Alasdair’s Engineering Pages © A. N. Beal 2016 www.anbeal.co.uk
www.anbeal.co.uk
Reprinted from Ground Engineering Vol. 1 No. 3 May 1968
by G. M. Cornfield M.Sc., A.C.G.I., M.I.C.E.
This article presents a new empirical dynamic pile formula for base
BSP Cased Piles
Cased piles are essentially concrete bearing piles, the concrete being placed in situ within a permanent thin steel lining or tube. They may be used in circumstances where bearing piles are required for the foundations of buildings, bridges, piers and other structures. The pile is formed by driving the steel casing to the required resistance making use of a special cylindrical drop hammer which operates inside the lining and strikes on a plug of earth
When the casings have been driven to the desired resistance they are filled concrete. No reinforcement is normally necessary when the piles are driven to full penetration for building or similar foundations. If however cased piles are used in jetty type structures, where the piles project above soil level, the upper portion of each pile is provided with reinforcement. In such structures it is also necessary to check the column strength of the exposed portion of the pile after tentative selection of a diameter for a given working load is made from Table A.
Pile
Base driving with an internal drop hammer is now the normal method of installing cased piles as much thinner casings can be used. While driving from the top was occasionally employed some years ago, experience has shown that base driving is the most suitable and economical method, and it is now invariably used. The casing is first pitched and temporarily supported in position while the concrete plug is placed. A vertical or raking casing can be supported quite simply, and four typical methods are: (1) to use a timber or similar trestle, with gates for the casing located at two levels; (2) to use gates at two levels supported by the bracing of a cofferdam; (3) to cantilever gates forward from piles already completed (applicable to a jetty type structure); (4) to use gates fixed to a pontoon or barge.
The internal drop hammer is suspended from the jib of a crane having a winch of appropriate size to operate the hammer. While it is possible to use a pile frame or hanging leaders, both for support of the casing. and operation of the hammer, these are not necessary and it is most usual to install cased piles in the manner just described. It will be observed, therefore, that the only item of pile driving equipment required is the internal drop hammer. This method of installation makes cased piles particularly appropriate for use in structures where the piles have to be driven over water, or for that matter in any situation where the access conditions are difficult, for example, within a cofferdam.
Immediately before driving is started a plug of dry concrete having a compacted height of about 2½ times the pile diameter is deposited within the casing. Experience has shown it is essential that the plug concrete, of 1: 2 : 4 mix, should be very dry with a water
The above Table is normally applicable for pile lengths up to about 50/60ft in average soil conditions. Where piles are driven in two lengths, the upper length may be of lesser thickness. For longer piles an increased casing thickness is required for the lower portion of the piles.
Where piles are driven to rock, dense gravel, etc., or when the soil conditions are precisely known and results of test loadings are available, it is possible to increase the stated working load for a given diameter.
(A separate check should be made of the column strength for the portion projecting above soil level.)
The casings are manufactured from mild steel by a process in which steel strip is formed into a continuous helix and the adjoining edges are butt
Stages in formation of base
Empirical Formula for Base
When base
The formula is essentially empirical in nature and is based on the results of experience to date.
Ru = 3.6W(3.0+h)/(S+0.5)
Where Ru= Estimated ultimate driving resistance 
W = Weight of internal drop hammer 
h = Actual hammer drop at final set 
S = Final set 
This formula is intended to be applicable only when the hammer drop at final set is between 4 and 6 ft (1.3 and 1.8m) (for vertical piles) and when final set is less than 0.2 in (5 mm) per blow, that is, equivalent to more five blows per inch. In addition the inclination of the pile must be strictly in accordance with the standard driving procedure applicable to these piles. As with any dynamic pile formula, it should only be used for piles driven to soils as sand, gravel, rock, hard marl, very stiff clay. A re
The following typical example of the use of the formula illustrates its simplicity in application. A base driven cased pile having an internal diameter of 16 in (40.6 cm) and a casing thickness of 9 s.w.g. (0.144 in) is fitted with a standard flat plate shoe. The pile is driven to a final set of 0.15 in (3.8 mm) per blow in a gravel stratum using a 2½ ton internal drop hammer operating on the recommended earth
Ru = 3.6 x 2.5 (3.0+4.5)/(0.15+0.5) = 104 tons
This pile would be satisfactory for a working load of 52 tons using a factor of safety of 2.0 on the ultimate load, subject to certain other factors referred to later.
Table B gives worked out values of Ru based on the formula, for different diameters, final sets and hammer drops.
Note:— Applicable to vertical piles 1 ton=2240 lb.
A typical working load in average soil conditions
B typical working load when final set obtained in dense gravel, dense sand, rock, hard marl, etc. for factor of safety of 2.0 on ultimate load.
Comparison of Formula with Test Load Results
Table C gives essential data relating to 37 B.S.P. cased piles which have been test loaded on a large number of different sites over the past ten years. These represent all the data available to the author at time of writing this article and no attempt has been made to exclude any case because of apparently unfavourable results except where some of the data was incomplete or doubtful.
As with all loading tests of bearing piles, there is the problem of estimating the ultimate or failure load from the test data. There are many different criteria for obtaining the failure load even when the piles have been clearly loaded to the apparent failure condition. The author has, however, selected as the criterion that load at which the residual settlement of the head of the pile, after removal of the load, is 1/3 in (8 mm). Unfortunately bearing piles are not always tested to full ultimate load, and in fact due to the cost of load testing the maximum test load may be well below the ultimate load. In an attempt to obtain as much data as possible for comparison with the empirical formula it was decided to use all available results even where loading has not clearly been carried out to failure and an estimate has therefore been made of the minimum ultimate failure for these cases. Based on experience and an examination of the results of many bearing piles of all types loaded to failure, it was considered conservative to estimate in the following manner. Where the maximum load applied resulted in a residual pile head deflection of less than 0.20 in (5 mm) but more than 0.10 in (2.5 mm) the minimum estimated ultimate load has been taken as 10 per cent greater than the maximum load applied during the test. When the residual deflection was 0.10 in (2.5 mm) or less the minimum ultimate load was estimated at 25 per cent over the maximum applied load. It is felt that these estimates are on the safe side and the actual ultimate loads may in some cases be significantly greater than the minimum figures estimated in this manner.
Using the results of Table C, Graph D shows minimum estimated ultimate resistance obtained by test loading plotted against the result obtained by application of the formula. The points on the graph suggest reasonable correlation with the formula. The scatter of the points is in fact no better or worse than that applicable to more known formulae, such as the Hiley formula, which are often used for top
It is interesting to observe that the accuracy of measurement of the observed driving data is in itself a source of scatter of results. The hammer drops can be measured in practice to an accuracy of about 10 per cent, and there may be some lack of accuracy measuring sets and so on 
Table C also records pile lengths and estimates of ultimate load obtained from tests and the formula respectively. Plotting these against each other showed no correlation of any sort even for a particular group of soils.
There tends to be a bigger scatter of results (Graph D) for cohesive soils than for sand, rock, etc. This is not unexpected and will be chiefly due to the sensitivity of cohesive soils and the effect of the time interval between driving and test loading.
General Comments
The base
It should not be assumed that a pile
Certain limits of applicability of the base
Conclusion
The comparison of test load results to the empirical base
References:
Ref. 1 Civil Engineering Code of Practice No. 4 
Ref. 2 ‘Cased Piles and their Application to some Maritime Structures’, G. M. Cornfield. ‘Dock & Harbour Authority’ February ISM.'
Ref. 3 "Further Examples of Cased Piles in Maritime Structures" G.M.C., ‘Dock & Harbour Authority’, August 1964.
Ref. 4 ‘Simplified Hiley Formula for RC Piles’ G. M. Cornfield ‘Engineering’ 14th July 1961.
Ref. 5 ‘Hiley Formula Simplified Without Graphs’ G.M.C., ‘Engineering’ 20th December, 1963.
Paper reproduced by kind permission of BSP International Foundations Ltd www.bsp

