S[z] =
 ListPlot[
   {{0.3451, 0},
    {3.61504, 14.5525503047423}, {6.43385, 28.1359640888397},
    {8.20271, 35.8693997314435}, {10.36884, 40.0596469567131},
    {12.70524, 40.7344299897916}, {14.6349, 49.3205653991629},
    {16.64968, 47.236883162213}, {18.57934, 53.6723782396654},
    {20.42387, 60.1963170848129}, {23.15755, 61.0961774098441},
    {25.49395, 67.8907159035738}, {27.74521, 66.7281232290898},
    {29.35326, 74.270969135446}, {30.72483, 69.3890375468614},
    {32.89096, 84.4267811980949}, {34.0166, 84.5491651221964}},
   Joined -> True]

data =
  {{0.3451, 0},
   {3.61504, 14.5525503047423}, {6.43385, 28.1359640888397},
   {8.20271, 35.8693997314435}, {10.36884, 40.0596469567131},
   {12.70524, 40.7344299897916}, {14.6349, 49.3205653991629},
   {16.64968, 47.236883162213}, {18.57934, 53.6723782396654},
   {20.42387, 60.1963170848129}, {23.15755, 61.0961774098441},
   {25.49395, 67.8907159035738}, {27.74521, 66.7281232290898},
   {29.35326, 74.270969135446}, {30.72483, 69.3890375468614},
   {32.89096, 84.4267811980949}, {34.0166, 84.5491651221964}};

S = Interpolation[data];

Plot2525 = Plotmeasured =
  Plot[S[z], {z, First[data][[1]], Last[data][[1]]}]
Labeled[
  Rotate[Show[Plot2525, PlotRange -> All ], 270 Degree],
  {"Depth m", " Unit shaft friction (KPa)"},
  {Left, Top},
  RotateLabel -> True]

ListLinePlot[Reverse /@ data]