The suitability of stony cohesive fill material

Transcription

1 The suitability of stony cohesive fill material Philip Jenkins, retired geotechnical engineer, and John Davidson, Dundee University 1. INTRODUCTION Stony cohesive fill derived from glacial till can be utilised to construct stable and durable embankments; in addition such fill can provide an adequate load-bearing platform for structures. Stony cohesive fills are generally classified as Class 2C Fill as defined by the MCHW Table 6/2 (Grading requirements for acceptable earthworks material). Since glacial till is generally over-consolidated, the in situ is usually close to or less than the optimum derived from a compaction test. However, the upper layer of many glacial tills has been subjected to weathering and consequent water ingress which has resulted in an increase in moisture content. Inclement weather during the earthworks phase can result in a significant increase in moisture content which may render the fill material unsuitable for engineering purposes. Relationship testing using routine laboratory tests can be used to assess the suitability and predict the performance of fill material at s in excess of optimum. 2. COMPACTION SPECIFICATION The Method Compaction Specification (MCHW SHW Volume 1 Series 6) requires compacted fill to receive a set number of passes by plant with a minimum static mass (kg/m) to achieve compacted layers of specified maximum thickness. Thus the earthworks contractor needs to ensure adequate coverage of suitable fill by compaction plant to meet the specification; the subsequent performance of the fill is not his responsibility. The end product specification requires the fill to achieve a minimum dry density, usually not less than 95% of maximum dry density, as derived from a compaction test (BS1377 Part 4), and with an air content not exceeding 5%. In situ testing is performed to monitor the quality control of the placed fill. The choice of plant is usually the remit of the earthworks contractor; taper-foot vibrating rollers are suitable for use on cohesive fill. Charles et al (1998) suggest that the 95% compaction criterion should be treated as a minimum requirement and not an optimal fixation. However, achieving these criteria do not provide assurance that parameters such as the undrained shear Figure 1. Particle size distributions for four Scottish glacial tills Percentage passing (%) FIGURE 1: PARTICLE SIZE DISTRIBUTION OF FOUR GLACIAL TILLS Laboratory simulated class 2c Scottish glacial till Glasgow Regional glacial till, upper and lower bounds Bradan Dam, Ayreshire glacial till, upper and lower bounds Particle size (mm) strength and California Bearing Ratio (CBR) values will be adequate. 3. SIMULATED STONY COHESIVE FILL The objective was to simulate a stony cohesive fill and perform a rigorous programme of laboratory tests to BS1377 (199). The particle size distributions for four Scottish glacial tills are shown in figure 1 (CIRIA 1999); also plotted is the grading for the simulated glacial till which was prepared by mixing silty clay (4%), sand (4%) and fine and medium gravel (2%). Figure 2 shows a ternary diagram (CIRIA 1999) which compares the simulated till with granular, granular matrix and well graded tills found in the UK. Figures 1 and 2 demonstrate that the simulated till is well graded and representative of Scottish glacial tills. 4. LABORATORY TESTING PROGRAMME The following laboratory tests were performed to BS1377 (199): Specific gravity Compaction using the 4.5kg rammer Moisture condition value (MCV) Unconsolidated undrained triaxial California bearing ratio Q September 215 GROUND ENGINEERING 25

2 FIGURE 2: TERNARY DIAGRAM (CIRIA 1999) Dry density (g/ml) Cohesive matrix tills Percentage fines Scottish tills North of England tills Welsh tills Southern English tills 2 1 Granular matrix tills Well graded tills 9 Granular tills Percentage gravel Midlands tills Sandy clay No 2 (Parsons, 1992) Well graded sand (Parsons, 1992) Gravel-sand clay (Parsons, 1992) Q The simulated sample was selected to ensure that the coarsest particles were less than 2mm which is the maximum size permitted in the compaction test. The 4.5kg rammer was selected as it is believed to be compatible with the compactive effort of modern earthworks plant, which can exceed 5,kg/m (Table 6/4: Method Compaction for Earthworks Materials: Plant and Methods). The compaction test was conducted at moisture contents between optimum -1% and optimum +3%. Samples for undrained triaxial and CBR were prepared to combinations of dry density and moisture content varying between optimum and optimum +3% Percentage sand FIGURE 3: COMPACTION TEST RESULTS PdryMax = 2.24 g/ml 95% PdryMax = 2.13 g/ml Moisture content vs dry density % Air content 5% Air content 1% Air content OMC = 7.