make sure that overlying fill layers after additional compaction meet the required condition of achieving the reference value of compaction parameter.
According to national standards and regulations [6,7,8,9], subsoil should be compacted to the value of at least Is =0.95, which corresponds with the value Evd = 15 MPa. In order to meet the condition (1) of empirical linear function: y = 1.150x + 15.50 in successive fill layers, i.e. Is = 0.97, the guaranteed value of dynamic modulus before additional compaction should be no lower than Evd > 30 MPa.

Concluding remarks

Construction of multi-layer embankments from mineral native soil is a very complex geotechnical process consisting of many stages. It is influenced by many factors that depend on the type of built-in material and competent decisions of the direct contractor.
Results of field testing and their analysis indicate that during building of earth embankments one can use the determined empirical relationships, e.g. y = 1.150x + 15.50. In practice, this formula means that it is not reasonably justified that every newly built-in layer of material with determined thickness should achieve 100% of measurement of the designed compaction, e.g.: deformation modulus E, dynamic modulus Evd. Practical verification of the correctness of building an earth embankment becomes under these conditions simpler and unambiguous in evaluation of the final condition of a multi-layer embankment.
Results and conclusions from field tests allow to indicate important geotechnical generalizations regarding forming of similar embankments:

  • there is a significant influence of compacting successive fill layer on additional compaction of underlying builtin layers, regardless of material used [1,2,3],
  • practical solution, not only in terms of costs, is building high embankments with the use of controlled effect of additional compaction of the bottom layers,
  • in practical cases, the main condition is earlier determination of correlations between compaction parameters, e.g.: Is = f(Evd), Evd= f(Evd0) at an appropriately high level of statistical significance in testing field conditions,
  • building of multi-layer embankments is possible without the necessity of compacting every built-in layer to 100% of the value of parameter required by the design.

Source: Geotechnical parameters of soil, considering the effect of additional compaction of embankment
Łukasz Aleksander Kumora, Maciej Kordian Kumorb*, Monika Kopkaa, – —– Geomatic and Geotechnical Department, University of Science and Technology in Bydgoszcz, 7 Kaliskiego Str. 85-791 Bydgoszcz, Poland Geotechnical Department, University of Science and Technology in Bydgoszcz, 7 Kaliskiego Str. 85-791 Bydgoszcz, Poland

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