The Flat Dilatometer Test (DMT) in Soil Investigations - Marchetti DMT

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c u = 0.22 σ' v0 (0.5 K D ) 1.25 (10) Eq. 10 has generally been found to be in an intermediate position between subsequent datapoints presented by various researchers (e.g. Lacasse & Lunne 1988, Powell & Uglow 1988). Example comparisons between c u DMT and c u by other tests at two research sites are shown in Figs. 22 and 23. Fig. 20. K 0 from DMTs and SBPTs in natural Ticino sand at Pavia (Jamiolkowski 1995) Kemigawa Ohgishima Fig. 21. Correlation K D -D r for NC uncemented sands (after Reyna & Chameau 1991, also including Ohgishima and Kemigawa datapoints obtained by Tanaka & Tanaka 1998) "cementation"). An inconvenience of the method is that it requires both DMT and CPT and proper matching of correspondent K D and q c . 11.1.4 Relative density D r (sand) In NC uncemented sands, the recommended relative density correlation is the one shown in Fig. 21 (Reyna & Chameau 1991), where D r is derived from K D . This correlation is supported by the additional K D -D r datapoints (also included in Fig. 21) obtained by Tanaka & Tanaka (1998) at the Ohgishima and Kemigawa sites, where D r was determined on high quality samples taken by the freezing method. In OC sands or possibly in cemented sands, Fig. 21 will overpredict D r , since part of K D is due to the overconsolidation or cementation, rather than to D r . The amount of the overprediction is difficult to evaluate at the moment. 11.2 STRENGTH PARAMETERS 11.2.1 Undrained shear strength c u The original correlation for determining c u from DMT (Marchetti 1980) is the following: Fig. 22. Comparison between c u determined by DMT and by other tests at the National Research Site of Bothkennar, UK (Nash et al. 1992) Fig. 23. Comparison between c u determined by DMT and by other tests at the National Research Site of Fucino, Italy (Burghignoli et al. 1991) 23
Experience has shown that, in general, c u DMT is quite accurate and dependable for design, at least for everyday practice. 11.2.2 Friction angle F (sand) Two methods are currently used today for estimating φ from DMT (see also Marchetti 1997). The first method (Method 1) provides simultaneous estimates of φ and K 0 derived from the pair K D and q D (Method 1a) or from the pair K D and q c (Method 1b). The second method (Method 2) provides a lower bound estimate of φ based only on K D . Method 1a (φ from K D , q D ) This iterative method, developed by Schmertmann (1982, 1983), described in Section 11.1.3.2 relative to K 0 in sand, permits the determination of both K 0 and φ. Method 1b (φ from K D , q c ) This method (Marchetti 1985) first derives K 0 from q c and K D by Eqns. 8 and 9, as indicated in Section 11.1.3.2 (K 0 ). Then uses the theory of Durgunoglu & Mitchell (1975), or its handy graphical equivalent chart in Fig. 24, to estimate φ from K 0 and q c . Method 2 (φ from K D ) Details on the derivation of the method can be found in Marchetti (1997). φ is obtained from K D by the following equation: φ safe,DMT = 28° + 14.6° log K D – 2.1° log 2 K D (11) Fig. 24. Chart q c -K 0 -f – graphical equivalent of the Durgunoglu & Mitchell theory (worked out by Marchetti 1985) As already noted, φ from Eq. 11 is intended to be not the "most likely" estimate of φ, but a lower bound value (typical entity of the underestimation believed to be 2° to 4°). Obviously, if more accurate reliable higher values of φ are available, then such values should be used. It should be noted that in cemented c-φ sands it is difficult to separate the two strength parameters, because there is an additional unknown. 11.3 DEFORMATION PARAMETERS 11.3.1 Constrained modulus M The modulus M determined from DMT (often designated as M DMT ) is the vertical drained confined (one-dimensional) tangent modulus at σ' v0 and is the same modulus which, when obtained by oedometer, is called E oed = 1/m v . M DMT is obtained by applying to E D the correction factor R M according to the following expression: M DMT = R M E D (12) The equations defining R M = f(I D , K D ) (Marchetti 1980) are given in Table 1. The value of R M increases with K D (major influence). It is to be noted that R M , being dependent on I D ,K D , is not a unique proportionality constant relating M to E D . R M varies mostly in the range 1 to 3. Since E D is an "uncorrected" modulus, while M DMT is a "corrected" modulus, deformation properties should in general be derived from M DMT and not from E D . Experience has shown that M DMT is highly reproducible and in most sites variable in the range 0.4 to 400 MPa. Comparisons both in terms of M DMT –M reference and in terms of predicted vs measured settlements have shown that, in general, M DMT is reasonably accurate and dependable for everyday design practice. M DMT is to be used in the same way as if it was obtained by other methods (say a good quality oedometer) and introduced in one of the available procedures for evaluating settlements. Example comparisons between M DMT and M from high quality oedometers at two research sites are shown in Figs. 25 and 26. NOTE: Necessity of applying the correction R M to E D – E D is derived from soil distorted by the penetration. – The direction of loading is horizontal, while M is vertical. – E D lacks information on stress history and lateral stress, reflected to some extent by K D . The necessity of stress history for the realistic assessment of settlements has been emphasized by 24
Page 1 and 2: International Society for Soil Mech
Page 3 and 4: The DMT equipment, the test method
Page 5 and 6: Fig. 3. DMT working principle Fig.
Page 7 and 8: Short cables are easier to handle,
Page 9 and 10: (a) Fig. 8. Lower adaptor connectin
Page 11 and 12: The operator will then use the shor
Page 13 and 14: NOTE: Frequence of C-readings (a) S
Page 15 and 16: Sharpness of electrical signal Usin
Page 17 and 18: Even in sand, which is usually cons
Page 19 and 20: SYMBOL DESCRIPTION BASIC DMT REDUCT
Page 21 and 22: Dilatometer E (bar) D 2000 1000 500
Page 23: Hence OCR in sands can be approxima
Page 27 and 28: A ( 600 400 200 Uo Tflex 0 0.1 1 10
Page 29 and 30: Fig. 29. Use of U D for discerning
Page 31 and 32: Figs. 32 and 33 show examples of DM
Page 33 and 34: guiding the selection of ρ are dev
Page 35 and 36: non-dimensional form: P P u ⎛ E
Page 37 and 38: epresentative results of such appli
Page 39 and 40: 14.3 ARCHING AND SENSITIVITY TO s h
Page 41 and 42: Dilatometer". ASCE Jnl GE, Vol. 116

The Flat Dilatometer Test (DMT) in Soil Investigations - Marchetti DMT

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