TABLE A 


Typical working load (t) 
Inside diameter of casing (in) 
Casing thickness swg (in) 
Weight of standard Internal Drop Hammer (tons) 
15 
10 
10 (0.128) 
¾ 
25/30 
12 
10 (0.128) 
1¼ 
40 
14 
10 (0.128) 
2 
50 
16 
9 (0.144) 
2½ 
65 
18 
8 (0.160) 
3 
80 
20 
7 (0.176) 
4 



TABLE B 



Internal diameter of casing (in) 
Internal Drop Hammer W (tons) 
Drop h (Feet) at final set 
Ru (tons) at Final Set S=0.20in (5 Blows/ in) 
Ru (tons) at Final Set S=0.1in (10 Blows/ in) 
Typical Working (Tons) A 
Range of Loads B 
10 
¾ 
4 
27 
31 
15 
20 


5 
31 
36 




6 
34 
40 


12 
1¼ 
4 
45 
52 
25 
35 


5 
51 
60 




6 
57 
67 


14 
2 
4 
72 
84 
40 
55 


5 
82 
96 




6 
92 
108 


16 
2½ 
4 
90 
105 
50 
70 


5 
102 
120 




6 
115 
135 


18 
3 
4 
108 
126 
60 
80 


5 
123 
144 




6 
138 
162 


20 
4 
4 
144 
168 
80 
110 


5 
164 
192 




6 
184 
216 