% 95% PdryMax moisture content = 9.3% Figure 2. Ternary diagram (CIRIA 1999) which compares the simulated till with granular, granular matrix and well graded tills found in the UK Figure 3. Summary of compaction test results in 1% increments. 5. LABORATORY TEST RESULTS Liquid and plastic limits of the silty clay fraction were determined to be: LL = 39 ; PL = 17 ; PI = 22 from which the soil is classified as a clay of intermediate plasticity. However, since this test is performed on the fraction finer than 425μm, its relevance to well graded glacial tills is uncertain. The compaction tests results are summarised graphically in figure 3; air content lines are based on a measured specific gravity value of 2.72 and the formula: [ ρ ( dry 1 w ) ] A = 1 ρ water SG + 1 x 1 where : A = air content ρdry = dry density of soil ρwater = density of water SG = specific gravity w = It is imperative that the specific gravity value of the soil sample be determined rather than assumed since minerals and hence rock fragments exhibit a wide range of values, as illustrated below: TYPICAL SPECIFIC GRAVITY VALUES (MINDAT.COM) Quartz 2.65 Augite 3.19 to 3.59 Montmorillonite 2. to 3. Illite 2.79 to 2.8 Muscovite 2.77 to 2.88 Feldspar 2.55 to 2.63 Hornblende 3. to 3.47 Olivine 3.27 to 3.37 An optimum (OMC) of 7% and a dry maximum dry density of 2.24g/ml was derived from the compaction test. Applying the suitability criterion of 95% of maximum dry density gives a target dry density of 2.13g/ml and hence an upper of 9.3% is applicable; material wetter than this would be rejected as unsuitable. The undrained shear strength results are summarised in figure 4 which shows that shear strength decreases steadily from 14kPa at optimum to less than 2kPa at optimum +3%. At an undrained shear strength of 5kPa, which is towards the lower limit of firm consistency, a of 9% applies. The reduction in undrained shear strength with small increases in is unambiguous and is postulated to be the primary cause of the difficulties 26 GROUND ENGINEERING September 215

3 FIGURE 4: RELATIONSHIP BETWEEN MOISTURE CONTENT AND UNDRAINED SHEAR STRENGTH Undrained shear strength (C u )(kpa) Lower limit of C u = 5 kpa y = 4.83x x R 2 =.9968 Upper limit content = 9% Figure 4. Summary of undrained shear strength results (left) Figure 5. Moisture contents correlate with CBR values of 15% and 5% (right) Average CBR (%) FIGURE 5: RELATIONSHIP BETWEEN MOISTURE CONTENT AND CBR y = 7E+1x R 2 =.9784 Lower limit of CBR before capping layer is needed = 15% Upper limit of before capping layer is needed = 7.9% Lower limit of CBR for trafficability = 5% Upper limit of for trafficability = 8.8% experienced when constructing earthworks during periods of inclement weather. CIRIA (1999) presents graphs of undrained shear strength versus moisture content for selected UK glacial tills; these graphs all demonstrate the reduction in undrained shear strength which occurs with small increases in (total) sample. Various authors, Head (1988) and Jenkins and Kerr (1998), have considered the significance of the matrix on the strength behaviour of stony fills. Formulae to adjust for the much lower water retaining capacity of the coarse granular material have been proposed. The net effect is that an increase in total of a sample results in a greater increase in the of the silt and clay matrix; this effect increases as the granular content increases. For the simulated fill tested, every 1% increase in total moisture content probably results in a 2.5% increase in matrix. Thus the upper limit of 9% for the whole sample equates to a matrix moisture content of approximately 15%. The CBR results demonstrate a similar trend; this is predictable since the CBR test is effectively a circular footing applying a vertical stress on the soil surface and its load carrying capacity is a function of undrained shear strength of the supporting soil and the bearing capacity factor NC. A similar exercise was performed to determine CBR and the results summarised in figure 5. Figure 5 has been annotated to show the moisture contents which correlate with CBR values of 15% and 5%. The graph demonstrates that the reduction in CBR at s between OMC and O MC+1% is dramatic. The variation condition value with increasing was investigated and the results summarised in figure 6. The test was devised by the Transport Research Laboratory as an aid for rapid assessment of earthworks suitability (TRL 1979, 1997 and 21). A moisture condition value (MCV) exceeding 7.5 is usually taken to indicate that a fill material is suitable for re-use. Thus for the stony cohesive fill investigated the MCV test predicts an upper limit of between 8.7% and 9.