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Dutch method of measurlng cone resistance, while the dynamie aspect uses a "standard hammer" (63.6 kg falling through 762 mm) to offect penetratlon.

(Sherwood and Child, 1974).

A static/cfynamlc sounding consists of a record of blow count for each 75 mm of penetratlon together wlth cone resistance recorded at 300 mm lntervals.

Sounding records are prepared by plotting both blow count and cone resistance against depth. The blow count for an inerement In which cone resistance is measured is usually ignored ln the reportlng of the results, although it is recorded in the field as it gives an indication of the effects of shaft friction in the individual soundings.

It is usual to termlnate soundings when a blow count of 50 per 75 mm penetration is reached, although in special clrcumstances blow counts of up to 200 have been sustained for relatively short interyals of depth wlth out excessive damage to the penetrometer.

3. INTER PR ETATION AND EYALUATION OF TEST RESULTS

The Interpretation of penetratlon tests depends both on soil type and depth of foundation. For convenience, soils have becn dlvlded into the following broad groups;

Gravels

Sands and silty sands Fine-grained non-cohesive soils Fine-grained cohesive soils Weak rocks

Each of these are discussed briefly below both for shallow and for piled foundations.

3.1    Shallow Foundations

3.1.1    Gravel8

In gravels, the dynamie cone penetration test (section 2.1) is generally used. Although experlence in sands show this test gives slightly higher results than the S.P.T., in gravel8 it is the generał practice to assume that the dynamie cone penetration test is equivalent to the S. P.T.

The Terzaghi-Peck relationship for sands (see next section) is aiso applled to gravels, although in well known gravel deposits local experience of foundation behaviour ofton influences the selection of allowable bearing pressures. Where heavy etructures require relatively high allowable bearing pressures on gravel deposits for economlcal foundation design, then plate-bcaring tests are carrled out in conjunction with dynamie cone penetration tests.

3.1.2    Sands and silty sands A. Sta tle Penetratlon Tests

Settlement of shallow footings ln sand are generally predicted by moans of the modified Terzaghi -Bulsman semi-cmpirical formula:

I

where p is total settlement,

p * is the effectWe overburden pressure at the mid-point of the layer,

Ap^ł is the inerease in effective pressure at the same point,

Z is the thicknesB of the layer,

E_$ is the equivalent Young's Modulus in compression.

The equivalent Young's Modulus is obtalned from the Buisman relationship, E_s =1.5 C_r, where Cr is the static cone resistance. Recently, the less conservative relationship of E₈ = 1.9 Cr suggested by Meyerhof (1956) has been moro widely used.

Until recently, De Beer and Martens (1957) method based on equation (1) was used. Schmertmann’8 (1970) strain influence factor elasticity method has gained some acceptanco, and it is interesting to notę that he obtains the equlvalent Young's Modulus from E_s = 2 C_r. confirming Meyerhof' s earlier modification of the original work.

Prellminary estimates of bearing capacity of shallow foundations are often based on the proceduro given by Meyerhof (1956).

B. Standard Penetration Tests

Prior to the publication of the research work of Gibbs and Holtz (1957), it was customary in the U. K. to interpret the results of the standard penetratlon test in accordance with the recommendations of Terzaghi and Pcck (1948). It should be appreclated that because of the limited field data. Terzaghi and Peck produced a conservatlve working solution, and the published relationship was only lntended to limit the settlement of the largest footing to one inch even if it rested on the loosest portion of the sand deposlt, and not for the predictions of the settlements of shallow footings founded on non-cohesivo deposits. If the standard penetration resistance (N) of a saturated flne or silty sand deposit is greater than 15, then the recorded N value is adjusted in accordance with the following recommendation of Terzaghi and Peck (1948):

Corrected penetration resistance = 15 + 0.5 (N-15)

(2)

However, oxperlonce lndicated that the Terzaghi and Peck relationship between the value of N, allowable bearing preBsure and predicted settlement gave conservative results. It was also apparent to practicing engineers in the U.K. that the S.P.T. carried out at very shallow depths gnve misleading values of rclative density.