from penetrating the soil disturbed by "prior anchors. The disturbed 

 area could cause a considerable reduction in holding capacity in a 

 sensitive seafloor soil. 



Plate Anchor Spacing. Plate anchors should be spaced according to 

 the relationships provided in Figures 55 and 56. 



Pile Anchor Spacing . A laroe amount- of data is avail able on the 

 behavior of single piles. The difficulty in applying required test load- 

 ings has resulted in little performance data for full scale pile groups. 

 This lack of group pile data has resulted in attempts to use single pile 

 data to predict group capacity. One approach has been to define a grouD 

 efficiency equal to the actual group capacity divided by the sum of 

 individual pile capacities. A typical efficiency formula is shown below. 

 This is known as the Converse-Labarre equation. 



- i nn v q D v (n-D(m) + (m-l)(n) (26) 



n - 1.00 - 57.3 F x 90mn 



where n = group efficiency 



D = oile diameter (m) 



b = pile spacing (m) 



m = number of rows 



n = number of columns 



The equation is based on overlapping of bulb stesses around piles which 

 causes a reduction of soil capacity. The Converse-Labarre eauation may _ 

 be used for approximate Drediction of friction pile group pull out caoaritv. 



Rough estimates of lateral load grouD efficiency may be obtained 

 from model tests (Poulous, 1975). No actual field data is available for 

 laterally loaded pile groups in soft seafloor- type materials. 



Practical considerations concerning spacing of driven piles were 

 stated by Terzaghi and Peck (1967); their observations appear applicable 

 to uplift or lateral resistina pile groups. They recommend that the dis- 

 tance between oile centers should not be less than 7h diameters. If 

 spacing is less then the heave of the soil is likely to be excessive and 

 the driving of each new pile may di solace or lift the adjacent Diles. On 

 the other hand, a spacing of qreater than four diameters is uneconomical 

 because it increases the cost of the pile cap without materially benefit- 

 ting the foundation/anchor. . 



For intermediate and final design of grouDS in particular soils a 

 detailed analysis usinq one of the rational techniques availab e (Terzaghi 

 and Peck, 1967; Focht and Koch, 1973; Murthy and Shrivastava, 1972) is 

 recommended. 



114 