%. Figure 7 graphically demonstrates the dry density values achieved when preparing the samples for the undrained triaxial and CBR tests; these values conform closely with the compaction test. In the MCV test significantly lower dry densities were achieved until the attained a value of 9% which is 2% above OMC and is coincidentally the upper limit of suitability predicted by the compaction test. The energy applied by the rammer to the soil in the MCV test is about a third of the energy applied in the 4.5kg compaction test and hence it is to be expected that the dry density values achieved will be lower. However no logical explanation can be presented to explain the relationship between dry density and for the MCV test. 6. TECHNICAL DISCUSSION Currently the DMRB and MCHW advocate the following suitability criteria for Class 2C fill material: 5% or less air content and at least 95% of maximum dry density Lower limit of undrained shear strength = 5kPa Lower limit of CBR = 15 before a capping layer is required Lower limit of MCV = 8.5 Based on the 95% of maximum dry density criterion, the upper suitability limit for this fill is 9.3% which is 2.3% above the optimum. At a of 9.3%: the undrained shear strength = 4kPa which is on the soft firm boundary the CBR = 3 the MCV = 6.4 On the basis that a minimum undrained shear strength of 6 Pa is required to permit trafficability (CIRIA 1999), this Class 2C fill, when compacted, will be suitable up to a maximum of 8.7%. Q September 215 GROUND ENGINEERING 27

4 MCV FIGURE 6: RELATIONSHIP BETWEEN MOISTURE CONTENT AND MCV Lower limit of MCV = 7.5 y = -.476x x R 2 =.9993 Upper limit content = 8.9% Figure 6. Variation condition value with increasing (left) Figure 7. Dry density values achieved when preparing the samples for the undrained triaxial and CBR tests (right) FIGURE 7: COMPARISON OF DRY DENSITY VALUES ACHIEVED FOR ALL TESTS Dry density (P dry )(g/ml) UUTXL test MCV test Comapction test CBR test Q At a to 9.% the undrained shear strength is 5kPa which is often regarded as being the lower limit of trafficability for heavy construction plant (CIRIA 1999). It is suggested that, based on practical experience, the 6 Pa limit may be slightly onerous. Research by Parsons and Toombs (TRL 1988) demonstrated that a moisture condition value of 7.5 represented the lower limit of trafficability for earthworks plant. For the fill investigated this implies a maximum of 8.9% and hence this fill, compacted at a of 9.3% or greater, would be classified as unsuitable with respect to trafficability. The MCV test has gained wide acceptance in relation to monitoring construction suitability of cohesive tills. When MCV testing has been carried out, the results frequently demonstrate poor correlation with other material suitability parameters (Rutty and Johnston, 212). Figure 7 which shows the relationship between dry density and for a variety of tests may be used to explain the vagaries of the MCV test results. It can be seen that the dry density achieved in the MCV test is significantly less than for the 4.5kg compaction test and exhibits a dry density trend which is inexplicable. Experience has demonstrated that foundations constructed on subgrades with a design CBR of less than 2.5% may be problematic. Adoption of the 95% of maximum dry density criterion for this fill will result in a CBR value in excess of the minimum requirement for pavement design. The shaded area in figure 8 shows the limits of fill suitability, based on a typical end-product specification. This work has demonstrated that the 95% of maximum dry density criterion, based on the heavy compaction test, is optimistic. For the soil investigated, compaction at a moisture content of 9.%, which is +2.% above optimum, will provide a fill which is suitable in all respects; this correlates with a criterion of 96% of maximum dry density. 7. CONCLUSIONS The compaction test endures as a simple and appropriate laboratory test for understanding the response of soils to compaction; however the subsequent performance of compacted fill cannot be deduced from the results of this test. The arbitrary criteria of 95% of maximum dry density and 5% air content has been found, in practice, to be a reliable guide as a limit of earthworks suitability but does not, per se, guarantee adequate performance of fills with respect to trafficability, embankment stability and load bearing platforms. This programme of relationship testing has demonstrated that the simulated Class 2C fill would be classed as suitable at s not exceeding 9.3% (OMC+2.3%) which is close to the 95% of maximum dry density criterion. In addition the fill would be expected to perform satisfactorily with respect to trafficability. The results of the MCV test indicate an upper value of 8.8% as the limit of suitability. Therefore it may be concluded that, in the absence of triaxial and MCV data, the extant criteria of 95% of maximum dry density and 5% air content should be reliable as an end-product specification. However adoption of a 96% of maximum dry density criterion will restrict the fill to a maximum moisture content of 9.% and result in a fill which should surpass all relevant extant criteria. It is recommended that undrained triaxial testing on samples compacted to combinations content and density derived from the compaction test is routinely undertaken at the site investigation stage to define the suitability limits with confidence. 28 GROUND ENGINEERING September 215

5 Dry density (g/ml) FIGURE 8: SUITABILITY LIMITS 95% PdryMax = 2.13 g/ml Moisture content vs dry density % Air content 5% Air content 1% Air content 95% PdryMax moisture content = 9.3% Figure 8. Limits of fill suitability based on a typical end-product specification Series 6 - EARTHWORKS - Series_6.pdf mchw/vol1/pdfs/series_6.pdf [Accessed 8 October 214]. mindat.org [online] Available from: [Accessed on 24 June 215] Rutty, P.C. & Johnston, T.P., 212. Optimum use of material: selection of limits for suitable earthworks fill: Irish experience. Geological Society. TRL Report 522 (1979). The Moisture Condition Test and its Potential Applications in Earthworks. Transport Research Laboratory. TRL Report 273 (1997). Use and Application of the MCA with Particular Reference to Glacial Tills. Berkshire: Transport Research Laboratory. TRL Report 13 (1988). Pilot-scale studies of soil by earthmoving vehicles: Transport Research Laboratory. (Parsons & Toombs, 1988) TRL Report 484 (21). Application of soil acceptability forecasts: Transport Research Laboratory. 8. REFERENCES British Standards Institution BS1377-2, Soils for civil engineering purposes Part 2: Classification tests. London: BSI British Standards Institution BS1377-4, Soils for civil engineering purposes Part 4: Compaction-related tests. London: BSI British Standards Institution BS1377-7, Soils for civil engineering purposes Part 7: Shear strength tests (total stress). London: BSI Building Research Establishment, The specification of fills to support buildings on shallow foundations: the 95% fixation. Ground Engineering Charles, J., Skinner, H. & Watts, K The specification of fills to support buildings on shallow foundations: the 95% fixation. Ground Engineering, 31, CIRIA C54 (1999). Engineering in glacial tills. London: Construction Industry Research and Information Association. DMRB Volume 4 Section 1 Part 1 - HA 44/91 Design and Preparation of Contract Documents - ha4491.pdf dmrb/vol4/section1/ha4491.pdf [Accessed 22 January 214]. DMRB Volume 4 Section 1 Part 5 - HA 7/94 - Construction of Highway Earthworks - ha794.pdf dmrb/vol4/section1/ha794.pdf [Accessed 25 January 214]. DMRB Volume 4 Section 1 Part7 - SH 7/83 - Geotechnics and Drainage. Earthworks. Specification for Road and Bridge Works: Soil Suitability for Earthworking: Use of Moisture Condition Apparatus - sh783.pdf [online] Available from: section1/sh7_83.pdf [Accessed 26 January 214] Head, K.H., Manual of soil laboratory testing Vol.1, Pentech Press, London. MCHW Volume 1 -Specification for Highway Works - Geosolve SLOPE Slope Stability Analysis & Reinforced Soil Design WALLAP version 6 Retaining Wall Analysis sheet piles - diaphragm walls - combi walls SLS / ULS analysis Soldier Pile analysis Soil Properties archive Comprehensive advice on EC7 design NEW Integral Bridge design according to PD 6694 GWALL Gravity Wall Analysis Contact: Daniel Borin MA, PhD, CEng, MICE Tel: September 215 GROUND ENGINEERING 29